Okay, I've been trying to figure out when she does this, but I can't find a constant... Usually when the engine is cold, but sometimes when it's at normal operating temp....
Anywho, what will happen is either A) I'll put it in gear (either reverse or drive) and it'll jump up to 1500RPM, then slowly drop back to 800RPM... On an excpetional day it'll drop back to 750RPM then jump back to 1500, float down, then jump up again...
On a really unlucky day, I'll be in drive, lah-dee-dah... come to a red light, apply breaks, and as soon as I start to come to a stop it will jump from 800RPM up to 1500...

It seems to do it less when I have a full tank of gas, but it still does it...

Any ideas what's going on? Is there some kind of sensor that revs the engine when the RPM drops below a certain level? It almost seems like there is such a thing and my idle is set below that level so it drops, alerts that sensor, revs back up, then drops back to the idle, which sets off the sensor again... But seeing as I don't know much about these trucks (yet), I'm anxious to hear any answer. I'm afraid one of these days it's going to rev stronger than I'm breaking and I'm going to rear-end someone...

 

possibly your idle air control valve needs to be replaced

 

Any more info on the idle air control valve? I see that autozone has em for $60, but what does it do exactly?

 

Dude I have the exact same problem!!!!!
Last time I had the codes read it said a problem with the thermactor valve which I
believe works with the air pump to mix fresh air with your exhaust.
But I have'nt yet fixed it because im not really shure that's it so I will keep checking your post to hope for other replies... Jeff
90 bronco (351)

 

the iac supposedly controls how much air bypasses the throttle body and controls the idle speed. There is a way to check it before u blow 60 bucks on a new one

 

Would a check engine light come on if the iac was bad?

 

i don't know,i would guess it wouldn't come on but first off, you should check everything for vacuum leaks and be positive you have none and then check the iac, and then check the tps

 

Odd one that, odder still was that a version of that plagued a Dakota pickup that I used to have. Was in the shop several times to try to 1) get it in while it did the nasty and of course, 2) fix the darn thing!!

There weren't any codes on that one, but when it FINALLY did it in the Service bay WHILE a tech was there.. it turned out to be a bad connector in that (or the Dodge version thereof).

Idle surged back and forth from 400 to 2000... and yeah, two feet on the brake pedal was the order of the day. Had the ugly vision of the brakes giving up while the pedxers were in front of the dang thing.. so didn't waste any more time than I had to..

Get her fixed pronto fellas...

Swit

'94 XLT 5.8

 

The idle air controller will not put a check engine light on. clean the iac with throttle body cleaner. you must remove the iac and clean all the carbon out of it and make sure the plunger moves freely.this will probably fix your erratic idle.

 

where is the iac? I'm looking it up in the chilton but with minimal luck...

 

its hooked up to the right side of your throttle body

 

high. When you start in the mornings, it idles at 1500, like its cold, but if you tap the throttle it never goes down. If you pump the brakes it goes up. As it warms it goes up. it runs pretty good, seems to lack the power that the 84 carbed 302 has. I just got this thing running ""see post new bronco now to get it running"" I know the donar was running, but i dont know how well, but i used the diz , and the whole TB assy. The beast came with a 351 but prior owner swapped in a 302 from an 89 van. i am happy so far, the idle is annoying, especially whenit gets up to 2500, and the power is disapointing, thought the efi would be better than my carbed motor.

 

What year is your Bronco ? On my 89, I had almost the same type problem and it was the fuel pressure regulator on the fuel rail near the firewall on the drivers side. Could also be the IAC as mentioned earlier, dirty throttle body or maybe even a faulty TAB or TAD near the fuel rail as well. If you clean the throttle body, make sur eyou do it with throttle body cleaner and NOT carb cleaner. Could also be a throttle position sensor problem. Either way, keep us posted. Also, a Haynes manual is much better than a Chilton's but both are handy to have.

Good luck,

 

I have a 90. Where should I start looking? (lovely that I have about 0 time for this this week...)

 

I had the same problem in my 88 302. It was my throttle positioning sensor. To me that is what your problem probably is. I think I picked mine up at the zone for $15. It is not too hard to fix. Just take off the throttle body flip it upside down and I think two phillips screws take it off. It has a round piece and a wire harness coming off it. Hope this helps!