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Need help troubleshooting Idle problem. Idling at 4,000 RPMs. What could cause this?

Tiny
BCHEEVER
  • MEMBER
  • 1995 TOYOTA TACOMA
  • 150,000 MILES
Truck idling at 4,000 rpms. I've check vacuum lines and replaced the throttle position sensor. Checked throttle cables and Throttle body for stuck components. Throttle is closed on body still idling at 4,000 rpms. Not sure what to check for next.
Friday, September 16th, 2011 AT 3:39 AM

13 Replies

Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,724 POSTS
Have you checked the idle air control valve?
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Friday, September 16th, 2011 AT 3:48 AM
Tiny
BCHEEVER
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No I'm reading on how to check it right now. Could it cause that high of an idle and constant? Engine runs good no misfiring but won't idle below that?
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Friday, September 16th, 2011 AT 4:00 AM
Tiny
JACOBANDNICKOLAS
  • MECHANIC
  • 109,724 POSTS
Yes, the IAC is designed to allow a fixed amount of air to enter the engine when the throttle plate is closed. If it isn't working properly, the idle is affected. It can idle high, low, up and down.
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Friday, September 16th, 2011 AT 4:06 AM
Tiny
BCHEEVER
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Tested the IAC with an ohm meter and it reads 22.4 which shows good. Is this the only way to test it?
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Friday, September 16th, 2011 AT 5:51 AM
Tiny
BCHEEVER
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Noticed that If I remove the air intake system the engine dies. Is this normal? Cleaned the Mass air flow sensor. Still having same problem. Next step I guess is to clean the IAC. Gonna take some time removing the intake manifold. Let me know if you have any more suggestions. Thanks
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Friday, September 16th, 2011 AT 6:32 AM
Tiny
KHLOW2008
  • MECHANIC
  • 41,814 POSTS
Yes, if there is any vacuum leakage between the MAF and throttle body, engine would stall or idle badly.

When engine idling speed increases, it is usually due to excessive air getting to the intake manifold. There should be a vacuum leakage somewhere and usually a faoult with the IAC would not increase the rp to 4,000, A stuck open IAC woulod most likely increase the engine rpm to 2.5 k maximum.

Not likely to be the MAF as well as you require more air to get to the manifolds and since throttle plate is closed, it is not going to increase up to 4 k rpm.

A leak with the intake manifold would be more likely. Was the TPS calibrated correctly?

Using carb cleaner and spraying them around suspected leaking areas would mosty likey result in changes in the rpm if any leaks are present. Ensure the PCV is good.
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Friday, September 16th, 2011 AT 1:16 PM
Tiny
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Not sure how to calibrate the tps. I did notice when first installed it made tthe engine fluctuate rpms up and down between. 2500 and 4000 so I adjusted it slightly and engine is back to 4000 constant. I will check pcv and test for leaks. Could there be a problem with the ecm computer? If so how do I test it?
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Friday, September 16th, 2011 AT 2:43 PM
Tiny
KHLOW2008
  • MECHANIC
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You can test the ECM only with the aid of a scan tool or by substituting it.

Which engine do you have?
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Friday, September 16th, 2011 AT 3:00 PM
Tiny
BCHEEVER
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Checked for leaks using carb cleaner and no surges of course kind of hard when it's already running that high. I'm scared to keep running it because of the high speed and the oil seems like it has some excess fuel in it. Smells like it. I'm really leaning towards a bad ECM. The engine is a 3.4l V-6 5VZ-FE.
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Friday, September 16th, 2011 AT 3:14 PM
Tiny
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The weird part is no check engine light during all of this.
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Friday, September 16th, 2011 AT 3:16 PM
Tiny
KHLOW2008
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IAC port is at lower portion of throttle body. Use a strong adhesive tape to cover up the port and reinstalle the air hose and restart to see if idling drops.
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Friday, September 16th, 2011 AT 3:48 PM
Tiny
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K tested IAC with tape. This made the truck idle down to 3,000 rpms. Took off intake and cleaned IAC. It was not stuck open and tested good with voltage applied. Any other ideas?
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Friday, September 16th, 2011 AT 6:08 PM
Tiny
KHLOW2008
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There is a vacuum leak sonmewhere after the throttle body.
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Friday, September 16th, 2011 AT 7:06 PM

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