Weird idle hunting problems

[Mattglas]

I'm having some weird idle hunting problems. When I start my car the rpm gauge fluctuates barely just above the 500 tick mark. Then as the engine warms up the needle fluctuates between 500-1000, the engine also shakes for a second and then stays still multiple times. The dwell reading, while it's doing this, is a steady 40. Then after about 2-3 minutes of running, the dwell drops to 10 and the engine has a steady idle around 750 and runs smooth. Any ideas?

[Jonathan]

During this first minute or two when your idle is low at around 500, use the rubber end of a pencil to push directly on the idle microswitch and report back with what happens. Do this on a cold engine if that is when you're having your problems.

[David T]

Verify the RPM's, the tack is not that accurate. Verify the idle is being controlled by the idle motor. If it is doing this when cold it sounds like the Thermister is not increasing the idle. The dwell doesn't start to fluctuate til the O2 sensor starts to work and the motor is warm enough. You may have to have your CPR rebuilt. That is what acts like a choke and affects the idle quality til the motor gets warmed up.

[Mattglas]

During this first minute or two when your idle is low at around 500, use the rubber end of a pencil to push directly on the idle microswitch and report back with what happens. Do this on a cold engine if that is when you're having your problems.
Nothing happens, and it doesn't click either.

[Jonathan]

Nothing happens, and it doesn't click either.

Do you think that is because it is already pushed in (by the end of the pin on the lever arm that is)?

Could be as simple as just switch problems.

You could test the switch by removing it from the car and then test for continuity with a handheld meter. Clip the leads from the meter onto the connections on the back of the switch and then while pushing the switch tab in and out, see if your meter picks up the change.

You can't drive the car while doing this, but if you want to see if the car is responding properly to the throttle spool being at rest, you could just connect the two wires from the car side that normally go to that switch (a black ground wire and the other one that's green IIRC). Then see if there is any change during that 500 RPM period.

[Mattglas]

Do you think that is because it is already pushed in (by the end of the pin on the lever arm that is)?

Could be as simple as just switch problems.

You could test the switch by removing it from the car and then test for continuity with a handheld meter. Clip the leads from the meter onto the connections on the back of the switch and then while pushing the switch tab in and out, see if your meter picks up the change.

You can't drive the car while doing this, but if you want to see if the car is responding properly to the throttle spool being at rest, you could just connect the two wires from the car side that normally go to that switch (a black ground wire and the other one that's green IIRC). Then see if there is any change during that 500 RPM period.
I'll do that test tomorrow to make sure it's working, but it's not clicking because it's being pushed in by the throttle arm.

[Bitsyncmaster]

The idle electronics do all the control on the idle RPM. The system has a lot of extra control keeping the RPM high but not so much keeping it low. So I'm guessing your idle system is not working if it's true you are only idleing at 500 RPM.

Turn your key to on but don't start the engine and feel your idle motor. It should be vibrating and you can also hear it humming. If not, you need to find the problem. Note if you have my idle ECU this test does not work since I shut down the idle motor power when the engine is not running.

[Mattglas]

So, I tested the idle micro switch and it works as it should. The idle motor is buzzing in the second key position and while the engine is running. Also, it turns out that my RPM gauge is wrong and the actual idle fluctuates between ~670 to ~800. Here's a video of what it looks/sounds like:

https://www.youtube.com/watch?v=7cr7LJeO3pM
The RPM reading for a 6 Cyl is the bottom numbering of the middle arc.

[Jonathan]

So Matt, was exactly is it that you're trying to solve?

Is there more to the idle hunting than what is shown in the video clip, either after the engine warms up or before? I ask because that doesn't look like too big of a problem if at all. The whistling sound you can sort of hear makes me think you have a vacuum or unmetered air leak of some kind though still somewhere.

[David T]

The spec is 775 +, - 50 RPM's. so the outer range is as low as 725 and as high as 825. It should not fluctuate much though. A large fluctuation could be from vacuum leaks or cylinder imbalance. You probably need a tune-up and you should do a compression test to see that all of the cylinders are balanced. You may need a valve adjustment. Your problem is not with the idle system. On one car I fixed that had a badly fluctuating idle it turned out to be the distributor. At some point in the past someone replaced the impulse coil but did not regap the reluctor. With the gaps all over the place the coil wasn't sparking correctly resulting in a very poor idle. Point is to have a smooth idle you must get all of the cylinders balanced as closely as possible so they all produce the same amount of power. If any cylinder is "weak" the motor will slow down and then speed up.

[Mattglas]

So Matt, was exactly is it that you're trying to solve?

Is there more to the idle hunting than what is shown in the video clip, either after the engine warms up or before? I ask because that doesn't look like too big of a problem if at all. The whistling sound you can sort of hear makes me think you have a vacuum or unmetered air leak of some kind though still somewhere.
I'm just trying to solve the idle fluctuation which I thought was idle hunting, guess I'm chasing the wrong thing. It happens when the engine is cold and usually fixes itself after it warms up. I'll look around and see if I can find any vacuum leaks.

[Mattglas]

The spec is 775 +, - 50 RPM's. so the outer range is as low as 725 and as high as 825. It should not fluctuate much though. A large fluctuation could be from vacuum leaks or cylinder imbalance. You probably need a tune-up and you should do a compression test to see that all of the cylinders are balanced. You may need a valve adjustment. Your problem is not with the idle system. On one car I fixed that had a badly fluctuating idle it turned out to be the distributor. At some point in the past someone replaced the impulse coil but did not regap the reluctor. With the gaps all over the place the coil wasn't sparking correctly resulting in a very poor idle. Point is to have a smooth idle you must get all of the cylinders balanced as closely as possible so they all produce the same amount of power. If any cylinder is "weak" the motor will slow down and then speed up.
Thanks for the info. I've given my engine a basic tuneup, but haven't adjusted the valves. I'm going to a tech event soon so maybe I can get it done there. I've also done a compression test, and the cylinders were all pretty much balanced. I'll check around for vacuum leaks.