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W126 1990 500SE (ASR) & W126 1991 300SE
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Discussion Starter #1 (Edited)
I wonder if someone can help me with an ongoing issue on my 1990 500SE (M117.965). I've had the car for about 2 years, and I've always thought it had a lack of power compared to other W126s that I've driven. The two main issues are: the accelerator pedal has to travel quite a long way before the throttle opens, and the other is that when I try to floor the car from a stand still, the engine has an extreme hesitation, basically going nowhere, as if it wants to go but then the throttle cuts out, then back in and out etc.

I haven't driven the car much since owning it, and it spent a long time in a bodyshop having other bits done. The diagnosis from an experienced friend was that it has a misfire under heavy load.

To further complicate things, this car has ASR, which basically means that the throttle is not opened in the usual way, by a mechanical link between the pedal and throttle linkages, but instead, an actuator reads the position of the pedal/linkages and then instructs the throttle to open accordingly.

To cure the 'misfire' I have replaced the spark plugs (ones with resistors were fitted), ignition leads, distributor cap, rotor arm. The car does seem to idle a little more smoothly. A compression test showed that all the cylinders are well within acceptable limits.

So, this past week I've spent a lot of time under the hood, and realised that there is a mechanical adjustment that can be made for the throttle:
2625443


So, I think this particular linkage is different from those vehicles without ASR, but basically someone had lengthened this. Both screws had been moved from a previous position, that can be seen marked in green. I should have taken a picture after I adjusted it, but basically I moved both screws to shorten the linkage, and now the car has excellent pedal response - as soon as you touch the accelerator, the throttle opens, just as with other W126s I have driven.

So, that's the first problem solved...but after driving the car today, I realised why this had been adjusted. Now that the throttle responds as it should, I can reach the 'hesitation' point much sooner than before - by pressing the pedal down about 3/4 of the way. So now I would like some help to diagnose this problem!

I decided to test the kickdown, by letting the car rech 4th gear, then letting the revs drop a little, and then at around 40MPH flooring the accelerator. When I tried this, the car basically did nothing....as if the throttle just closes, and the car just floats along. I then thought it must be an issue with the kickdown switch, but I was able to simulate the same thing, without reaching the full travel of the pedal (and therefore not making contact with the kickdown switch). Could it just be that the car cannot take the sudden acceleration when it's in a high gear at low revs?

If I accelerate more gradually, the car has good power, and will accelerate well. I don't think it's a 'misfire', although I'm not sure. There is no smoke or backfiring. I'm think that the next natural step would be fuelling....checking injectors, fuel pressure regulator etc? But is there anything else I can try while driving, to try and get more information to diagnose the problem?

I think the kickdown switch could still be an issue, because if I manually select 1st gear before attempting to floor the car from a standstill, I have managed to spin the wheels, whereas trying this with the transmission in any other position means that the car is in 2nd gear when stationary and has to kickdown.

One thing I noticed when working on the engine is that the metering plate seems to be sitting at an angle:
2625444


This is the view from the side. Is it normal for that plate to be sitting like that - slanting down towards the windscreen? Or should it be sitting perfectly level? Just a thought.

Sorry for the long post, but I wanted to describe the problem as accurately and as thoroughly as possible, to help identify what's wrong. I really appreciate any advice you guys can offer!
 

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1989 560SEC, 1989 560SEL, 1995 E420
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You might want to follow the factory service manual steps to confirm all settings. I'd just start at the beginning and check everything. See section 073-121.
 

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1987 560 sel ,1990 560 sec,1988 6.0
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Throttle position sensor or air position sensor could be your culprit. They are 2 diffrent things This could be either bogging down your engine by fuel starvation or not enough air to keep a proper mixture. Or the mixture screw has been turned. But then you could smell gas in the exhaust.....and idle issues
31bzfwHw7xL._SL160_.jpg

The second issue is air flow sensor I posted a link that may help Air Flow Sensor Potentiometer Question - PeachParts Mercedes-Benz Forum happy hunting and good luck. Hope this helps.
 

