308QV idle fluctuates, sometimes dies

I've finished doing the major on my '84 QV, now I'm on to troubleshooting an idle problem. This problem existed before the service, so it doesn't have anything to do with the work that was done. Here are the symptoms:

The car starts well, hot or cold, without any application of throttle. On cold start-up it settles into a pulsating, sinusoidal sounding idle. It is right around the desired 1000 rpm, but bounces up and down maybe 150 rpm. Sometimes it will smooth out a bit after a minute or two, but it never goes away completely.

Now for the annoying/embarrassing part. When I pull up to a stop sign or light, and let off the throttle completely the engine will slow way down to the point where the alternator light comes on. At this point sometimes it dies outright (maybe 1/2 the time). The the other times the engine seems to catch, speed back up, and start pulsating again. When this happens the swings seem to be slightly larger than the start up pulsations.

Any ideas? The car is a euro, so no lambda, O2 sensor, etc. Any advice would be appreciated. Nothing worse that having your car die at a light.

Let me add that at all other speeds it runs great and pulls hard. Even accelerating off of idle isn't an issue.

 

Symptoms were present before I did the work (belts, check valve clearance, degree cams). No change after the work was done. Cam covers were off for a couple weeks. Injectors have not been removed or serviced.

 

I took a quick look at the AAV. Mostly closed even though the garage was fairly cool. I'll take it off tonight and experiment further.

I definitely don't have a faster idle when cold, which would point to the AAV not working, but at same time the idle fluctuation problem occurs when hot or cold so I don't think the AAV is specifically to blame even though it may not be functioning.

Vacuum leak is probably the best guess. I'll try to pick up a gauge and a tester today and start testing lines. I figure I need to rule out a vacuum leak before I start messing with anything else.

 

I did some more trouble shooting last night. All vacuum lines are tight with no leaks. While running the car I did discover a few things. At first I had a vacuum gauge plugged into the port that typically connects to the oil separator canister on the crank case vent system. The car idled worse than usual, surging, and dying if throttle was not applied to catch it. Realizing that this port would usually pull from the cank case and the air intake box (before the metering head), I unplugged the gauge let it suck unmetered air directly into the plenum. The idle speed increased as the mixtured leaned out, as you would expect, but also became steady. When I plugged the line to the canister back in, it revented back to the original condition of moderate surging.

Based on these observation and the smell of the exhaust, it seems to be running too rich at idle and not getting enough air. Creating an air leak seemed to really improved things. I suspect I need to adjust the idle mixture, but I am hesitant to touch it until I'm sure everything else is right.

At this point the big unknowns are the condition of the injectors and the function of the WUR. I know the AAV isn't working right which would also improve the cold start idle.

 

Each line (ignition advance, brake booster, both lines to WUR) was unplugged at the plenum and vacuum tested with a hand pump. The AAV line was plugged with a stopper at one end and vacuum pulled on the other. All lines held vacuum.

 

I pulled the AAV tonight to check it out. At first I thought it was faulty because it was partially closed when cold with the hole 1/2 to 2/3 blocked. After pulling it I found the I could blow plenty of air through it. I did the freezer thing and it stayed the same, opening maybe a tiny bit more. Then I put it in the over and brought it up to 200F. At this temperature it closed off completely and I could not blow through it.

Next I tested the resistance and measured 45 ohms. I then hooked up a 12V power supply for about 5 minutes, and the valve closed. Not bubble tight like when I heated it in the over, but just a wisp was getting through with a hard blow.

Based on what I have read the valve functions as it should. My next step is to try to adjust the idle mixture, or order a fuel pressure tester to check out the WUR. Since the car runs well at speed I assume the WUR is fine and is at least putting the right control pressure once warm. I think I'll try the mixture adjustment first.

 

Latest progress is that I tested the WUR (hot/cold system and control pressure), and all was OK. At this point I think a mixture adjustment in all that is needed, but I've been socked in with snow and haven't had a chance to play with it.

 

I got a digital tach to check my idle speed. The factory tach was reading about 300 rpm high, so my idle speed was set quite a bit lower than it should have been. I adjusted the warm idle speed up to 1000 rpm, and my problem has all but vanished. There is still a little fluctuation at cold start, but no more stalling. I still think I am running a bit rich, and will fiddle with that when I have some time. The roads here are still not great.