Hi folks,

I am trying to do a basic fuel pressure differential test on my '91 560 (KE Jetronic). I have the inexpensive Harbor Freight master fuel injection pressure test kit. I also have the factory service manual; this is part of test job 07.3 - 120, which I think many here know well. I'm hoping someone can comment on a) whether I'm thinking about this test correctly and b) whether I'm hooking the test kit up correctly. I did do some searches but didn't quite come up with the answer.

Based on the searching I did, I know the Harbor Freight kit I bought has decidedly mixed reviews. This thread indicates it works OK when connected directly to upper/lower chamber; this thread indicates it's really problematic. The first time I used my own kit, I had reason to be skeptical - the air bleed valve seems to spew gas everywhere when operated. Unfortunately I think I bought this from HF a while ago, so I'm stuck with it.

A) Concept: Essentially we're testing system pressure and working (lower chamber) pressure, and then calculating the delta. System pressure must be high enough (6.2-6.4 bar); if not, check fuel pump, lines, pressure regulator. Then the delta between system and working (lower chamber) pressure should be 0.4 bar lower than system pressure, testing at idle with a warm engine. If delta is off, adjust EHA and check for its proper functioning.

B) Connections: With the HF kit, you get a set of lines with a ball valve in it, designed for CIS systems. I think the way this operates on the HF kit is: with the valve closed, the gauge will read the pressure from line A only. With the valve open, both lines A and B feed the gauge. So, I'm assuming A should be hooked up to the lower chamber via the lower chamber test port on the fuel distributor, since this will be the lower pressure; close the valve and it should read lower chamber pressure. I'm assuming B gets hooked up to the upper chamber via the cold start injector line, since that's the higher pressure; with the valve open, that higher pressure would be what the gauge reads.

View attachment 2629232

I'm also wondering if this fitting actually isn't right for CIS-E, if my assumptions about its design are right. E.g. maybe I don't use the valve at all and simply do two tests: one with the valve hooked up directly to the upper chamber, then a separate test with it hooked up directly to the lower chamber (no valve).

Input appreciated!

Thanks,
Robert

Appreciate the headsup to that negative review by me of the HF gauge set. I've updated that thread since I figured out how to make the gauge work.

Necessary mods: I had to replace the oring inside the air bleed valve. It was cut! Now that works fine. I also had to remove the Schrader valves at each connection. They weren't working properly and would cause inconsistent readings.

A) Yes, you're testing delta between system and control (lower chamber) pressure. But I don't think you want to jump right to adjusting the EHA until you've proved all other components are functioning properly. Same idea as not wanting to go right to adjusting the lambda screw.

B) Correct.

I think you can take the measurements as you state in your last paragraph but I still want to recheck system pressure and delta under various conditions and the ball valve makes that super easy.

 

@luckymike, thanks for the thoughtful reply!

I will have a closer look at the bleed valve assembly. I bet mine has a similar problem to what you saw with yours, since it spews fuel everywhere. I think replacing the schrader valves is beyond me though.

It sounds like you're also saying the valve assembly from HF can be used, even if it's a bit different from the specialized valve in the service manual. I'm not totally sure, but it looks like the M-B one is an actual switch, i.e. choosing between input A or B exclusively, whereas the HF valve is more like A+B or A exclusively.

I'll have a look at modifying the bleed valve, will give the test another go, and report back. And yes, definitely aware that there are some other variables here besides EHA.

Appreciate the input as always!

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@luckymike, thanks for the thoughtful reply!

I will have a closer look at the bleed valve assembly. I bet mine has a similar problem to what you saw with yours, since it spews fuel everywhere. I think replacing the schrader valves is beyond me though.

It sounds like you're also saying the valve assembly from HF can be used, even if it's a bit different from the specialized valve in the service manual. I'm not totally sure, but it looks like the M-B one is an actual switch, i.e. choosing between input A or B exclusively, whereas the HF valve is more like A+B or A exclusively.

I'll have a look at modifying the bleed valve, will give the test another go, and report back. And yes, definitely aware that there are some other variables here besides EHA.

Appreciate the input as always!

Sent from my moto x4 using Tapatalk

Removing (not replacing) the Schrader valves is not beyond you! Unless you're kidding. You just need a tire valve remover. Something like this:

You're just unscrewing the valve core, like you would on a tire valve. Piece of cake.