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All my similar problems were traced back to the cruse control unit (image attached) the grease in the actuator may have gone hard and this screws up the system . I’ve known of good cars scrapped because the problem has never been diagnosed. The throttle ,ASR ,Cruse Control are unique to the last series built and with ABS are all interlinked. You have to remove the unit and it’s cable and disconnect the throttle rod. Open the throttle side up and inspect the series of gears inside , re grease (I used a lithium based grease) and re assemble. no need to open the other half where the cable enters.
I have written the whole problem up so it should be in the archives .
Raoul 3A0969D2-2701-491E-858E-CBF6151AA7A3.jpeg 5FDF5265-E050-4853-94E4-CED050E59DE6.jpeg 5FDF5265-E050-4853-94E4-CED050E59DE6.jpeg
 

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The images show The coupling to the throttle position sensor, remove plastic cap and unlock. The other is the late model cruise control. The big mistake MB made was to have no way of turning the system off. They fixed that for the next model ,the w140.
Raoul
 

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W126 1990 500SE (ASR) & W126 1991 300SE
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Discussion Starter #8
I would start out in first gear and change up manually and see how the engine behaves to eliminate any issues the trans may have.
I can confirm that having today tried this, the problem is not to do with the car changing gear - thank you for the tip!

All my similar problems were traced back to the cruse control unit (image attached) the grease in the actuator may have gone hard and this screws up the system . I’ve known of good cars scrapped because the problem has never been diagnosed. The throttle ,ASR ,Cruse Control are unique to the last series built and with ABS are all interlinked. You have to remove the unit and it’s cable and disconnect the throttle rod. Open the throttle side up and inspect the series of gears inside , re grease (I used a lithium based grease) and re assemble. no need to open the other half where the cable enters.
I have written the whole problem up so it should be in the archives .
Raoul View attachment 2625587 View attachment 2625588 View attachment 2625588
That's interesting! Thank you for the suggestion, I will check this out! So, are you suggesting, that when I have this problem, it could be the ASR pressing back against the throttle and closing it? I don't get the ASR light on (or any warning lights for that matter) when the problem occurs. Is the cruise control attempting to alter my pedal input?

For anyone curious about the long throttle rod that the ASR cars have, it's called a redundancy rod. When I first bought the car, and took it to a mechanic to diagnose the poor pedal response, he observed the linkages moving while the pedal was depressed, and found that at the front end of this long rod, it was not pushing down all the way to open the throttle. His diagnosis was that I needed a new rod lol. Turns out, this rod is only in place to provide a mechanical linkage from the pedal, so that if the actuator that opens the throttle breaks, you can still drive the car - but only with 3/4 of the throttle. I eliminated this rod from my diagnosis by removing it from the car and then going for a drive! I suppose it is a pretty easy mistake to make, for a mechanic who is not familiar with the cars, when he sees the pedal only opening the throttle 3/4 of the way - mind you, I took it to a mechanic who was supposed to know these cars, but that's another story.

Thanks again for this, I will dig out your original post and take a look.

Edit: Just had a thought - could I pull the cruise control relay to rule out this being the cause?
 

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Discussion Starter #9
The images show The coupling to the throttle position sensor, remove plastic cap and unlock. The other is the late model cruise control. The big mistake MB made was to have no way of turning the system off. They fixed that for the next model ,the w140.
Raoul
Ok, this answers my afterthought about cutting out the cruise control by pulling out the relay under the dash. :)
 

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Hello Martin
Yes the ASR/ Cruise Control is limiting the throttle because the system is getting the wrong info.
Second paragraph answer is yes. My son is Porsche trained an an Audi tech and he could not figure it out.
The car should start and run no problem .Select drive and the ASR box should should light up. I’m not sure why your ASR dose not light up.The car can be driven at no more than 70 km and the throttle pedal will feel unresponsive and heavy. After a while you may need the brakes or squeeze the throttle and the engine will stop. The steering will become very heavy and brakes next to useless.
its Worth persevering with as it’s a superb system and way ahead of its time.
Raoul
 

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1991 500SEC 55K mi. 1987 560SEC Now 153K mi. 2020
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Hey Martin,

In addition to the contributions above, I offer something from my direct experience of a couple of years ago.

