Now this is confusing! Runs good at idle, misfires above 1500 rpm, and POOR power!

GBEMandrelGBEMandrel Member Posts: 4
edited May 2020 in GMC
Got van used from a guy. He stated it ran great after he replaced tune-up parts. Cap/rotor/plugs/wires/fuel filter. But 2 weeks later, vehicle was bucking so he parked it. I towed it to my shop, and the CEL was on. The ECM and the Eprom didn't match according to the CEL. I found that the ecm was a reman for a TBI and I have CPI. It took 3 weeks, but I found 2 used ECM's. I crossed the eproms with the correct GM service manuals to find the right ECM. There is ZERO aftermarket reman's for this vehicle.

92 GMC Safari, CPI with non electronic 700r4. Most are TBI with 700r4 or CPI with 4l60E. Guess this is a rare bird. I checked fuel lean conditions and it seemed like the injector spider and/or non repairable regulator. I replaced it and it runs the same, no change.

Runs good at idle, but when pressing medium to heavy pedal, it takes a bit for the engine to catch up to my acceleration. When in gear, it runs ok, but under accelration like leaving a light or stop sign, it's gutless. After getting up to freeway speeds, slowly.. say about 45 seconds, the van feels as if it's misfiring and it feels as if someone is grabbing your shirt by the neck and holding you back. Scanned for codes, ZERO. Seems to be a lean misfire.

So I decided to take it to an electrical specialist business who has never failed me.. until now. He stated after testing primary and secondary spark controls, fuel this and that, blah blah blah, he said he beleives it's the fuel injector assy. and most likely the regulator. (sound familiar?) So, after replacing it a 2nd time it still ran the same. so, that's the original, first and second replacements and it still runs the same. so, it's not the injector, period. 3 injectors doing the same thing, so it cannot be that.

I exchanged the ECM with the other known good and it runs the same. I pulled the ditributor and checked the drive gear and it looks great. I replaced the ignition module with anew one and it still runs the same.

Here is what has been checked or replaced:
1.) plugs-new iridium for CPI injection
2.) plug wires, new lifetime
3.) cap and rotor, new
4.) fuel filter (I also checked to see if it was installed backwards and
5.) ignition control module
6.) fuel injection/spider (2x's and the original)
7.) bosch fuel pump (awesome upgrade)
8.) fuel lines under intake plenum
9.) ECM (have 2)
10.) oil changed to synthetic
11.) added seafoam to fuel tank-2 cans, no difference and fuel tank been filled up 3x's with new fuel)

All peramiters look great on the scanner, but it shows it leans out under power...

Any ideas?


  • thecardoc3thecardoc3 Member Posts: 5,669

    What is the fuel pressure KOEO (Key On Engine Off), idling, and when trying to drive it while it is acting up? What specifically are you looking at that has you feeling it is lean under power, the O2 signals, or the block learn and integrator data pids? What are they reporting under idle noload, 2500rpm no load, 1500 rpm against the brakes and on the road during the failure?
  • GBEMandrelGBEMandrel Member Posts: 4
    From what the electrical shop stated, the fuel pressure is awesome under KOEO. But when it is running especially under power, the pressure drops below specs required. that's why I replaced the fuel pump with new. Without the fuel pump or injectors being replaced again for no reason, what would be the cause of that?
  • GBEMandrelGBEMandrel Member Posts: 4
    Fuel pressure dead headed is 80 psi on KOEO. When we get the lean condistion, the O2's go lean and the intergrator goes lean as well. When driving, the pressure drops down to 50 psi and stays there until acceleration is removed from the equation. When doing RPM test with brakes on, the van doesn't show signs of misfire or lean. But when I am about 1800 and accelerating, thatys when the misfire and lean condition reapply's itself. I called Auto/Electric Specialties, also ASE Master techs, and they reconfirmed my suspision that the regulator is not functioning. (that makes 3 in a row) So, I just ordered a new adjustable regulator and I will install that Monday and see what happens.
  • thecardoc3thecardoc3 Member Posts: 5,669
    Do you have a digital storage oscilloscope and a low amps current probe? If you have those available can you capture a current waveform KOEO dead headded, at idle no load and then under acceleration during the loss of pressure? I would also like to see system voltage to the pump (at the tank connector) and the ground both referenced to battery negative.

    Having good pressure with no demand is one thing, and even being able to max out the pump with it deadheaded is another but that doesn't mean that the system can provide volume under proper pressure. Get me the above data and we can move on from there.

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