High idle started out of nowhere

[DMC3165]

Hi all,

Today I was using the car all day. First time in a week. Did a little over 120 miles. By the end of the night I was putting the car away and when I switched off the A/C the idle jumped to 1250 and stayed there. I shut the car off and started it again and still 1250 idle. If I switched on the A/C it went back to around 800 but if I turned off the air it jumped back up. I was looking around under the hood and the only thing I found was if I disconnected the idle speed motor the idle went back to normal. So what's up with my car?

[Bitsyncmaster]

This is a common problem that shows you the idle system is at the end of it's control range. The idle motor is fully closed and the engine has no load so it's getting to much "gas" at idle speed.

First thing is to be sure the air bypass screws are fully shut. Don't force them or they break off. Then back out the "curb idle screw". That is the screw below the idle switch screw. Make sure the idle switch screw is also adjusted properly.

If you still have the same problem, make sure the ignition advance is not active at idle. You can test this by pulling the connector off the solenoid at idle and the you most likely will get a higher idle RPM. Put it back on and the RPM should drop. That would tell you the advance is working correctly.

A vacuum leak will also cause your problem by letting more air into the engine just like not closing the throttle plates would do.

[DMC3165]

I will check that stuff later but all of that was gone through very recently. I know anything is possible so I'll look it over. Isn't there a switch somewhere that tells the idle speed motor to pump more gas when the A/C is on. Perhaps that is stuck or went bad?

[Bitsyncmaster]

I will check that stuff later but all of that was gone through very recently. I know anything is possible so I'll look it over. Isn't there a switch somewhere that tells the idle speed motor to pump more gas when the A/C is on. Perhaps that is stuck or went bad?

Not in the original idle ECU.

I did incorporate the AC idle speed change into my new ECU.

[content22207]

A vacuum leak will also cause your problem by letting more air into the engine just like not closing the throttle plates would do.

Vacuum leaks are unmetered as far as fuel is concerned (not closing the throttle plates meters more fuel along with the additional air). A better analogy is hitting the O2 trigger on an acetylene torch. Vacuum leaks cause the fuel/air mixture to run lean, which means hotter and less controlled combustion. If the leak is bad enough, you can actually melt the spark plugs or pistons.

Bill Robertson
#5939

[DMC3165]

It is defiantly too much fuel. On a cold start it ran fine for a few minutes. As it warmed up the idle steadily increased to around 1100. With the A/C switched on the idle fell back around 750. Turn it off and back up it goes. If I disconnect the idle speed motor with the A/C on the engine stalls. Could my mixture suddenly just need adjusting?

[Bitsyncmaster]

A rich mixture will increase the idle speed (open loop) a little but I bet all you need to do is back out the curb idle screw a little.

When you change the curb idle screw you may have to readjust the screw on the idle switch also.

[DMC3165]

I adjusted both the mixture screw (very slightly) and the idle screw. In the process I used throttle body cleaner which did seem to help. But now it seems like its idling a little low (around 600-700) but running ok. Now I have to wait until the motor cools to see how it starts cold.

[DMC5180]

Low Idle May indicate the ISM is binding or stuck. The internal bearings can rust up due to moisture over time Or excess carbon soot build up from recycled Crankcase blow-by passing through it. In this case of 600-700 the valve would be stuck at a near closed position. 1250 after cold start frictions are over come would indicate the Valve being stuck open more than needed for normal 775 hot engine operation. If the valve was stuck closed the engine would not run with throttle closed it needs some air. You can for free movement of the motor by removing it and rapidly twisting it Axially back and forth you can hear the valve click open or closed from the rapid movement.

Btw: Did you verify the micro switch is closing at idle.

REDO after rereading Orig Post; Perhaps Vacuum advance stuck on is the issue.

[Bitsyncmaster]

I adjusted both the mixture screw (very slightly) and the idle screw. In the process I used throttle body cleaner which did seem to help. But now it seems like its idling a little low (around 600-700) but running ok. Now I have to wait until the motor cools to see how it starts cold.

Does the idle motor buzz? You should only have 775 RPM no mater what you adjust until the ISM is at the end of it's range of full open or full closed.

