Haunted Bosch K-jetronic?

[bassie]

My car has some weird behavior. When starting cold is start immediately. It runs great and idle is about 800 rpm. When I shut it down and restart it after a short time it starts immediately but idle is higher (1000-1100 rpm) and it won't come down. However when I restart it after let's say 1 hour the idle is 800 rpm? I can drive for hours and the idle is about 800 rpm. Only on restart you get this strange behavior. Has someone any idea what is going on?

Car is a Euro 930 -1987.

[JFairman]

Sounds like typical mechanical k-jetronic. It is not consistant on all starts: cold, warm, sorta hot after a 15 minute shutdown, and hot restarts.

There are two systems that adjust warm up performance. The control pressure regulator/warmup regulator and the cold start auxilliary air valve. They both use mechanical bimetallic thermostat type strips of metal to operate them according to ambient temp and heatsoak transfer temp. Both bimetallic strips also have 12 volt wire wrap heater elements on them that are connected to the 12 volt supply voltage from the rear fuel pump relay.

Your high idle is caused by the aux air valve bypassing more air around the throttle body and the slight air fuel ratio changes from the WUR cold start control pressure changing during the time after you shut it off and then restarted it and whatever the ambient temperature is.

You'll never ever be able to make k-jetronic totally consistant like perfectly set up and tuned EFI. If it had a computer or ECU controlled idle control valve bypassing air around the throttle body at idle like EFI it would have a consistant idle at all times including when AC is turned on but it doesn't. That stuff wasn't invented yet.

Your's doesn't sound all that bad. You can add on a mechanical work around in the throttle body air bypass hose using a BMW 320i heater valve and generic choke cable to operate it and adjust cold start idle speed yourself from inside the car but thats not for everybody.

[RarlyL8]

You say the 1200rpm idle will not come down; is this after driving a fully warmed engine?

[spuggy]

Sounds to me like someone already swapped out your aux air for an SC one, which apparently was a popular mod back in the day because of the obnoxiously high 930 idle on a warm restart...

My 930 gives an 1800 RPM idle on warm restart, and takes a few minutes to start dropping. But I don't start the motor until I'm ready to drive, so don't let it idle immediately after starting anyway.

Only effect I could see is that it used to make it tougher to select reverse without clanging on a 915 (no synchro on that gear). The 12 second rule applies...

[bassie]

I have the car now for almost 14 years, and this is something of the last 3 months. The high idle is after restart with a fully warmed engine. With a restart after 1 hour the idle is normal. When it started with this behavior the idle was about 1300 rpm and some black smoke. Now the high idle is about 1000-1100 rpm. The high idle is still there after driving it (for 5 minutes). I restart the car now as little as possible.
The car is fully stock.

[mark houghton]

Bassie, do some simple diagnostics of your AAR to rule out some other possible air leak that may be causing your high idle.

Pull off the hose between the AAR and the IC, and look inside the AAR to view the notched window opening to see how far it's open. Do this under these conditions: (1) stone cold, (2) fully warmed up and idling correctly, and (3) fully warmed up but stuck at idling high after restart. Take a mental picture of the position of the opening...especially noting any differences between #2 and 3. As well, the next time she's stuck at high idle when fully warm, pinch the hose completely closed and note if the idle drops. And finally, with the hose off, reach inside with a pencil and try to move the internal disc. It should move freely against spring tension and return back to its starting point.

All that will give you an idea if the AAR is working correctly. Could be that it's gummed up and sticking. It will (or should) open up after you shut down hot and cause a higher idle, but it should also close back down after a minute or so after restarting and drop the idle. If everything appears normal, then perhaps you've got an air leak somewhere.

[JFairman]

From your trees it looks like you're in a cool climate this time of year. The car will do this more when it's cool or cold out.

A rag wrapped around the AAV hose for some protection and needle nose pliers work well to pinch it and cut off possible air flowing through it while checking for idle speed change.
Needle nose vice grips work good to hold it pinched while checking things out. If the hose is dried out and cracks while doing that you needed a new one anyway.

[RarlyL8]

What year is your Euro?
Start the engine dead cold then pull the RED hose off the distributor while the engine is running. Note if the rpm goes UP or stays the same.

[JFairman]

Be very careful and keep your fingers away from the engine fan if you do that.. It could wack some of em off in fraction of a second.

Then you'll be driving a car with an automatic transmission and sell the 930.

[bassie]

Thanks for the answers! I will do some testing and will let you know what the result is.

[tops911]

JFairman, I did that once. I was timing my car (new distributor) after a hernia surgery and was on percacet and beer. I got the end of my thumb in the fan (motor running) and I remember thinking "man, if I was not on the pills mixed with beer this would have hurt". LOL
sorry for the off topic...

[JFairman]

Ouch! hope your thumb is ok.

Beer does kick in the percosets nice... makes going to the fridge for another beer more fun.
I had some of those after a motorcycle crash followed by shoulder surgery in '89.