High Idle When Engine is Warm

[oak]

Re: 80 SC. When fully warmed up and at a stop, the idle will "slowly" increase to about 2K. Background: last Winter I tore the engine down to replace the lower studs (2 were broken); at this time I very carefully inspected all of the hoses and replaced the following: injectors with seals, fuel distributor; cold start valve, warm up regulator, frequency valve, auxiliary air regulator, fuel filter, fuel pressure regulator, intake manifold gaskets, and probably a few things that I forgot. I also had the pressures set and passed the smog test with ease. The car has a "updated" air box with a pop valve. Within the last 50 miles the idle problem appeared and I subsequently replaced the 2 auxiliary air valves. I also tested the current to auxiliary air regulator. I also took one of its hoses off and plugged up everything - but still the problem exists.
According to the Pelican parts diagrams, the 80-83 cars do not have a de-acceleration valve but have the 2 auxiliary air valves with different plumbing.
I am now thinking that I have a cracked air box - but I don't know.
Any thoughts out there?

P.S. I also bench tested the aux. air regulator

[Walter_Middie]

When the idle climbs to 2K, can you lower it with the idle adjustment screw? If not, you are getting air leaking in somewhere. An air leak is the most likely problem - however, if the springs in your distributor advance weights are old and worn out - or if the little plastic washers under the spring perches are disintegrated, the timing could be advancing on you at idle, and raising the idle.

[oak]

The idle screw is all the in and I put in a new distributor - so yes, I have a leak - now the problem is how to find it.

[911SC520]

funny, my 1980 sc was doing the same thing, I cleaned out the air box and check all vac hoses and my idle went back down. For a while I was thinking it was the weather, over thanksgiving temps really dropped here in tucson

[mca]

If you have your decel valve (vacuum limiter) connected, pull the vacuum line off of the decel valve nipple and plug with a golf tee. If that solves the problem then you need to fix/replace your decel valve or leave it unplugged. You can fix by pulling it out and squeezing it slightly in a vice (search on "decel valve vice").

The decel valve is located towards the right rear of the engine (rear = toward firewall). A bad decel valve will cause your idle to "float" - you come to a stop and the idle slowly begins to increase. When working correctly, the decel valve prevents rapid rpm drops in between shifts - it also prevents popping, smoking, on rapid deceleration.

BTW ... I have an 82 SC and it DOES have a decel valve.

[Hard-Deck]

+1 on the Decel Valve, let us know.

[BRP83SC]

Originally Posted by mca

If you have your decel valve (vacuum limiter) connected, pull the vacuum line off of the decel valve nipple and plug with a golf tee. If that solves the problem then you need to fix/replace your decel valve or leave it unplugged. You can fix by pulling it out and squeezing it slightly in a vice (search on "decel valve vice").

The decel valve is located towards the right rear of the engine (rear = toward firewall). A bad decel valve will cause your idle to "float" - you come to a stop and the idle slowly begins to increase. When working correctly, the decel valve prevents rapid rpm drops in between shifts - it also prevents popping, smoking, on rapid deceleration.

BTW ... I have an 82 SC and it DOES have a decel valve.

I am having a similar issue and currently am troubleshooting...

question on the above: plug the vacuum line with the tee or the decel valve nipple?

[mca]

Vacuum line.

[jpgroth]

Thread bump, experiencing high idle currently. To confirm, I should remove the T from the top of the decel valve and plug the short vacuum line?

I also have a 'braided' vacuum line from the bottom of the decel valve routed towards the firewall, on the driver side, that isn't connected to anything. It pulls vacuum but when I plug it with my finger while running it has no affect on idle.

[Jtt84911]

Oak
If you quickly “blip” the accelerator pedal with your foot does it drop back down to normal idle?

I had a similiar issue after dropping the motor im my 84 a few years ago..... and it turned out to be the accelator rod off the trans was binding very slightly....against the small slot in the sheet metal....where it passes through then connects to the throttle linkage bracket near the air manifold.

I readjusted/realigned the throttle rod....problem fixed.

JTT