High idle
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High idle
1990 F150 5.0 So I have a follow up to my post a few weeks ago. I recently took my truck to a mechanic (I am at college and away from my tools) because the idle would surge up to 2000 rpm and wouldn't start after some highway driving. Anyway, he diagnosed it as a bad TPS sensor. Anyway, the truck idles as it should around 800 rpm sometimes when I first start it. However after some driving, it idles at 1500 rpm at lights and such. It doesn't matter if the clutch is depressed or not. New TPS sensor, throttle body blades are clean, throttle's not sticking, IAC valve is clean, new throttle cable, distributor cap, pugs and wires have maybe 2000 miles on them. Ideas?
Fleet Owner
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From: Kentucky
You put a used IAC and TPS on the truck a few weeks ago with pretty much the same results. I do not recall if you have unplugged the IAC when the idle is high to see if it returns to normal. If it does that points to a bad sensor. Maybe not bad enough to trigger a code. If the idle remains high you have a vacuum leak somewhere or you have a throttle body issue (Sticking, warped, ?)
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You put a used IAC and TPS on the truck a few weeks ago with pretty much the same results. I do not recall if you have unplugged the IAC when the idle is high to see if it returns to normal. If it does that points to a bad sensor. Maybe not bad enough to trigger a code. If the idle remains high you have a vacuum leak somewhere or you have a throttle body issue (Sticking, warped, ?)
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Yes I did. One of the 2 vacuum hoses going into the throttle body is a little torn on the end? Could that be it?
Fleet Owner
Joined: Oct 2005
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From: Kentucky
Best way to find a vacuum leak related to something on a vacuum circuit is a hand held vacuum pump/gauge. Or a simple way to narrow down which one may be causing the vacuum leak is disconnecting each vacuum circuit on the manifold vacuum tree one at a time. Of course you need to plug the now open port.
Keep in mind you will need to keep a vacuum source for the MAP sensor, you may inadvertently trigger an increase or decrease in idle RPM when removing it's vacuum supply. Run a temporary line to it for testing.
If the high idle persists then you are probably looking at an intake manifold gasket or secondary AIR system leak.
This is also assuming the throttle blades are not being held open by a mis-adjusted throttle stop screw.
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No go on that question.
Best way to find a vacuum leak related to something on a vacuum circuit is a hand held vacuum pump/gauge. Or a simple way to narrow down which one may be causing the vacuum leak is disconnecting each vacuum circuit on the manifold vacuum tree one at a time. Of course you need to plug the now open port.
Keep in mind you will need to keep a vacuum source for the MAP sensor, you may inadvertently trigger an increase or decrease in idle RPM when removing it's vacuum supply. Run a temporary line to it for testing.
If the high idle persists then you are probably looking at an intake manifold gasket or secondary AIR system leak.
This is also assuming the throttle blades are not being held open by a mis-adjusted throttle stop screw.
Best way to find a vacuum leak related to something on a vacuum circuit is a hand held vacuum pump/gauge. Or a simple way to narrow down which one may be causing the vacuum leak is disconnecting each vacuum circuit on the manifold vacuum tree one at a time. Of course you need to plug the now open port.
Keep in mind you will need to keep a vacuum source for the MAP sensor, you may inadvertently trigger an increase or decrease in idle RPM when removing it's vacuum supply. Run a temporary line to it for testing.
If the high idle persists then you are probably looking at an intake manifold gasket or secondary AIR system leak.
This is also assuming the throttle blades are not being held open by a mis-adjusted throttle stop screw.
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Fleet Owner
Joined: Oct 2005
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From: Kentucky
Another method is to remove the throttlebody so you can verify the blades are just short of touching/completely closing against the body casting with the blades pulled back to the Idle Stop Screw.
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Here is the update. I checked all the vacuum lines around the tree and throttle body. All are good. I checked them by unplugging the truck and seeing if the idle would change. All were good. I found that the throttle body is not closing fully. I was able to manually adjust the rpm to 800 without it touching the idle stop screw. I sprayed white grease on it. I think a new thottle body may be in order. I have a spare I had ordered but it has an extra coolant port where I have no coolant, fuel or vacuum connectors. Am I right?
Have a great Thanksgiving!
Have a great Thanksgiving!
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Joined: Dec 2007
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From: Edmonton, Alberta
its free floating... so that would mean the springs in the tb are toast.
time for a new TB
if you want i have mine off my 92 you can have, just send me your address, and you pay shipping
time for a new TB
if you want i have mine off my 92 you can have, just send me your address, and you pay shipping
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But I am interested but want to make sure we have identical throttle bodies