IROC343

91 firebird, 305 tpi, auto, 98,000 mi. About three weeks ago car quit, towed home, convinced myself it was the fuel pump (original), replaced fuel pump (ac delco) now has 40psi. Cranks fine would not start. since then I've replaced the following, map sensor, tps sensor, oxygen sensor, injectors, distributor including module and pickup coil, three different ecm's, coil, Cap and rotor, plug wires and plugs are recent. I had ordered a jet prom and chip, but the prom was out of stock. Replaced the chip with a jet and engine started. We're making progress. Runs but not well, driveable but that's about it. Has a hesitation off idle. If you powerbrake it it'll die. I've got myself convinced now that it must be the prom (AXXB), but I'm not having any luck finding a replacement and I'm not totally convinced that's the problem, but what else could it be? I've replaced nearly every replaceable part and I've run out of ideas. Help me please.

red rock

iTrader: (2)

Have you cleaned and set your IAC. Have you set your TPS at between .54 and .75?

IROC343

The TPS is set to .54 and I know the IAC is working fine, because when I powerbrake it, if I can catch it before it dies the idle goes to 2000rpm before it settles back to about 750rpm.
I'm wondering if it could be EGR related. As I said it runs great once you get going. It's just off idle that it seems to stumble and once it stumbles it wants to die. And the fact that it's hard starting sometimes. Cranks fine but doesn't want to catch.

red rock

iTrader: (2)

Do you get any codes?

IROC343

No codes no SES light. Light does work.

red rock

iTrader: (2)

If the EGR was acting up, you should see a code. The only thing i can think of, seeing you have replaced so many parts, is maybe you have a small leak in vacuum, which would affect an idle problem.

kevinc

Throwing parts is an expensive way to troubleshoot as you're finding.

With the electronic timing control disconnected, whip out your timing light and tell us what advance you're seeing at idle.

Air_Adam

Kinda sounds like a vacuum leak to me....

IROC343

With the est disconnected the timing at idle is 6 degrees btdc per factory specs. Some of the vacuum lines are original, a little soft maybe, I'll change them and see if that helps. Thanks for the suggestions.

kevinc

OK, now with the EST connected are you seeing the timing jump around in the 10-20deg BTDC range while the engine idles?

IROC343

Yes, the advance seems to work fine. Somebody at work suggested maybe the knock sensor is faulty. Could the knock sensor retard the timing enough to make the engine falter to the point it wouldn't want to run?

red rock

iTrader: (2)

Again, with a knock sensor problem you should see a code 38 or 39.

kevinc

You'd be seeing knock retard on your scanner, hook it up and see what you're getting.

And while you're scanning, what BLM value are you seeing while the problem is happening and at idle?

mightymaro94

I know it was said before but it seems like a vac issue. dies when you power brake it ... really makes me think vacuum

kevinc

Good point, vacuum leaks usually show up as the IAC at zero on the scanner. ECM closes the IAC fully, trying to get the idle down to the desired RPMs, fighting the air leak.

IROC343

I think the problem is solved. Going over things I found two bolts loose on the ram tubes to the lower base. I evidently missed them during the tightening process when replacing the injectors. They were the two end bolts. This must have created a vacuum leak as some suggested.
Starts and runs much better now. Thanks for the help.