The HF version can totally be used. Yes, it's A+B but since B overrides A, it's like A or B anyway. The MB version is overkill.

 

Would someone please post clear photos of the connection locations for the system and control pressure readings. The factory manuals are such low resolution I cannot figure it out (380SEC).

I have the HF kit as well, and I think it a adequate for the purpose, noting luckymike's remarks.

 

Here you go, nugent. My thumb is pointing to the lower chamber connection and my finger is pointing to the upper, or system pressure connection (this is the cold start injector line that RobertD mentions). As the manual states, be careful removing the lower chamber plug since your wrench can damage the plastic potentiometer mounted nearby.

 

Maybe a foolish question but with the gauges being relatively cheap, why not just two gauges and use them over a range of RP, note both, then calculate the differences.

 

@luckymike - Ah, I didn't realize removing the valves was that easy. Thought it might require special hydraulic line tools.

@nugent - Happy to post clear pics. My car is a gen 2 560, and I believe the gen 1 cars like your 380 may be a bit different. The upper chamber is the same - connect to the port for the cold start valve (gen 2) / warm up regulator (gen 1). But I think I picked up the gen 1 cars don't have a lower chamber test port? Perhaps someone else can clarify.

@Stutz - good idea. I guess the only risk would be some inaccuracy between the two gauges, if they were different brands. Solid idea regardless.

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@luckymike, could you share more details on how you fixed / improved the HF bleeder fitting? I tried again with the set today and that bleeder device is even worse than before, with gas streaming down the vent tube even after fiddling with the bleed valve. Appreciate you sharing your experiences!

Cheers,
Robert

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@luckymike, could you share more details on how you fixed / improved the HF bleeder fitting? I tried again with the set today and that bleeder device is even worse than before, with gas streaming down the vent tube even after fiddling with the bleed valve. Appreciate you sharing your experiences!

Cheers,
Robert

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Robert, it's been years since I dismantled that bleeder valve. I remember breaking off the push button while learning it had to be unscrewed to dismantle the valve. I'm pretty sure all I did was replace the oring or orings inside. There's a shuttle valve, like in a brake master cylilnder, I'd say. It had at least one cut oring on it. I'm pretty sure that's all there was to the repair.

We are talking about the push button leaking, right? When you say 'after fiddling with the bleed valve', have you dismantled it?

And did you unscrew all the Schrader valves?

 

I'm following up with where I landed on this, in the hopes it'll help someone in the future. Thanks again, @luckymike, for the modification ideas. Details on my mods and the testing process (on a '91 560) below.

When it comes to the Harbor Freight kit in short, I'd have to conclude it's not a great solution, given that at least two of us here have had to significantly re-engineer it for proper functionality. Now that I've done the re-engineering however, it worked fine for the testing, including the A / A+B valve.

Modifications to the kit:

• Per LuckyMike's suggestions, I removed the Schrader valves from the fittings I would need to use. Indeed, a simple valve stem tool fits in over the valve and you can easily unscrew them. Here's a pic of one fitting, the removed Schrader valve, and the tool I got:
  
• If you remove the Schrader valves, you must also remove the valve in the quick-couple fittings on the hoses that you plan to use. (The tip of the Schrader valves opens the valve in the quick-couple fittings.) This is easy - you just need two 14mm wrenches to undo the tip of the fitting, and then the valve comes right out. The pic below shows the valve removed, and the red arrow points to where you undo the tip of the fitting.
  
• Now, in my case, the additional problem was that the bleed valve unit in the HF kit was malfunctioning. Unfortunately fitting a bigger o-ring did not solve the issue; in my case the valve itself was defective and would not close completely, allowing gas out under pressure and also preventing an accurate reading. My solution to this was to buy a different fuel pressure gauge with a built-in bleed valve - an Actron CP7818, which appears identical to the Bosch 7818 but likely with cheaper components (~$30). Then, I removed the supply hose from the original HF bleed valve and replaced the Actron supply hose with it (same fitting to the gauge). What this does is enable the Actron gauge to work with all the fittings in the HF kit. It's a simple matter of unbolting the hoses and swapping them. The Actron gauge seems good enough, but the one disadvantage I see is that the needle is kinda thick and the gauge kinda small, making very fine measurements a challenge. The result of my mods:
  

For the test itself:

• Since we're working with fuel under pressure and the manual says to do this with the engine at operating temp, get your fire extinguisher ready. Also have a stack of absorbent rags to place under the fittings as you work with them and connect/disconnect. Since all the Schrader and check valves are removed, gas will leak out whenever you disconnect.
• Disconnect the electrical connection to the EHA.
• Disconnect the fuel line to the cold start injector, at the injector itself. (You can also do this at the fuel distributor, but I prefer not messing with those fittings.) Note that there will be fuel pressure present unless you've bled it some other way, so have a rag underneath / around the fitting when you break it free. Then, attach fitting #20 from the HF kit to the line and snug it down. This is where you measure system (supply) pressure from.
  
