The car is an '89 Firebird 305 TBI completely stock setup. No bypassed items or missing items. I know, rare right? Anyway, I'm more of a carb kind of guy usually, but my '89 has got this ever so slight rough idle once in awhile. It starts perfectly every time, fast idle is normal and it idles down to normal when warmed up. Acceleration is perfect. There is no power loss at all. Car seems ok otherwise. However, while sitting at a stop light, foot on brake, car in drive, the idle will drop 50 RPM quickly, then back to normal and it's random. Sometimes, it will act like it has a faint misfire at the same time it drops 50 RPM. This car has always idled at 550 in drive/reverse, so I'm dropping from 550 to 500 quickly and then back again at random times and sometimes not at all.

It's never stalled and I as soon as I hit the gas, the power is there with NO hesitation. I've kind of ignored it for a few weeks since it doesn't seem to affect the car. It's just annoying that it won't sit there and idle perfectly smooth. I've chased vacuum leaks to death. I haven't done any work to the engine other than change the oil recently, but it was already doing this before I changed the oil. I swear, sometimes, I can get in it, go to town and back and it doesn't do it at all. It's got to be something "about" to go bad on me.

 

What fuel system checks have you performed?

 

Check the rubber lines going from the engine to the charcoal canister, from canister to plastic valve, and from plastic valve to fuel tank vent. Mine were rotted from age but still there. Leaked like a sieve.

 

Finally, I've gotten back on this with the warmer weather and some semi-spare time. My job CONSUMES me. Anyway, it's still doing this and it stalled on me once last week. Fuel pressure is normal and consistent even when the idle surges while in drive foot on brake. The cone shape spray does not change and is also consistent. I did replace the fuel filter about a month and a half ago and that thing was nasty. Every 10,000 miles my eye. I think it was factory. However, that did not help the issue, although it needed to be replaced. I did find that the PCV hose was not sealing perfectly to the PCV valve causing some vacuum leak, but again, not the problem. I've checked all other hoses in the bay and even replaced some that were a little too stiff for my liking.

I was running the engine tonight with the air cleaner off and re-checking spark with the light, then I get a code 23 which I assume is only because I had the air cleaner off sensor unhooked for a bit. I wasn't getting any codes before. I reset it and I'll check that again for sanity tomorrow night after I drive it.

It's definitely an idle only problem which acts like a vacuum leak since drivability is not affected. It does not stumble at all on acceleration and the power is there. It just doesn't like to idle perfectly sometimes and it seems to be getting a little worse. At this point, I'm not sure where to go. I'm going to have to get me a scan tool that's for sure.