azurite300e

Thanks for the tips. Yesterday I spoke to someone that might have access to a fuel pressure gauge test, so I will try to get the chamber differential pressure test done in the next days.

azurite300e

Found this post at the Mercedeslist group private archives. It seems to describe my exact same situation. I believe this confirms my suspicion that the fuel distributor differential chamber pressure setting is VERY critical indeed.

================

[[quote original text]]
(Name deleted for privacy) recently described an interesting experience
with adjusting the mixture on his 560 engine. When he adjusted the 3mm
mixture adjustment screw so the idle was right (on-off ratio about 50%
and EHA current ranging around zero from +2MA to -2MA), the mixture at
2500rpm was way rich (on-off ratio around 10% and EHA current at -10MA).
When he adjusted the mixture to be right at 2500rpm, the idle was rough
and way lean, confirmed by an on-off ratio of 90% and EHA current of
+10MA). The literature suggests that this likely is caused by vacuum
air leaks or a bad EHA valve. Bob found some minor air leaks in the
brake booster line and corrected these, and it helped but did not solve
the problem. He eventually measured the pressure in the fuel system and
found that the pressure differential in the lower chamber was off by (a
mere) 0.1 bar. He adjusted the EHA valve (there is a small adjustment
screw under a screw-off cover in the EHA valve--it has to be removed
from the fuel distributor first) to restore the correct 0.4 bar pressure
differential and voila! The mixture adjustment could be made right at
both idle and 2500 rpm. Car now starts, runs and idles right, with the
on-off ratio and EHA current working at spec. Bob had bought a new EHA
valve and found that it did not improve things, but once adjusted to
produce the correct pressure differential the (new and old) EHA valves
worked perfectly.

What is interesting about this experience is (i) how a very
small fuel pressure difference from spec makes such a huge difference,
and (ii) that the manual makes no mention of the adjustability of the
EHA, or its relationship to the requisite, critical pressure
differential. It seems that the pressure in the system should be
checked first thing, and if the differential is off, the EHA can be
adjusted to restore the correct differential. Then the mixture screw
can be adjusted to produce the correct on-off ratio and EHA current.

From Bob's experience, it would seem that the pressure differential
should be checked and re-set with an EHA valve adjustment whenever the
difference in the on-off ratio at idle and at 2500 rpm exceeds the spec
(plus or minus 10% for post-87 cars).

Does the manual overlook the adjustability of the EHA valve
because of emissions rules that prevent user adjustment of the mixture?
Or because the "right" thing to do would be to buy a new fuel
distributor and other associated parts at vast expense. I note that in
Bob's car the EHA valve had to be adjusted to work "leaner," although in
many cars it appears that the right adjustment likely would be to make
it slightly richer.
[[unquote original text]]

crr1612

problem solved i suppose...tell us how it works out for you

azurite300e

Not yet, but I'm close to getting my hands on a fuel pressure set. Hopefully sometime this week I'll be able to make the EHA adjustment.

YNVDIZW124

So, it sounds like your fuel distributor is going bad and the EHA has to be adjusted to compensate between the differential pressure of the upper and lower chamber. I would buy another fuel distributor that's been tested and guranteed from a newer car. Adjusting the EHA is a temporary fix but i think those gremlins are going to come back to haunt you.

The CIS-E or KE-Jetronic system is a very simple system but a lot can go wrong fast. Like i said in my post above when the fuel distributor goes bad the plunger in the middle of the fuel distributor will usually be lose. The systems pressure is determined by the tension of the spring reacting against the plunger. To compensate for this and keep the car running you will have to adjust the idle screw which may be close loop at idle. Once you step on the gas and the EHA wants to add more gas for acceleration it will give you a full rich mixture. Maybe everything you adjusted now is fine at idle but the differential pressure of the fuel distributor is different when revved up..

