M104 mercedes 6 cylinder engine fitted to E280, E320, C280, C36, E36
This is a great thread guys (sorry I'm a newbie poster!), I suspect I have a similar idle fluctuation issue but not sure so thought to describe the symptoms...
When I am idling in drive gear (never happens in neutral), there is the slightest hint that the engine idle rpm fluctuates up and down. If I look at the tach, the rpm fluctuation doesn't actually register, it's that narrow of a range. There are only 2 noticeable symptoms, the first is a slight resonant vibration in the cabin (no plastics shaking but a low resonant vibration) and the second symptom is the oil pressure goes from around 2.1 to 2.3 / 2.4.
I cleaned out the throttle body with throttle body cleaner b/c it definitely was gummed up a bit difficult to move before the cleaning, but now moves freely. I also checked the OVP fuse and the fuse seems OK. However, the 2 symptoms are still present, but the idle rpm's sound very consistent....
So what else could it be? If its the OVP relay, is there any way to check before replacing for another one? Or I'm thinking is it something more mechanical like the engine mounts?
hi, dont know if this is too late!
i got the exact same problem!
its not the OVP. just bought a brand new one from mercedes. its now for sale if any one wants it! its the new updated one. same one as already on the car. my car is 1996. i will knock £20 off!!
i cleaned the throttle body, it made some improvement.
i changed an ignition coil over from my E36, actually changed all of them over, found that 2 of them were bad! this made a huge improvement!! now just cycles now and again. only done this today, so i will put brand new coils on soon!
i changed over the idle control actuator it is a reonance flap simular to the butterfly valve on the throttle body... the other one broke! this fits inbetween the 6 plastic air intake pipes, it looks like a black box about 2" x 3" AND has 4 hex bolts. 2 on the outside and 2 hidden underneath rubber caps.
my old M104 (300E 24) didnt have one of these it used a different kind of valve!
ignition coils should be changed every 70k miles.
this is the probable cause!
i have an lpg conversion on mine, and started thinking that the resonance flap sensor on the intake had broke due to a back fire on LPG gas and then clogged the air intake causing it to cycle. may be wrong!
i can run the car great now on petrol with the coil packs from my e36 amg, but if flick onto lpg gas it cycles slightly and car judders slightly at idling and when i drive at 30-40 mph and take my foot off the throttle the revs drop from about 1300-1500 rpm to 700 even sporadically down to 0(this is on lpg) on petrol it seems to be almost perfect except 10% of the time the revs drop about 500 and then quicly cycles back.
i think its just a case of putting new coil packs on!
almost forgot to mention i swaped the MAF over from my E36 and this did not fix the problem..
so ignition system has to work perfect.
spark plugs, air filter, coil packs, leads, clean throttle body with carb cleaner and check the idle resonance flap on the intake tubes!
if still no joy! then definetly an air leak, but i also heard that the EGR valve gets gummed up also. so may be worth cleaning with carb cleaner.
but i think your problem is definetly coil issue!
i covered everything i just learned about my car, but some peoples cars may also have the engine harness problem fitted to cars manufactured between 1993-1995..
hope this helps! if theres any thing i have left out please let me know, im now intrigued to learn as much as i can about these cars!
the problem car above is a C280 which is my wifes and drives on lpg with 154k miles on it.
and i just bought a rare E36 amg 1996, just learned this issue today by swaping coil packs over, since 2 were bad on the c280, i mean really bad! and one is a tiny bit bad on the e36. the e36 sometimes cycles up and down when idling only about 2-3% of the time. this car also has 151k miles on it.
