Random idle surge problem
 

Hi guys,

My car is 1980 928S euro model (manual, K-jetronic).
I have this strange problem in which sometimes the idle simply goes up until 2000 RPM or the RPM doesnt drop to idle and stays around 2000RPM sometimes for 20-30 sec or can be few minutes.
I searched the forum but I couldnt find specific info with regards to K-Jetronic systems.

I noticed that this happens more often under the following conditions and always when the engine is hot:
1. If I adjust the CO level to be too high
2. If the CO is normal this happens more if the radiator electric vent is on (!?) Noticed this few times but can be a coincidence.

I checked the WUR and it seems to be ok. Perhaps this can be related to the electric pump/fuel filter? The filter was changed about 6 years ago but the car only had about 30K KM since than. The fuel tank was also replaced 2 years ago due to licking strainer.

Thanks for your help!

I've had that issue on my '79 and I assumed it was the distributor vacuum retard wasn't kicking in. My car only has one vacuum connection and it retards the ignition as part of the pollution control system. If I pull the vacuum hose off the distributor the engine speeds up to about 2000 rpm. Also if I have the idle screw down a little too tight, the engine slows down until the vacuum drops, the retard goes away and the engine speeds up just to repeat the process with about a 4 second cycle.

Key item for engine RPM increasing is air, something is leaking a LOT of air.

Guys, thanks a lot for your replies! Is it possible that it is the WUR? or most likely one of the vacuum lines?

I had similar issues with my 1980 928S 300bhp UK car a few years back, and it was sorted once I replaced all vacuum lines with silicon, (and I emphasise every line) not just one or two, they all rely on each other I found.., also sprayed some wd40 into the WUR and let it set a couple of days, only other issue I had in the meantime was an overhaul of the fuel distributor , it all seems fine now touch wood.

My understanding is the WUR changes the air/fuel ratio during the the warm up period by changing the pressure on top of the fuel distributor. I don't think that would cause the rpm to surge. Do you mean the auxiliary air valve? As Danglerb suggested, you are looking for a major source of air.

..., yeh, sorry my bad !

Thanks guys! I checked most of the vacuum lines and they look good. When I disconnect the vacuum from the distributor the idle indeed surge but reconnecting it make it normal. Maybe there is indeed an air leak somewhere that causes this phenomenon. I will focus on the auxiliary air valve and see how it goes.