82 911-SC has pulsing idle

[brshaw911]

My 82 911-SC runs fine while driving, but sometimes and not always, when you stop at a long stop light or get stuck in trafic the idle starts to fluctuate a small amount and eventually gets worse and starts to hunt down to 200 rpm and up to 1200~1800 rpm with a cycle of 10 to 20 seconds between the highs and lows. It also sometimes stays at a high idle of about 1200 rpm and just failed California smog because of the high idle. It has been doing this, idle thing for months but it usually does not bother me as I live in the country and I get in the car and go until I get to my destination.

Does anyone have any ideas of how I can fix the problem?

Thanks for your interest,

[Paulporsche]

Hi and welcome.

For questions like this, usually you can find what others have done by using the "search" function.

In a nutshell, though, fluctuating idle is usually a result of a too rich mixture, which can be leaned out by adjusting the fuel mixture screw in a counterclockwise direction in small increments(about 1/32 or 1/16 of a turn) . Take the allen wrench out, rev the engine, and see if your idle steadies.

Do not adjust too much without the aid of an exhaust tester, as 1 complete rev is approx equal to 8%. Most of the time about 1/16 to 1/8 turn will be enough to fix your idle.

Good luck and let us know if that worked.

Paul

[vash]

i would check cis pressures first. i bet wur press are low making you rich.

[rickeolis]

Look for vacuum leaks after the air cleaner cover, like tubes that may have poor seals, or right at the cylinders, this problem could be from those leaks.

[brshaw911]

Thanks for your suggestion's. Could you please tell me where to find the fuel mixture screw or where to find some photo's of where it is.

I would like to see if that corrects the pulsing problem and then take it in to see if it will pass smog.

Thanks for your help,

Brian

[Paulporsche]

Brian,

Try a search. The mixture can be adjusted with a long 3 mm allen wrench placed down in the hole on the top of the mixture control unit.

Vash and Rickeolis may be right. You might ultimately find that leaks or an out of adjustment WUR is the culprit. I am suggesting trying the easy, cheap way first. Many times, the mixture is simply too rich. If that slight leaning works, then you're OK. If not, then the gauges will tell you exactly where your control pressures are. You will also find some suggestions on how to check for vacuum leaks.

If you get it sorted out, let us know what it was.

[randyrap]

I have to agree with Paul. I had the exact same problem and did what he suggested and it cleared up the problem.

Randy

[pjv911]

1980 was the last year the mixture screw was left open without the pressed in aluminum block off cap. Meaning if yours was never touched you will need to drill it out and remove by threading in a sheetmetal screw and pulling. Honestly it sounds like you will get into further trouble if you start playing around. I would suggest you have a shop check it out for you. Use the above suggestions while discussing it with your wrench to let him know your somwhat educated. You may leave with your problems solved for under $100.00 if its only an adjustment.

Kurt Williams

[Paulporsche]

I think for any of the suggestions you get on this board, if you don't feel comfortable doing them yourself, get a pro to do it. As pjv911 says, you will at least be knowledgeable about the problem. Maybe you will be able to tackle it later yourself if it ever comes up again.

I see you are in Oakland. There are lots of CA guys on this board. Maybe some of them can recommend a good wrench.

[brshaw911]

82 911-SC has pulsing idle

Thanks to everyone who has helped with my problem.

I agree it might be better to take the car into a good wrench, can anyone recomend someone in Oakland or Alameda. Also I am looking for a good mechanic or shop to make sure it passes smog before I take it in and have it fail again

It also sometimes stays at a high idle of about 1200 rpm and just failed California smog because of the high idle (I think the guy doing the test did not let it warm up).

I checked on the pulsing idle problem and have a better description of the symtoms, maybe that will help with the solution.

My 82 911-SC runs fine while driving, but when the engine is warm, and you stop for a stop light or get stuck in trafic the idle starts to fluctuate a small amount and after a minute it gets worse and starts to hunt down to 200 rpm and up to 1400 rpm with a cycle of 2 to 3 seconds between the highs and lows. If I push down on the gas pedal slightly I can get it to hold a steady idle of 1000 rpm. It has been doing this, idle thing for months.

Thanks,

Ciao! - Brian
~~~_/) ~~~

82 911SC Targa

[fstar]

All my idle issues, including the one you describe were eliminated when I disconected my O2 sensor.. I'm offering this as information only, the guys above know what they are doing, I do not. Good luck.

[Paulporsche]

Brian,

Since I'm not in CA, I don't know for sure, but wouldn't the car fail the smog test if the tech noticed the O2 sensor were disconnected?

In the short term, that might be a good fix. Also maybe the sensor could be disconnected but still put back in position to look stock at the test.

Don't forget, there were a couple of other possible causes mentioned.
It's probably best to get it fixed right and then go on from there.

Let us know what you did.

[randyrap]

Paul.

What was that trick mentioned in a prior discussion on how to adjust the mixture? Had to do with pushing up and pulling down on the air flow sensor.

Thanks ............................ Randy

[Paulporsche]

Randy,

I think what was said was that if you push the air flow sensor plate up and the engine speed goes up, you can go richer. If you pull it down and the revs rise you should go leaner. If you push up and if you pull down and ther evs drop each time, then you are in the sweet spot.

If you try this, let us know if it works and if you see/feel any benefit.

[randyrap]

Thanks Paul. I was searching for that discussion thread but just couldn't find it. I'll give it a try and let you know.
Randy

[wswilburn]

I found on my car that when the mixture is too rich, the O2 sensor system slowly leans the mixture until it acts like it hits a stop (30% duty cycle), then jumps back to default setting (50%). It does this about once a second or two, causing a pulsing idle. Disconnecting the O2 sensor would stop the pulsing, but the mixture was still too rich. Adjusting the mixture cured it. Reading other threads, though sounds like there are other causes of pulsing idle as well.

[vesnyder]

My '83 SC just starting doing the same thing, but only briefly when I start it cold - it stops after ~30 sec??

[scedmonds]

Mine does it too -- and has for a year or so. It only gets driven once a week, now, so it doesn't bug me like it used to when the car was a daily driver.

I'll get my wrench to adjust the fuel mixture --

Cheers!

C.