Hopefully this topic hasn't been beaten to death too much, I did a pretty extensive search and found some things that were helpful, but still have a problem. And this is my mom's car (I have an '01 WS6), so it's hard to determine what's normal on this car

'91 Firebird 305 TBI, 62k miles, all stock. It's run flawless for the past 10 years, all of a sudden started idling rough and acting like it wanted to die. I start it, it idles fine for about 10-15 seconds, then starts acting up. In Park, idle would fluctuate between 500-1000rpms, and would often die. Put it in D or R, almost guaranteed to die. No pinging, ticking, or foul smells, no SES light, all guages appear normal.

I replaced the fuel filter, PCV valve, and fuel pump relay (just to be safe). Now the car will idle arounf 750rpm, occassionally dipping to 500rpm at startup. Give it a couple revs, idles perfect at 750rpms. In D and R, idles perfect around 500rpms. As soon as I start to accelerate, it feels like the engine's bogged and seems about 750rpms below what it should be (under WOT). It shifts into 2nd, chirps the tires (timing catching up?), then does the same thing in 2nd (only not as bad).

It feels like the timing is off, but it idles fine now. I tried to check the EGR valve by pushing up on the valve from the opening on the side of the casing, and nothing moves and the car doesn't die. Any suggestions? Like I said, it's strange that this hit all of sudden and after replacing a few things, part of the problem cleared up. Thanks!

Jason

 

Thrasher,

Welcome aboard!

Your symptoms could be caused by a sluggish IAC stepper motor os clogged IAC air passages. Try cleaning the passages with a spray-type carb cleaner (sensor-safe, of course). If the situation improves, you might want to remove, clean, and lube the IAC unit. I have some directions on that if you get that far.

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Later,
Vader
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Justice and Freedom will Prevail
Adobe Acrobat Reader

 

Thanks Vader, most of the information I got to get me this far came from a lot of your posts. I'm right with you, I brought the carb cleaner with me when I worked on it, the car seemed to run a lot smoother after each shot of carb cleaner down the TB (and a lot of black junk out the tailpipes). I'm just not sure if I hit the IAC unit. If you've got some info on that, I'd appreciate it.

Thanks.

 

Update:

I started it up today, and it was a repeat performance of yesterday's initial problem. Apparently when it's cold, the idle fluctuates between 400-1000rpms, when the car's warm, the idle is pretty smooth at around 700rpms with occassional dips to 400rpms.

I pulled the IAC and cleaned it as best I could. The plunger/screw was a **** to get out (and back in) the unit. Then I reinstalled it, and no difference in idle. I disconnected the IAC, started the car, and it idled at 1500rpms (no more, no less). I tried adjusting the idle (torx) screw, and no matter how much I backed the screw out, the idle never fell below 1100rpms - BUT the idle was consistent and didn't fluctuate with the IAC disconnected. I reconnected the IAC, reset the ECM (disc the batt), and it still runs like it does when warm (occassional dips to 400rpms).

I also found a slight leak around the front of the TB, but I think the real problem here is probably the IAC. Unfortunately none of the parts stores in the area have them in stock. Anything else I should try?

 

If you can't get the engine to stall by backing out the throttle stop screw, either your throttle blades are binding open slightly or you have a vacuum leak. I'm sure a TPI will not run at all with the throttle screw backed out all the way, and I'd say the same is true of a TBI engine. Someone correct me if I'm wrong. The IAC is trying to compensate for the leak and it is apparently not successful. Get some carb cleaner and start spraying around looking for the vacuum leak.

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1990 IROC 350
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"It's better to have and not need than to need and not have."

 

i have a similar problem. first my car (90 5.7)wouldnt idle and i found that problem to be the EGR valve. But now it idles fine however i have strange acceleration it wont go over a certain RPM, kicks in 2 and 3 rd gears and sounds like somthings holding my engine back. someone said my cats might be clogged so i had new cats installed and my exhaust is perfect, all free, yet i still have these acceration problems. when i romp on it it goes nowhere just all hesitation. i have the TPI though , so who knows

 

I'd be willing to bet that your problem is your fuel pump. I've had the same exact symptoms occur three times between two Firebirds I've owned (V6 MPFI & TBI) and both times it was the fuel pump. I even sent my L03 to the dealer to diagnose and they said I had perfect fuel pressure but then I sent it to my mechanic who replaced the pump and the car runs perfect now.

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~Luke
'91 Firebird :: L03/700R4 ::
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