1989 Firebird Formula, originally with a 5.0 TBI system.

In October, I swapped out the entire drive train with an 89 Formula 350 with
TPI, and the 9 bolt Borg-Warner Rear. I also replaced the computer, and entire
engine wiring harness and all necessary components. The engine is a ZZ4 crate
engine, new as of Oct, 1999, now with ~5500 miles on it.

Engine mods put on are: K&N filter system, SLP 58mm throttle body, Accel coil and 9 mm
wires, Edelbrock headers, SLP stainless 3" exhaust, and a Random tech 3" hi-flow cat.
I also partially ported the plenum when I had it apart in October.

The car was running perfectly for October and November. Last month, I was pulling out
from a light under somewhat heavy acceleration, and I felt the engine stumble and start
to break up. I pulled to check it out, and the car would not idle. You need to constantly
keep playing with the gas to keep it running. It feels like it stops running when the
RPM's get below 2K, and you have to feather the throttle to 2-3K to get it to run.
When the idle gets low enough (below 500 rpm) it backfires through the plenum.
I limped it home, and started checking it over.

No vacuum leaks.
So far, I have swapped out the distributor, distributor module and coil (no effect).
Checked fuel pressure: 45 PSI at idle, 37+ under throttle.
One interesting note: the car will almost run when it is cold, but as soon as it
warms up, the symptoms worsen dramatically
Cleaned the MAF (Previous owner had cut the screens out, etc). No difference.
Noticed the TPI was loose. Retapped the holes and put in the right screws.
Reset it to spec. No difference

I borrowed my friends scanner (He's a Snap-on dealer) . No codes, but it is
reporting that the O2 is lean, and there is no O2 cross count. I haven't replaced
the O2 sensor yet, cause of the 2 feet of snow on the ground, but I am also not
convinced that this is the problem.
The computer that came with my donor car was a remanufactured one, and though I don't
remember the name of the rebuilder, it was not one I recognize.

I am extremely curious as to what the problem is. Although I don't plan on driving it
until the snow melts, I do want to have it running so I can move it around from time to time.

Anything I neglected to mention?

Any comments or suggestions?

Thanks,

-Lee

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89 Firebird Formula
ZZ4 Crate, WS6 package, 9 bolt rear, Hotchiss and Lakewood suspension
Edelbrock TES Headers, Random Tech 3" cat, SLP exhaust...

 

Mine was doing something similar to your's. It turned out to be what I thought it was all along: the fuel pump. This solved the problem even though I was also getting perfect fuel pressure, go figure. I had the same idling problem, stumbling, hard starting when WARM, not hot. When I replaced my O2 sensor it did nothing to fix it.

------------------
"The world would have no problems if everyone had a presence of mind."
'91 Firebird
305/TBI(L03)/700R4/T-tops
Mods: Catco high flow cat - B&M Shift kit - TA tail light conversion - K&N - MacEwen white faced guages
Stereo: Pioneer DEH-P8200R - Pioneer 6x9's - Kenwood 8" Tube
'87 Firebird
LB8/700R4/220K - Twas a good car until the accident.
Firebird parts - Now with an '80 Firebird for sale!
Moderator of tranny board over at TransAmGTA.com

 

Well, you've got a scan tool and that's a good start. I assume you've been all over the ignition by now so let's think about that O2. You say the car runs OK until it warms up. That can happen because the ECM/engine begin running in closed loop mode once the O2 heats up enough. Then the ECM starts looking at the O2 voltages and adjusting mixture to bring it in spec. If the O2 is lying to the ECM then the car's drivability, power and economy can go in the toilet pretty quick.

I'd check for exhaust leaks (loose header bots, blown gaskets, cracked welds, etc.). Keep checking for vacuum leaks, too. After that I'd try replacing the O2. Keep using that scan tool at every step of the way to see if you are making progress or not. Sometimes multiple small related problems can act like one big one.

In most cases, when the fuel pump goes bad the motor breaks up at high RPM at WOT before the idle and part throttle start to suffer.


[This message has been edited by Damon (edited January 02, 2001).]

 

Just reread your post- did you say the fuel pressure at idle is 43 and 37 under load? That's not right. With the vacuum line disconnected from the fuel pressure regulator it should hold 43 PSI under any and all conditions. Witht the vacuum line connected it should read 1 PSI less than 43 for each 2" of manifold vacuum.

Maybe you should look at that fuel system a little closer. Taping a fuel pressure guage to the windshield (temporarily!) and taking a spin can sometimes be very revealing.



[This message has been edited by Damon (edited January 02, 2001).]

 

Just a couple more suggestions I haven't seen listed yet. Try cleaning your IAC motor and its passages and readjusting it. Also, check your TPS voltage. If your IAC is stuck open from being dirty, or your TPS is reading incorrectly, either one of these could cause the 02 sensor to get a lean reading. I'd check into these before spending money on a fuel pump, especially since it seems like you're getting adequate pressure from it.



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89RS w/350 TPI; 69RS/SS w/450 HP 350/Muncie 4-Speed "Too weird to live, too rare to die."

 

Between this board and other guys I have talked to, I seem to have a lot to check out! Fuel pressure, IAC, MAF and MAF relay,
O2, vacuum...

I will keep you all posted when I get some more diags done. Thanks for all the hints!

-Lee

------------------
89 Firebird Formula
ZZ4 Crate, WS6 package, 9 bolt rear, Hotchiss and Lakewood suspension
Edelbrock TES Headers, Random Tech 3" cat, SLP exhaust...