I've had this problem for quite some time now. When I'm at a stoplight and I hold the brake and give it a little gas, or when I'm going up my driveway and I do the same, the rpms fall to almost 300, then surge back up. I know it's not a vacuum leak in the manifold, which I've had off several times, and I'm checked the other obvious places for a leak.

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

 

I'd also like to know why this happens. It happened on 3 occasions to me. The first time i was low on gas so i thought is was that but the 2nd and 3rd i had plenty of gas, but it didnt start to bog until the car was drven for about an hour. It was the same thing though....when i hit the gas off the idle it would bog real bad the RPM's would drop and then pick up all the sudden.

 

TA,

You're certainly on the right track. A vacuum leak would cause a lean condition, and it sounds like that is your problem. It would be best to verify the low vacuum condition with a vacuuum gauge.

If you've checked all the gaskets and hoses, you might also need to look at the PCV valve and power brake booster. You should also check the throttle minimum air position and IAC operation. An incorrectly adjusted throttle position can cause the condition.

If the vacuum is still low, it's possible that the timing is incorrect, either ignition timing or more likely valve timing. Late ignition timing will contribute to low power and low vacuum, but incorrect valve timing will affect vacuum much more. It's not as likely that the problerm is related to the timing, but possible.

If the vacuum is O.K., check the fuel pressure, injector spray patterns, and flow capacity of the pump. Low pressure and/or flow could indicate a failing pump, regulator, or a clogged filter.

You could also check the operation of the TPS sensor, since a "flat" or "dead" spot in the sensor can cause similar symptoms. Your sensor should be the non-adjustable type, but it still needs to operate correctly. Minimum throttle position will have an effect on the TPS signal as well.

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Later,
Vader
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"I'm'a do Things My Way - It's My way or the Highway."
Adobe Acrobat Reader

 

Do I need to check several different places for vacuum, like each hose, or is there a particular one I should check first? Also, other than shaking it, is there another way to check the pcv valve? The ignition timing is fine, but what do you mean by the "valve timing"?

Thanks,
Brandon

 

btt

 

I had this same problem and it turned out to be the IAC. It was loose to the touch which was probably allowing air to get in...also, it was raising my RPM's and causing hesitation right off idle. Remove it and clean it up with some TB cleaner (the pintle on the end of the IAC and clean out the passage..mine was full of carbon)

Steve