M104 E320 miss at idle, hesitation
 

Hello gurus,

I've spent quite a bit of time wrenching on older diesels, but I'm pretty new to the M104's.

I've searched the archives, and seen a lot of discussion about misfires on the M104 engined W124 E320's, but nothing that really seems to address my problem.

Car is a 1995 E320 with 155Kmiles. Car misses at idle when warm, and sometimes will hesitate on acceleration. Once things get above idle, or the hesitation stops, everything is good to go. Runs smooth at speed, and gas mileage seems to be OK.

Engine wiring harness is only about 25Kmiles/3 years old. All spark plug wires have been replaced. Plugs are the correct non-resistor BOSCH's, and I've double checked the gaps. I have built Arthur's code reader tool, and have pulled the codes several times. After replacing an obviously bad oxygen sensor, it looks like I'm left with only one recurring code. I'm still getting a code 11 on pin 14. Does this sound like the electronic throttle actuator wiring issue? Anyone know where in the FSM the holy gods of Mercedes have outlined R&R of said beast? I see lots of CIS stuff on the CD, but not much mention of the HFM system. Could it be a bad coil or two? Not sure how to test these short of throwing parts at it. I do have access to an oscilloscope, and could probably cobble up an inductive pickup, but I'm not sure if this would allow me to "see" the miss...

Any help would be greatly appreciated. Thanks.

The first thing I would try is the spark plug connectors under each of the three coils. They fail commonly and will cause a misfire or hesitation. I think they're about $5 each or something like that. I notice you said you replaced the spark plug wires, but as you didn't specify the connectors on those three cylinders, I thought I'd mention it. Some kits don't include the connectors.

Jonathan

Jonathan,

The plug wires, including the connectors under the coils, were the first thing I tried. Got a brand new set in there, but it made no difference.

I just wanted to make sure. I saw that you did the wires, but you didn't specify the connectors and the wire sets don't always come with the spark plug connectors. The Bosch kit has them, but with the other kits require you to buy them seperately. I was crossing my fingers that it was a cheap fix for you. I hope someone can point you in the right direction.

Car is a 1995 E320 with 155Kmiles. Car misses at idle when warm, and sometimes will hesitate on acceleration. Once things get above idle, or the hesitation stops, everything is good to go. Runs smooth at speed, and gas mileage seems to be OK.



Any help would be greatly appreciated. Thanks.[/QUOTE]

Hi -
Ignition coils are a common problem on these cars with higher mileage. They too, will cause a miss at idle or under a load.

Scope will show a misfire.
I would also tap the OVP while I had the scope on the primar..see if it drops power when tapped...

Thanks, Arthur. I'll take a look at the OVP and see if I can borrow a scope.

Had identically sounding symptoms and suffered for 2+ years while unsuccessfully trying out most things mentioned in various forums. Finally tracked it down to a very slight build up of oily cr@p inside the dizzy cap. Resolved it by a quick spray of a solvent called something like 'Electrical Contact Cleaner' and wiped round entire cap and rotor arm 'til squeaky clean. Problem immediately disappeared and hasn't returned (4+ months now!).

Someone once posted an explanation that the oily film coating the inside of my cap was probably vaporising at a certain temperature and somehow affecting the sparking. This was why my engine would idle perfectly from cold but with a kind of 'hiccup' as temp hit 60-70deg.

Hope your problem turns out to be something so simple, and you don't go through $$s (as I did) to find the answer.

Best of luck,
Neil

<<Finally tracked it down to a very slight build up of oily cr@p inside the dizzy cap.>>

Chassis in question has DIS ignition ... no distributor to be concerned with.

The missing at idle could be related to collapsed motor mounts. Have you had your air/fuel mixture checked?

Haasman

The first thing that you should check is that you have the right OVP relay. Lots of posts - even recently - about the OVP and the correct part number including a very funny post by Arthur about testing the OVP that is worth the search by itself!

Two words for you....cam timing. I'd check the engine vacuum and then check the cam timing if it is low. Fixed a 119 with a misfire at idle just this past week with an exhaust cam 15 deg late.:cool:

Just wanted to follow up with a thank you to all who chipped in their expertise, and make sure the solution got into the archives for anyone else in my situation.

Broke a rear shock about a month ago, so I just parked the car until I had a chance to work on it over the holidays. (Never seen a Bilstein shock come apart like this one did, but I digress...)

I borrowed an oscilloscope from work, and spent a little quality time trying to get a look at the inputs and outputs to the three coils. One appeared to have a slightly lower output than the other two. Not enough I would have necessarily expected the misfires, but enough to notice. Sooo, I very scientifically decided to throw parts at the problem. Installed three new coils to go with the new plug wires and connectors. Figured if one was bad, I'd just as soon get them all.

Problem is GONE! Yes, coils do go bad. Good to have things back the way they were meant to be. Much more fun to drive a properly running W124 than the beater W123 diesel.

May I inquire how you were able to use an oscilloscope on the secondaries? Do you have some sort of inductive pickup? Was is a standard electronic oscilloscope or is it an automotive specific scope?.

I have an oscope but I'm not sure how to go about picking up the secondary voltages on the coils. I'm not having much luck finding an inductive pickup to use with the scope.

Thanks,
Bob

Bob,

Used a standard oscope, nothing auto-specific. My inductive pickup consisted of a couple of coils of 20ga wire wrapped around each plug wire and terminated to about 6 ft of 50 ohm coax. Had to experiment a little with the wire to get enough coupling to be able to see anything, and not so much I saturated the front end of the scope.

It was crude at best, which is why I really hesitated to draw any conclusions from what I saw. Measurements were not very repeatable.