My very stock 1993 968 runs great except for a slightly annoying pulse in idle rpm speed. As read via my Durametric, the rpm alternates between 800 (sometimes 780) and 840 (sometimes 880) rpm when warmed up. Just a constant pulse between the rpms and resulting slight roughness from a very smooth 4 cylinder otherwise. Not enough to cause problems but annoying to me at lights.

Is this normal or any idea where to start?

1993 968 with 100k miles with recent belts, solid state DME, new distributor, plugs, new fuel system (pump tank lines) after sitting for 10 years, but I have close to 1000 miles on it since recommissioning. It always cold starts after 3-4 rotations.

 

Not normal. You can finagle the ISV out from under the intake and clean it, or maybe clean it via one of the lines to/from it at the throttle body. PB blaster or an acetone/ATF mix perhaps.
https://rennlist.com/forums/924-931-...s-up-down.html

 

Thanks - given that these seem to be > $600 new - I'll opt to try to clean it first.

Finagle is a good word - I've taken off the MAF and still trying to figure out how to get the ISV out from under the intake without removing too much.

 

The part is quite common, used in Saab, Volvo, etc. One of my 968s came to me with an E30 BMW ISV in it, which worked fine despite have the wrong hose arrangement. I have always been able to get them out eventually, try not to break the plastic elbow under the MAF boot.

You can also try to clean them in place, that is a common 928 thing as they are pretty buried on the 928.
-Joel.

 

Sorry to disagree with other opinions, but this slight surge at idle is common. It is the Motronic unit slightly increasing and decreasing fuel delivery so to achieve the optimum average fuel mixture. This is typically an indicator that your O2 sensor is alive and well.

As the O2 sensor reading starts heading rich, the M unit starts decreasing the duty cycle (pulse width) to the injectors. As the O2 signal then heads lean, the M unit then increases duty cycle. This constant “up and down” change in fuel mixture results in a slight idle surge that a perceptive operator will notice.

If you wish to confirm this, then unplug the round O2 sensor located behind the intake manifold. If the surge stops immediately, then it’s completely normal operation or perhaps your O2 sensor is slowing down a bit and your surge has become more noticeable. It the surge remains, then unplug the idle valve and see if that remedies it.

Be very careful running chemicals through the idle control valve. Bosch did not intend for it to be cleaned and over the years I have seen some ruined by being flooded with some sort of solvent. Make sure you hold the unit so that no cleaner can run into the motor area. I hope this helps

Cliff

 

Thanks @chudson. My surge is only noticeable as the engine is very slightly rough when it dips to 800 rpm and then pulses to 840. It’s totally livable. I did attempt to remove the ICV but got nervous as it would have meant cutting the (original?) hose clamp(s) on the rubber hoses under the ICV to remove and I could see no way to cut or reinstall without removing the intake.

I am also nervous about spraying cleaner in there as I would have had to cut the same clamps.

I’ll see if the O2 sensor is part of it as you suggest but maybe this is just part of my cars character. I would have loved to of had a perfectly smooth idle.