SC Idle Confusion

[mercury26]

So I recently purchase a '82 911SC and working on getting the car baselined. I took in for service this past week, valve adjustment, new spark plugs, fuel filter, etc ( a good old tune up). I picked up the car today and drove it home. I did some spirited driving, but nothing crazy but never went above 5500/6000 RPM when winding it out.

I parked the car and went into my office for maybe 30 minutes. I came out and turned on the car, noticed the idle was about 650 RPMs. Also the idle would bounce occasionally bounce up and down, maybe a couple hundred RPMs but nothing major. The car idled fine, no rought running or anything just the idle was low.

I adjusted the idle with the idle screw and bumped the idle up to about 950 RPM. I still see the little bit of bouncing, but does not occur often. I am just a bit perplexed at the low idle.

Any thoughts? I am happy to have my 911 and enjoy the car. I look forward to all kind of fun.

Cheers,

Chuck

[Vereeken]

If it stays at 950 rpm after you increased the ilde you are ok.

The low idle after restart and the up an down bouncing on an SC is due to the fuel mixture being too rich at idle.

What you have done is added more air to the mixture when at idle.

If the car is now behaving as it should you just solved it.

If not you will need to check WUR control pressures and mixture but that is a lot of work involving some test equipment. Look for WUR control pressures.

Michel

[LUFTMAN]

that model is a frequency valve model check fuse for oxs system in front fuse box. same fuse for interior lights. do dome lights work? if so those models are very sensitive to any air leak injector o-rings,vacum hoses,fuel injectors live in a plastic insert that is staked into each intake manifold runner there are 2 o-rings that are usually hard as a rock never replaced. after car is warmed up remove oil tank cap while engine is running does car idle change? run lower it should. a few things to check first

[mercury26]

I will continue to monitor. Though one thing changed from the time of tune-up at the shop and me leaving with the car from the shop.

The car was all wrapped up (tune-up completed) and was sitting. I was chatting with technician and both discussed our dislike of K&N filters (which my car came with). I decided to put a stock paper filter in the car and the removed the K&N filter. I then left the shop and headed home. Would a K&N to paper filter change enough of the air flow volume to alter the idle?

Thanks,

Chuck

[Vereeken]

Could very well be. It makes sense. The hunting at idle is usually a sign of too rich. Before with the K&N you had more air going through -or so they claim- now with the paper one, less air is coming through but he volume of fuel is unchanged.

So there is soem logic behind it.

[mercury26]

I have another hunch also, which I am going to check out tonight. When I had the major service done on the engine, I had them install a Pop-off valve. I am wondering if I might have a sealing issue around the pop-off valve. I am going to try to peak my head in the airbox tonight and see what's going on.

[Vereeken]

Also possible. check with carb cleaner around the pop off valve. If it leaks the engine will surge.

Most pop-off valves have a O ring under the lid. It can also be that it is not properly seated., check as well.

[rusnak]

I concurr with Vereeken. I would hook up an afr gauge and cis pressure gauge to baseline it. No guesswork.

[mercury26]

Issue resolved (sorta), took my car back to garage that did service and had them look into the idle. Essentially they did a bunch of work on the throttle linkage (to get things right) and also found problems with the relay/computer for mixture adjustment. We disconnected the oxygen sensor for now, which sorted things out a bit. So while the issue is not completely resolved, the idle is a lot better.

[Joe Bob]

The 02 sensor is important and a wear item, disconnecting it will cause rich conditions and bad mileage.

[mercury26]

OK, not resolved. I had the same low idle issue after I stepped out from work this afternoon.

Also, I understand the O2 sensor and bad mileage. We are still in diagnosis mode.

Thanks,

Chuck

[rusnak]

Check fuel pressure while in failure mode. Then check air fuel ratio with a gauge/ wideband tester.