Need help diagnosing this problem!!!
 

I have a problem that I need help troubleshooting. My car is a 1979, and idles smooth-as-silk from a cold start. For about 10 minutes the car runs great, oil pressure is normal. When the car is being driven, it runs fine. But when it is idling at stoplights, etc., the idle starts becoming erratic, and oil pressure drops to where the dash warning light comes on. The car will also shake a bit, and sometimes you can hear some clanking noises from under the hood. But when the car is actually driving, it is fine. It never stalls, either. So.....for the first ten minutes, the car idles and drives great! It then progressively gets worse to where the car idles like absolute crap when at a stoplight. Your thoughts / ideas / suggestions are greatly needed and appreciated!!!

WHat's the indicated idle speed when it's running so rough? This sounds to me like the idle is too low when warm, the low oil pressure being an RPM issue not a mechanical one. You may need to use an external tach to test for accurate idle RPM, as the dash guage can be a little off.

If the RPM is too low, there's an adjustment screw at the front of the throttle body, turn this out a little at a time to get the right setting. The screw is a bit hidden, but has a rather large head on it so it's hard to miss.

You'll also want to do the vacuum leak search of all the rubber hoses underhood, and probasbly find yourself replacing some (if not all) of them. Running some fuel injector cleaner thru the next couple tanks may clear up and stabilize the idle as well.

The question in my mind is, "is the low idle causing the problem of low oil pressure"......or........."is the low oil pressure causing the low idle".

If the the motor heats up, clearances can, indeed, increase and cause rough idling, in which case, if it idles down far enough it can cause low oil pressure and perhaps some clunking.

Your car is a CIS (continual injection spray) type, I believe, so it's different than my 83 with L-jetronic, but the vac still is important, and there are lots and lots of old brittle vac lines under the hood that have been cooked for years. When I replaced spark plugs a while back, I touched some of them and they broke upon contact.

Probably the best thing you can do is to take it in to a shop familiar with the 928, and let them use the diagnostic tools available to find out what the issue is.

My car is set to idle just below 1000 on the tach, maybe around 800 or so which seems a bit high but it runs and drives well at that point.

Here is some moral support for you, buddy, all the best. I hope its a simple fix for you.

Regards, P

When cold the control pressure regulator (WUR) provides extra fuel so a vacuum leak would be masked until the engines warms up. There are lots of hoses to check and some may require removing the top of the air filter housing and upper air plenum but it appears the task is overdue.

Dennis

Thanks for your responses. Looks like I've got my work cut out for me!