CIS-E Experts in here! Lower chamber pressures too low
 

I am completely out of ideas on this...

Problem:
During acceleration, it almost stalls unless I go easy on the gas. At speed, no issues.

Lower chamber fuel pressure is not to spec (sometimes), not always. Will go back to 4.9 Bar sometimes.
http://w124-zone.com/downloads/photo...mble/lower.jpg

Tests done:
Upper chamber fuel pressure: 5.4 Bar
Lower chamber fuel pressure: 4.6 Bar <--- too low!! Sometimes will go back up to 4.9 bar. Its random, I can't tell why. - Swapped the EHA with a spare one...SAME PROBLEM, so I can probably rule the EHA out.
EHA current fluctuates (-)3.6mA to (-) 2.2mA. Ignition On/Engine Off: (+)20mA.
Duty cycle is 50% fluctuating

Fuel Volume test out pressure regulator: ~1.5L/40sec.
Coolant Temperature Sensor checks out: 1.4k Ohms at 36C ambient. 290 ohms at Operating temperature.
Potentiometer reads: 7v +0.1v.

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I'm stuck here...What could be the issue here that is causing the lower chamber pressures to be whacky?

Replace fuel filter, be sure both fuel pumps are working if you have two.

And check for internal fuel system leak - accumulator or damper can be leaking gas back into fuel tank. The test for that is to see if it holds pressure for 5 minutes or longer after turning off the engine.

If I have 1.4L/40sec fuel volume, how could anything back there be an issue? MB spec is 1L/40sec minimum. If i had a blockage or a dead pump, I wouldn't have adequate volume. I would also assume I would see problems at wide open throttle at speed, which I do not. This is strictly an issue from idle to 10mph, then it clears up. Also, system pressure is 5.4bar, so pressure from the pump is fine. The lower chamber pressure is the problem.

Fuel filter is 22k miles old anyway, so its not that old.
Accumulator holds 2 bar for about 15mins. I also have read the accumulator is culprit for HOT start problems, which I do not have.

Sounds like a weak ignition coil, that will cause a hesitation at low speed ie load at low speed and will seem to clear at high speed, pretty easy to check spark should easily jump a 1/4 gap and spark should be bright blue not orange or red

Re: Lower chamber pressure

The Lower chamber pressure is constantly in flux once the O2 sensor is at operating temperature. At idle with the engine warm, and O2 feedback, the lower chamber pressure will typically be .3-.5 bar below system pressure, and will continuously oscillate in response to the continuously changing current through the EHA. In the warm idle condition, look for EHA current to oscillate between 8 & 12mA. Blipping the throttle should produce an increase in current up to ~50 mA, depending on how far the throttle is opened and for how long. If the EHA current values are not being achieved, look to the air-flow sensor potentiometer and/or the ECU.

Quick update - I ran the MB resistance test on the coil between the two terminals and it came to 0.9 ohms and spec is 0.3-0.6. I ordered a new coil and waiting to get it in.

Hopefully that will do it. If the issue is an ignition miss on all cylinders it's going to screw up the O2 sensors readings and throw off the EHA. Will report back soon!

You were dead on the money - new coil and everything is driving great again. Thanks!

Great news , helps too to work at mbz dlr, I had a car once also that started to bog and blow black smoke after extended idle , that too needed a coil , as spark got weaker it couldn't light off the fuel ,lambda went rich according to gauge , o2 was showing rich, all due to bad coil

nulu-good catch. i've replaced a lot of coils on that gen ignition but never saw 1 affect fuel so bad. eventually, i would've scoped the ignition, seen the coil fail, replaced it and crossed my fingers on this 1. again, good catch.

ps2cho-is there any flash over on the outside of the old coil? white stuff on the bracket? any mb coil that i see that has that evidence, gets a coil, it's dead. thanks, chuck.

I had a '96 Ford Exploder with the 4.0L pushrod V6 that had a spark plug simply fail one day while driving. You talk about a wild ride! "Check Engine" light on, galloping idle, crazy acceleration behavior, you name it. I thought I dropped a valve or something the way it was acting, but no, just a dead spark plug.

When O2 sensors are involved, any unburnt fuel will cause the computer to attempt to lean out the mixture to "correct" the problem, only to go too lean and wind up swinging the other way. The result is erratic operation, stumbling, and poor power. Makes sense that a weak coil would cause similar behavior at lower RPM's.