Tab92bird

My car starts up fine, idles fine at first, but once the idle goes down to 550-600 the engine starts to stumble. I also noticed that when it does this sometimes it will cycle back into closed loop for half a second.

I've done all the major tune-up stuff and nothing has solved the problem (although cleaning the plugs helped it). Please help!

Thanks,
Brandon

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White '92 Firebird w/ 305 TBI. Open element w/ non drop base, increased fuel pressure, TBI spacer, Hedman Hedders & y-pipe, no cat, cat-back exhaust

formula90

i hope you meant going back to open loop and not closed. how do you know it went back to open loop? things to check, Coolant temp sensor, map sensor, and maybe egr,even though you arent getting codes it still may be operating in its parameters but still effecting idle and acceration. if not check for intake mani leak or hose leaks. good luck

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Tab92bird

btt

JP84ZROC

The MAP sensor is a good place to start. If you have a scanner check the voltage and see if it is stable or if the voltage jumps around quite a bit at idle, mine would fluctuate quite a bit and it turned out to be bad. It was not steady on the tenth's scale ie: it would keep changing from about 1.3 to 1.8 volts at idle. Also you can tap on the MAP sensor with a screwdriver and affect the idle quality if it is very bad, much like they do with the MAF to check it in worst case scenario.

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"84Z28/'91 305TPI comp cam flowmaster TB coolant bypass/shutoff valve to heater core MAT relocated Ram-Air(home-made) gutted air-box IROC ground effects/wheels GY245/50's 700R4 superior shift kit/ corvette servo(excellent shift kit BTW) Rebuilt engine and transplanted it myself. Love this engine!
LOW performance people should drive low performance vehicles!

Vader

Brandon,

I hope that the "major tuneup stuff" included cleaning the IAC passages and valve pintle. You should also have cleaned or replaced the PCV valve, since it controls vacuum to some extent.

A leaking EGR solenoid vavle or leaking EGR valve can also cause poor idle quality. What is the intake vacuum like when this occurs?

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Later,
Vader
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Tab92bird

Yes, I did clean the iac passage with carb cleaner. The pcv is working fine, but I sprayed it with carb cleaner just to be sure. The MAP voltage stays at 1.5 volts. I forgot to mention that there is a distinct popping noise coming from the exhaust when this happens. it's not loud like a backfire, and it's not noticable unless you are standing right behind the car.

Tab92bird

btt

Tab92bird

Can anyone else help me???