Hi

Can anyone please advice on this issue:

Cold start engine, the idle will rise up to appx 2000 rpm, then drop down to 1000 rpm and run up to 2000 rpm again, on and on untill warm, then it is stable at appx 1000 rpm. Not able to adjust idle rpm down to 700-800.

Any ideas?

Tom

__________________
1974 MB 230 4 cyl auto Sold
1973 MB 280SE #12004
1970 MB 230/8 #53280
1970 MB 300SEL 6.3 #4509 sold
1968 MB 300SEL 6.3 #0335
1972 MB 280SE 4.5 75k miles sold
1971 MB 230/8 60k miles sold
1972 Volvo 142 60k miles sold
1969 Opel Rekord 1900 sold
1974 L508D Camper

Tom,
That is a common problem on the D-jets. It is caused by an air leak. This leak can be external, like a loose hose that is connected to intake manifold, or it can be an internal air leak. The internal air leak, would be a stuck Aux Air Valve (AAV).

If you get in the car and drive it right away until it is up to temperature, does it still cycle up and down in rpm? If not, the AAV would be the cuplrit.

If it still does it when warmed up, I would look for a loose or disconnected hose. Possibilities, would be:
- hose from the back of the intake manifold to the Manifold Pressure sensor (MPS)
- tubing that starts at the throttle housing and goes to distributor via a small switchover valve (several connections there to check)
- tubing that comes off back of intake manifold, passenger side and connects to transmission and in same area, another connection that goes to chassis systems like door locks etc.

But my guess would be a sticking AAV. If the el shaped hose that comes off the AAV is still flexible, try squeezing it to close it off while the engine is cycling. If you have the idle screw all the way in, the engine should stall. Another possibility, is to remove the air cleaner and block the port that leads to the AAV/idle screw. That should cause the engine to stall if there are no external leaks.

Another thread on same subject:

http://www.peachparts.com/shopforum/sl-forum/254408-all-sudden-reving-up-down-when-cold.html

If it is the AAV sticking, let us know. They are expensive but there are ways if freeing them up.

__________________
Graham
85 300D,72 350SL, 98 E320, Outback 2.5

Graham,

Thanks a lot for info, I have forwarded it to my friend who owns the car now (my old car...).
Will return with result ;-)

__________________
1974 MB 230 4 cyl auto Sold
1973 MB 280SE #12004
1970 MB 230/8 #53280
1970 MB 300SEL 6.3 #4509 sold
1968 MB 300SEL 6.3 #0335
1972 MB 280SE 4.5 75k miles sold
1971 MB 230/8 60k miles sold
1972 Volvo 142 60k miles sold
1969 Opel Rekord 1900 sold
1974 L508D Camper