E320 - Why do the revs fluctuate on prolonged tickover?

[andyrob]

I have a 1993 E320 (124-series)
I've noticed that if I leave her ticking over for ten minutes or so the revs suddenly rise, runs a little rough then after 30 seconds to a minute settles down again.
What is happening?
(Has the catalytic with the air pump if that makes any difference).
More a matter of curiosity than concern, runs fine normally, I just happened to notice while I was warming up the engine after changing the anti-freeze.

 

[Nello]

I also have an E320 1996 inline 6.

The idle on mine is rock solid for about 3 minutes then about every minute it dips by about 300rpm and then over revs to about 900rpm then settles back down.

Wierd.

when running though its fine.

 

[mjtray]

Nello

The surging idle you describe is normally caused by the butterfly flap in your throttle body being dirty, which in turn causes a slight air leak which in turn causes confusion within the ECU, ie; it momentarily overfuels or underfuels the engine and causes it to surge.

It's easy to clean.......as you look at the engine, follow the induction trunking over the engine and as it plunges off the right hand side it enters the throttle body. Undo the jubilee clip holding on the trunking and pull off the throttle body.

Inside the throttle body is a butterfly flap. Spray some carb cleaner in here and then get a long screw driver with a rag tied around the end. Push down on the butterfly flap outer edge and it fill move down. Clean the edges that mate with the throttle body and you will notice a load of black crud will come off. Do the other side until its all nice and clean and reassemble.

Next on the top right hand corner of the induction trunking there is a sensor. It's a push fit. Pull it out...its likely to have a coating of oil mist on it. Spray some carb cleaner on it and carefully wipe clean. Push back in.

You should now find that your idle behaves itself.

Cleaning the butterfly flap should be a standard maintenance item, but more often than not it never gets cleaned.

Hope this helps.