Fast Idle Revisited

[JJG911]

I have checked several things on my 79 911 SC that were suggested by the list. Today I took the dist cap off and the lash checked good. Cleaned the rotor and cap and reinstalled after finding no problems.

Then I decided to check for vacuum leaks using starter fluid with a fire extinguisher standing by. Something really strange happened as I was spraying around the intake boot. When I sprayed in a certain spot the engine would suddenly sputter and cut off. I could crank it back up, spray again in that general area and it killed the engine. I took the air filter housing off to try to locate the trouble spot which seems to be near one of the two clamps that attach the intake boot to the intake.

Why would spraying kill the engine rather than increasing RPM? Does this indicate a problem at the connection or something deeper into the intake structure.

Always learning,
Jim

[steely]

oxygen starvation? still sounds like you found a leak.

[JJG911]

I am still totally baffled why the engine kills when I spray starter fluid around the intake boot. I'm trying to find if there is a vacuum leak causing the fast idle problem, but this has me stumped.

Please any help!

[Targalid]

Just a thought. If it is already adjusted rich with an air leak, feeding extra fuel through the leak might cause it to stall.

[JJG911]

That is a possibility Targalid. The exhaust seems to be very rich, especially during warm up. Any ideas from experience where the problem lies? Should I just reseat the intake box and tighten the clamps or is there a seal between the intake air box and intake?

Thanks!

[JJG911]

Another question from this newbie. When Targalid says "adjusted rich due to air leak" does that mean that the computer system has adjusted the mixture to the rich side. If I find and repair the leak will the computer make corresponding adjustments? I'm trying to picture if this is a manual adjustment or a computer adjustment.

Thanks

[steely]

Originally Posted by steely

oxygen starvation? still sounds like you found a leak.

I guess I should have explained myself better - when you used the starter fluid, you replaced the air at the intake with more fuel, and that caused the stumble/rpm reduction. Targalid said it better.

As leaks may have developed over time, a previous owner may have manually adjusted the mixture to compensate. If you find a leak and stop it, a re-adjustment may be needed. I don't know that you have a computer. If your 79 is stock, it may be CIS, and you can check out this tech article. I hope this helps.
Pelican Technical Article: Tuning and Adjusting CIS Fuel Injection - 911 (1974-83) - 930 Turbo (1975-89)

[theFONZ]

Your SC will have a manual adjustment on the fuel distributor. No computers for 79 CIS.

You obviously found a leak so you will need to fix that first. That leak is causing unmetered air to enter. I would guess the fuel distributor has been unknowingly adjusted to mask that problem, so you may have to adjust it after the leak is fixed.

[timmy2]

Do you have a pop off valve? Check that the seal on it is good both under the lid and where it is epoxied in. Check all of your rubber isolators between the airbox and runners fit tight and have no cracks. You might want to do a low pressure air test with soapy water to check for leaks. Remove and check every vacuum hose, they can be split or cracked and not be visible. I pull them off and do a suction test... I just found the 1" long reducer adaptor for the vaucuum hose where it attaches to the WUR was leaking.

[rusnak]

You could be running way too rich.

Start by checking your fuel pressure and CO% mixture.

[Targalid]

JJG911,
All the CIS Gurus will tell you the same thing. Before you adjust the fuel mixture, find the air leaks. For certain get a fuel pressure gauge kit and measure your basic fuel pressure readings. Once you do this you will not be flying blind. Hooking up the gauges is easy and the readings are important for diagnosis of the CIS. Likely the previous owner compensated for an air leak with an adjustment to the fuel mixture screw and you are suffering the consequences. I had the same problems with my "new" car. Look for loose or split hoses as well as the leak you have already found. Fix these simple problems, measure the fuel pressures so you know the basic components are OK and then adjust the fuel mixture. It is kind of fun and very rewarding to get your car running really well.

[Mike Kast]

Pelican Parts - Product Information: 928-110-158-01-M260
I have changed out all of these on my car twice. The left side is easy, the right side is not and if you don't do at least a partial engine drop you risk breaking the air box and or dropping all kinds of things down the intakes.
That being said, I didn't drop the engine. These sleeves get holes in them. I bet that is your problem. You must polish the manifolds if you remove them. It is mandatory. To reassemble, I applied dielectric grease to the inside of the rubber sleeve and slid it all the way over the manifold so it was flush with the end with the clamps loosely in place. Then I bolted the manifold back in place and easily slid the sleeve in place.

[JJG911]

Timmy, Targalid, Mike and others who contributed. Thanks so much for the valuable information. To have been a Porsche owner for only 5 days I fell as though I've learned a wealth of information in such a short time. Yes, I do have the pop off valve and it looks good.

Today I took a closer look at the area where I sprayed starter fluid that had killed the engine. I had thought that the problem might be the air intake box to intake fit, but don't think that is really the problem.

Just below where the left side of the intake air box clamps to the left part of the intake are two vacuum fittings about 1 inch apart. The right hand fitting has a hose about 1 foot long that I removed and inspected and plan to replace although it looked OK. The left hand vacuum fitting was plugged with a piece of vacuum hose with a screw (or something similar) in the end. Having worked around motors for years, this usually means some mechanic wanted to mask a problem rather than fix the problem. After checking the plug for leaks I drove the car and it ran fine past normal warm up, but eventually resorted back to high idle but now it is down to about 1200 RPM. I'm not there yet but in 3 days the fast idle has come down from 3000 to 1800 to 1200 RPM.

Question:

What's the plug for? Is this some common method to conceal the bigger problem? As usual, any advice is appreciated. Next week I plan to check the fuel pressures and the vacuum system, but I'm really curious about this plug.

Always learning,
Jim

[timmy2]

If the "plug" is on the side of the airbox higher than the intake runners and more towards the alternator it opens into the airbox intake on the filter side and really shouldn't affect your vacuum. Take your filter off and you'll see what I mean.

The other hose is your vacuum line to your power brake booster. The vacuum line to the booster has been known to crack where it goes up and over in the shock area. Could have other leaks in that first section too.

[timmy2]

Or...... if there are 2 vacuum ports side by side below the "throttle body" then someone may have changed out your airbox for a newer one. '78 and '79 should be the same. I just looked at my old one and there is only one large port for the vacuum boost to the brakes, the other is higher and on the intake section.

Can you get a camera in there and take a picture for reference?

[JJG911]

I'll take and post some pics tomorrow. The two vacuum ports are both about 1/8 inch. I would think these are too small to be going to the brake booster.