M104 miss
 

Hi folks,
Car is a 1995 E320 wagon, W124.092. Currently has 207,xxx miles and the upper engine wire harness was replaced about 85K miles ago (4 years ago).

Here's the scenario, last weekend I flushed the brake system...that went very well, but I spilled a bit of brake fluid in the engine compartment. So, I sprayed it with a degreaser and then proceeded to hose the engine down. I've cleaned the engine like this many times and have never had a problem but now the engine misses at idle and throughout the rev band.

Checking through my maint. records, I noticed it had been about 45K miles since I changed the spark plugs, so I went ahead and put in some new Bosch Copper Plus plugs. The miss was still present so I changed the ignition wires and under coil boots...the car had the originals so I figured they were due. The miss is still there.

Around the same time I did the wire harness, I replaced all three ignition coils to fix a similar miss and rough running. Since I kept the old ones, I tried swapping them in one at a time to see if I could isolate the current problem to a bad coil. I didn't come up with anything conclusive.

My question is, does this sound like a coil(s) problem and do you think I should just replace all three coils again? Is there an easy way to test the coils?

Thanks,
Glen

Go here:

http://www.peachparts.com/shopforum/showthread.php?t=107630

Build this LED tool and go to pin 8 for codes . If a coil is suspect , you should see a code 21/22/or 23... one code for each coil..you can then move the coils location and see if the code follows the offending coil.

More likely it just needs a good drying out...

Thanks Arthur. I have a code reader and will check codes tonight. When I checked right after it started missing, there were no relevent codes but since we've driven it a few times since then, hopefully there are new codes present. I'll post results this evening.

The only code stored right now is on pin 3, diagnostic module. The code is 4: Air injection system faulty hot film mass air flow sensor with hot wire

Is this a definitive MAFS problem? Or is there something else I may have gotten wet to trip this code?

Glen, a bad coil pack in our 1995 E320 caused a serious engine shake that could be felt throughout the car and caused an engine miss. You may want to buy one new coil pack and try that one in the three positions ... I doubt that all three coil packs would go bad simultaneously. If so, the engine may not run at all. In my case the rear coil pack was bad.

Thanks Fred. I'm starting to think it's not a coil pack at all. I do have some used coil packs that I swapped in one by one in a semi-methodical pattern with no change in "miss". I think my next test will be the MAFs...

Is there a test procedure for the MAF sensor?

You should be able to trust your OBD to tell you whether it's bad - in your case code 4 on pin 8. More on testing why it's bad on page 4 here - http://www.ms-motor-service.com/ximages/pg_si_0079_en_web.pdf

Thanks!

Well, I think it's fixed...finally. It turns out that during the engine wash a few weeks ago, I somehow damaged one of the coils AND the MAFS. Changing one or the other didn't cure the problem...

This is odd as I've washed the engine many times before without incident, maybe it was just a coincidence. Anyway, thanks for your help everyone!