Well, that's what i get from this whole thread...I may be wrong...

shdoug

I wouldn't say the fuel distributor is going bad, just that it has excessive clearances that the EHA can't compensate for. Changing the EHA base adjustment should correct the problem for a long time.

azurite300e

According to the information I gathered, it seems that the EHA base mixture setting is extremely critical, and any miss adjustment will produce EXACTLY the same symptoms my car has now. I suspect the setting was not right to begin with as someone (probably the previous owner) had tampered with it (the screw cover shows obvious signs of tampering) and possibly without the aid of a fuel gauge, so it would have been easy for him to leave it at the wrong setting. Apparently just 0.1 bar away from the optimum setting is enough to trow things things completely off.

Still, the car now runs great, only rich, and it has been like that since I own it, now over 4 years) so I don't think the FD is bad, or even close to that. Maybe just the normal wear expected at 117KMi. From all the info I have now, it seems that the logical deduction is that the system just needs the EHA setting checked/corrected. In any case that would be a mandatory step at this point in order to rule it out as the obvious cause.

Hope you're wrong, but we'll see what happens after I check the upper and lower chamber pressures. I bet the differential will be way off 0.4 bar (or the magic setting of 0.45 bar which is probably what the previous owner was trying to attempt - see the "magic 1/4 turn screw" described on other treads on this and other forums). If I'm not able to bring the differential pressure within spec, then of course the first suspect would be the EHA, and if a replacement doest cure the condition, then and only then I would suspect the FD.

azurite300e

I concur with you that the base mixture adjustment will most likely cure the rich condition at off-idle RPMs, but not necessarily that there is "excessive" FD wear. Just whatever clearance there would be expected after 117KMi, which should easily be compensated for by increasing the differential pressure up from 0.4 to 0.45 bar. This should be about 1/8 CW turn on the EHA.

But of course first I have to make sure the differential pressure is actually in the ballpark, which is what I would hope to accomplish today.

YNVDIZW124

Well, if the fuel distributor is within the ballpark then adjusting the EHA would be the best option. The next thing to do if that doesn't work is to replace the fuel distributor. I guess we are on the right track but you also have to take care of any air leaks or vaccum leak before anything.

azurite300e

In regards to detecting vacuum leaks...

If it is of any help, my "Economy" Vacuum gauge needle points right over the "o" of "Eco..." when in traffic, engine at about 85c, shift in D, A/C on, and at idle ~650-700 RPMs. When the engine is started cold and warm up RPMs are at about 900 RPMs, the needle is pegged to the left most portion of the scale. Not sure that would tell anything useful, but a friend mechanic said that needle's position can sometimes be a tell-tale sign of air leaks.

Haven't checked the injector seals, was going to use WD40, but I got conflicting information about possibly damaging perfectly good seals with sprays like carb cleaner or WD40 as it could cause them to eventually dry out, and then start leaking air for sure.

Any thoughts about that?

azurite300e

I had finally access to a fuel gauge and was able to check the fuel pressures. I warmed up the engine to about 82c and measured. Results where as expected, with my earlier tinkering of the screw:

Initial: Upper 5.7 bar, lower 4.85 bar. Dif: 0.85 -- Way off!

Then I started adjusting the screw CCW in 1/2 turns first, then 1/4, and last 1/8 of a turn to fine tune. While doing so the lower pressure chamber started to come up gradually, narrowing the differential. After each EHA screw adjustment I also readjusted the CO mix screw to bring back the EHA current as close as possible to zero mA within +/-2mA, which was done without any problem. In the end, after about 9-10 EHA adjustments I ended up with these readings:

Upper 5.65 bar, lower 5.2 bar. Dif: 0.45

At that point I once again adjusted the CO mixture screw to bring back the EHA current to close to zero. Then I revved the engine to a about 2500 for a few seconds and the current briefly went to -3mA, then a second later back to zero mA, and then started jumping around a bit but always within 10% or so from the idle value. Lower chamber pressure also reminded stable, it did not change from the idle value. So great, things improved a lot! Then I went for a short test drive, and found that there was an appreciable accelerator hesitation/lag when going from slow to suddenly pressing on the accelerator almost WOT.

