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Just inherited an 88 420SEL that's been sitting for 6 years, just replaced the fuel pumps and filter. It was running fine, smooth idle and temperature fine. I went to drive it to get more gas because it was completely drained besides the 2.5 gallons I put in. Then when I'm leaving it starts to stumble , and at a stop it has little power to take off, by the time I made it back it was smoking from the exhaust and black smoking on acceleration. I was at my uncle's and decided to make a run for my house across town and I made it but just by a hair, after heating up it was bogging down at every stop, had to get to at least 2k rpms to get it to move. I'm new here and I really need some advice on what to change/check to fix this !! (I also put in regular gas not knowing it took premium till I looked at the cluster, don't know if that's a problem) I also just put in new spark plugs
 

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W126 1990 500SE (ASR) & W126 1991 300SE
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Welcome to the forum Zack

I think black smoke means the mixture has too much fuel/not enough air. Easiest thing to check should be the air filter box, and that the air intakes at the front of the car are not blocked. On your engine, these are the two big plastic pipes running from the air box towards the front of the car, and they open up at the front grille just inside each headlamp.

When you changed the fuel pumps and filter, did you also change the fuel pressure regulator? It's often called the accumulator, and is part of that same package housed with the pump(s) & filter. Not that it neccesarily should need changing, but In theory, it could be another cause if it's not connected or not working properly
 

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1991 500SEC sub 50K mi. 1987 560SEC Now 150K mi. 2018
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Hey Zack,

Welcome aboard BW!!

How frustrating to have all those issues straight off!

I had had some rodent activity in my 560 air filter housing over one winter. It was a mess in there.. The mice came up the air intakes Martin refers to.. Built a big nest and chewed on the filter media.. I can envision a mouse nest up in there, definitely worth a look.

As far as accumulator and fuel pressure regulator. The accumulator is located in the fuel pump package. It is purely a mechanical design and its main function is to maintain sufficient fuel pressure in the system during a hot/warm start condition... [meaning somewhat less than an hour between starts]. The fact it started after the fuel purchase indicates the accumulator is functioning properly.

The fuel pressure regulator resides up near the fuel distributor on the engine. In that the engine idled & ran smoothly would indicate this component is likely to be OK at this point.

Keep us posted on what you find & good luck.

MBL
 

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W126 1990 500SE (ASR) & W126 1991 300SE
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As far as accumulator and fuel pressure regulator. The accumulator is located in the fuel pump package. It is purely a mechanical design and its main function is to maintain sufficient fuel pressure in the system during a hot/warm start condition... [meaning somewhat less than an hour between starts]. The fact it started after the fuel purchase indicates the accumulator is functioning properly.

The fuel pressure regulator resides up near the fuel distributor on the engine. In that the engine idled & ran smoothly would indicate this component is likely to be OK at this point.
Ahhhh, I didn't know that! I thought they were both the same thing referred to by different names. Here's what comes up on Google:

Fuel Pressure Regulator
Accumulator

I think my confusion could be down to the fact they are similar looking? Or maybe both searches bring up photos of both items. Thanks for the clarification though - it will be helpful when I'm trying to fix my own fuelling issues!
 

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350SDL, '17 GLS450, "Grandpa's Roadster" Project Car
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I have seen this (red hot exhaust) when there were leaky injectors. The fuel makes it to the cat and combusts there. Be careful! You can set the underside of the car on fire. (Don't ask me how I know this...)

There should be a heat shield, but they do rust out, especially if the exhaust is red hot...
 

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1990 420 SEC
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If you attempt to fix the car DIY style this would be worth reading:

When engine warm you could try disconnecting the cable to EHA (a black box at the side of fuel distributor) and see how the car behaves. If it works better there is something wrong with electric systems. If it works worse then there is a mechanical fault. To start searching for mechanical fault you have to find out how to jump fuel pump relay and undo the lines from fuel distributor to injectors (without burning the car). When fuel pump is running no fuel should come out from distributor ports. Even amount of fuel should come out from all ports as air meter plate is deflected and fuel pumps are running.
 

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Discussion Starter #9
Wow I love this forum already, after work today I will check it out !! Thanks guys I will definitely keep you posted. Btw this is all diy.
 

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1985 MB 500sel (Euro)
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I hesitate to suggest this because Zack has probably already done so and probably also knows the car's history, but after six years of sitting in a garage somewhere, the first thing I would do would be to find an experienced W126 mechanic and give the whole car a once over. Or, if you are an intrepid DIYer (I am not) also read everything you can about Lambda adjustment as Heikkif suggests, including H.D.'s extensive tutorial.

