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Ask Ironmike Your Own Question
Ironmike
Ironmike, Ford Technician
Category: Ford
Satisfied Customers: 3377
Experience:  30 years ASE Master tech L1 . Ford master
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My name is ***** ***** have a old 1992 ford f150 with a 4.9

Customer Question

Hello my name is ***** ***** have a old 1992 ford f150 with a 4.9 liter engine with a/c manual
transmission, i have a racing issue at times sometime its normal sometimes when shifting it starts to race if i turn off the key real quick and turn it back on it will idle normal again, i changed the idle control valve,replace the throttle plate gasket because there was a vacuum
leak but it still the same, if i remove the wire on the iac valve the idle returns to normal,i checked for dtc's only found rpm too high
Submitted: 7 months ago.
Category: Ford
Expert:  Ironmike replied 7 months ago.
greetings, can you check for trouble codes? did you check the throttle body for excessive movement/wear? did you check the vacuum canister or other places for leaks?
Expert:  Ironmike replied 7 months ago.
you might have a vacuum leak from the EGR, vacuum canister for the A/C and the throttle body shaft may be worn causing the throttle plates to stick open. I would check these first and if you can get codes let me know what you find
Customer: replied 7 months ago.

hello Mike, the first thing i did was check for a vacuume leak and found that the throttle body gasket was leaking and i replaced it,and yes i checked for dtc's the only one was rpm's to high, there is no vacuum leaks now something is triggering the idle air control to open up if you turn off the key real quick while driving and turn it back on the issue is gone the same as removing the wire off the idle air control unit,there is no binding of the of the throttle plates if there was the rpm's would not go down when removing the iac wire

Expert:  Ironmike replied 7 months ago.
if the EGR is leaking(stuck open) it would cause a vacuum leak internal. Is the map original and how is the hose to the map? I have seen on older fords idle issues caused by the coolant temp sensor.
Customer: replied 7 months ago.

coolant temp sensor reading coolant temp within specs, there is no vacuum leaks, short fuel trim 1-2 engine idles beautifull, no black smoke,

no rough idle, like i said something is trigging the iac solenoid to engage.

Expert:  Ironmike replied 7 months ago.
how is the fuel pressure? did you check the fuel pressure regulator? they can leak and the vacuum hose will suck fuel into the intake. if you remove the vacuum hose and it is wet it is bad.
Customer: replied 7 months ago.

if that was was the case thetruck would be running richtheat not the case.

is there any thing that would cause the ecu to command the iac solenoid to energize?

Expert:  Ironmike replied 7 months ago.
02 sensor fault, ECM fault. most common would be the vacuum leak.
Expert:  Ironmike replied 7 months ago.
did you check the canister purge valve. if stuck open could mess the idle.
Customer: replied 7 months ago.

mike most of the problems you talk about would be there all the time, like i said something is trigging the ecu or computer to energize iac solenoid,

redid koeo test and koer test with no dtc's the problem even happened while the test was going on, the rpm's were going north of 3000rpm's had to remove the wire going to iac solenoid.

Expert:  Ironmike replied 7 months ago.
does it race or surge when hot and cold? trying to see if the ECM is waiting for warm up to take input from a sensor like the 02 sensor before it starts acting up.
Customer: replied 7 months ago.

it does it when it wants to hot-cold don't make a difference

Expert:  Ironmike replied 7 months ago.
did you check the PCV valve? you can plug the hose and see if that drops the idle.
Customer: replied 7 months ago.

checked all vacuum connections coming off the intake manifold, there again it should do it all the time, right now if i go and start that truck you would not know that it had a problem, drive it and it might act up and it might not.

Expert:  Ironmike replied 7 months ago.
how did you check for vacuum leaks?
Expert:  Ironmike replied 7 months ago.
I like to use carb/brake spray cleaner(not the non chlorinated type) and spray around all suspected gaskets. sometimes things like the PCV and EGR need to be removed because they can leak internally.If you replaced the throttle body gasket did you look inside the bore and check the TPS. I have seen the TPS fail many times on older fords and cause all kinds of idle issues. I am running out of ideas. I keep coming back to a vacuum leak I guess because that is mostly what I have seen and done for repairs on these engines and the 5.0 in these older trucks.
Expert:  Ironmike replied 7 months ago.
oh yeah, try disconnecting the brake booster when the idle jumps high. I have seen them leak inside.
Customer: replied 7 months ago.

sorry Mike i tried all that, look don't feel bad i ran out of ideas too thats why i called you guys, i hoped you could tell me something i did'ent know

or try.

Expert:  Ironmike replied 7 months ago.
I am sorry I will opt out and see if another expert has seen something other that what you have tried. good luck
Customer: replied 7 months ago.

THANKS FOR TRYING, JUST TOOK THE TRUCK OUT AND COULD NOT RUN ANY BETTER, I DONT UNDERSTAND ETHER.

Expert:  Ironmike replied 7 months ago.
if the trucks runs good now it could be an electrical/harness issue. You might try a wiggle test. grab the harness in several different spots and wiggle it so see if the idle changes. I have seen connectors to components with terminal to pin fit problems(loose) cause intermittent issues. When found either I repair the wire or tweak the terminals with a small pick/screwdriver.