Doesn't SOMEBODY know why 400E's misfire
 

Has anybody resolved the mystery of why these M119 engines, specifically in the 400E, start to misfire when hot? It isn't the obvious stuff like wiring harness, caps, rotors, wires and coils or CPS and ETA. Somebody must have solved the mystery. Share it....please. This is driving me crazy. Trouble codes don't help. Thanks

There isn't one big mystery, lots of things it could be. What do you mean when you say misfires - won't run, runs roughs, lopes? Trouble codes won't help means what? On what pins are you getting one flash?

What is oxygen sensor control doing during the event?

I've looked for codes on the 38 pin connector on socket 4,6,8,14,17,19,30
On socket 17, I got a code 8....Trans Overload switch
On socket 6, I got code 32....CAN data to left/right igntion module.
This seemed the most promising as it has to do with the ASR, which this car has.
No code for the 02 sensor but when it was on the scanner it seemed to be working OK. (I had a MT2500 SnapOn)
So, here is what happens.
Get into a cold car and it starts right up and purrrs.
Drive to the hiway ramp, 3 miles stop'n go, purfect...no problems.
Get upto hiway speed and it is flawless. Lottsa power, USUALLY no issues.
Once, however, when I floored it to pass it did start to miss but then seemed to settle down, until I got off.
After driving about 10 miles at hiway speed and getting off it may start to miss with rough idle as I come to the stop sign just after exiting...BUT NOT ALWAYS.
Usually it will idle fine. I stop in at a small shop within 1/2 mile of exiting the hiway.
Get out and shut the car off, remember it is still running fine at this point, usually.
I return to the car after 15 minutes and as soon as it starts it misses, rough idle, shakes..like it is running on 4 cylinders.
I can drive the car, takes awhile to get it upto speed, but it will get moving.
It is NOT in Limp Home Mode.
Sometimes, as the RPM increases it will suddenly surge ahead as if it wants to run normally, but it will revert to missing as I backoff on the gas.
If I apply to much gas while it is missing it will back fire, regardless of RPM.
So, I can limp back onto the hiway, slowly getting upto hiway speed, backfiring all the way.
It will run upto 60/70 MPH eventually, not that long.
And then...mysteriously, it will start to run OK again, sometimes, but not always.
As I exit the hiway after running at 60 for 15 minutes it MAY be just fine....no misses or rough idles.
But it could also be acting up.
In this case, and driving through town, I have to keep the RPM up with the gear selector to get it moving.
Again, this is the weird part, if I run it this way with high RPM (2000/3000 in 1st/ 2nd) it is just as likely to loose the miss and run normally again.....and stay that way for the rest of the 5 mile drive home.
Now tell me that isn't frustrating.
I have a friend with a 400E so I have swapped all the modules incl. ignition, temp sensors, cam sensor, replaced the CPS and swapped coils. Replaced coil wires and completed inspected both caps/rotors/spark guard for streaks.
One is almost new and the other is fine.
Test everything with high voltage.
All the vacuum lines are good or new.
Replaced the ETA and it has the new upper engine harness.
Plugs are fine...tested each one seperately.
Obviously it is something getting hot.....but what's left to check?
And it doesn't do it consistently the same way all the time.
None of the codes suggest ignition OR fuel problems...so what ever it is, it ISN'T throwing a code. A clue???
The wires on these engines (all mid 90's Mercedes...unbelievable) are soooo bad it could be anyone of the other harnesses.
But if that was the case then I wouldn't be the only one with this weird problem....I don't think.
I've spent hours on the forums reading and did find a similiar problem with a 92 SL500(same engine M119)....lots of talking back and forth....but no answer....the forum just ended. 2 years ago.
Geeez....that doesn't help when solutions aren't posted.
Jim Forgione suggests that the fuel pressure reg. should be changed...which I will do...but I doubt that is it....but then who knows. Jim does seem to know his stuff.
Thanks for your combined brain power boys, I do appreciate it.
Someone MUST have run into this and fixed it.......hope the symptoms help jog the 'ol grey cells.

Any reason you haven't checked pin 7? I'd just keep reading, clearing, reading the codes - seems like those symptoms should be consistently throwing a code or two.

and what does Oxygen sensor control look like during the event.

and what purpose does swapping coils do, did it move misfires to the other four cylinders? Or did you swap them for know good ones. Changing which cylinders misfire doesn't do much unless you can view the misfire change through testing. Of course a scope would have defined the issue without moving it.

if your issue isn't mechanical (and it probably isn't) then it is either fuel or ignition related. Replacing the pressure regulator could solve issues of leaking or pressure issues. These things can be tested cheaper than by replacement. But of course an answer to my original question could eliminate fuel issues anyway.

Oh.... and BTW, I doubt you will find any codes that do much good here.

While you are monitoring O2 sensor control you might as well also tell us what the burn voltages and burn times are also.

Thanks....I no longer have that SnapOn scanner but do remeber that the voltage was within the speicified range. Unfortunately I don't have the numbers.
I swapped coils between the left and right bank then put an known GOOD one in the mix and did the same thing. I tried to get the misfire to move with the coil.
At one point I did have DTC 34,36,38,40 show up on the DI, pin 17. But I think I created that by pulling the coil wire on that side since it hasn't returned and the misfire is still there.
What about DTC32 on pin 6 ?

What voltage? The firing voltage?

What about O2 sensor control, integrator?

PIN 7 is a single blink as are all the others except PIN 6 DTC 32 and PIN 17 DTC 8

Didn't record the voltage.

I just used the MT2500 diagostic software when testing. Don't know the protocols.

Are you suggesting a bad 02 sensor would present these symptoms?

Well if its clogged it will throw the fuel/air mixture out of whack.

Im not an expert on this engine, but i had a similar hot start problem, It was caused by having the 4 wire temp sender in the front of the engine plugged in incorrectly, It will only take you a few minutes to test this, so it might be worthwile.

I'm having some very similar issues with my 1994 e420. In the morning, when the engine is stone cold, it cranks immediately. The rest of the day, you have to turn the engine over several seconds before it will start and sometimes it will stumble & die - then crank again and it will start.

Based on the recent comment, I'll check the plug of my 4 wire temp sender. Been chasing this problem for awhile...

as suggested earlier it is possible that fuel pressure regulator becomes defective when hot.as steve has said if we knew whether the o2 sensor was reading too rich or too lean this would be very useful.get hold of a fuel pressure gauge and attach it to the rail.then set the engine up for test[condition seems to be repeatable]get the miss going and stop car open hood and check fuel pressure.if you took reading when cold you will know if it changed.