I am unsure if this will assist or not, but here we go.

In 2004, on my NON-ASR 560SEC, finding that the throttle would not open fully when accel pedal was floored, I adjusted a similar link to achieve full throttle valve opening.
WOT Nirvana!

More recently, on my '91 500SEC [Euro with ASR], I had a situation where the otherwise perfectly running car would refuse to take full throttle.
This manifested also as the car would have extreme difficulty maintaining highway speeds if any hill was involved.

If I eased into the throttle, I could get to say 70 mph or so, but if I stepped into it at all at that speed it was as if the engine had been shut OFF, come back ON, then go OFF within seconds.
This condition was repeatable at regular road speed if the engine was pressed to perform. It acted as if the key was shut off multiple times during acceleration, only returning to normal if I drove like an 'aunt Bea'. It was a violent transition from full ON to full OFF, to say the least, and very unsettling to me as I thought the ASR had packed it in [and then what would I do?].

It turned out that one of the fuel pumps had failed, and when demand for fuel was high, the engine just died, pulsating from full ON to full OFF... Demand LOW, all is well once again.

The pump & fuel filter were replaced and I can say the car has been silky smooth and a willing road partner for the last 15,000 miles.

I am unsure if your situation is exactly the same, but on the face of it, there appears similarity.

Stay safe man,

M
 

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Discussion Starter #12 (Edited)
Just to clarify on the ASR light - the light does work when needed, so when the ASR cuts in during normal operation, the light does come on when it should. But when I experience this issue with the loss of power, the ASR light does not come on.

Wow! I really have so many suggestions to get me going!! I must say that MBL's issue seems to display the closest symptoms to my own...actually EXACTLY the same symptoms.

The fuel filter, I did change recently, so there is definitely no issue with that. I had wondered about the fuel pumps earlier, and I think I looked into testing it, but couldn't find the information I wanted. Also, to replace one of those is pretty expensive (and we have two on here), so it wasn't a part that I was prepared to buy unless I was sure it was the culprit.

What I'd like to understand, is how they actually work. Does fuel get routed through both pumps during normal operation, or only when more fuel is needed? Could I disconnect one of the pumps and see how the car runs, then switch it over with the other one, and see how it runs?

Thanks a lot to everyone!
 

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1991 500SEC 55K mi. 1987 560SEC Now 153K mi. 2020
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Hey Martin,

Let me suggest then, given this additional information, that yours is likely, if not very likely, a fuel supply problem as mine was.

Fuel strainer in the tank*, upstream of the pumps.. or insufficient fuel delivery from one or both pumps [M3 below].
Possible failure of the Fuel Accumulator [57 below] diaphragm bleeding off pressure into fuel return..**

According to the MBCDs on the subject, a single fuel pump should deliver 1 liter in a maximum of 40 seconds.
Again, from the manual, a fuel pump should deliver between 2 & 4 bar pressure. If below 2 bar, the fuel pump needs replacement.

Martin, the fuel pumps are routed in series. I 'spose one could disconnect a pump and see if the symptoms got worse... The theory being if there were no change in performance, you might have found the culprit. If there were a huge change in operation, you found the best one. That said it seems kinda hit or miss to me as without checking fuel delivery/pressures it's not an absolute resolution of the root cause. I mean it is possible that one pump is bad and the other is marginal, you know?
At least that's what is so in my little mind here.

In addition to the above, when you changed the fuel filter was there a buffer/insulator sleeve between the filter & its mounting bracket?
Some sort of insulation is very important between the dissimilar metals to prevent a corrosion/premature failure in the fuel pump package.

M

* Personally, I have not had an issue with an in-tank fuel strainer in either of my SEC cars during 17 years & 110,000 miles of 126 bliss.

