High CIS Engine Idle

[86 911]

I know there are a million of these high idle posts but everyone's high idle issues happen a bit differently than mine, so I greatly appreciate the help

Car: 1978 911 SC 3.0 - stock engine.
Issue: Idle hangs up at ~1600 RPM for about 10-15 seconds when the engine is at operating temperature (when oil temp gauge needle is in the middle). This problem progresses as the engine gets warmer.
Chain of events: I start the car and engine struggles at 600 RPM for a few seconds and then idles fine at 900 RPM (pretty normal). When the engine gets up to 50% of operating temperature, the idle *almost* hangs at 1600 RPM but decides to drop back to idle. As the engine gets warmer, the idle will hang at 1600 RPM for longer. If it is a REALLY hot (100+ degree day) and engine temp is high, the idle can hang at 1600 for up to 30 seconds before lowering down to 900.
Began when the Porsche mechanic readjusted my mixture. The car was running super lean to pass smog and the mechanic readjusted it to run richer. High idle problem began immediately with the richer mixture. This did not happen at all when mixture was super lean.
Only Solution is if I leave the car in gear and press on the brake when I come to a stop so that the engine is forced to go below 1600 RPM. ***When I get it below that point and I release the clutch, the idle will drop naturally and not increase again until I accelerate above 1600.
Things I've Checked:
1. Blocked off and plugged vacuum to Decel Valve (does nothing),
2. Distributor advance is working and vacuum hose is new (not a problem)
3. Took off oil filler cap and engine struggles to stay running (normal)
4. WUR and Aux Air vale have been replaced about 10K miles ago, so I'm not suspecting them.

What do you think it is? My suspicions are the cold start valve.. I'm guessing it is not a vacuum leak because once I force the idle below 1600, the idle will stay at 900 and not escalate. Mechanic is not local so going back to him is not an option.

Thank you!!

[Bob Kontak]

Originally Posted by 86 911

My suspicions are the cold start valve.

Nope (betcha). You thinking it is spraying while the car is running?

[86 911]

Hmmm - I would assume it's still spraying but I really don't know enough about it to be confident that this is the case. Well if it's not the cold start valve, then I am relieved because it looks like its very hard to get to.

What baffles me about this problem is that it began when the mixture was adjusted to a richer setting.... what gives?

[Bob Kontak]

Originally Posted by 86 911

Good suggestion - I forgot to mention that I've tracked the linkage and found no hangups.

I pulled the reference to linkage/mechanical after reading your first post more thoroughly.

[Bob Kontak]

Doesn't the 78 have two Aux Air widgets? The valve and regulator?

Read failure mode on AAV & AAR in CIS components. Not saying it's the answer.

http://members.rennlist.com/jimwms/CIS/CIShome.html

[86 911]

Hey Bob - I edited my last post too. Two Aux Air widgets? Are you referring to the Aux Air Valve and the other valve in the back of the engine? If so, I only replaced the one on the side of the engine. The other one has not been checked due to it lack of accessibility with the engine in.

Edit: Thanks for the link. for the AAV, the failure mode is independent of engine heat.. my problem is dependent on engine heat, so I don't think that valve is the issue. And I don't think is the AAR because when I force the idle back down to 900 (or when it drops to 900 after a few seconds) it does not escalate again. I would assume that a constantly open AAR would keep the idle at 1600 all the time. Can someone confirm my logic on this?

[Targalid]

Could the throttle plate be hanging up on the way back to its rest position? This could be due either to an obstruction (piece of gunk) or lack of lubrication on the pivot point. Just guessing here. Try moving it with your hand and check forl a rough spot in its travel.

[Bob Kontak]

Originally Posted by 86 911

Hey Bob - I edited my last post too. Two Aux Air widgets?

Cool - thanks on the edit.

I may be thinking of the auxilary air device, which is your decel valve (also called the vacuum limiter).

Let me take a look at the parts on your car.

[Bob Kontak]

Originally Posted by Targalid

Could the throttle plate be hanging up on the way back to its rest position?

