TBI 305 running like crap. Help me diagnose.

1992 Chevrolet Camaro, 305 (5.0L) with TBI (NOT TPI). The problem initially occurred when I was driving to work. The car ran fine all the way there, but then unceremoniously cut out when I pulled into the parking lot. Literally coasted the last few yards into a space. It would restart but the idle was very uneven, almost dying. It goes like this: Crank a few seconds, it starts and idles fine for three seconds or so. Then the RPM will drop nearly to zero, but it catches itself and surges back to "normal" idle speed, then immediately drops back to zero. So, it will just oscillate like that nearly dying and then catching itself. I changed the fuel filter over the weekend, still runs like total crap. So, anyone seen this before? The car is old but has low miles, so it has done a lot of sitting.

Could have a number of causes. Loose distributor causing bad timing. Bad MAP sensor. Partially clogged injectors. Vacuum leak.

Had that issue with my 350 TBI Silverado. Sounds like your MAP sensor is bad, they do not like to idle when one goes out. Junkyard replacements are often better than aftermarket. If you can find a Delphi or AC Delco that would be the best.

EGR valve stuck open? Hint- It shouldn't burn your fingers at idle.

Assume I know nothing. How do I test or confirm any of these things? Will plugging the vacuum line to the MAP sensor cause a change and confirm? How about the EGR valve?

Does the engine have an idle stabilizer switch or idle control valve within the throttle body?

When these fail the symptoms are just what you're describing. Worth looking up on Google for sure.

WilD wrote: Assume I know nothing. How do I test or confirm any of these things? Will plugging the vacuum line to the MAP sensor cause a change and confirm? How about the EGR valve?

On fords you can unplug the map sensor vacuum and see if it changes. Thats the easy way to test them. Same symptoms my f150 had with a bad egr though. It was stuck open.

disclamer: Not sure if either test is the same for gm products

My highly unexpert opinion is the fuel pump. Pumping just enough to run a little bit and die. Usually when it dies like that, but will still start and run a little, it's the fuel pump. Now get out there and prove me wrong.

I had a somewhat similar situation with my '87 g20 auto cross winning conversion van (same tbi engine as yours) several years ago. I replaced cap/rotor/wires which were due at the time anyway. That didn't solve anything. Turns out the culprit was a bad ignition control module. Think a new one was around $35 at the time at the local parts store. Start with the least expensive parts first.

For what it's worth, that's the way my car ran when I inadvertently failed to reconnect the MAF to its plug.

Egr quick test is to run the car for a minute or so while its running badly, then touch the cast base of the valve with the tip of which ever finger you don't need to use for the next week or so. If the finger blisters, its stuck open...

I have printed a crude diagram of the vacuum routing and sensors and will start poking around under the hood. They shouldn't have let Rube Goldberg design these things.

My initial thought was fuel pump, but that is hard to get to, so I am crossing my fingers that the culprit is something easier to reach.

Did it throw any codes? I know that it's OBD1, but the computer might have something useful to say.

Would OBD1 illuminate a check engine light on the dash like OBD2? It hasn't done that, so I had not considered trying to pull codes.

IIRC it should, either the engine graphic or it may just spell out "Service Engine Soon" in orange letters. On my truck the code for failing MAP sensor is 33 and 34. GM OBD1 is all 2 digit trouble codes.

NINJA EDIT: If you do have a light, here's a list of the codes OBD1 Codes