** I highly doubt this to be the case as you do not cite extreme difficulty with a warm/hot engine restart condition.


Screen Shot 2020-03-28 at 7.47.36 AM.png


IMG_4376.JPG
 

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Kind of a hassle but you could run a set of wires from each pump to a couple bulbs in the cockpit and monitor when and if each pump quit running.
 

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There is a ball at the end of the throttle linkage and if it's gone (broken), you cannot get to full throttle. Simple to look and see if it's in place and in good condition.
2625686
 

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When trouble started the first thing I did was to replace both fuel pumps. No change. But now i can rule that out. It’s like I have had this bad idle problem I’m working through , turned out it was a cracked ceramic on the spark plug. I only picked it up from small puffs of ceramic coming from the leed. But fuel pumps are good. Pull the trigger and get a new set.
 

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Discussion Starter #17 (Edited)
There is a ball at the end of the throttle linkage and if it's gone (broken), you cannot get to full throttle. Simple to look and see if it's in place and in good condition.
Ok, first things first...my balls are good. :) Actually that linkage in your photo is substituted on my car with the one from the first photo in my original post (perhaps this is normal for ASR cars). But there are other linkages that have a ball and socket, and I removed and lubricated them all a few months back, and they're all ok - thanks for the suggestion, it's definitely maintenance that gets overlooked.

Pull the trigger and get a new set.
That would be my normal strategy, but I checked the price with MB a few months back and it was £270 for ONE. If I can pick up the correct ones from somewhere else, I might just do that. I've seen Bosch ones for around £90, which is definitely more acceptable.

In addition to the above, when you changed the fuel filter was there a buffer/insulator sleeve between the filter & its mounting bracket?
I took some photos at the time:
2625717
2625718

I don't see any sleeve where you mention. I'll definitely see if I can get one! The pumps do have their insulation in place at least. Could the lack of a protective sleeve on the filter have caused these issues? It also looks like one of the fuel pumps is newer than the other. And the fuel accumulator looks like it's been there at least since the car was built, if not before.😁 But, as you say, no issues with hot start - in fact no other issues or symptoms of any kind really.

Thank you so much for the worksheets and diagram! The short version is that I'm getting plenty of petrol through, but I don't have the equipment to check the pressure at the pumps. :( The long version follows, especially for anyone else who needs to attempt the same diagnostic!

The diagram was really useful for me to get my head around how the fuelling works. I did not have a clear idea of how fuel gets routed around the car, and it's great to see it laid out like that. Perhaps I should look through the manuals more often huh? So, as you mentioned, the pumps are connected in parallel, which I suppose means that both of them are supposed to be pumping at all times, as opposed to one cutting in when needed? Wouldn't it have been better to have one large fuel pump? I suppose if a pump is able to fail, and still allow fuel to pass through it, then it has backup value. So, fuel is drawn out of the tank and then pumped to the accumulator, and also through the filter. I remember you telling me recently that the accumulator maintains pressure in the system for hot starts. So, from there the fuel goes to the distributor. At that point, I don't have the knowledge to follow the diagram fully, because there are a lot of devices leading off that I never knew existed! 😊 We could really do with a sticky that explains all the components, and goes over all the routing, because having this understanding makes it easier for people to diagnose a problem, and also to know exactly what you more experienced members are talking about, without having to keep asking! Now all I want to do is ask 'What does this do? What does that do? hahaha. But still, I was able to recognise that petrol flows back to the tank via the fuel pressure regulator and a cooler. I didn't realise that there was a constant loop, I just thought petrol was pumped to the engine and used as it was pumped. Then there's another exit from the tank, going to a purge valve? Ok ok, I could write a book of questions, but I'll try to stick to the point!

So, I tried to work my way through the instructions you sent me. First step is to test the power consumption on the Fuel Pump Relay. I was glad to have an excuse to use my new toy - a multimeter:
2625720

I noticed it says on the front, that the multimeter is fused up to 10A! That's cutting it a bit fine really. Is that normal for a multimeter? Anyway, this should be between 6A & 10A and it sat at around 7.95A - 8.1A.

