Increased idle with AC

[Jonty]

Hey all

Just a small thing I wanted to understand. My car is in perfect Kjet tune and starts and runs great. All fuel pressures and dwell is right in range and my idle switches and mechanical connections work. My car idles at 750 and has not hunting. The only thing I can’t figure out is when I use AC after a period the car idles between 900 and sometimes up to 1000. The idle micro switch is engaged.

Any ideas why that might happen and it is common? If I turn off the Ac after a few minutes it back to idling at 750

[David T]

It would have to be something not working correctly with the idle system. The idle system is supposed to maintain a constant idle speed no matter if the A/C is on or not. The additional load of the A/C should not cause the idle to increase. It should cause it to decrease and the idle system is supposed to compensate by allowing more air (and more fuel) to the motor to maintain the idle speed. My first thought is that you have a vacuum leak somewhere and the idle system doesn't have full control. Start by inspecting the "O" ring seal to the tube in the bottom of the mixture unit. Best if you could find someone who could do a smoke test on the whole induction system to verify there are no vacuum leaks and if there are, where they are.

[Elvis]

Push manualy on the butterflies when idle is high and report what it does.

[Jonty]

It would have to be something not working correctly with the idle system. The idle system is supposed to maintain a constant idle speed no matter if the A/C is on or not. The additional load of the A/C should not cause the idle to increase. It should cause it to decrease and the idle system is supposed to compensate by allowing more air (and more fuel) to the motor to maintain the idle speed. My first thought is that you have a vacuum leak somewhere and the idle system doesn't have full control. Start by inspecting the "O" ring seal to the tube in the bottom of the mixture unit. Best if you could find someone who could do a smoke test on the whole induction system to verify there are no vacuum leaks and if there are, where they are.

Thanks David - I have a smoke machine and it’s tested all fine for vac leaks

[Bitsyncmaster]

My idle ECU can bump the idle speed an extra 100 RPM (with AC on) if you have it. Check if you have a blue wire coming off the idle ECU.

[David T]

If you are convinced you have no vacuum leaks there are a few other things we can check. Make sure the adjustment screws on the throttle arm are all done correctly. Make sure there is no play in the quadrant arm ends. Make sure the springs on the poppet valves in the throttle butterflies are not distorted. Make sure the air sensor plate is centered and doesn't touch the sides of the venturi. Make sure the air screws are lightly seated closed. Make sure the throttle spool and the throttle cable are moving freely and return all the way to idle setting the idle switch to activate the idle system.

[Jonty]

My idle ECU can bump the idle speed an extra 100 RPM (with AC on) if you have it. Check if you have a blue wire coming off the idle ECU.

Thanks Dave - no it doesn’t have your handy ECU

[Jonty]

If you are convinced you have no vacuum leaks there are a few other things we can check. Make sure the adjustment screws on the throttle arm are all done correctly. Make sure there is no play in the quadrant arm ends. Make sure the springs on the poppet valves in the throttle butterflies are not distorted. Make sure the air sensor plate is centered and doesn't touch the sides of the venturi. Make sure the air screws are lightly seated closed. Make sure the throttle spool and the throttle cable are moving freely and return all the way to idle setting the idle switch to activate the idle system.

Thanks will try this next. It is weird the idle is perfect with AC off but it can’t handle it with AC on. Could it also be a dodgy idle motor? It’s a Hervey motor from what I can tell

[David T]

Thanks will try this next. It is weird the idle is perfect with AC off but it can’t handle it with AC on. Could it also be a dodgy idle motor? It’s a Hervey motor from what I can tell

I guess it could but they don't typically fail that way. It can't hurt to take it off and clean it and make sure it moves freely.

[Bitsyncmaster]

If it was just the added engine load and your idle system could not keep up that with the AC your idle would drop. So my guess is the problem is electrical. What is your car voltage when the AC is running? One thing that kicks the idle up if the idle system can't compensate is if ignition advance is on at idle. But I can't think of an electrical problem that increases idle with AC on except a low car voltage.