I have an 86 Bronco II, V6-2.9 fuel injected, automatic Trans, 4WD with
70K miles on replacement engine. When the vehicle is started cold or
prior to full operating temperature, it runs up to 1800 rpm for about 40
seconds, it will then slowly role back to 800 rpm and kill.
Restart the vehicle and it will run great for approximately 30-40
seconds, it will then drop out of high idle and dies again.
Press the accelerator and it runs extremely rough and bogs out.
If I continue to restart and play with the accelerator until it comes up
to temp. It runs and idles great.
This is what I have done up to this time;
1.) Replaced spark plugs, cap, rotor, & wires.
2.) Replaced fuel filter & air filter
3.) Replaced oxygen sensor.
4.) Removed catalytic convert and insured there is no excessive
5.) Replaced the fuel pump in the fuel tank.
6.) I took it to a local shop for a diagnosis he replaced the TPS and
said I need a power break booster and a fuel pressure regulator, I
replaced both those items.
7.) Replaced the air temp sensor.
8.) Replaced the coolant temp sensor.
9.) Checked fuel psi, manifold vacume, initial timing and spark advance
all were within spec.
10.) Replaced the MAP, MAS and idle air sensor with used parts from a 88
that runs fine, they made no difference, and I put the old ones
back on. Now when I check codes none come up.
Where should I continue my quest to find the trouble?
Fords of that era are very difficult to diagnose. A mechanic with a great deal of experience working on similar models using a good scan tool can likely find the problem but finding that mechanic could be very difficult. There was a notorious problem with high idling caused by the idle air control valve but it didn’t quite fit your description.
Next step is to change the engine control computer. Price Check on Bronco Two!!! It could be cheap compared to one of those other components. An insane computer won’t tell you that it is raving. Those year computers may not even have the capability to implicate themselves. No witness, no crime. Sometimes the idle speed control motor (I call it a throttle positioner) might be working on fast idle and tries to go back to idle and goes all the way back. See if you can adjust it farther out so that when it’s back all the way the engine will idle.
Thanks for the info. but I can’t find the idle speed control motor either on the car or in the book.Where schould I look?