I though maybe the fuel gauge might be slightly off, or there was a small reading error on my part, etc, so I once again adjusted the EHA to increase differential chamber pressure an additional 1/8 CW turn (which gave me a differential reading of about 0.5). Very slightly readjusted CO for zero mA at EHA (2500 RPM was also still fine) and went for another test drive. The accelerator hesitation was now much shorter when going to almost WOT, but still present.

BTW, one thing I really forgot to do is disconnect the regeneration valve hose before making the CO adjustments, not sure if that could have been a factor leading to a lean mixture adjustment. I understand some fuel fumes could be sucked in from the charcoal canister, etc.

One additional thing I noticed, is that the fuel pressure in the system drops very rapidly, once the engine is shut off. It takes just about a minute for the pressure to drop from 5.7 bar to about 1.1 bar. Probably a bad fuel accumulator? I checked under the airflow plate for any fuel leaks, and didn't see any wetness at all after shutting off the engine. I checked again 15 seconds later, still bone dry. I also thought about the other possibility: a leaking injector? But then I assume I would not been able to get the positive results I described earlier? There is no other indication (fuel smell) that there could be any external fuel leaks.

I guess the quick pressure drop explains why it takes now about 2-3 seconds of cranking before the engine starts, as the pump has to restablish system pressure from almost zero.

It was getting late, so I left things there. Since I made a small mark on the 2mm hex screw on the EHA, now I have a good point of reference, and I've fully documented all my previous adjustment steps, so in case further fine adjustments might be needed, I could eventually do them without the fuel gauges.

Other then the slight accelerator hesitation described before, the car feels good, as soon as the transmission downshifts after WOT, it takes off like a devil. Well it always does that anyway.

Now about this morning. The engine started within 3-4 seconds of cranking, but initial idle was extremely unstable, as I had to help it by slightly keeping my foot on the accelerator, otherwise I'm sure it would have died. After 30 seconds to a minute it kind of stabilized, but was still very sensitive to sudden normal acceleration, like when you are leaving the driveway. I would say the car wasn't decently driveable until the engine reached 50-60c, and then it still had some random stumbles at idle.

Obviously all the earlier findings seem to point to a lean mixture. Maybe an air/vacuum leak? I starting to believe I really have to double check those idle valve connecting hoses, which feel kind if stiff, although I wasn't able to detect any cracks on them. Then again its difficult to get at the ends of them, specially the manifold side just under the cold injection valve. Also planning on checking the injector seals for air leaks by spraying them with carburetor cleaner.

OK, so that's about it. Making good progress, but still a couple of loose ends there. Any insights will be of course very welcomed.

azurite300e

OK so I tested all the injector seals and rubber hoses for air leaks by spraying them with carb cleaner. No leaks found.

Since Saturday I was plagued by a cylinder not firing, not sure which one. On that same evening I went out for a drive and after a while the missing cylinder problem went away by itself. I though it might have been that some water got into some place and finally dried out. The next morning it was again the same thing, but this time I noticed the problem went away as soon as the engine reached about 90c and all was fine. I know this because i was driving around for a while and watching the temp which would not go above 85c, but as soon as I started climbing a long road the temp reached 90c and the problem cleared itself. A while later left the car standing at idle just for about 5 minutes, the temp went back down to about 82c and the problem was back when I resumed driving. That afternoon on a almost cold engine (about 50c) it did not present itself, neither was there any missing cylinder this morning on a completely cold engine (less then 40c).

What gives? Maybe a spark plug or cable, definitively electrical. Will wait for it to reappear and will buy an extra spark plug and rotate it around the cylinders with a cold engine to see if I can pinpoint a faulty one. Once I eliminate this problem will resume adjusting the mixture.