This is a 1990 F150 4WD pickup with 5 Liter eng. Manual Trans. Fuel Inj. Gasoline. I bought this truck in October and it ran well. I spent a month rebuilding the brakes, and front 4WD axels and numerous other things, but nothing to the power train other than oil change. So, I have not driven it real long yet. However it ran great.

About 2 weeks ago I was driving it when the engine suddenly went from the normal idle speed around 700rpm. and jumped up to around 2000rpm. Eventually it went back down, but then it went up again, and this repeated.
I should mention that we have had a severe cold spell in the last few weeks, and wonder if that could have any effect. Ever since this keeps happening every time I drive it. It runs normally until it warms up, then it surges and sometimes gets up around 3000 rpm.

A mechanic told me to check the IAC. When it was in high idle I knocked on it with a metal tool, which did nothing. I removed it, and sprayed it with PB Blaster and replaced it. That did not fix it. I do not see any loose vacuum hoses, although in this severe cold weather its sort of hard to thoroughly check everything. There is one hose that was plugged near the firewall on the pass. side. I do not know where it's supposed to go, but the truck came that way. There is one other problem I am aware of, the O2 sensor is broke off. I would have replaced it, but the exhaust pipes are bad and that is a warm weather job, not to mention the lack of funds right now. However, that was also that way when I got the truck and it ran fine then.

Because it surges up and down, it seems to me that it's not the IAC. I can leave it idle and it will climb to 3000, then drop back to 2000, and go back up, or back to normal without me touching the gas pedal or anything else
A mechanic agrees with me that it's not the IAC but some other control. He said that on a 90 model, a code analyzer will not do much of anything to determine this problem.

I'm at a total loss what is causing this. This is also my first Fuel Inj. vehicle since all the others I have owned were carb. It's dangerous driving it like this, especially on winter ice and snow. I am a DIY person and hope I cna find the problem.

Can anyone shed any light on this? I am lost. I do not have any costly analyzers. What can I unplug or bang on, to test for the problem?

(The gas pedal is not sticking).

Thanks

George

Besides posting to the group, and email would be appreciated.

 

If I remember correctly the 90 F-150 was speed density. If so your surge could be caused by a vacuum leak.

JMC

 

Fix the O2 sensors first.

 

I had something simular to this on my Mustang 5.0L and it was the TPS ( black thing bolted to the top of the throttle body ) But my 5.0 was Mass Air and I'm not too sure where or if it would be on your truck.

 

I think I know which part you are talking about. There is a piace behind it that has small holes all the way around, looks like a small filter of some sort. The TPS has a plug with several wires going to it. I am not sure how to test that thing. I was reading something on a website that explains that part as a variable resistor (potentiometer). It would make sense that the part is bad. because if the IAC was sticking, it would not keep speeding up and slowing down again, without me touching the gas pedal or anything. This only happens when the truck is warmed up. It runs fine when cold.
Not knowing how to check the thing, even though I do have a volt/ohm meter, I am wondering what would happen if I unplugged the wires when it went into high RPM mode? I guess I can give it a try.

If anyone knows how to test this part with an ohm meter, or the voltages, please let me know.

Now I got another problem. I parked the truck and fortunately I left something in it. A half hour later I go to the truck, it's shut off, key removed, and the fuel pump is still running in the tank. The ign switch is OFF.
What the heck??????

Of course it's 5 below zero. I just pulled one of the battery cables for now.
Maybe I should post this in another message....

Thanks

George