Ok, step 2 is to check the pump delivery rate. I just need 500mm of hose, a pipe with a sealing flare, and a union nut. Yeah, because I just happened to have all that in my pocket. 🤣 Well, I don't know if it's advisable, but I decided to take a 14mm hose and cable tie it around the connection like so:
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2625723
2625724

I hoped that the upward path of the hose wouldn't be an issue, but I knew it would have been better to route it downwards, it just wasn't that convenient.

I noticed that when getting the amperage with the multimeter, it had the effect of jumping the 7 & 8 pins and started the pump running, so I basically used the same method to jump the pins. So, fuel seemed to come out at a pretty good pace, but once the fuel filled the hose, and began filling the jug, I noticed a leak occuring and stopped the pump straight away. All the fuel left between the pressure regulator and the top of the curved pipe then began to leak back through the pressure regulator!!
2625725

So, it seemed to be leaking from the diaphragm. I checked around to see if it was leaking from somewhere else (A or B for example), and just dripping off the FPR from the lowest point, but no. It didn't leak from my connected hose.

I was confused at this point, because I was pretty certain there was no leak coming from there under normal circumstances. I was worried that perhaps I had loosened something when I cracked the return line nut. The only other explanation, was that perhaps the upward pipe direction was creating a back pressure on the FPR and causing it to leak. 2nd attempt:
2625726

2625727

This was as level as I could get it. Started up the pump and it didn't leak from the FPR! I had my 1 litre of fuel in about 28 seconds, so, well within the 40 second limit. I put everything back and ran the engine - no leak from the FPR. I'm surprised, but apparently it was the back pressure of the rising fluid that caused the leak. Unless someone has a better idea? Apparently, if the required volume of fuel is achieved, this rules out a problem with the tank strainer.

So, the next step is to test the Fuel Pressure at the pump. The good news is that the way this testing is described, it makes it very simple to find out, in one step, if a pump is defective, and also exactly which one it is! The bad news is that I cannot 'fabricate' a bloody gauge to read the pressure. This is not an episode of the A-Team! Plus, there is no chance of contacting Mercedes to borrow or buy a tool, because they're all shut. Is there a general automotive tester for this kind of thing? How will I know which fitting the tester will need to go on the pump? Didn't the first test prove that the pumps were working? Or did it just prove the fuel pressure regulator was working?

Ok, so that's where we're at. Thanks again to everyone for their kind assistance. I haven't ruled anything out yet - one or more of the suggestions here could be responsible!

Oh, there is something else that I should mention, as it could well be relevant. I have an after market alarm that was fitted by the previous owner (3-4 years ago). I press the fob, and it unlocks all the doors and removes the immobiliser. If you have not started the car within 30sec (happens quite often, especially if I happen to turn the car off while I'm sitting in it, and then later on go to start it), then you must insert and turn the key to position 2, press the fob again, and this gives you another 30sec to start the car. As we all know, when turning the key to position 2 the fuel pump will engage for 1-2sec and then stop. In the circumstances above, the fuel pump does not start when the key is turned, but only once the fob is pressed, it will then continue to run for 30 seconds!! I can understand the pump not coming on if the car is immobilised, but the pump staying on? Does anyone else have an alarm wired like this? It's the first car alarm I've had, so I have no idea.
 

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1991 500SEC 55K mi. 1987 560SEC Now 153K mi. 2020
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Hey Martin,

What a nice & most detailed write-up!

I am unsure if you did this or not [fuzzy here as I have been up since 3AM or so.. Can't sleep Uff]

The fuel volume spec I mentioned is for EACH pump, one at a time. Please forgive me if I misunderstood/or was unclear.

You are correct: strainer seems to be good.

As far as a gauge goes, maybe Amazon UK? Amazon.co.uk: Fuel Injection Pump Tester
There seems to be a good selection on the site...

Good luck.

*If your car is acting as I described mine was, the fuel thing has to be pretty blatant, one would think.
I mean to say, mine was really just horrible when pressed for any kind of fun acceleration, and one had to drive as if a raw egg was between the foot and the accel pedal to maintain smooth running on the highway, especially if a hill was encountered. Exceed those parameters, and the engine would act exactly as though the key was shut off.

