930 Hunting Idle - what causes this?

RarlyL8 · RarlyL8's Garage

Back at the motor mess this evening.
The auxiliary air valve (a.k.a. throttle bypass valve) has been removed and the lines plugged.

Here is what's left of the CIS:

Fuel Head
Air Flow Sensor Plate
Warm Up Regulator

That's it.

I Adjusted the idle screw from all the way shut to 2 turns out.
Hit the key and BAM she starts and idles at ~1500 RPM. Good deal.
In about 3 minutes the WUR kicks off and the idle shuts down to ~1000 RPM. I'm getting excited at this point.
Then it starts.

Vroooooooom POP Vroooooooom POP Vrooooooom POP

What the hell is that????
The idle starts cycling from 400 - 1500 RPM. I'm thinking this is a mixture problem so I back off the fuel mixture screw about 1/4 a turn. Hit the key and idle is at 1000 RPM. Cool.
So I get in the car and rev the engine. As soon as the rev comes down...........Vroooooooom POP Vroooooooom POP...... again.

So what's going on? Have I removed too much stuff and effected the idle mixture? Will another session with the gas tester and allen wrench cure this? What the hell is the "POP" noise? It is very mechanical / metalic sounding.

John Rogers · 09-05-2002, 06:41 PM

If you "backed off" the mixture adjustment or turned it to the left, then the mixture gets leaner; At least on our 1987 930 it does. Sounds like the mixture is way out and I'd recommend an air fuel meter to get it set correctly. The shop that does ours gets it warm by driving it, then going hard (full boost) and then checking the mixture. I thought hunting was caused by lean mixture, which could be air leaks such as cracked lines, intake spacer blocks loose, etc. Good luck.

dean · 09-05-2002, 07:32 PM

The pop sounds like the compressor bypass valve closing. Maybe

1bar · 09-05-2002, 10:36 PM

My car used to do this. The metallic popping sound is the pop-off valve, it just does that. The rubber bumpers may be worn inside, allowing the piston in the valve to bang into the metal end caps - but only go there if your sound is really a sharp metal on metal "smack" when the valve releases.

My problem was a bad Aux Air Valve. It stayed open all the time, so to compensate I had to adjust the idle screw very low. When I let off the gas, the idle would transiently drop to 200RPM (the "true" idle), then recover (POP) and (vrooooom) back to 1500 and repeat. Especially embarrasing when I restarted the car at a gas station.

I've been assuming it's something to do with the length of the Aux Air Valve hoses, and a stable limit cycle oscillation? I replaced the Aux Air Valve, got the electrical hookup squared away, readjusted the idle screw and then the mixture, and now it works well.

On my '77 the idle screw has a LOT more than 2 turns of adjustment range - don't know if it's a different pitch than on yours, or if you have more adjustment to play with...

Good luck!

RarlyL8 · RarlyL8's Garage

The auxiliary air valve on this engine ('78) has been removed and the lines blocked off.
I'm thinking that the mixture may still be too lean. The original setting on the fuel head was done for the smogged engine. Now it is stripped and has a Euro exhaust. Won't know until the gas tester is used, it was too dark to mess with it last evening (I don't like running cars in the garage).
The idle screw does have more movement than 2 turns. That is just a guess placement that I chose, the screw was full in previously due to the AAV staying open - like yours did.
Did you happen to adjust your timing when the AAV was replaced? Where is your timing at now?