Surging on start up

BroncsRock · Post by **BroncsRock** » Sat May 13, 2006 4:07 pm

My 89 Bronco II has had this weirdness for about a year. When first started, it fires right up, idles a bit high, but then about 5 seconds later (if that long) begins surging. Usually never dies, but drops rpm to about 500, then right back to about 1100, and does this for about the first whole minute until warm. Once warm, usually runs excellent.
However, it also takes these spells where it will die at a stoplight idling, and also has died on trails going down hill, and in situations when on the brake not giving it gas. But only at weird intervals, not every 10th time, it can go weeks without dying like that.
I've had it scoped, supposedly ran for trouble codes and nothing is returned.
My check engine light only comes on briefly as it is going to die, and I can never get a time when it is on and stays on.
I've heard you can only get an accurate trouble reading if something is currently wrong and check engine light comes on? Is this true?
It had the fuel pump and filter replaced about 4 yrs ago.
Also, on last smog check, it failed horribly on first try. So was sent to another place, and they said it passed fine without them doing anything to the engine.
Something is acting up, but not all the time.
I think it is some kind of air/fuel sensor but my chincy Haynes manual does not help me enough to know what to try or change.
I can't even get a shop to find a problem!! Any ideas? Otherwise what a great little 4x4!

tekatlarge · Post by **tekatlarge** » Sat May 13, 2006 10:41 pm

I would be suspesious (sp) of the IAC valve. Lots of things can cause what you describe. There are several tsb's on the intake drivability problems. check em out if you have availibility.

BroncsRock · Post by **BroncsRock** » Sat May 13, 2006 10:48 pm

Sorry, what is the IAC valve (the long name) and where is it located? And what is tsb's?

eisle89 · Post by **eisle89** » Sun May 14, 2006 7:22 am

The IAC ( Idle Air Control ) valve is cylindrical and mounted on the pass. side of the upper intake. Take the solenoid off the end ( the part with the plug ) and clean the valve body out, there will be a gummy residue in there that hinders the valve sliding back and forth. You also say that the fuel filter was replaced 4 years ago, that's a long time. I replace mine annually with the tuneup.

BroncsRock · Post by **BroncsRock** » Sun May 14, 2006 10:34 am

Thank you, this is a great reply and photo is most helpful.