The reason I pulled my thoughts on the throttle linkage is due to the only thing done was making the mixture more rich. I am sure you have done what targalid recommends.

[86 911]

Hmm I'll keep the pivot point in mind. When I took the intake boot off, I cleaned any gunk out and the plate seemed to go up and down fine. Where should I lube it? Again, the odd thing is that this problem immediately began when mixture became richer...

[Bob Kontak]

After your car warms up, in your driveway, just "force" the linkage closed for a check when idle is high

You may have false air leaks you don't think you have (because of the RPM dropping when you pull the oil cap).

Can you screw the air bypass screw in all the way and have the car run after it warms?

I recognize the oddness of the only change being richening.

[86 911]

By air bypass screw, are you talking about the screw used to increase/decrease idle speed on the throttle body? If so, the idle drops/rises when I turn the screw and the engine is warm, so it is responsive as if the engine was cold. Another oddity is the nature of the hang up at 1600 - it will only hang up at that point but if I force the engine speed (by keeping the car in gear) to, say 1400 RPM, the idle will drop to normal speed.

Is there a smoke test I can do to find any air leaks?

[Bob Kontak]

One step at a time.

First replicate the condition in your driveway and see if you can force (manual assistance) the throttle linkage to it's resting point and drop the idle - assuming it is not doing this. Eliminate that variable.

Next test - screw the air bypass screw in all the way clockwise (when warm in the driveway) and see if the car stalls when warm. If it does not then we can talk about air leaks.

Do you have any fuel pressure gauges?

Talk tomorrow - bed for me. Later.

[boyt911sc]

Originally Posted by 86 911

Hmm I'll keep the pivot point in mind. When I took the intake boot off, I cleaned any gunk out and the plate seemed to go up and down fine. Where should I lube it? Again, the odd thing is that this problem immediately began when mixture became richer...

Matt,

Talk to your mechanic and ask him why he tweaked the air mixture screw in the first place. He must have a good reason why he did it. Did he use a gas analyzer? If you are using a mechanic, why not bring it back? BTW, many good young mechanics are not familiar with this antiquated technology.

This is a very common mistake I see today when people are so eager to change or alter the air mixture screw during troubleshooting. That's the very last thing I would touch specially for starting problem. That's has a very sensitive setting and should only be changed after verifying the absence of vacuum/air leak in the system. Once you tweaked it, you have added more variables to the problem. It is adjusted for fine tuning and not for making the engine run good.

A change in the fuel mixture ratio could be attributed to two (2) things:
1). Fuel supply variation.
2). Air supply variation.
Or a combination of #1 & #2. But it boils down to either fuel or air. For fuel supply you use pressure gauge and for air (???) use vacuum/air leak detection methods.

If I were in your shoes, I would do the following:
1). Check ignition timing, valve setting, good spark plugs, etc.
2). Check fuel pressures (system, control, & residual)
3). Check for vacuum/air leaks.
4). Tell your mechanic it is not running well.

Keep us posted.

Tony

[86 911]

Hi Bob and Tony - thank you for your info. Bob - I'll get back to you tomorrow about what I find out.
Tony - I don't have any of the fuel pressure gauges, just a simple vacuum gauge. The reason why the mechanic tweaked it was because I told him the mixture was leaned out to pass smog and I wanted it to run a bit richer. The mechanic I took it to has a good rep, been in service for 30 years and I think Pelican hosts meets at his garage in Riverside, CA. He's a great guy and even lent me specialty tools when I was doing DIY work. He adjusted the mixture by feel, so that may be the problem. I'll keep everyone posted.. since I don't have the gauges I will take it to a trusted garage in the LA area and have them do some tests, hopefully, at a graduate student-friendly rate. Haha.

