1994 4.3L vin W surging. Service Bulletin recommend upgrade to BPLC.

[craigger]

It would take a lot of investigating before I altered VE tables to "fix" a problem if the engine was stock. You're saying that the exhaust readings are showing rich, and you've reduce the delivered fuel volume? Is it about 10% on average? Because that's not that much... But if I thought there was a problem I'd confirm the rich readings using a wide band O2 or a three or four gas analyzer. You have to prove the sensor is correct. And I wouldn't be afraid to run open loop for a time as a diagnostic step. We had a six liter truck engine here yesterday which was adding fuel at idle. It had the slightest misfire which was sending unreacted O2 past the sensor, but it was very subtle with the pcm adding fuel and the desired idle at 550 rpm. Forcing the engine into open loop and clearing the long term trims made it much more obvious. Next step is basic mechanic stuff; diagnose a misfire.

I don't know if 'stock' as I don't full history, but I assume stock. The previous owner said he had put in about $2500 in parts is all I know.
So, ~10% change might not be of concern. If my math is correct, my 89.5 to 77.7 VE cell change is ~13 %.
When I first changed the injector flow to shift all the cells in the VE table for the rich issue, I changed from 119.4 gm/hr to 153 gm/hr to get most of the trims to average ~128. I think that was about a 28% increase.
See any worries from these numbers?

I don't have a wide band O2 or gas analyzer. I had LT trims maxed neg (2 O2 sensors produced same), rich smelling exhaust, plugs fouling black.
But, if I understand what you're saying, a subtle misfire might, cause my narrow band O2 sensor to not get proper reading of 'richness' due to the unreacted O2 (causing lean bias) with additional fuel add to compensate, thus my richness.
Is that correct?

On your truck with misfire, when you forced it into open loop with trims cleared, can you tell me what were the 'more obvious' symptoms you saw? Were they something I might observe without wide band O2 or gas analyzer?

Thanks for your help.

[1project2many]

"More obvious" means the vehicle was noticeably shaking from the severity of the misfire.

Rich plugs and black exhaust are not acceptable. Are all the plugs turning black or just a couple? Where is the fuel pressure currently? If the regulator has been replaced it may be adjustable.

[craigger]

All the plugs were turning black.
I didn't record the fuel pressure, but I remember it was in spec and I think ~60psi. Leak down test passed.
No adjustment on the fuel regulator. 2 spider assemblies (1 new) both fuel pressures were good. The fuel regulator is replaced with the spider assembly.
I had the truck at a shop where the mechanic says he has had ~10 years GM experience. He said the engine is a little rough but is still within 'normal' from his experience.

[billygraves]

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[craigger]

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I have not tried to measure that O2 ground with the motor running. The sensor ground is through the threads.
My pcm ground to back of engine, which I can't reach. Have tested by running a separate ground from batt to pcm, no change.
I have cleaned all grounds I can find, except the 2 on the engine between it and the firewall. My hands do not fit. Testing, I have run a separate ground from the battery to the engine, and another time from ground to pcm. No change.
I have tried another pcm, no help.
Fuel pressure tested at engine off (fuel pump ran), engine idling, engine reved. All good. Not driving down road. Not sure how to do that, my fuel pressure gauge line too short to expose for viewing while driving.
Was using cheap autolite plugs during much of troubleshooting. Recently changed to CR43TS, gapped at .045. No change. Tried new Delco cap, rotor, wires, just to make sure. No change.
My O2 runs full time. Tried 2 different new O2 sensors, same results.
Current status is with it running with good looking trims, just the light surging around 2k rpm (+- 500rpm), at light to moderate throttle.

[1project2many]

Watch scantool and correlate readings with surging. O2 swinging to lean or rich while it's happening? Timing advancing when it's happening?

O2 grounds to exhaust, ok, but what connects exhaust to engine? Sometimes you need a good ground strap there.

Rings OK? No oil usage? External oil leaks? O2 "compares" exhaust with fresh air. I once ran into a rich engine caused by oil dripping on exhaust and smoking. Smoke accumulated around O2 at idle and caused the O2 outside the exhaust to be less than inside and O2 sensor reported that as a lean condition to the ecm.

Also, silly question but check to make sure vent tube in valve cover is working. Vacuum in crankcase can draw hydrocarbons past rings.

Any chance you've got an older engine??

[craigger]

Stared at the scan readings for hours looking for any correlations. Can't see any correlation, positive or negative, other than rpm 2000 +/- 500.
No O2 correlation that I can see, spark advance can be going up or down.
Ground of exhaust has not been checked. Will do that.
Haven't run it enough to have data for any internal oil usage. Known, small oil leak from pan (leaving for future fun), not leaking onto or near exhaust and the O2 sensor.
Compression all within 15%.
No oil leak from around intake manifold, but the intake manifold gasket was replaced on forlorn hope, and just to check for possible vacuum leaks and any cracks. All ok.
Oh ya, the PCV was replaced on gp.
Previously tested for internal vacuum issue (at oil dipstick tube), none found.
Hmm, haven't checked for older engine. Suggestion on how to do that? If not original, I assume it can't be much older as it has the balance shaft.

[billygraves]

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