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Premium Member
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Discussion Starter · #1 ·
I just replaced my motor mounts and now appear to have a very high idle speed after doing the passenger's side. I followed the excellent advice on this forum and removed the intake manifold to get access to it (including removing the throttle body from the manifold and attached sensors), five hours later I had it all back together (the driver's side took two hours). When I started the car, the tachometer jumped to 1700 RPM and won't drop.

I checked obvious things like making sure the throttle was reattached correctly (and not stuck partially open), but nothing seems to be amiss. From a visual inspection, all four plenum hoses seem to be on snuggly. All vacuum hoses are attached (I installed silicone ones a few weeks ago). Is there anything obvious I should check? Is there a suggested method for locating leaks? Could something other than a leak be responsible (e.g., damaged sensor)?

I was debating just adjusting the idle speed, but that seems like a bad idea, because something is clearly wrong. I assume it's a leak somewhere. Any advice on things to check would be most appreciated.
 

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But Mad North-Northwest
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You've got to let us know what year your car is.

Typically a vacuum leak will cause the engine to die at idle, not rev high.
 

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Administrator
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Under the plenum is a bell crank to turn the horizontal motion of the throttle rod to vertical. Make sure nothing is preventing full motion of the bell crank & the rods.

I agree that a vacuum or intake leak would cause a poor idle - not a fast idle.

Are you sure your tach is accurate?
 

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Premium Member
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Discussion Starter · #5 ·
Well, the tach should be as accurate as it was when I started the project this morning, right?

Before this project, idle was set correctly at 1100rpm, which was verified when I took the car in for a smog test last month (I live in CA). It definitely sounds a lot faster too, so I'm pretty sure it's still accurate.

The bell crank seems fine. As I said, I verified that the throttle is fully closed at idle.

I just read somewhere else to reset the ECU when replacing the throttle sensor; I figure since I removed/disconnected the sensor (though did not replace it), maybe a reset will help. I just disconnected the battery and am waiting the requisite 45 minutes.
 

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Discussion Starter · #6 ·
I doubt this is relevant, but I also sprayed out the throttle body and intake manifold with carb cleaner while they were off the car. They were pretty clean, but I figured it wouldn't hurt. I assume this has nothing to do with the problem (i.e., the carb cleaner should have evaporated a while ago)?
 

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Administrator
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I am not aware of any need to reset the ECU.

And I would assume the carb cleaner would be gone.

When you removed the plenum did you disconnect the hose at the Idle Speed Adjuster? If so, perhaps in removing the hose you inadvertantly adjusted the idle speed?
 

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Premium Member
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Discussion Starter · #8 ·
I've set the idle before, so I'm familiar with it.

I just gave it a double check: the lock nut is still on there completely solid, so it seems very unlikely that it got moved. The main reason for starting this thread is that I wanted to eliminate other possibilities before adjusting the idle speed manually, since it idled fine before I took apart the intake manifold (thus, I'm worried there's a larger problem).

I just reset the ECU, which seemed to have helped; the idle is down to 1300-1400 (from 1700+). But it's still not where it was before I started the project. Any other suggestions, or should I just manually adjust the speed and hope for the best?
 

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The only other thing I can think of is maybe there was an air or vacuum leak before and when you R/R'd the plenum you fixed it. So now the idle is faster for that reason.

I'd say if you are certain the throttle is operating correctly - nothing interfering with its return to idle - then adjust the idle speed using the idle speed adjustment.
 

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Discussion Starter · #10 ·
Hmm, I was about to give up and adjust the idle speed, but first took the car on some errands The engine surges in first gear (like someone who has never driven a manual transmission), but this is only when in first. I started looking this up, and all the posts I've found suggest an air leak...
 

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Check the air duct across the top of the engine, all the associated joints and especially the accordian section of the duct near the AFM (Air Flow Meter). While you're at it, check that the flap in the AFM moves smoothly. And that the wire harness connector is securely attached to the AFM.

Usually we hear about the air duct/accordian section getting pulled loose due to saggy engine mounts - not the other way around...
 

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Discussion Starter · #12 ·
It turns out the air duct was completely detached on the other end (away from the accordion / AFM side)! It must have pulled loose when I jacked the engine up.

Anyway, the surging is fixed and the idle speed is now down to 1300rpm. It could be that when I did the ECU reset with the air duct accidentally loose, it calibrated itself incorrectly. I'll see if it sorts itself out now, otherwise I'll just adjust the idle speed manually.

Thanks a lot for all the suggestions!
 
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