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.

this sounds like a fuel pressure problem...either with the pressure regulator or the actual fuel pump is going bad..

follow up - I had checked my fuel regulator in the past, but it did not appear to be leaking. Checked it again recently and sure enough, there was fuel in the vacuum line that connects to the regulator. I changed the fuel pressure regulator, and she now starts on the first crank every time - hot or cold. That was a week ago, I'll post if there are any updates.

Feburary, did you ever find a solution to your problem? The symtoms you described are an exact match to my problem.

Thanks in advance for any help

February, did you find a solution for your 400E's running problem(s)? They sound very similar to problems I'm experiencing with my '92 400E. Thanks.
-Forrest

Finally Fixed:

My INDY solved the problem:

Bad Fuel Pressure regulator and fuel injectors.

A VERY long Drama but email if you have any specific questions.

thanks

-andy

Cool. I suspect the same are responsible for the somewhat shaky starts and occasional low-power feel my M103 has....

misfiring intermittently
 

I had the same 400e problem. After many months of checking it out by dealer, it turned out to be a throttle control unit. Pretty expensive but they become bad on 400e model. Also check out if one of the injectors is intermittent which I had which gave same symptoms as intermittent missing.

misfire hot
 

I JUST FIX A 95 S420 CAR STARTS COLD. DRIVE CAR FOR 5 MIN MISFIRE STARTS WILL NOT RESTART HOT MUST COOL DOWN ABOUT 30 MIN.
I FOUND WIRES TO CAM SENSORS INSULATION DISINAGRATION MAKING CONTACT WHEN HOT HOPE THIS HELPS

m119 95 E420 misfire
 

I have a 1995 e420 w/140k miles which I purchased in late Sept. On the 2 hour ride home the car ran flawlessly. It did not reach the optimal temp (80-85) on my test drive, (1 hour) or on the way home so I knew it needed a new t-stat.
I figured I would drive a few days and make a list of todo’s, more extensive than the original test drive list.
My first day commuting everything was smooth sailing until I hit bumper to bumper traffic. Upon creeping the engine temp remained around 60, and a misfire commenced anytime I was creeping. As I accelerated I smoothed out. It only occurred with the engine was warm.
I consulted the web and found that many people have experienced the same problem with many suggested solutions. I knew it was either spark or fuel delivery related.
I have the unfortunate advantage of recently totaling my 92 500E. There are many parts sitting in my garage, including the venerable m119.974 5 liter. I swapped many sensors (CPS, coolant temp), and distributor caps and rotors. Nothing resolved the problem, until I swapped the fuel pressure regulator (FPR) , Viola!.
As someone mentioned, it the best 2 minute fix ever completed. $40 bucks a ********AZ or ********.
Here are some DIY links part # included. Thanks for all the heavy lifting.
Photo DIY-Fuel Pressure Regulator R/R M119.981 Engine - Benzworld.org - Mercedes-Benz Discussion Forum
HOW-TO: Replacing M119 / M104 fuel pressure regulator
If the regulator is not your problem Good luck. Check the link below.
Everyday Diagnostics: Mercedes S-Class Misfire