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High idle


spark plug

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I have the 4.0l ohv v6 I had a idle of 12-1500 rpm but I have sense replaced the vacuum modulator on the transmission and immediately after I started it and the idle dropped off at 8-900 rpm within a minute or so but now I have an idle of 980-1k rpm I have the evaporator box out but all vacuum ends are capped off so no possible vacuum leak that I can think of and I also replaced the vacuum hose for the modulator the tach needle sits in the middle and towards the top of the 1k mark anyone have any ideas?
 


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dennisinaz

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My 2000 does the same thing. It comes down after about 2 minutes though. Since it goes away eventually, I haven't killed myself trying to chase it down. Figure yours out yet?
 

SenorNoob

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Are you sure the tach is right? They can be pretty far off from the factory.
 

RonD

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After engine is warmed up, idle should be 650 on manual trans vehicle, 750 with automatic, AC off.

If not then after engine is warmed up, unplug the wires from the IAC(idle air control) valve.
Valve will close and engine idle should drop down to 500 or engine may even stall, either is good and means no air leaks.
If idle stays high then you do have an air leak
 

RonD

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My 2000 does the same thing. It comes down after about 2 minutes though. Since it goes away eventually, I haven't killed myself trying to chase it down. Figure yours out yet?
That is normal, all cold engines need a high idle until they warm up, 5 to 8 minutes depending on outside air temp, but idle will slow start to drop during the warm up
 

spark plug

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After engine is warmed up, idle should be 650 on manual trans vehicle, 750 with automatic, AC off.

If not then after engine is warmed up, unplug the wires from the IAC(idle air control) valve.
Valve will close and engine idle should drop down to 500 or engine may even stall, either is good and means no air leaks.
If idle stays high then you do have an air leak

I cleaned the iac and tps yesterday and pcv and no avail got to work this morning unplugged the iac it went down but it still at 1000 rpm any thoughts?
 

RonD

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With engine warmed up and IAC unplugged find the vacuum leak, that is the problem.

Common leaks on 4.0l is lower intake manifold gasket, the bolts loosen up.
On the drivers side rear of upper intake is the vacuum manifold, remove each hose, and caps, one at a time and cover port with your finger, if idle drops to 500, that hose/system has the leak.
Spray starting fluid(ether) around lower intake and throttle body, if RPMs change there is a gasket leak.
BE CAREFUL, ether is flammable, which is the point of using it, exhaust manifolds are hot and spark plug wires and coil can have arcing sparks
 

spark plug

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With engine warmed up and IAC unplugged find the vacuum leak, that is the problem.

Common leaks on 4.0l is lower intake manifold gasket, the bolts loosen up.
On the drivers side rear of upper intake is the vacuum manifold, remove each hose, and caps, one at a time and cover port with your finger, if idle drops to 500, that hose/system has the leak.
Spray starting fluid(ether) around lower intake and throttle body, if RPMs change there is a gasket leak.
BE CAREFUL, ether is flammable, which is the point of using it, exhaust manifolds are hot and spark plug wires and coil can have arcing sparks
Can I use anything else for that
 

RonD

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Some use propane and a hose, others carb cleaner spray.
Soapy water in a spray bottle can cause idle to change if it is sucked in.

You can also use a cigar and scotch on cold engine to do a Smoke Test
Remove air tube from intake, seal that opening with food wrap and tape, or ??
Pull power brake hose off of the power booster.
Light cigar
Get some good smoke going and blow it into the power brake hose and into the intake.
Watch for smoke coming out of a vacuum leak point.
Have some scotch
repeat smoke into hose
have some more scotch

Some say the scotch is not required, non-professionals usually.

This test can take a few hours, depends on how much scotch you have :)
 

spark plug

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Some use propane and a hose, others carb cleaner spray.
Soapy water in a spray bottle can cause idle to change if it is sucked in.

You can also use a cigar and scotch on cold engine to do a Smoke Test
Remove air tube from intake, seal that opening with food wrap and tape, or ??
Pull power brake hose off of the power booster.
Light cigar
Get some good smoke going and blow it into the power brake hose and into the intake.
Watch for smoke coming out of a vacuum leak point.
Have some scotch
repeat smoke into hose
have some more scotch

Some say the scotch is not required, non-professionals usually.

I did the test with the vacuum ports all but pcv and cruise the cruise works well, no change in the idle, then I covered the maf inlet with my hand it bout clasped the intake tube before stalling this morning I tried it again with iac un plugged it literally died it wasn't trying to stay running no effort to stall
 

RonD

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Engine needs to be warmed up when IAC is unplugged, if testing for high idle from vacuum leak.
Cold engine needs high idle or it will stall as soon as you unplug IAC valve and it closes.

In your first test, with engine warm, when you unplugged the IAC valve did the idle change, but just stay higher than 700rpm?
 

spark plug

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Engine needs to be warmed up when IAC is unplugged, if testing for high idle from vacuum leak.
Cold engine needs high idle or it will stall as soon as you unplug IAC valve and it closes.

In your first test, with engine warm, when you unplugged the IAC valve did the idle change, but just stay higher than 700rpm?
With engine warmed up and running ot dropped off from 1500 to 1000
Shut truck off n restart after a few it runs aprox 800

Also with engine cold iac unplugged it doesn't wanna run ot runs bad and wanted to stall when u Crack the throttle but it does run with it unplugged barely but does
 
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enjr44

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With engine warmed up and running ot dropped off from 1500 to 1000
Shut truck off n restart after a few it runs aprox 800

Also with engine cold iac unplugged it doesn't wanna run ot runs bad and wanted to stall when u Crack the throttle but it does run with it unplugged barely but does[/QUOTE

Disconnect the battery and let it sit for a half hour or so. Let the computer relearn the engine. Got nothing to lose.
 


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