Hello everyone, I have a 89 F350 with a 351w. When I first start the engine, it idles normally but after it gets completely up to temp and only if I apply a lot of throttle the idle will be stuck between 1500-2000 (it'll be at 2000 if I went to full throttle). I'm not sure if this is a TPS or IAC problem. The TPS reads .95 volts at idle even when the truck is doing this high idle thing. Half the time when the truck is stuck in high idle as soon as I touch the volt meter to the signal wire on the TPS, the idle comes down to normal. Also when it's stuck in high idle and I unplug the IAC, it dies. Today I removed the IAC solenoid to move the IAC by hand. It moved freely and didn't hangup or stick at all. Any ideas?

 

Unplug the iac and see if there is a difference. If it idles lower, get a new IAC valve or at least clean it really good

 

It stalls if I unplug the IAC

 

Unplug the TPS and examine it to be sure that all of the "pins" that mate up to the connector are straight and true.

It also could be that the TPS plastic casing is broken (around the "pin" area(s) causing an intermittant connection, or, one or more of the "pins" has broken from it's seat (internally) in the TPS.

The connector that plugs into the tps can be disassembled (and in your case probably should be) and examined for any possible loose connection/corrosion problems too.


That's what I got to offer you..........

Bob

 

Clean the iac and possibly replace, it could be that. But check the TPS first as Bob said

 

I took the throttle body off and found that the TPS doesn't have a plug, it's hard wired. All the wires and connections look good. I reinstalled the TPS 180 degrees from where it was. Idle voltage now shows .67 volts. Today I took it for a drive. Once it was up to temp again it's idle went to 2000 rpm. I took a reading on the IAC plug and it showed 3.3 volts. If I unplug it, it will stall right away. This leads me to think that the computer is asking the IAC for this high idle. Does anyone know what the IAC volts should read or how to properly test it? I turned the truck off, let it cool for 5 mins, started it up and idle was normal

 

See,

 

Everything in the TPS circuitry is backwards now that you installed it 180 degrees.
You need to re-wire the TPS correctly now.
It would appear that the "signal wire" and the "vref wire" need to be switched.
The computer is trying to close the IAC only to have it open further.

Bob

 

So, what was the outcome ?

 

So I was tinkering with the truck yesterday and I found something interesting. When I started the truck, it was idling at 1100 rpm. With the truck running, I unplugged the map sensor partway and the idle would go down. When I completely disconnect it, the idle would go back to 1100 rpm. Would this point to a faulty map sensor? Also, when I unplug the IAC valve, the idle goes down low, around 500 rpm. Is the IAC valve bad?

 

Don't know about the MAP sensor but, everything I've read over the past few days indicates that if you disconnect the IAC, the engine should stall, and that this is normal operation.