[lonewolf]

Originally Posted by 86 911

I know there are a million of these high idle posts but everyone's high idle issues happen a bit differently than mine, so I greatly appreciate the help

Car: 1978 911 SC 3.0 - stock engine.
Issue: Idle hangs up at ~1600 RPM for about 10-15 seconds when the engine is at operating temperature (when oil temp gauge needle is in the middle). This problem progresses as the engine gets warmer.
Chain of events: I start the car and engine struggles at 600 RPM for a few seconds and then idles fine at 900 RPM (pretty normal). When the engine gets up to 50% of operating temperature, the idle *almost* hangs at 1600 RPM but decides to drop back to idle. As the engine gets warmer, the idle will hang at 1600 RPM for longer. If it is a REALLY hot (100+ degree day) and engine temp is high, the idle can hang at 1600 for up to 30 seconds before lowering down to 900.
Began when the Porsche mechanic readjusted my mixture. The car was running super lean to pass smog and the mechanic readjusted it to run richer. High idle problem began immediately with the richer mixture. This did not happen at all when mixture was super lean.
Only Solution is if I leave the car in gear and press on the brake when I come to a stop so that the engine is forced to go below 1600 RPM. ***When I get it below that point and I release the clutch, the idle will drop naturally and not increase again until I accelerate above 1600.
Things I've Checked:
1. Blocked off and plugged vacuum to Decel Valve (does nothing),
2. Distributor advance is working and vacuum hose is new (not a problem)
3. Took off oil filler cap and engine struggles to stay running (normal)
4. WUR and Aux Air vale have been replaced about 10K miles ago, so I'm not suspecting them.

What do you think it is? My suspicions are the cold start valve.. I'm guessing it is not a vacuum leak because once I force the idle below 1600, the idle will stay at 900 and not escalate. Mechanic is not local so going back to him is not an option.

Thank you!!

Am I missing something here?
Car ran fine before you had mixture adjusted. problem arose right after you had mixture adjusted.
Why are you looking at all of the other parts of the engine ?
Always go back to before the problem started.
What was touched?,adjusted?,turned?or?
In your case the mixture was adjusted and then a problem arose.
Go back and have the mixture adjusted properly
Now that you have touched the idle bypass screw everything is out of wack.
the idle bypass and mixture screw have to be adjusted together and it's not as easy as just turning screws.
they are finicky and you really have to understand the system and what you are doing before you adjust anything.
If you ever do it with the assist of an analyzer you will see how the tinyest movement of either screw makes a big diff in air/fuel ratio.

[jjrowe]

Did you check that you have power to the AAV? There is a connector on it (at least on mine) I had similar issues when mine wasn't getting 12v. When the engine is warms up its not closing as fast as it should. I was also able to kick mine down sometimes but then it would happen again at the next stop light.

Jesse
76 911s

[Bob Kontak]

Originally Posted by lonewolf

Now that you have touched the idle bypass screw everything is out of wack.
the idle bypass and mixture screw have to be adjusted together and it's not as easy as just turning screws..

Argh - And I told him to do what you should not do - turn in the idle bypass. I see Tony said the same thing (no touch).

Another deserved lump on the forehead.

What I have never done is use an analyzer. Always a dwell meter to get within spitting distance. I have really focused on the mixture screw as the primary driver of AF ration and have been pretty casual about the idle bypass.

[86 911]

Oops - looks like I should have checked the forums before touching it! Anyways, I ended up turning the idle screw all the way in when the engine was at operating temperature and the idle speed dropped to ~600-700 RPM (Is this good or bad?). I turned the screw back to its original setting to get the car to idle between 900-1000 RPM. Next thing I'll do is check the AAV for power and to make sure it is closing all the way.

Lonewolf - The only thing the mechanic touched was the mixture screw and the problem mysteriously began... it's just hard to believe that a richer mixture would cause this behavior so I'm guessing there must be another cause contributing to it.

[lonewolf]

Bob. I actually never read Tony's post till now but turning the bypass screw in richens it up.
To me Matt typed out the answer .
I can see Tony and I are on the same page.
Matt ,not sure how close you are with the mechanic .If he's a guy you can call and ask questions or just a place you took your car and asking is not an option.
The mechanic may have just turned the mixture screw a CH or so clockwise and to solve your problem it may be as easy.
Matt, just read your post as I was typing. doesn't sound like your setting is far off. Don't panic about touching the bypass ,sounds like it's working fine,just return it to it's position.