W124 E220 1993 irregular idle, hi revs, returns to normal on restart

[jacodupreez]

Hi,

just bought a 1993 E220 W124, and in the last week it has been giving me a bit of a head ache.
it would for no reason start shuddering, as if its not firing on one cylinder, but if the car is switched of and restarted, it runs normal.
Revs also pick up to 1500 rpm.
have changed the spark plugs today, as i thought one might be faulty . ( disconnected each plug lead while it was in its "shuddering state" ) and found one to be "dead" cylinder no 2

But the problem returned a couple of hours later. this time on cylinder 1 and then on cylinder 4.

What really confuses me is that, once the car is restarted, it returnes to normal, for a time.... have noticed that if sudden braking / stopping is done it also "misfires"

 

[grober]

Sounds as if you might have a problem with the OVP relay. quite common -they can often be repaired by resoldering the connections on the relay small internal circuit board. Try cleaning up the throttle body throat with solvent also- they get dirty and this effects idling. try doing a search on here for details.

 

[jacodupreez]

thanx grober, will clean the throttle body today, because i noticed something else this morning, when ever it does it "misfiring / shuttering " thing and i floor the accelerator, causing the car to drop a gear or 2, pick up revs, problem solved... for a bit.

checked the relay fuse, its fine, plus when i disconnected the fuse, i could hear a reaction.

 

[Vlad]

The OVP relay may have an OK fuse, but internally the soldering starts to break down casuing erratic faults. Very common and the new OVP relays have a slight redesign. I personally would take the plunge and buy a new one.

 

[balge]

Hi
this might be useful, if it is the OVP

OVP - basic function and test - Benzworld.org - Mercedes-Benz Discussion Forum


tbh sounds like ignition trouble to me though... if that's the only symptom?

Try starting it up in the dark and check for sparking?

cheers

 

[neilz]

I could be completely wrong so don't take my word for it, but I had this exact problem on my W124 E220 from '95 (which I've just sold) and it just needed new HT leads and ignition coils.