'92 5.8 Fast Idle
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New User
Joined: Jul 2004
Posts: 5
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'92 5.8 Fast Idle
I'm expierencing what was once a periodic problem but now has become a regular problem. My '92 F150, 5.8, has an idle problem. Sometimes at start-up, the idle is fine (about 600 rpm) then increase to 1500 - 2000 rpm. As I drive and have to brake, I have to hold hard pressure against the brake pedal to keep the truck from moving. I can put the trans. in neutral and the rpm reading is 2000 - 2500 rpm. I may drive for several miles with this condition then for no apparent reason, the idle returns to normal and all is well. This behavior is becoming the norm lately and I know it's using more gas then needed along with the extra wear on the brakes. Could use some help determining where the problem lies. I'm assuming there is some type of sensor that has gone bad, but we all know about assumptions.
Thanks.
Thanks.
Elder User
Joined: Sep 2004
Posts: 941
Likes: 4
If the TPS is bad or intermittant, the computer may think you are still cruising down the boulevard, and not return to the correct warm idle strategy. This would leave the IAC wide open, giving a fast idle. A TPS problem should be detected during a KOEO self test.
If the IAC in flaky, sticking, or partially blocked, it may not react when the computer tries to close it down. Try a KOER test to give to computer a chance to exercise it.
Pull the hoses off and inspect the throttle body bore. Make sure nothing is interfering with the ability of the throttle blades to close properly.
Check for severe vacuum leaks, old and stuck PCV, etc. Sources of air besides the throttle body/IAC will mess up the computer's idle control.
If the IAC in flaky, sticking, or partially blocked, it may not react when the computer tries to close it down. Try a KOER test to give to computer a chance to exercise it.
Pull the hoses off and inspect the throttle body bore. Make sure nothing is interfering with the ability of the throttle blades to close properly.
Check for severe vacuum leaks, old and stuck PCV, etc. Sources of air besides the throttle body/IAC will mess up the computer's idle control.
Cargo Master
Joined: Sep 2004
Posts: 2,691
Likes: 6
I was cleaning my intake the other day. I disconnected the PCV hose from the PCV and poured seafoam in it. To get the hose off I disconnected two little vacuum lines that were in the way so I wouldn't break them. I forgot to reconnect them and when I started the truck it idled very high. Once I plugged the lines back in the idle went back to normal. My point is that a vacuum leak can cause this.
