1991 Ford F-150 Idle Speed

Tiny
VANGREENE@CHARTER.NET
  • MEMBER
  • 1991 FORD F-150
  • V8
  • 4WD
  • MANUAL
  • 206,000 MILES
I have been fighting a idle speed problem for a long time. I can cold start the truck and it will idle at 1100 rpm until it warms up and the idle will go down to 750 rpm. It will idle at this rpm for a few minutes and then will go to 3000 rpm and I haven't touched the truck. The upper plenum body was leaking air and I replaced the gasket. I have replaced the throttle position sensor, the air bypass control valve twice and the map sensor. I have replaced the plugs, wires, cap and rotor. This is a new crate engine with 6000 mil;es. Could this be the o2 sensor? I thought about changing the computer but I couldn't find it under the dash. Please help
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Saturday, January 3rd, 2009 AT 11:36 AM

6 Replies

Tiny
BLUELIGHTNIN6
  • EXPERT
Hello
Thanks for the donation.

The erratic idling is a symptom of a faulty idle air control valve. You say you already replaced this? It may also be a vacuum leak somewhere in the engine compartment. It is a new crate engine with 6k miles so I am guessing it was recently installed into your vehicle? If so then whoever installed the engine may have missed a vacuum hose somewhere.

Are there any engine codes popping up? If so, scan and post codes.. I do not believe the culprit is an O2 sensor, but if the problem does lie within the O2 sensor then the engine light would illuminate.

Hope this helps.
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Sunday, January 4th, 2009 AT 1:34 AM
Tiny
VANGREENE@CHARTER.NET
  • MEMBER
Thanks for the response. I have replaced the idle air control valve twice. If it was a vacuum leak wouldn't it do it all the time? I disconnected the battery negative cable to reset the computer and restarted the engine. It will idle normally for a few minutes but seems to be running rough and then it's like the computer tells the air bypass valve to open and the idle goes to 3000 RPM to try and correct a bad fuel mixture or something. I don't have a device to check for codes but it seems like I going to need one. What do you recommend. Thanks for the help.
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Sunday, January 4th, 2009 AT 10:37 AM
Tiny
BLUELIGHTNIN6
  • EXPERT
Since you DO have codes on, the first thing to do is to get those codes. You can have your vehicle scanned for free at most local part stores such as AutoZone, O'Reilly Auto Parts and Advance Auto Parts. Since the issue is only itermittent then it does seem like it would be more on the electrical side then mechanical, ruling out the vacuum leak. Post the codes you receive and I will follow up.

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Sunday, January 4th, 2009 AT 10:50 AM
Tiny
VANGREENE@CHARTER.NET
  • MEMBER
I went to advance to check the codes and was told they couldn't check any vehicle before 1996. I tried to purchase a code reader that was reliable and the only model they had that could read my 1991 truck was a model CP9015. I did a check and the codes were 53 which indicates a problem with thethrottle position sensor and 67 which I'm not sure about. I've already replaced the throttle position sensor could it be installed incorrectly?
Thanks,
Van
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Tuesday, January 6th, 2009 AT 7:27 PM
Tiny
BLUELIGHTNIN6
  • EXPERT
Great. Thanks for that info.
First off, the code 67 is - Neutral Drive Switch Circuit Open. This probably is indicating that the neutral drive safety switch is faulty and needs replaced. The purpose of the switch is to prevent activation of the starter (by creating an open circuit) when the transmission is in any gear other than P (park) or N (neutral).

Possible causes for the code 53 are the TPS is not seated properly, faulty TPS sensor (not likely since your sensor is new), faulty ECA, or VREF circuit shorted in vehicle harness. Double check the wiring and installation of the TPS and post your results. If you need some installation instructions for this component just let me know and I will post for you.

Hope this helps.
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Wednesday, January 7th, 2009 AT 1:02 AM
Tiny
VANGREENE@CHARTER.NET
  • MEMBER
I took a meter and ohmed out the TPS and it checked okay. The resistance increased as the throttle was opened from.3 to 3. I cleaned the connectors and started the truck. It was idling okay for a few minutes but it then went to idling about 3000 RPM again. I ohmed out the TPS again and it was very high. It appeared to be affected by heat when the truck was running. I replaced the TPS again and it seems to be working fine. I disconnected the battery to reset the computer and cranked it up then checked for codes again. I got a 1 1 1 which I believe means it know passes the test. Just for curiosityI ohmed out the TPS again and the resistance reduced as I opened the throttle. I drove it for about 25 miles and it seems to be working okay for know and I hope it's fixed. Thanks for the help and great communication.
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Saturday, January 10th, 2009 AT 3:05 PM

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