Oh,and welcome to the forum.
 

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1988 560SEL
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It does sound like a very rich fuel problem. Can you check your plug condition?
 

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Discussion Starter #12
It does sound like a very rich fuel problem. Can you check your plug condition?
Checked the plugs, the wires are corroded inside which also is a problem but as far as I can tell no misfires have occured . Need to order a new set
 

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Discussion Starter #13
If you attempt to fix the car DIY style this would be worth reading:

When engine warm you could try disconnecting the cable to EHA (a black box at the side of fuel distributor) and see how the car behaves. If it works better there is something wrong with electric systems. If it works worse then there is a mechanical fault. To start searching for mechanical fault you have to find out how to jump fuel pump relay and undo the lines from fuel distributor to injectors (without burning the car). When fuel pump is running no fuel should come out from distributor ports. Even amount of fuel should come out from all ports as air meter plate is deflected and fuel pumps are running.
Is it bad that when the engine is at idle and I press that plate down it kills the engine?? Even the slightest push kills it.
 

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I have seen this (red hot exhaust) when there were leaky injectors. The fuel makes it to the cat and combusts there. Be careful! You can set the underside of the car on fire. (Don't ask me how I know this...)

There should be a heat shield, but they do rust out, especially if the exhaust is red hot...
What would I be looking at doing/checking on to fix this? Btw engine light isn't on but does work could there be a fault or does it not read very much on the system?
 

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What would I be looking at doing/checking on to fix this?
With hard injector lines it's tough to diagnose. First thing I'd do is pull the spark plugs and see if you can ID the cylinder(s) causing the problem. Look for a black, maybe wet plug. If you can find that, pull that injector. If you have another one, stick it in. Otherwise, you're probably going to have to pull all the injectors and have them tested.

All of the cars I worked on back in the day had flexible lines. We used to pull the injectors out of the manifold and put them into glass bottles, then pull the air flow plate up (down in your case) and eyeball the spray patterns. I suspect you'll find one or more that are streaming instead of spraying, maybe even without moving the plate.
 

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1990 420 SEC
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Pushing down the measuring plate kills the engine if it already runs rich no doubt. Why it runs rich is the question. There is the mechanical side of the system and electrical. Mechanical side of the system when in order is able to run the car so and so when warm but better that yours without any aid from electrical system. Did you try disconnecting EHA cable? When it is disconnected mechanical system works without aid or disturbance from electrical system. On mechanical side there is pressure regulator, fuel pumps, fuel distributor, EHA-valve and fuel lines from pumps and from regulator to tank and injectors. To test fuel distributor you could jump fuel pump relay so that pumps run and check that no fuel comes from distributor ports to injectors when measuring plate is not deflected. When plate is deflected same amount of fuel should come from every port.
 

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Agreed: if you push on the plate you're adding fuel without opening the throttle. If you already have a couple of cylinders running way rich (not running) and you enrich the others, it will stall.

I don't think the system is running too rich so much as one or more cylinders are just dumping fuel. The amount of fuel necessary to get the converter to glow red is too much to keep the engine running, so, IMHO, at has to be an issue with one or a couple of cylinders, not the whole metering system.
 

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1977 450 SEL 6.9
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Careful with all this dumping of fuel into the cylinders while testing. You can easily hydrolock the motor which can very easily bend a rod resulting in it being landfill.
 

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Discussion Starter #19
So I ran the car today and on start up it ran fine, still black smoking but about 5 min in the exhaust started spitting out black, pretty sure fuel is just being streamed in there , still need to check each injectors.
 

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Pushing down the measuring plate kills the engine if it already runs rich no doubt. Why it runs rich is the question. There is the mechanical side of the system and electrical. Mechanical side of the system when in order is able to run the car so and so when warm but better that yours without any aid from electrical system. Did you try disconnecting EHA cable? When it is disconnected mechanical system works without aid or disturbance from electrical system. On mechanical side there is pressure regulator, fuel pumps, fuel distributor, EHA-valve and fuel lines from pumps and from regulator to tank and injectors. To test fuel distributor you could jump fuel pump relay so that pumps run and check that no fuel comes from distributor ports to injectors when measuring plate is not deflected. When plate is deflected same amount of fuel should come from every port.
I unplugged the eha and it was still spitting out black liquid (pretty sure it's all the fuel) and thanks for the heads up I'll check the injectors here soon
 
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