• Remove the bolt that plugs the test port on the lower chamber of the fuel distributor, being careful not to damage the potentiometer on the side of the fuel distributor with the wrench. There will be fuel present here too, so place a rag underneath. Then, attach fitting #17 from the HF kit to the test port and snug it down. This is where you measure control (working) pressure from.
  
• Then, attach the A / A+B hoses and valve from the HF kit to the fittings. The hose end that includes the shut-off valve (B in the pic below) is for system pressure and attaches to the cold start feed. The hose end that does not pass through the shut-off valve (A in the pic below) is for control pressure and attaches to the test port. Make sure the quick couplers are firmly seated. Then attach the gauge (with the HF bleed contraption if yours isn't busted) to the middle coupling. The hose + valve enables you to switch between reading control pressure (A only, B shut off) and system pressure (A+B, but since B is the higher pressure, it wins out and you essentially get a reading of B).
  
• With the hose valve open (A+B), turn the ignition to '2' a couple times to build up a bit of pressure without starting the engine. Check for leaks / spraying gas at all your connections and address. Also, there's enough pressure that you can then use the bleed valve to bleed off any air, making sure the bleed relief tube (clear plastic) drains to a suitable container for gas.
• Then, close the hose valve (A only going to the gauge) and start the engine. Let the engine stabilize. Note that your Check Engine light will come on, since you have the EHA disconnected. Double check you don't have gas spraying / leaking anywhere! Once everything appears safe and stable, take readings.
• With the valve closed (A only going to the gauge), you'll get control pressure (the lower of the two readings). With the valve opened (A+B going to the gauge), you'll get system pressure (the higher of the two readings). The spec is 0.4 bar difference between the two, i.e., (system - control) = 0.4. (Note that when you open the valve on the hose to read system pressure, the engine will bog down as you're overwhelming the control pressure via the test port.)

My own results:

• There are several things that can influence the pressure differential here, and I'll leave those to other threads. My main goals for my situation were to validate that my system pressure is high enough, and then adjust a new-to-me EHA valve to hit the 0.4 bar difference.
• System pressure on my car was spot on spec at 6.4 bar. BTW, just out of curiosity, I also read system pressure on my car without the hose + valve contraption, i.e. directly from the cold start line with no other equipment involved. I got exactly the same reading as with the hose valve in A+B position, so that validates that the HF hose valve contraption works fine for this.
  
• Control pressure on my car originally read too high, at a bit under 6.2 bar, say 6.18. The spec I'm shooting for is a delta of 0.4 bar. I believe my initial reading means my car was running lean.
  
• Then, I adjusted my EHA. It ended up being about 2/3 of a rotation clockwise on the adjustment nut to bring it into spec. I adjusted it a little and then checked, then adjusted some more and re-checked. Eventually I hit 6.0 bar control pressure, which was the magic number for 0.4 bar delta.
  
• That's it for the testing. When you're done, bleed what fuel you can via the bleed valve - this will release the majority of the pressure. But remember, there's still fuel in those hoses, so be thoughtful with rags and such as you disconnect everything. With the Schrader and check valves gone, there is some mess...but in the end, it's better since you can actually drain all of the fuel out of the hoses before packing the kit away.
• Don't forget to reconnect your EHA and reset your Check Engine light.

And that's that, at least until someone speaks up and reveals that I've made a huge mistake here. And if that is the case, please do speak up! This was a satisfying project, mainly because there are a number of diagnostics that require fuel pressure measurements, and this was my 101 course in it. I hope I passed.

Thanks to all again who helped with this. Such a great community here.

 

Okay, nicely done, RobertD. Really thorough and will be super helpful for others ready to tackle checking their own fuel pressures.

