High Idle RPM

[B.C. Bob]

I have finally got my inline 6 (chev 250) to start and run with megasquirt. It has a very high idle at between 1800 and 2200 RPM. I have read thru the "tuning your megasquirt" and have checked for vacuum leaks and adjusted the throttle stop. I do not appear to have a vacuum leak and the throttle stop OK. Is there any other things to check and would there be any other causes for such a high idle? This is a milestone for me to get the car finally running and would appreciate any help.


Bob

[B.C. Bob]

I thought I would ad some additional information. I did have the PCV connected to the intake manifold and thought that it was the cause of the increased idle. I disconnected it from the intake manifold and plugged the hole, but the idle is still high. I also tightened all the intake/exhaust bolts hoping to seal any possible intake leaks, with no change in the idle.

I will go thru the configuring again tonight to see if I have made any errors.


Any help would be appreciated.

Bob

[vinister]

It really has to be a vacuum leak. There is no way the engine can run with no air, so if you block all the holes, and it is still running, then there is still a leak, guaranteed.

Unhook EVERYTHING that hooks into vacuum, including your brake booster, EGR, literally everything except for the MAP line to the MS. If the throttle body is closed and there are no other air vanes open, the engine should not be able to run, if it does, it should idle VERY low.

[B.C. Bob]

Thanks. I will go thru my engine again and look for the leak. I purchased a turbo exhaust manifold from Brazil that does not fit well and it might be the cause. I will switch it out for the original OEM exhaust manifolt and give it a try.

Bob

[B.C. Bob]

Hello, I still cannot get my RPM to go down. I pulled the intake put in a new gasket sealed everything as good as I could get and I now get around 1800 RPM. Could I have a problem with the idle air control (stepper motor). When I unplug it nothing happens. Could I possible have it wired incorrectly? The car ran great with the carb and it idled a lot slower then it does now.

I was thinking of removing the idle air control and covering the area on the throttle body and run it to see if it makes a difference. | don't think it is a leak in the manifold?

Please help.

Bob

[vinister]

Hello, I still cannot get my RPM to go down. I pulled the intake put in a new gasket sealed everything as good as I could get and I now get around 1800 RPM. Could I have a problem with the idle air control (stepper motor). When I unplug it nothing happens. Could I possible have it wired incorrectly? The car ran great with the carb and it idled a lot slower then it does now.

I was thinking of removing the idle air control and covering the area on the throttle body and run it to see if it makes a difference. | don't think it is a leak in the manifold?

Please help.

Bob

Bob,

Of course it is the ISV then. Here's what I would do:

- Take off the ISV and plug the hole, leave the ISV connected to ms
- use the throttle stop on the TB to get the engine to idle at 1000rpms or wherever it is comfortable
- Look into the ISV with a flashlight, if possible run the wires to wherever you have the laptop, or bring the laptop to it, so you can see both
- Using the idle target table, set the rpm for, say, 500rpms across the table. This should cause the valve to close, because if it is idling at 1000rpms, with a target of 500, ms will command it to close up.
- Now set the target table for a high value, like 2000rpms. This should cause the valve to open up, because the current rpm is well below target.

If the valve is not responding like it should, especially on the 'closing' cycle, then something is wrong. If the valve doesn't move at all, obviously something is wrong. By looking into it with a flashlight while you mess with the settings, you will get a good feel for what it is/isn't doing, all while your engine idles calmly.

Once you get the valve working properly and responding to MS's input, you should be able to hook it back up. Some idle valves don't close all the way up, there will be a certain 'minimum' opening. Usually then you leave the throttle body completely closed and all the idle air comes from the valve. Other valves will close completely, in this case you may wish to set the TB to a very slight opening (~500rpms) in case the valve ever fails on you, so you can still run the engine

When you take the valve off, and cover the opening with your thumb, the engine should die immediately. IF it still manages to run, then it is still getting air from elsewhere (although it sounds like you have looked quite well).

[B.C. Bob]

It was the idle air control valve. I had it wired incorrectly. It will now idle at around 750 RPM. Now on to the rest of the tuning.

Bob

[vinister]

nice!!

Good to hear, enjoy the rest of the show ;)