Intermittent high idle
Thread Starter
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4wd Low
Joined: Dec 2024
Posts: 14
Likes: 3
Intermittent high idle
Hi all
i have a 1990 f150 with the 5.0, manual transmission, 4x4.
it is stock except that I have removed the vacuum pump (because it seized) and capped the hard vacuum line that runs parallel to the side of the engine.
i have been having issues with an intermittent high idle (started quite some time after I removed the vacuum pump)
i have thrown the parts cannon at it:
- idle reset procedure
- new ICV
- remanufactured ECU
- new EGR position sensor (because there was an OBD code)
next I’m looking at MAP sensor and TPS?
there are no obvious vacuum leaks and I figure if there were, I’d have an always inconsistent idle. Right now it is either perfect at 600rpm for several trips, or it starts rising up to 2000rpm (like every 4th or 5th time I start the truck). I can usually stop and restart the truck and it’s fine for a while.
it happens with a warm truck usually.
any ideas from all y’all?
i have a 1990 f150 with the 5.0, manual transmission, 4x4.
it is stock except that I have removed the vacuum pump (because it seized) and capped the hard vacuum line that runs parallel to the side of the engine.
i have been having issues with an intermittent high idle (started quite some time after I removed the vacuum pump)
i have thrown the parts cannon at it:
- idle reset procedure
- new ICV
- remanufactured ECU
- new EGR position sensor (because there was an OBD code)
next I’m looking at MAP sensor and TPS?
there are no obvious vacuum leaks and I figure if there were, I’d have an always inconsistent idle. Right now it is either perfect at 600rpm for several trips, or it starts rising up to 2000rpm (like every 4th or 5th time I start the truck). I can usually stop and restart the truck and it’s fine for a while.
it happens with a warm truck usually.
any ideas from all y’all?
4th Gear
Joined: Nov 2024
Posts: 4
Likes: 0
I have a 89 300 EFI so just speaking from my experiences. Have you put a voltmeter on the TPS leads and verified that it’s still in range when acting up? Idle voltage should always be around 1 volt… and these things are hyper sensitive. A tenth of a volt makes a difference.
Also check your timing when acting normal vs acting up. Almost sounds like maybe the ignition control is over advancing the timing when it gets warmer.
Also check your timing when acting normal vs acting up. Almost sounds like maybe the ignition control is over advancing the timing when it gets warmer.
Fleet Owner
Joined: Oct 2005
Posts: 20,789
Likes: 1,753
From: Kentucky
Run the Key On Engine Off (KOEO) test and Stored Code display. You can use a paperclip to count the flashes from the Check Engine Light (CEL) or invest in a very capable code reader such as the Innova 3145. If you get the System Pass Code (11) for both tests, then move onto the Key On Engine Running (KOER) tests.
Hotshot
Joined: Jul 2020
Posts: 10,562
Likes: 2,674
From: Near Salt Lake City
I would first check the throttle linkage and the condition of the throttle plate (how dirty is it? And is it binding up?)
Next
I would be all for checking the actual TP voltage and watching the voltage sweep as you move the throttle plate
RLA advice good and straight out of the book
But
Some TP's pass a self test just fine but are faulty
But, agree, do the self tests if you have a star tester or equivalent
Quick and easy and if you get a TP code, you need no further testing most likely
Time IS money
Next
I would be all for checking the actual TP voltage and watching the voltage sweep as you move the throttle plate
RLA advice good and straight out of the book
But
Some TP's pass a self test just fine but are faulty
But, agree, do the self tests if you have a star tester or equivalent
Quick and easy and if you get a TP code, you need no further testing most likely
Time IS money
Thread Starter
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4wd Low
Joined: Dec 2024
Posts: 14
Likes: 3
Smoke machine ordered ($50? If I had known they were that cheap I would have ordered one long ago)
so this weekend:
- KOEO
- Smoke test
- measure TPS voltage for normal idle and if I can get it to happen also at high idle
so this weekend:
- KOEO
- Smoke test
- measure TPS voltage for normal idle and if I can get it to happen also at high idle
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Tuned
Joined: Apr 2023
Posts: 468
Likes: 67
From: Near SLC Utah
I know this sounds crazy but I had the same issue. It turns out the computer freaked out because the connector for the thermactor solenoids wiggled itself off and for some reason the computer decided to bump up the idle to 2000 rpm occasionally. I doubt that's the issue but it was something that happened to me. I would also check the TPS voltage. I made a connector out of an old one and some wire with solder at the end of it. It plugs into the tps and then you connect the wires to the engine harness and then measure on the exposed wires. It's also possible the IAC got stuck in the open position. Just my $.02.
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Thread Starter
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4wd Low
Joined: Dec 2024
Posts: 14
Likes: 3
Smoke test showed a leak at the IAC spacer, the egr had smoke coming out of the underside, and the vapor canister was smoking.
I’m gonna assume the vapor canister is fine, replace the EGR and get a gasket for the IAC spacer.
what do y’all think?
i checked the thermactor solenoid connectors and they seem to be connected.
didn’t see a good way to read the TPS
I’m gonna assume the vapor canister is fine, replace the EGR and get a gasket for the IAC spacer.
what do y’all think?
i checked the thermactor solenoid connectors and they seem to be connected.
didn’t see a good way to read the TPS
Fleet Owner
Joined: Oct 2005
Posts: 20,789
Likes: 1,753
From: Kentucky
Code 31 (C): EVP Circuit intermittently below minimum voltage.
Fleet Owner
Joined: Oct 2005
Posts: 20,789
Likes: 1,753
From: Kentucky
Use a DVM set to DC voltage, back probe the TPS connector or use pins. Turn the key to the Run position. Verify the TPS output voltage is between 0.6 to 1 VDC. Anything in between is fine. Then watch the meter and slowly open the throttle to wide open, the voltage should smoothly increase to near 5VDC. Anything above ~4.5 is usually good. Anything less at wide open throttle is a sign of a bad TPS.
Thread Starter
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4wd Low
Joined: Dec 2024
Posts: 14
Likes: 3
since I just replaced the EVP sensor, it may be a bad EGR, right?
i bought a new one, along with a 1-1/16” wrench and soaked the nut in PB blaster. We’ll
see if it wants to get loose tomorrow.
i bought a new one, along with a 1-1/16” wrench and soaked the nut in PB blaster. We’ll
see if it wants to get loose tomorrow.