95 Bronco 5.8 idle problems

any idea where a vacuum leak might be? I 've looked around, and don't see anything.

All the hoses check OK

Actually, that is what it is supposed to do. It is programmed to do that for the purpose of heating the O2 sensor to get it warm enough to go into closed loop operation ASAP. On a cold day, it may stay at high idle as long as 90 seconds before it starts getting the proper sig from the EGO sensor. The cold start sequence is based on engine coolant temp. As long as it does not go below the set point, the cold start sequence does not kick in. On a cold day, it may go into cold start sequence after only a few minutes. It is common that they slowly get to a point that they do not go immediately to high idle, if at all, after cold because of normal gumming of the IAC. The PCM is constantly and incrementally adapting to the way your engine operates. If the PCM was not cleared after replacement of the IAC, it is simply seeing a startup condition that it has not yet adapted to and does not perceive as normal for your engine. Therefore, a CEL flash alerting you to a change on startup. If, on the other hand, the CEL comes on for 3-5 secs after cold start, that is as it should be to let you know the system is functioning.

I wouldn't waste any more time looking for a high idle "problem". Reset the PCM by disconnectiong the neg battery cable for 10 minutes or, have someone clear it with a scanner. After it is cleared, start the engine with all accessories off and not touching a thing including the throttle. Let it idle for 5-10 minutes and then move the trans into each gear selectiona couple of time allowing the idle to settle down in each before moving to the next. Drive the vehicle a few mile for it to start relearning you and your engine. It will take a few start/drive cycles before it completely adapts everything but, you should see an improvement. Don't bother doing a lot of worrying or tinkering before doing this. The only thing I would do first is clean the MAF sensor to make sure there is no problem there. There are istruction with pics all over the internet on how to do thi as well as having been posted numerous times in this group if you want to search it out.

Get some sleep and enjoy your Bronco.

Lugnut

You're right Lugnut, this thing was really bugging me. Well, I disconnected the battery to reset the PCM (like you said), and it worked! I just can't understand why it would just start doing that high idle thing out of the blue. Also, the idle problem started a couple of days after the ABS light came on. I figured the worst case senario is the computer(s) had gone kooky.

Thanks to you, I only have one current problem with the Bronco. The ABS light still stays on. I replaced the rear speed sensor and the light briefly went out, but it's back on all the time now. I was hoping the ABS computer would have reset too, but no. Do you have any ideas about it?

Thanks again, John

Don't be totally surprised if the high idle returns when cold weather returns. The high idle is determined by how cold the engine coolant and the air intake charge is in the engine in addition to how long it takes the oxygen sensor to become warm enough to provide a good signal.

The ABS system is one with which I am not terribly familiar. the main thing to know about the ABS is that it will take itself completely out of the system such that your brakes operate as if no ABS were installed if it senses a failure. I am attaching a link below for you to take a look at the general troubleshooting procedure and the fault codes for the ABS system.

You might need a wiring diagram for the system, which you may also be able to find on the AutoZone site as they now have some if not limited service materials.

Lugnut

Thanks for the good info, and the ABS link. It did it a little this morning again (the idle thing), but I'm not freaking out about it.

Take care, John