Cold engine running problems

[SC Saab Owner]

My '85 Saab 900S runs fine once its warmed up, but when the engine is cold, it feels and runs like it is not firing on all cylinders. I had the oil changed back in January and the mechanic who performed the service adjusted the idle (without my permission). About a month after that, the problem started and I've had it ever since. When the engine reaches normal running temperature, everything seems fine.

I took the car in to be checked out at a Saab repair shop and the first thing they suggested to fix the problem was--a valve job. I don't think the valves are the problem.

What really pisses me off is that I wasn't having any problems with the car at all until that (damn!) mechanic messed with the idle (and possibly disconnected a vacuum hose or two). So right now I don't know who to trust. I don't as a rule trust mechanics to tell me the truth, Saab or otherwise, especially when the first thing I'm told is to have the valves adjusted for a $170!!!.

Can anybody help me diagnose this problem? I think it's a combination of improper idle adjustment and possibly some hoses that haven't been connected properly. Since I am not a mechanic, I don't know where to start, but like I said, the problem only materialized after I got the oil changed.

Any suggestions that might help me diagnose and fix this problem would be sincerely appreciated.

[Willy Wonka]

Where to start? Valve problems if on the intake cause vacuum fluctuations. If they suggested an adjustment, that's different than a valve job, which is replacement, machining etc. Adjustment is setting the gap by shimms or something on overhead cams. Look to your plugs for answeres. If they are all even colored, and their specific colors. If black sooted then the mixture's too rich. If yellow burned at spots the the mix is too lean. If white tipped with "gradual" shift to black then passible. The mechanical fuel injection is a pain and I'm not qualified to advise. It works with an air lifted plate that drives a fuel metering valve. Sometimes to set the idle some mechanics change the mix. Check the plugs!

[SC Saab Owner]

Thanks. It was a valve adjustment that was recommended, not a valve job as I incorrectly stated in my first message.

Does the fact that it runs normally though after the engine has warmed up suggest any other possible causes in your opinion? It always takes the same amount of time, about 5 minutes, for the engine to reach what appears to be normal running temperature. I judge this based on where the needle stops on the engine temperature gauge (or is that the oil temperature gauge?).

FYI, some time ago I was told that I have a compression problem due to a leak in this big round-shaped component that is attached to the firewall on the driver's side. I forget what it is called.

Nevertheless, I will check the plugs first before considering any other causes.

Many thanks for your help and best regards from Southern California!

[ProfZ]

It could be that something in the cold start system is either not connected, or has failed. The Haynes manual gives several simple tests you can do to check this out.

Definitely begin by examining your spark plugs, as suggested above. Look for any difference between plugs, as well as their color.

[gerryp]

My guess is the mechanic adjusted the idle with the throttle stop screw rather than the air bypass screw. This will have stopped the throttle sensor switch closing and that will stop the ECU from using the AIC to control your idle when the engine is cold.

What you have to do (if I'm right) is back off the throttle stop screw until you can hear a light spring-click just as the throttle closes. You won't need to adjust it much.

Good luck 8)