Air Bypass Valve is what my 91 Ford shop manuals call it, another name is Idle Air Control Valve.

1991 F150 4x4 with 5.8 and E4OD. Truck has been running great for weeks now with zero codes (only code 11 in KOEO, has a code 41 for HEGO in CM that always clears but comes back).

Yesterday it kicked up to 3000 rpm and held while I was driving through town. This is scary!

I pulled into a parking lot, turned it off, and checked the throttle linkage and looked for obvious things. Started the engine and it went over 3000 rpm this time, probably because it was not in gear. Turned it off and went under the hood and worked the throttle manually. This time when I cranked it up it idled normally so I drove it home.

This afternoon I was going to work on the door panels. When I moved the truck over to the shop it had the high idle condition again. But this time it was only 2000 rpm so I was able to leave it running and go under the hood. Throttle was closed and working it open manually didn't have any affect on engine speed unless I approached WOT. So I unplugged the air bypass valve. RPM dropped to normal, around 800. Whenever I connected the air bypass valve it would shoot back up around 2000 rpm. AHA! I found the problem, I thought (which was wrong).

This is where I should have pulled codes, but I didn't. Lesson learned: always check codes.

I started checking the air bypass valve, figuring it was sticking. After an hour spent cleaning it (it did need cleaning) and checking the solenoid resistance and the voltage with KOEO (12 volts), I decided to check the voltage with it unplugged and engine running. It was 14.5 volts, which I believe is full voltage when engine is running. I have another truck with a 5.0 and it read 11 volts with KOEO. With nothing to lose but wasted time I swapped the air bypass valve from the good truck to the problem truck but problem was the same so I swapped them back.

Now I pulled codes, should have done that first. Had code 23, for TPS out of range. After I bought this truck I found the wires from the TPS had all their insulation broken off, like wires got hot and insulation cracked and came off. So the TPS was replaced in the last year. I checked the TPS with a meter for resistance and it was within spec.

Disconnecting and reconnecting the TPS seems to help the problem, once I did this the code 23 went away and the truck idled fine. But while idling and me holding the throttle up around 2000 rpm to warm the engine, the mysterious high idle returned, up around 2500 this time with tranny in park. Unplugging the air bypass valve returns the idle to 900 again.

Next I cut into the wires coming from the TPS to allow me to use a meter to check for voltage coming out of the TPS going to the ECM. It was 1.0 volt with throttle closed and 4.95 with throttle wide open, within spec. I can't get to the wires going to the ECM, they come from under the throttle body, but everything looks good on the TPS side of the connectors.

Once again, disconnecting and reconnecting the TPS seems to help the problem, once I did this the code 23 went away and the truck idled fine (again). But I'm pretty sure there is a problem with the connector at the TPS, or something along those lines.

It appears to me when the TPS signal is lost the computer uses the air bypass valve to ramp the engine up to 2000-3000 rpm, perhaps to allow you to drive it from highway speed, to get off the road? That part has really thrown me, its pretty scary. Does the computer use the air bypass valve to run engine at high speed when TPS signal is lost?

For right now the truck is drivable without the air bypass valve connected. Any pointers on what to do next?

 

It was stated on here that if the idle air control valve slows a fast idle, you have a faulty sensor. Beings you got the code for the TPS, I would GUESS your TPS or the wiring to it is faulty. Was the TPS you replaced recently a Ford part or other?

 

It wasn't Motorcraft brand, but it was a quality part from Rock Auto, don't remember the brand name. It tests perfect, I don't think it is the TPS itself.

 

Drove it a little today, first with the air bypass valve connected and no codes, and had no issues. Went into town to drop it at the paint shop. Didn't have any problems but didn't want any - could just imagine the 3000 rpm idle and them dealing with it. So when I got to the shop I disconnected the air bypass valve before leaving the truck with them. See it in three weeks with the paint done. I have been saving and waiting for a year and now the body shop I use is slow, just what I've been waiting on. I'll post pics when it comes back. Maybe the high idle gremlin will disappear in the meantime (NOT).

On the '87 with the 5.0, with engine running if I disconnect the air bypass valve the engine dies. I trust this symptom more then what I'm seeing on the 5.8 because the 5.0 truck is a one owner with code 11 for the three years I've owned it. The 91 with the 5.8 has continual annoying problems. Food for thought.

 

You have a bad TPS or a problem with the TPS wiring.

 

Thanks Conan.

 

Update: it was the base timing. You guys always preach the basics and I am kicking myself for not following your advice.

I don't have a timing light, I've invested in equipment for these old trucks, vacuum gauge, vacuum hoses, code tester, fuel pressure gauge, but not for a timing light. Finally took it to a local guy who's known as a Ford guru. First thing the guy does is check the base timing and finds it off, sets it. Drives the truck and its fine. Charges me $25. What a guy.

Only saving grace is what he charged is less than the cost of the d*** timing light!

 

How did the base timing change? If the truck has been "running great for weeks now with zero codes" what changed? I'm not sure about the 351W in '91 but my '87 302 had the nylon timing gears and it left me stranded in the middle of the road when the cam gear(?) teeth wore out. The truck shut off in the left turn of a 4 lane road and wouldn't restart.