Hey guys.

I usually hang out on LT1 tech having a 94Z but need some advice about a 90 vette w/ a ZF6. It has an on and off miss to it as you increase the rpm. It seems to clear up some when the car is warmed up and at WOT. The motor is a 383 with a hotcam kit. Everything else is stock except for an adjustable fuel pressure regulator. I know that it's a SD system but how does that work in closed loop or does it have closed loop? Does it just take readings from the MAP sensor?
Without doing fuel pressure, vaccum, and ignition checks, I know this question leaves alot open. I just want to get how SD works alittle. The car was trailered to Ed Wright and had a custom dyno tune for this setup and ran fine before this problem.

Jeff D.

 

When you're wide open the ECM reverts back to open loop. It reads the fuel and spark advance values straight off it's pre-programmed tables in the chip. It doesn't look at the O2 sensor and very little learning happens. It reads RPM, throttle position, MAP, and a few other things.

As such, there's no way it could have "learned" it's way into bad full throttle behavior, with the possibly exception if it seeing "false knock" from the knock sensor and pulling back the timing. This would generally just kill power long before it caused any kind of popping or hesitation.

Part throttle is a different situation since it looks at the O2 sensor. i.e. it's in closed-loop mode. If the sensor is reading rich for some reason it could lean things out to the point it doesn't run so great. Or vice versa.

I have also found a quirk to this. I thought that the ECM saved it's long term fule trim values (block learns) from start to start. Spend some time with a scan tool and you'll see that it doesn't. Everything resets to 128 (baseline) every time you turn the motor off and remove the key. So.... it's possible that it could run OK for a little while after a restart and then go downhill the longer it runs as it learns again from the O2.

If it used to run OK under all conditions with your current chip for some relatively long amount of time (days, weeks) and then it jsut started acting up one day then it's not your chip's programming. Something has gone wrong somewhere. Could be a bad sensor or some mechanical or electrical problem, etc.

 

Doesn't the ECM run off the programmed tables at cold and adjusts at closed loop? If so, the problem is in the programming. Before you go any further, maybe the ECM isn't reading the correct cold temp? If that is fine, the problem is in the programming because it ran find before the ECM chip from Ed Wright.

What is the fuel pressure regulator set at and is it holding the correct pressure?

 

Thanks for the info. So, SD is pretty much the same as MAF then.
After mechanical and sensor checks are done, trying VE master may be an option.

Jeff D.