Is this a random misfire at idle only. I have found on my 500SEL that if I get a random misfire it will be plugs or plug wires, or lean condition. The lean condition can be temporally corrected by adjusting the mixture to a rich condition and just see if the misfire goes away. Problems I have usually found with the plugs are the wrong plug for the engine. Don't depend on what the dealer or any other store sells you, but find the original type plug for your car. Also, check each plug wire with an OHM meter and make sure they are all reasonably close to each other. On my 500 they will be aprox 5K OHM. If the above doesn't show up anything, then replace the CPS. The signal from the CPS is lowest at idle, and increases with RPM. If it is getting a little weak, it will cause your problem. If I was working on this car, I would research the plugs first, then replace them even if they are the correct ones in the car. I work on cars with this type problem every day and most misfires turn out to be plugs or wires.

Well with quite a bit of experience of my own 190s, the only time I've encountered this condition, and it's very repeatable, is when there's an air leak somewhere. On my 2.0 for a while the rubber pipe came loose off the valve cover (it goes to the air intake). This made the whole idle incredibly shakey but once over 1200rpm it ran as brand new.

I can do the same with my 2.5 - unplug the air pipes for teh air enveloped injectors and it's instantly huppity wobble jigga jigga.

Yes I have the OEM H9DCO plugs in her. I have been through tons of plugs too before I got my valve stem seals fixed. So I'll check the ignition wires then replace the crank position sensor.


I had the car smoke tested...so I have no leaks.

OK, not sure what that is but you say it's a test for vacuum leaks but I'm talking about actual intake leaks - extra air getting in and it being unmetered by the air flow flap?

While we're there - does the air flow flap sit in its rest position correctly with engine off? Should slide up into its hole, not catching on the edges at all.

Yeh its fine.

http://i135.photobucket.com/albums/q...alveSea2lE.jpg

^ You can see how good the engine is. Its gotta something small. Once I have checked out the wires and fuel pressure that CPS is going first!

ooh ooh ooh.... Throttle position switch? Either the one on the linkage with the wheel, or the one one in the intake but with the pins at the side which you can test for full throttle or idle throttle positions. Got to be said that you should test connection through to the ECU as well, have heard of problems with wires between ECU and sensors.

incidentally engine will run without ECU.

You're right. I believe all I need to do is pop off the OVP and that turns off the ECU. Last time I tried that was when I was checking for vacuum leaks using the O2 sensor...it still misfired but I've progressed alot from then so I should give it a shot again now.

Take a look at these wire readings:

1) 1.266 k Ohms
2) 1.271 k Ohms
3) 1.248 k Ohms
4) 1.263 k Ohms
5) 1.227 k Ohms
6) 1.107 k Ohms

They look pretty linear except Plugs 3 and 4....Are they within spec or could they be suspect? These plugs are about 6 months old since I bought them new. OEM Bosch ones.

What do ya think?