Stay safe man, these are unprecedented times..

M
 

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Discussion Starter #19 (Edited)
The fuel volume spec I mentioned is for EACH pump, one at a time. Please forgive me if I misunderstood/or was unclear.
You mean that each pump should deliver 1 litre in 40sec? But I followed the manual, and there was nothing about removing or disabling a pump during the test, and they quote the same numbers as you.

As far as a gauge goes, maybe Amazon UK? Amazon.co.uk: Fuel Injection Pump Tester
There seems to be a good selection on the site...
Thanks for that I didn't even think of looking there. I've got one on the way!

Stay safe too!!

And if that huge write up didn't send you off to sleep, nothing will! 😁
 

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Discussion Starter #20 (Edited)
So, continuing on with this issue, I got two new fuel pumps and this testing kit from Amazon. Although I could just change the pumps, I'd rather go through the testing, just for my own curiosity. So, the only combination of adaptors in that kit that would go from the pump to the gauge were these:
2626697

Actually it was quite convenient having a new pump to test the connections with, before attempting to fit the gauge on the car. Here is what it looked like:
2626698

2626699

2626700

When I started the pumps, fuel sprayed out of the first connection point (red arrow). I had tightened it a lot, but just to be sure, I tightened it as much as I physically could! Fuel still sprayed out, and this wasn't some dripping, it was a huge spray. So, what's the deal? Surely there should be O-rings or washers or something? But then there is no washer in place normally. The filter has copper washers, so why not the pumps? Unless I missed a trick, and there is another way to hook up this tester, it will be going back!

Ok, so the only other thing I could do, was to test the voltage on the pumps. The manual says it should be 11.5V minimum - and this was during a test in which the engine is off, and the fuel pump relay has been jumped using pins 7 & 8. So, I tested the voltages with the pins jumped and they were 11.3V & 11.4V. I also tested them with the engine running, and the readings were 12.9V & 13V. But the manual seems clear about 11.5V, so should I be concerned about this?

I then went on to disconnect one of the pumps, i.e. I disconnected the wires, not the fuel line. The engine started and ran Tried the same thing with only the other pump connected - engine started and ran. No detectable difference between those two setups, although I did not drive the car.

I was worried about getting the acorn nut back on, without the pump leaking. I put it on as tightly as I thought possible, but there was a slight drip with the engine running. So, I heaved on it some more, until I eventually managed to get it tight enough to stop any leak at all. I get quite nervous under there, because the fuel package is on a rubber mount, which doesn't lend itself to things being tightened up. It flexes, even when I use two spanners...one to hold the unit and one to tighten.

All of this put me off changing the pumps. I get concerned about things not going back properly if they haven't been moved for decades. At least with the filter, you can fit new crush washers, and feel that it will make a secure connection. If I can't get things to go back on, the car will be out of action until a new part eventually shows up (if one is available). Plus, for the type of severe issue I have with acceleration, I was expecting a chronic fault with a pump, like one of them not working at all. But this doesn't seem to be the case.

Ok, hopefully someone can answer these questions for me! With the tests that I've carried out so far:

  • Can I rule out the Fuel Pressure Regulator being at fault?
  • Can I rule out the pumps?
  • How about those voltages?
  • MBL87560SEC, what is the origin of that sheet you provided? When I look at my own copy of the workshop manual, my 07.3-130 looks very different from the one you posted! I don't even remember where I got mine from, I think it was a CD from eBay years ago. The pdf is entitled 'W126 - Engine m116 v8 - 380se 500se sec' but I don't know if this was the original name, or if I renamed it. EDIT: Ok, my engine is the 117, not the 116 - that's why they're different! 😊
So, the plan now is to rule out fueling...I think I can rule out ignition, although I have not tested the coil, and that leaves air. I also want to go back and try flosshilde's suggestions, but I am trying to work through this as methodically as possible.
 
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