High Idle '79 3.0 when warm

[tadel001]

I have a stock '79 3.0 RoW engine in my 76 911 coupe. When cold the idle settles down at 1,000. As it warms, it creeps to close to 2,000 RPMs. I have done some research on this site and not coming up with answers.

1) Air Leak - When I remove the oil cap, the idle drops and the car will usually stall after 10 seconds.

2) Aux Air Valve - I removed the hose and looked at the opening (it was open), ran the car until the idle was high, removed hose and it was closed.

So, I am assuming it is not an air leak or aux air valve. Any other ideas?

[mca]

Does your engine have a decel valve?

I'm not familiar with the set up on the euro. If you DO have decel valve, you can pull the vacuum line off of the nipple and plug it (the line) with a golf tee. This simple test will let you know if the decel valve is failing. When it fails, it causes high idle as the engine warms.

[Bob Kontak]

+1 on mca's comment

I saw a mechanic take a big pair of channel locks to my decel valve and squish it thinner to make the idle drop - it worked - I can still see the marks. I have no idea what the basis was for the fix but it worked.

[mca]

Originally Posted by Bob Kontak

+1 on mca's comment

I saw a mechanic take a big pair of channel locks to my decel valve and squish it thinner to make the idle drop - it worked - I can still see the marks. I have no idea what the basis was for the fix but it worked.

Yep. That is how they are fixed. Probably better to put in a vice to prevent from compressing too much.

[tadel001]

can you explain where the decal valve is and what it looks like. I was under the impression the aux air valve and decal valve were the same.

[mca]

Originally Posted by tadel001

can you explain where the decal valve is and what it looks like. I was under the impression the aux air valve and decal valve were the same.

On a 79 it has been described as "it is an acorn looking thing behind the throttle body."

The decel valve is much different on the 80-83. It is pancake shaped - right side of engine.

[Bob Kontak]

The aux air valve has a different function than the decel valve. It can cause high idle if it does not close after start up.

Read the components section of this CIS primer for more info. The aux air valve is painfully expensive.

CIS Primer for the Porsche 911

[tadel001]

So I found the decel valve. I am going to pull the top hose today to see if that is the cause. I did a lot of searching but couldn't find too much in the details of pancaking the decel valve if that is the problem. MCA, I saw a post that you did this.

I have the acorn style decel valve. It is the one on the throttle body, not the one that looks and is near the AAV. I assume you cannot "pancake" this style. Is there another fix (rebuild) or just buy a new one if it is the problem?

Thanks, Tim

[sobamaflyer]

I've been going through this exact problem myself recently (also have a '75 with a '78 motor). I defeated the decel valve and it seemed fixed for a week or 2, then same thing started happening again.

I too have been reading bits and pieces about this issue and noticed people mentioning the AAV sticking and would be helped by gentle whacking w/ a wooden screwdriver handle. So I've tried this and twice so far it's dropped the idle back to normal and that day's activities went on just fine.

I read a post somewhere about eliminating the DV, AAV and one other component and re-hooking up our hand throttle for startups. I will probably do a bit more research then may attempt this as the next step. Given the cost of replacing these parts and the debatable need for them I'm starting to think this is the way to go.

Sorry I post with not too much new info, just moral support as someone going through the same thing you are slowly trying to fix it, as it's not been a huge problem just yet.

[mca]

Originally Posted by tadel001

So I found the decel valve. I am going to pull the top hose today to see if that is the cause. I did a lot of searching but couldn't find too much in the details of pancaking the decel valve if that is the problem. MCA, I saw a post that you did this.

I have the acorn style decel valve. It is the one on the throttle body, not the one that looks and is near the AAV. I assume you cannot "pancake" this style. Is there another fix (rebuild) or just buy a new one if it is the problem?

Thanks, Tim

Not sure if there is a fix for the acorn style decel valve. If plugging the vacuum line solves your problem, you could start a new thread asking how to fix your type of valve.

I think a lot of people run without them anyhow. The main function is to slow the drop of the engine speed when you lift your foot from the throttle. Some like em ... some don't.

[Vincent Hill]

I had this on my 1976 and do not remember having a problem with it! On my 1982 All it seems to do is hang the rpms up high, so I disconnected it and pluged the vacuum line.

[tadel001]

Vincent,

What lines are you plugging? Remove the hose out of the top and plug hose? Do you then plug the hose entering the decel valve as well?

[crustychief]

Just Remove and plug the top hose to identify your problem. You may be able to fix it by gently squeezing it in a vice or with a pair of pliers, as stated earlier. If not then you will have to get a new one.

[tadel001]

I removed the hose from the top of the decel valve and plugged it with a golf-tee. This solved the high idle problem. So, is there any reason to replace it or just leave it plugged? Do I need to anything more to secure the system? I have attached a picture for future people.

[crustychief]

A lot of people leave theirs plugged with no ill effects ( unless your state has a visual inspection for smog ). All it does is delays the engine rpm's to make smoother shifts. I am of the belief of fixing the problems, not the symptoms though.

[mca]

Glad to hear it worked out for you.

Might want to do some searching to see if there is a fix for that type of valve. Or roll the dice on a used one from a parts supplier.

I don't think there is any harm in leaving it disabled.