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Old 07-11-2016, 09:05 PM   #1
SaturnStick
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Default Vacuum Leak and Idle Speed Weirdness

I've been doing some light troubleshooting on an idle speed that likes to go to 1500rpm all by its lonesome. This post isn't directly about that.

The idle speed will be fine for a while as I drive, then pull the 1500rpm trick.
Usually if I stop the car (like going into a store or something), when I start it up, we're back at the normal 800-1000ish (depending on temp etc). On occasion it is 1500rpm if I restart after a short stop on a hot day or something, it's not consistent.

While I suspect my MAP, the question hit me of a possible vacuum leak.

If I have a leak allowing unaccounted-for air into my car (like a vacuum leak), that would affect the car as soon as I start the engine, wouldn't it? It's not like the car will 'wait' to go to the higher rpm, right?

Thanks :-)

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Old 07-11-2016, 10:04 PM   #2
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Default Re: Vacuum Leak and Idle Speed Weirdness

Search for worn throttle position sensors causing intermittent high idling. Cycling the engine off and back on resets it until high idling returns. The cure is a new position sensor.

In the meantime you can try testing for vacuum leaks and will probably not find any, even during high idle.

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Old 07-11-2016, 10:08 PM   #3
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Default Re: Vacuum Leak and Idle Speed Weirdness

Thanks, I've replaced it twice, hence my thought that it's the MAP. As I said - I'm lightly troubleshooting. Not overly concerned at this point.

But my question is "if I have a vacuum leak, then my idle will be high as soon as I start, right?"

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Old 07-12-2016, 12:26 PM   #4
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Default Re: Vacuum Leak and Idle Speed Weirdness

Sometimes there's no rhyme or reason to erratic engine behavior no matter what the symptoms are with straight forward answers. An intake manifold gasket leak has caused vacuum leaks but I can't recall the exact symptoms that may or may not fit your situation. You'll have to search for threads or a member will know or provide a link. Some here found aftermarket tps' not reliable with OEM the only cure.

Mileage, maintenance history?

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Old 07-15-2016, 10:11 PM   #5
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Default Re: Vacuum Leak and Idle Speed Weirdness

Actually, I posted the original question in a more specific forum.

In this forum, I'm hoping to learn/confirm a bit about the relationship between vacuum leaks and high idle.

The question is: If I have a vacuum leak (not intermittent leak, but a constant one) causing a high idle condition, is it reasonable to expect that it would cause the high idle as soon as I start the car, or are there situations where it might consistently wait a while?

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Old 07-18-2016, 07:39 PM   #6
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Default Re: Vacuum Leak and Idle Speed Weirdness

Any vacuum leak means the engine is sucking in air and not thru the throttle body. This leak means leaning out the air/fuel mixtures and usually results in a higher idle. Think of opening the throttle a little - its allowing more air into the engine so the rpm's go up. Throttle plate opening is manually controlled. A vacuum leak is not controlled.

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Old 07-18-2016, 09:43 PM   #7
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Default Re: Vacuum Leak and Idle Speed Weirdness

Ah, coolness. thanks for the explanation. Unfortunatey, the forums won't let me click up your reputation as I need to 'spread the love' before I do again.
Makes sense.. but limits the amount of credit you get in the face of all the help you provide.

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