Replaced ECU, now it doesnt idle on its own.

[Guest sublimewhatigot]

My ECU was fried so I bought a new one and I rebuilt the busted one so I have 2 good ecu's now. I had pulled the plenum off trying to figure out the problem took apart the AAC motor and put it back together.

 

Now it wont idle on its own it wants to die out unless I give it gas, the whole time its missing. If I unplug the MAF it will rev from 1800 to 2800 all on its own. I checked and rechecked everything I unplugged when I messed with the plenum. So sick of this ready to pay someone the big bucks to fix it but I really cant afford to. I get code 55 on the ECU, meaning normal. I turned on the AAC idle screw a few turns with no change. What the heck else could I have messed up??

 

Thanks.

[94extreme]

plenum? planum? what is it?

[Guest sublimewhatigot]

Intake plenum... its on the 90-95's I think?

[94extreme]

ok, i think those go with TBIs.. my 94 is injected.. regardless.... kind of what i was thinking anyway.. since you dis-assebled that recently i am thinking it's related to it but first did you try both ECUs.. maybe the new one is bad too.. it's possible..

 

otherwise you probably have a leak around the plenum. spray some soapy water on the areas around where you took things apart.. if the truck's running temporarily smoothens out then that's you found your problem..

[Guest sublimewhatigot]

Well the ecu before was completely fried, the car wouldnt do anything. Both ecu's do the same now so I'm pretty sure they are good.

 

It runs the exact same with the intake plenum bolted down as it does just sitting on top, the engine vaccum actually sealed it good enough to run without really leaking. So I dont think thats the problem.

[94extreme]

did you run diagnostics/codes on the ECUs?

[Guest sublimewhatigot]

Ya I said above that I got code 55 on both, meaning normal. Although this ODB1 crap doesnt tell me much, it doesnt even tell me when I unplugged my MAF or various other little sensors.

[94extreme]

sorry, i didn't read it again.. hope someone else chimes in while i am thinking..

[Precise1]

If the ECU isn't showing any error codes, I'd start by checking out all the ignition stuff. Dizzy, cap, rotor, wires and plugs...

 

B

[Guest sublimewhatigot]

I checked out all that kinda stuff, although it does sound like its not running on all 6. Im gonna try and check the timing and fuel tomorow.

 

Does anyone know what happens on a good running vehicle when you disconnect the MAF? It seems like its a combo of things, maybe something to do with the AAC motor the way its revving up and down.

[94extreme]

iirc, the truck started running crappy when i disconnected the maf.. i think the rpms dropped.. i could be wrong. it's been a while since i messed with it.

[mws]

AAC - is that what is commonly called the Idle Air Control? That is a prime suspect...

 

On (most) fuel injected engines, the idle circuit is totally different than on carb'd vehicles. Instead of the throttle plate being held open a little bit, the FI system uses a valve that lets air in under the throttle plate. And that valve is the IAC. If it is not working properly, it will let in the wrong amount of air. The IAC is controlled by the ECU.

 

At the same time, the ECU is adjusting how much fuel to squirt to match the air. And it adjusts both simultaneously to adjust RPM.

 

So I would call the IAC a primary suspect for your symptoms. And as mentioned, any vacuum leaks will screw things up royally. The amount of air and fuel needed at idle is tiny - so things are quite critical to keep it all in balance.

 

What happens when the MAF is disconnected? Any number of things, depending on the logic used by the ECU programmer. With the MAF disconected, it will send one of two signals to the ECU: A) That there is a HUGE amount of air going through, or there is NO air going through. Either way, it's sending a grossly bad input signal to the computer. How effectively the computer can use all the other signals to determine the MAF is giving it bad info and compensate depends on the logic. Most don't do well at all... The ECU just gets hopelessly confused and will revert to an open loop limp home mode.