intermittent high idle at cold startup

[teamsmith]

This has happened 3 time so far.

I go to start up truck, hold the clutch in, don't touch the gas, and turn the key. Truck starts right up but stays at high idle (2500rpm) and doesn't come down. I turn the truck off, let it sit, start it again. Same thing. On 3rd restart it settles down and behaves. I looked at the throttle linkage which looks fine (and shouldn't be in place anyway since I'm not touching the gas pedal). Would it be something int the throttle body? One of the sensors?

 

Thanks for any help. So far it's not a huge deal since it settles down after a minute.

[AeroNautical]

I've had this issue for years, I narrowed it down to the IAC getting stuck open, thats the only way enough air can be sucked in to get a 2500 rpm, if the butterfly isnt open. Its not a leak, because it's intermittent. I went through 6 iac sensors, some new, some jy finds. All fixed the problem for a short while. My only guess is a bad ground or voltage for that sensor.

[omega_rugal]

idle stepper motor gets stuck

[teamsmith]

Thanks for the replies. I guess my next step will be to go through all of the grounds and re-check them. It's only happened about 5 times out 30 or so starts. I haven't been driving it often that those 30 starts are maybe stretched out of 6 months. 

[cruiser54]

Thanks for the replies. I guess my next step will be to go through all of the grounds and re-check them. It's only happened about 5 times out 30 or so starts. I haven't been driving it often that those 30 starts are maybe stretched out of 6 months. 

Your TPS is likely bad and/or has a poor ground. 

 

Do Tip 5 at www.cruiser54.com and report back.

[AeroNautical]

 

Thanks for the replies. I guess my next step will be to go through all of the grounds and re-check them. It's only happened about 5 times out 30 or so starts. I haven't been driving it often that those 30 starts are maybe stretched out of 6 months. 

Your TPS is likely bad and/or has a poor ground. 

 

Do Tip 5 at www.cruiser54.com and report back.

 

Are you saying the IAC is not related at all? If the TPS is giving more fuel to the engine than it should be, I understand the idle might be a little higher, but there is hardly any air getting into the engine, I'm surprised the idle skyrockets like it does. 

[cruiser54]

 

 

Thanks for the replies. I guess my next step will be to go through all of the grounds and re-check them. It's only happened about 5 times out 30 or so starts. I haven't been driving it often that those 30 starts are maybe stretched out of 6 months. 

Your TPS is likely bad and/or has a poor ground. 

 

Do Tip 5 at www.cruiser54.com and report back.

 

Are you saying the IAC is not related at all? If the TPS is giving more fuel to the engine than it should be, I understand the idle might be a little higher, but there is hardly any air getting into the engine, I'm surprised the idle skyrockets like it does. 

 

TPS tells ECU throttle is open. IAC pulls back.

 

This is a very common issue. That's why you do Tip 5 to check the TPS and other sensor grounds. 

[omega_rugal]

 

 

TPS tells ECU throttle is open. IAC pulls back.

This is a very common issue. That's why you do Tip 5 to check the TPS and other sensor grounds.

 

how does the ECU know how much air is getting in? only throu the TPS? how about the MAP? how come cycling the ECU on and off several times eventually fixed the problem?

[cruiser54]

Yes.

MAP measures load via vacuum.

I'm not sure. Seems to be the M.O. of a failing TPS.

 

Unplug the tPS and see if the idle ever flares.

[Rockfrog]

This was what happened when my stock TPS failed. Exactly like this.

 

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[cruiser54]

This was what happened when my stock TPS failed. Exactly like this.

 

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Yup. Always check the sensor ground circuit per Tip 5 first, but odds are the tPS is taking a dump.

[Ωhm]

TPS/TPS wiring is usually the cure for high idle on start up. Listen to cruiser. But while you're at it check cruiser tip 14. Throttle plate angle adjustment is a calibrated air leak and once properly set allows for TPS (closed throttle) and IAC (pintle position) to find the happy zone. That is, the best position for TPS to be adjusted and centers the IAC motor for optimum performance. Also make sure linkage always returns to the idle stop screw and no slop with the throttle plate.

 

 

TPS tells ECU throttle is open. IAC pulls back.

This is a very common issue. That's why you do Tip 5 to check the TPS and other sensor grounds.

 

how does the ECU know how much air is getting in? only throu the TPS? how about the MAP? how come cycling the ECU on and off several times eventually fixed the problem?

 

 

ECU also uses engine speed (RPMs) with TPS and MAP to determine the amount of air during open loop.

[teamsmith]

 

Thanks for the replies. I guess my next step will be to go through all of the grounds and re-check them. It's only happened about 5 times out 30 or so starts. I haven't been driving it often that those 30 starts are maybe stretched out of 6 months. 

Your TPS is likely bad and/or has a poor ground. 

 

Do Tip 5 at www.cruiser54.com and report back.

 

 

So this is what I get for relying on my memory for the protocol of the tests. I put my multimeter on A & B of the TPS connector and got 0.3 ohms. I just re-read your tip and it said to touch the negative battery terminal. I'll go back and re do it. I've driven the truck about 10 more time since I posted this (amazing what a working A/C will do for driveability) and the problem hasn't cropped back up. I will still perform the check and report back.