[DMC3165]

The car was tuned up over the winter. Plugs wires cap rotor. At the same time I had a new water pump and some other odds and ends done when the manifold was off (vacuum lines and such). From that point on the car has performed flawlessly until yesterday.

The idle motor is buzzing when running. The microswitch is closing at idle but I have no way of verifying if the switch is working. If I disconnect the ISM now the car will almost stall. I started it tonight and it is holding a reasonable idle, but it seems like its idling rougher then it was before any of this started. I may try to drive it tomorrow and maybe pass by PJ Grady if he has time to look at it. Changing starters and alternators is no big deal but I know nothing about k-jet and I'm not really crazy about messing with it.

[Bitsyncmaster]

If it's idling at the correct 775 and your idle was higher before than it may just seem to not be running as smooth.

[DMC3165]

I only have the dash tach to go by so judging the idle rpm is a little difficult. It is generally halfway between 500-1000 +/- some occasional minor fluctuations. In fact on a cold start this morning it idled better then ever with absolutely no hunting even for a few seconds at all so all looks good there. Under normal operating conditions everything seems fine. The only thing I noticed is at a stand still if I mash down on the throttle there is a very slight hesitation. If I press the pedal normally all is good. Not real sure what that's about. Maybe a vacuum leak? That may also explain why it seemed to run rough last night although it did not seem like it was running rough this morning.

Suggestions?

[Bitsyncmaster]

I only have the dash tach to go by so judging the idle rpm is a little difficult. It is generally halfway between 500-1000 +/- some occasional minor fluctuations. In fact on a cold start this morning it idled better then ever with absolutely no hunting even for a few seconds at all so all looks good there. Under normal operating conditions everything seems fine. The only thing I noticed is at a stand still if I mash down on the throttle there is a very slight hesitation. If I press the pedal normally all is good. Not real sure what that's about. Maybe a vacuum leak? That may also explain why it seemed to run rough last night although it did not seem like it was running rough this morning.

Suggestions?

When you have hesitation, the problem is usually the mixture is wrong or your ignition timing is off.

You would want to adjust the mixture with a dwell meter after you get a fully warmed up engine.

[Soundkillr]

I only have the dash tach to go by so judging the idle rpm is a little difficult. It is generally halfway between 500-1000 +/- some occasional minor fluctuations. In fact on a cold start this morning it idled better then ever with absolutely no hunting even for a few seconds at all so all looks good there. Under normal operating conditions everything seems fine. The only thing I noticed is at a stand still if I mash down on the throttle there is a very slight hesitation. If I press the pedal normally all is good. Not real sure what that's about. Maybe a vacuum leak? That may also explain why it seemed to run rough last night although it did not seem like it was running rough this morning.

Suggestions?

Slight hesitation you say? Try this, check to make sure you have no vacuum on the wur after the engine is hot. My car would run great when cold, but once it got hot the switch under the intake wasnt killing vacuum on the wur, and it was causing the car to load up, and bog once warm. Im not saying this is your problem but you never know. When the engine is cold the wur should have vacuum, but when hot should have none.

[kings1527]

Hi all,

Today I was using the car all day. First time in a week. Did a little over 120 miles. By the end of the night I was putting the car away and when I switched off the A/C the idle jumped to 1250 and stayed there. I shut the car off and started it again and still 1250 idle. If I switched on the A/C it went back to around 800 but if I turned off the air it jumped back up. I was looking around under the hood and the only thing I found was if I disconnected the idle speed motor the idle went back to normal. So what's up with my car?

Hi Chris. I'm going through a very similar issue right now and scouring the archives for answers. Did you solve this problem? Was it the ISM? Thanks!

Alex

[DMC3165]

Mine was a simple fix. It must have been built up carbon near the throttle body. I sprayed it with throttle body cleaner and adjusted the curb idle screw and its been fine since. It even solved my idle hunting! It's been good for about a year now, hopefully it stays that way. :)

[kings1527]

Mine was a simple fix. It must have been built up carbon near the throttle body. I sprayed it with throttle body cleaner and adjusted the curb idle screw and its been fine since. It even solved my idle hunting! It's been good for about a year now, hopefully it stays that way. :)

Wow, I really hope this is my issue. I'll clean out the TB when I do the water pump. Thanks!