I didn't know you could attach at the end of the cold start valve line. Good job.

Apologies, it's been long enough that I forgot about having to remove the little valves in addition to the Schraders.

 

I have a question concerning an odd issue I'm having with system and working pressure on a 1991 560SEL. I am confident the gauge assembly is connected properly as described here, but I continue to get a system pressure reading of 4.48 bar and a working pressure of 4.20 bar. After originally finding these readings, clamping the return line generated approximately 10 bar, so a new diaphragm pressure regulator was installed. After installation the same readings were obtained. Now, additionally after this I set up the old DPR on my injector tester to check what pressure it would hold and found it would pop open at around 3.5 bar?? Maybe you can't test a DPR this way?? Any help is appreciated.

 

I have the same problem on my 1988 560SEL. New pumps, new DPR, New injectors, New EHA and still only reading 3.8 Bar control pressure and 4.2 Bar system pressure. Occasionally it will read 2 Bars higher. But repeating the test will show lower readings again. Car runs OK. Gauge has checked out OK. This is a mystery to me. Have you been able to find a solution?

 

Maybe someone has installed wrong type of pressure regulator. K-jetronic and 6 cyl .engines have different fuel pressures.

 

I can only think of three things that would do this after a new pressure regulator. Bad pumps, blocked screen in the fdu fuel inlet. Blocked screen in the tank. Of course a bad gauge too. Mine is a little low at 6.2 bar pressure side but I have original pumps

 

Thanks Stutz,
The pumps are new and fuel delivery is great. 1 litre in 20 sec. Factory spec is max 40 sec per litre. Screen in FDU has recently been cleaned. Fuel filter is 6k old. I don't think the in tank screen is blocked with the pumps doing 1 litre in 20 seconds. Gauge is checking OK. Car runs and starts great. I can get the fuel pressure to rise to 6+ Bars by pinching the fuel return line. I put my old Fuel pressure regulator back in this afternoon. I get exactly the same values I'm getting with the brand new unit. I can't believe that both units are faulty. I really don't know what else to do. Since the car is running great, I guess I'll just keep driving it.

 

Is the new pressure regulator correct for the car? When fuel pump relay is jumped one should hear fuel circulate trough regulator and distributor. If not there is something wrong with pumps, accumulator or some kind of blockage at tank or filter or screen. If fuel is circulating but the pressure is not correct the regulator is faulty. Accumulator can be tested for a leak by pinching its bleed line to tank. Also one can test pinching the return line from regulator to see if fuel pressure goes up that way.

 

Thank You Heikkif,
Yes, the new pressure regulator is the right one. Fuel return is working fine, bridging terminals 7 & 8 at the pump relay. Pinching fuel return line increases pressure to 6+ Bar. The new pumps have been checked for delivery volume and are showing great results. Accumulator has tested OK. Car runs fine and spark plugs look healthy. Checked gauge again and all good. I should have listened to my father: "Old cars and young women - nothing but trouble".

 

It still could be accumulator leaking. It has a leak/ breather hose to tank. Did you check the flow by undoing pressure hose from distributor? How about measuring the return flow from regulator? When pumps work without system pressure a leak at accumulator could be closed or it otherwise could be insignificant. When there is system pressure the flow could escape trough faulty accumulator. Some of fuel flow bypasses the regulator via EHA too. That is normal.

Other way to test if accumulator is leaking is to pinch its breather line and see if that affects fuel pressure.

 

Done this before and just done it again. No difference. System pressure 4.2 Bar, Control pressure 3.8 Bar. I'm surprised that the car runs fine with such low out of spec pressure.

 

Are the injectors supposed to open with 4.2 bar?

 

Here is a copy of the injector specs ex the service manual. Judging by this, the car should not be running. At least not as well as it does. Very strange!!!!!!!!!

And there is another reference I found giving lower values

 

Might be that year 86 is K-jetronic on gen. 1. cars. Except it says CIS E. Anyhow more than 4 bars is needed to open the injectors reliably.

 

Thanks Heikkif,
The publication is definitely covering the KE-Jetronic system. It is not a Mercedes publication and appears to mix up values between the 4 cylinder, 6 cylinder and 8 cylinder models. Hence I'm going to ignore these limits. I rather trust the limits shown in the original factory service manual. I will make an appointment with the local Merc specialist to have him check the pressures. Should be interesting. I'll keep you posted.