I have recently acquired a 1989 f150 4x4 5.0 5spd 150k. I thought I was suffering from the notorious high idle, now I'm not sure. The problem is when I crank the truck it goes to high idle then comes down to around 750rpms. However if I touch the gas pedal it will rev to around 1200 to 1400rpms and stay there. While driving no matter what rpm I change gears at it will rev to 2500 to 3000rpms with the clutch in. If I stomp the gas pedal it will fall down to normal idle. With the help of this forum I replace the IAC, MAP, TPS(which I set the voltage to .989v for idle),and cleaned the TB. I also replaced the vacuum reg. vlv (not sure if this is the correct name). For 2 days everything was perfect now it is doing the same thing again. I unplugged my IAC and the truck died instantly. Could this be a vacuum leak? My passenger side manifold is cracked, could this have any factor? If not what could it be? Any help will be greatly appreciated. Also would a vacuum leak make the idle high or low? Thanks

 

Vacuum leak will make it low. Unless your throttle body is stuck it sounds like the IAC is not working right. Is the throttle body clean?

 

Quick check on the IAC is to start the truck and let it idle, unhook the connector to the IAC and if the idle changes your IAC is functioning if the idle stays the same head to autozine and get a new IAC.

 

he said it was clean, and he also said it dies when the IAC is unplugged.......no soup for either of you....NEXT!

Sounds like the throttle cable is sticking. Take the cover off the top of the TB and chedck out the plastic sheath and spring where the throttle cable hooks up. You will probably find the sheath cracked and inhibiting function, if so, replace it. Should be a 15 min check and fix.

 

The crack on your passenger's side manifold could be the problem I am not familiar with the '89 setup but your O2 sensor should be mounted right before the cat and any exhaust leaks before the sesor will throw off your idling and your mileage. Also I would replace the cracked manifold as soon as you can it will only cause bigger problems in the future, loss of backpressure is not good!
Good thing I don't like soup !

 

Thanks for the replies, I failed to mention that I replace the acceleator cable as well. This had no effect on the problem.

 

In that case then I would replace the cracked manifold or for a temp fix try and patch it to see if your condition gets better without the leak.

 

The cracked manifold will have nothing to do with the high idle. There are a ton of these trucks runnin around with cracked manifolds and run just fine. A vaccuum leak will actually make it idle higher, and very unsteady. It would also throw a check engine light. Do you have a check engine light? If so, pull the codes using the instructions under www.fordfuelinjection.com.

The only way an engine can idle that high is if air is leaking somewhere, or if the EEC is really ramming the fuel through it. While at high idle, does black smoke flow heavily out of the exhaust, especially if you smash it to the floor? If not, that eliminates that. But with it coming back down to 750 RPM, this tells us its not a constant leak. This leads me to believe that your IAC valve may be sticking open a little, allowing more air than normal to be passed through the throttle body. With it dying out instantly when unplugged, I think the IAC could be faulty. Reason being, is because it's supposed to open a little to allow air through when unplugged. If it's not working, it will shut, not allowing any air through, thus killing the engine.



I say pull the codes if there are any, and let us know what you get. I know this was already posted, but make sure the butterfly valve is closing all the way.

 

I pretty much had the same problem on my taurus.Might want to unhook the battery for a half hour and let it rethink.It worked for me at least the first time.