After All These Years... Idle Speed Increases?

[scotricker]

First I thought the idle would come back down when the car warms up a little. But today even as the temp gauge came up, the idle speed was higher than normal, like 1800 or so, or more.

So I can go set the idle speed lower, but that got me to thinking maybe I should find out why it has increased? Or am I over thinking this, and should just go fix it.?

[fanaudical]

Binding throttle linkage?

[Reiver]

A lean condition often raises the idle....vacuum leak?

[JohnJL]

Vacuum leak. Search for various methods of finding it. I like using an unlit propane torch, when you find the leak, the gas makes the motor surge. Could be at any gasket surface, union or along any vacuum line or in a component like your AAR, WUR, Brake booster, etc. Good luck!

[DRACO A5OG]

On my buddy's 86, his vacuum line was disconnected going to the Fuel Pressure Regulator, reconnected and it came back to normal levels.

[universeman]

Try pulling up on the gas pedal with your foot to isolate linkage issues from everything else. If the idle drops, you've got a linkage issue.

[cabmandone]

If you really want to isolate the linkage, unhook it from the throttle body. If the idle settles down, your problem is between the pedal and throttle body. If your idle changed, it changed for a reason. I wouldn't simply make an adjustment on the idle because you're most likely masking a problem. When you remove your oil filler cap, does the idle somewhat noticeably drop?

[boyt911sc]

Originally Posted by cabmando

If you really want to isolate the linkage, unhook it from the throttle body. If the idle settles down, your problem is between the pedal and throttle body. If your idle changed, it changed for a reason. I wouldn't simply make an adjustment on the idle because you're most likely masking a problem. When you remove your oil filler cap, does the idle somewhat noticeably drop?

Scott,

Make the troubleshooting easy and simple. Before going crazy for these hard to find vacuum leaks, try the above suggestion first. Then proceed locating other most likely culprit. If the by-pass adjustment screw by the throttle body does not change or affect the idle speed, you have unmetered air going into the system.

Tony

[scotricker]

I removed the oil fill cap and there was not much difference. Previously, at the correct idle setting, removing the oil cap would induce wildly fluctuating oscillating idle speeds.

Now at the high idle situation, removing the oil fill cap creates a slight reduction in idle speed, but not nearly as dramatic as before.

Must find vacuum leak? But where to start?

I resisted the urge to try to just adjust the idle speed screw.

[Bob Kontak]

Originally Posted by scotricker

I resisted the urge to try to just adjust the idle speed screw.

Idle screw only adjusts the amount of air allowed to bypass the throttle plate. It is not something to be afraid of.

I suggest screwing it in clockwise until it stops. On an SC with no vacuum leaks, if you do this, the car will stall or just chug at 200 rpm.

I bet your car will still be running above 500 RPM.

Also, a car with no false air leaks will simply drop 150 RPM's and hold there when you pull the oil tank cap off. Absolutely no "wild oscillations".

You have work to do.

[scotricker]

Idle most days is now around 1100, unless it goes (and stays) higher for awhile.

Idle speed screw is turned in clockwise utill all-the-way bottomed out, no more to go. I thought it might be that pesky spring that wouldn't let it go in all the way, but that was not the issue.

Air filter cover is off, and heater blower motor is out of the way. I sprayed starter fluid around, and also tried un-lit propane torch, but no change in idle speed.

When I take off the oil cap I get a noticeable 100-150 rpm drop.

With the engine running, and the air cleaner box off, I can push up and/or down on that air flapper intake and get the idle to go down, to almost choke/stall.

But the little tiny brass adjuster screw with lock nut doesn't affect idle.

I'm gonna start getting out the hammers soon.

[timmy2]

Check the vacuum lines all around, could be a split one.
The air screw not affecting idle is a big clue that you have unmetered air coming in somewhere.

[jason2guy]

for the idle to increase you need metered air not UNmetered air. think about it- unmetered air would cause a lean condition leading to a misfire not increased idle.
i didnt think about this much until it happened to me and after thinking about it - it made sense. i too am familiar with the rising idle issue with SC's as they heat up and remains kind of a mystery. i guess one exception would be that if the car is running very rich UNmetered air could cause the idle to increase. i would look at all that plumbing involved with AAR system- thats the metered air side. the UNmetered side is the airbox, intake gaskets and related parts outside of the vacuum circuit

[juanbenae]

Originally Posted by Reiver

A lean condition often raises the idle....vacuum leak?

that's what I thought.. has the car been sitting for a while?

[scotricker]

No, has not been sitting. It's my daily driver.

I'm going to check in and around the AAR . To see if that half moon thingie is stuck open. and the related vacuum lines.

[groovydude]

If your air box is original, it could be that the glue sealing the lower portion has failed. Starter fluid sprayed around the runners might reveal this as the source of your leak.

[Cooper911SC]

Just another thought, Cruise Control cable connection at Throttle linkage ok?

Plastic connector/seat breaks and hangs throttle open...

Common cheap "fix" is 2 zip ties crisscrossed over connector and metal braket.

Cooper

[juanbenae]

im pretty sure there was no cruse option in 78-79? please advise?

spitballing here.... fuel filter?

[Cooper911SC]

Cruise started in 1976 and was called "Tempostat" by Porsche. Made by VDO.

Cooper

[timmy2]

Originally Posted by jason2guy

for the idle to increase you need metered air not UNmetered air. think about it- unmetered air would cause a lean condition leading to a misfire not increased idle.
i didnt think about this much until it happened to me and after thinking about it - it made sense. i too am familiar with the rising idle issue with SC's as they heat up and remains kind of a mystery. i guess one exception would be that if the car is running very rich UNmetered air could cause the idle to increase. i would look at all that plumbing involved with AAR system- thats the metered air side. the UNmetered side is the airbox, intake gaskets and related parts outside of the vacuum circuit

To test your theory, Disconnect the AAR hose on the outside of the device while your car is idling and see what it does to the idle. Pure unmetered air entering below throttle body into the air box. Report back...