I have a 350 Vortec in a Firebird with the 746 ECU. The car would not idle with the original 305 TBI PROM so I got Tuner Cat and tried several different setups trying to get the thing running OK. (It wouldn't run without backfiring and surging at any throttle position.)

The engine was surging and stopping sometimes during idle. I kept adding to the Base Pulse Width until it was at stock times 1.5

That got it to idle OK but there was a huge flat spot from just off idle to 30-40% throttle. When I push down on the accelerator enough to get the speed to 1000-1500 rpm, instead of speeding up the engine would die or backfire. WOT would be OK as long as the pump shot lasted, then it would run rough but would make some power.

I tried adding to the vol eff table II until it is at stock * 1.5 too. Now the thing idles smoothly (but is rich as hell) and I still have the flat spot.

I hooked up a meter to the O2 sensor and am getting the following readings.

All readings are warm engine - O2 sensor is new.

Idle .950 volt

10-30% In gear .1 or less (hunts around some too.)lays down, backfires.

40-100% .400 or more. Gets to .8 at 60%

WOT .8 to .85

When holding 50mph on level .1 to .4

I've checked the throttle position sensor (.6 volts at idle - no way to adjust) and the MAP sensor.

I'm getting no codes.

Do I have a hardware problem somewhere?

What is going on?

HELP!!!!!

 

Sounds like your spark timing is off. Check your distributor and make sure it's not 1 tooth off and set to the proper base timing.

Nick

 

Which way would it be off? I set it at 8btdc with the jumper disconnected.

 

Also consider adjusting your Pump Shot table. Is it an off-idle backfire / deadspot? If so, what you might be experiencing is the sudden inrush of air when you FIRST open the throttle blades. Carbureted cars address this through the use of an acclerator pump. The 746 ECU system addresses this through the use of the Pump Shot table (accelerator enrichment vs. change in TPS).

Tim

------------------
TRAXION's 1990 IROC-Z
Best Time = 12.244 @ 112.51mph (1.778 60' / 7.819@88.32mph in the 1/8)
All Natural. No Force. No Drugs. Stock Bottom End. Stock Body Panels.
Gunning for NA 11's after installing Hooker LT Headers and reducing weight.
-=ICON Motorsports=-
Moderator: PROM board at thirdgen.org

 

Thanks for the replies.
The engine idles rich (.95-1V). If I push down a little on the accelerator (enough so on a "good" car the rpm would go to 1000-1500) the stumble/backfire happens.

I do it so slow I don't think the pump shot is active. If I go from idle to 20% quickly, the engine is OK for a fraction of a second, then starts running rough. I'm assuming that's because the pump shot ran out.

The O2 goes to .004 volts or something like that everywhere except idle or above 70-80% throttle right at first. It moves around after the rpm's pick up but it never goes above .5