M104 Running Rough. Thoughts?

[JamesDean]

Hey everyone,

A few days ago my buddy brought up his sister's m104 (3.2) 300E to have me talk a look at how rough it was running. I'm pretty sure my om617s run smooth than this thing!

When I took a look at the engine bay I found that the water pump had nearly failed and the tensioner shock was on its way out as well.

My buddy got the water pump replaced but didn't have the parts and the car was needed by the end of the week.

So yesterday we went for a ride in it and its still pretty bad. I shot some videos of it and thought I'd post them. I havent had a chance (or the car in front of me) to pull/check codes.

MVI_5607_zps6bda2f16.mp4 Video by JamesDean59 | Photobucket

MVI_5608_zps38d57841.mp4 Video by JamesDean59 | Photobucket

In retrospect these videos might not reveal much. Theres a lot of wind noise from the fan. I also didnt realize the 4,000 rpm limiter existed.

[97 SL320]

You didn't say what year, this should have 3 ignition coils on plugs and 3 wires running to the other plugs. These coils fail and will cause roughness. Also, make sure the wires are routed to the proper cylinders. I'd run drive the car, shut it off then pull plugs and look for carbon / wet with fuel.

If the plugs look clean but slightly oily, look for a injector that is not firing.

Another way to test is to start a cold engine then use a heat gun to measure temps at each exhaust port. A dead miss will be cold . ( you can feel for heat if you start with a _COLD_engine that has not run more than a minute. )

[JamesDean]

Yeah that what we initially suspected was a bad coil pack. This is a 1993 MY.

We replaced the spark plugs not to long ago.

We also found that the 6th plug which is connected directly to the coil pack doesnt seem to "click" in as well as the others.

[hookedon210s]

The three spark plug connectors that connect three spark plugs directly to each coil frequently fail and defy detection. It is always a good idea to replace these whenever the plugs are replaced. Also, one coil can fail for either one or both cylinders that are served by this coil. It is easy to diagnose which cylinder(s) are misfiring by disconnecting the electrical plug to each fuel injector while the engine is idling and noting any change in engine roughness and then reconnecting the injector plug and moving on to the next cylinder. Once you have identified the misfiring cylinder, you can swap coil packs and retest to see if the cylinder miss has moved. Mark

[oldsinner111]

A dirty MAF if it has one will cause rough idle to.I clean mine twice a year.