Further idle diagnosis....

[Roadster]

I can understand what you are saying, but what I don't understand is why the throttle angle is always at 0% at high or normal idle. Wouldn't that indicate that the blades are completely closed?
I appreciate all the comments and suggestions, but it looks like another plenum pull is in order to make sure everything has stayed attached. Could be that there is a slight vacuum leak. Not sure as you and many others have way more experience than I do with the LT5.
Will have to wait for now as I'm out at the 74....alot of work to be finished here..lol


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[Roadster]

Not sure if this will have any bearing on my idle problem or not. The vacuum pump is for the secondary operation if I understand that correctly. Just did a test with the key on and the pump came on for about 4 seconds. Then as is the pump came back on after 20 seconds for a split second. Then each time the pump came back on, it was a 1/2 second difference lower. I let it happen about 4 times and it was down to about an 18 second difference. So I imagine that there is a vacuum leak in the system. Is that a correct assumption???
Thanks for any additional info.....

[tpepmeie]

Originally Posted by Roadster

I can understand what you are saying, but what I don't understand is why the throttle angle is always at 0% at high or normal idle. Wouldn't that indicate that the blades are completely closed?

The ECM doesn't actually measure the throttle angle as "zero". Rather it sets as zero the lowest voltage registered by the sensor at various intervals. In other words, it assumes the lowest voltage, say .52v, is closed throttle and therefore sets that as zero. It is updated from time to time, but frankly without my notes, I can't remember the exact logic.

I think the suggestion that the throttle plates might be hanging slightly open, then get shut by the extra vacuum when the TB is blocked off, might be the most likely. Can you re-do the blocked off TB test when the idle is high? Just to check where the air source is (TB or after).

[Roadster]

Originally Posted by tpepmeie

The ECM doesn't actually measure the throttle angle as "zero". Rather it sets as zero the lowest voltage registered by the sensor at various intervals. In other words, it assumes the lowest voltage, say .52v, is closed throttle and therefore sets that as zero. It is updated from time to time, but frankly without my notes, I can't remember the exact logic.

I think the suggestion that the throttle plates might be hanging slightly open, then get shut by the extra vacuum when the TB is blocked off, might be the most likely. Can you re-do the blocked off TB test when the idle is high? Just to check where the air source is (TB or after).

Thanks for the explanation. I just thought that since the Tech 1 was reading the Throttle Angle @0% and showing .54v for the TPS at any idle and anytime my foot is off the accelerator that the blades would be fully closed.
So if what you and others have suggested about the throttle blades, would a possible solution be to undo the idle stop and make sure the blades are fully closed, then adjust that stop to that position???
When I had the air horn off to change the gasket, there did not appear to be any additional area to where the blades could be adjusted more closed than what they already are...but I imagine it's worth a try.
Won't be able to do the test for a day or two, but will do it again.
Have to find a different book, as it almost sucked in my FSM.....lol

[tpepmeie]

It's likely not the stop screw if it can actually idle down to normal. If it's sticking it's in the mechanism (shaft or blades) my guess.


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[Roadster]

It just doesn't feel like the blades or the shaft are sticking, as throttle response is non-existent. When first starting the engine or doing a hot restart and revving the engine with normal idle, it returns down to normal.
If the throttle is or was sticking, then why would it not stick on a restart after shutting down the engine with a high idle? If the throttle is sticking then wouldn't it still be sticking at high idle on the restarts, which it never does?

[Paul Workman]

Ya gonna have to TEST your theories. Can't assume anything; only trust what you VERIFY!

PS:

Because the vacuum pump does shut off, and you're not reporting any codes/secondary issues, the check valve to the plenum IS working. Your Idle problem is NOT coming from the SPT system.

However, besides gaskets, there ARE several other vacuum circuits that could be introducing air into the plenum. You might want to verify the MAP vacuum is not leaking, and then block off the other plenum vacuum circuits, e.g., the purge line, PVC line...see what happens when these other "blank checks" are eliminated...

[XfireZ51]

Take a strip of paper and stick it between the primary throttle plate and bore. Pull it through. Is there any drag? Do it also after opening the throttle.

[tpepmeie]

Do I understand right that the high idle occurs after you've been into the throttle a good bit? I thought I saw that earlier somewhere in this thread. If that is the primary symptom then would point me towards the larger throttle plate mechanism hanging up. The 59 (or. 63mm) bores don't open up under light throttle. It's a progressive linkage.... the small primary plate opens at the first several degrees of throttle position.


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[conesare2seconds]

Fwiw, I had a high idle after the engine was warm. Marc found the throttle blades were hanging up. Had him recondition the throttle body.