Questions on 90 Bronco...

Yeah, so i posted this on the big bronco forum, but i havent had any replies, maybe someone here can help me. I have a 90 Bronco with a 351 and an auto tranny. My problem is when i go to start it, it revs up to 1400-1600 rpms for a few minutes, then dies down just above the 500 rpm mark. then when i go to put it in reverse it revs up to 1500 rpm again, and goes into gear rather hard. If i go into drive it will go into gear smoother, but the motor will rev up to 1000 rpm and stay there for a few seconds, then go back down to just above 500 rpm. It will also do this if i am coming to a stop, it will stay around 1000 rpm, then die down to 500 again after a few seconds. I dont think it is harming anything, as i have yet to find any metal particles in my tranny fluid, but it is really annoying to have to jam on my brakes sometimes to get it to stop, and the clunk sound that it makes going into reverse isnt to pretty to hear either. any help is greatly appreciated! thanks!

yo Drew,
Have you checked for Diagnostic Codes yet?
A typical problem for what you describe is:
...Diagnostic trouble code 123 throttle positioning sensor (TPS) circuit
here is one article on this:
http://autoclinic.net/article/ford/e250_tps.htm
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or the TPS connector as decsribed in this message thread:

http://www.v8sho.com/SHO/highidle.html
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A code for the TPS may not mean the TPS is bad. It may be reacting to another problem, such as a bad TPS ground or the connector.
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A vacuum check is also a consideration and is cheap and fast check too.

No, i havent gotten around to checking the codes yet. As long as time allows i plan on doing it this weekend. Id really like to get to the bottom of this, mainly just because it annoys me, and it probably isnt that great for my brake or motor. Thanks for the articles, they should prove to be very helpful!

Yeah, a bad TPS is the first that comes to mind. If that checks out O.K., then it may be a bad IAC (Idle Air Control valve). You can clean this to make it last longer, but it's often best to replace it.

Bad TPS will also cause you to shift at randome times ... it's a weird device

Quote:

Bad TPS will also cause you to shift at randome times ... it's a weird device

[quote]

Well I back probed the TPS and I got good solid readings across the board, so nothing wrong there. Im gonna move onto cleaning/replacing the IAC. The only problem is its so freakin cold here, we just went 31 days without a temperature above 32 degrees... and it just snowed 4 inches this last saturday, ugh, I cant wait till spring

you could just unplug the IAC while the truck is running and if you hear your rpm's change when you do that, you know your IAC was working when it was plugged in.