97 K3500 fresh engine runs great in open loop and horrible inj closed loop.

[suspect racing]

1997 7.4 K3500,fresh stock build no cats JBA stainless headers, no cats 2.5” dual exhaust with X pip[e back to rear axle then trough a flow master two in one out muffler. Pulls like a train in open loop..about 1.5 miles then switches to closed loop where it falls on its face. Surges and misfires . Can drives it part throttle but if ya try to make it work it falls on ir’s face. Coukld this be a new tune issue? No codes.

 

[Schurkey]

Is this a stock tune, or modified?

Is this the original computer?

What does the scan tool show for sensor data, and misfire counts?

Verify EVERY sensor
Verify all the computer outputs--EGR, spark advance, etc.

How old are the usual "tune-up" items--cap, rotor, plug wires, plugs, fuel filter, air filter, PCV system checked, spark advance checked, EGR system checked, fuel pressure checked, etc?

 

[RichLo]

Can you connect a scan tool to see the O2 measurements on each bank? And read the fuel trims? Sounds like a stuck injector something where one bank is fighting the other.

 

[suspect racing]

Is this a stock tune, or modified?

Is this the original computer?

What does the scan tool show for sensor data, and misfire counts?

Verify EVERY sensor
Verify all the computer outputs--EGR, spark advance, etc.

How old are the usual "tune-up" items--cap, rotor, plug wires, plugs, fuel filter, air filter, PCV system checked, spark advance checked, EGR system checked, fuel pressure checked, etc?

The engine is fresh, all the sensors are new and on a scan tool all are working. At last check all the misfires were on #5, changed spark plugs and wires, new cap new rotor, new coil, new module, tested three know working PCMs one with a tune for header, no cat and 2.5 " Xpipe exhaust which is what the engine has. All of these created the same result First pull closed loop or open loop pulls like a train. Second pull starts to misfire and gets worse. Fuel pressure is 55 to 62. 93 octane fuel tried stack injectors and fresh Bosch injectors. Everything looks correct on the scan tool. No codes. I am stumped.

 

[Schurkey]

Any vacuum ports on the #5 runner?

What is the cam/crank sensor correlation? Spec is + or - 2 degrees at 1100 RPM.

Are the plug wires routed EXACTLY like the service manual shows--specifically, LOTS of distance between #5 and #7?

 

[suspect racing]

Ok it has been a while. I am still having this problem. In open loop this truck hauls the mail. In closed loop it misfires under load. Scan tools shows everything is right where it should be. Zero codes. Fuel pressure is between 55 and 60 psi all the time. There are no codes. I have replaced every sensor. I have checked voltage to all the sensors. I have tried 3 computers, the original, the reprogrammed stock unit and a friends reprogrammed unit that works fine on his. All produce the same results. I even introduced extra propane into the intake to see if richening it up would help...it was maybe a bit better. I have been working on cars for over 50 years. While I no longer do it for a living I know my way around a scan tool. I have even let friend in the business try to solve the mystery. Everyone says there is a problem...No one can find it. This particular 3 year run of truck does not all me to record the data and play it back. Can a programmer just put this damn thing in open loop and let me pull my race trailer to the tracks 4 or 5 times a year? If i did not have the 4L80E gearbox it would already have a carb and HEI on it. Any ideas? My wife says she would have parked it in the back pasture and burned it to the ground by now.

 

[Schurkey]

If #5 misfires, what is different on #5 than any of the others?

Which of those differences would be affected by open-loop/closed-loop operation?

I asked about plug wire routing, vacuum nipples on #5 runner, and some other stuff, and never got an answer.

 

[Komet]

There should be some magical combination of cables and software that will get you a datalog. That would help the most.

 

[suspect racing]

There should be some magical combination of cables and software that will get you a datalog. That would help the most.

I wish, I have already tried to have the data recorded to send out for a tuner to have a look. When I told him the year he said, 1996 to 1998 will not allow data to be recorded. I cannot record real time data from this computer. Now I just want to make it stay in open loop and let rip. It runs great in open loop. Runs like **** in closed loop. 3 computers and all the sensor have be replaced and all look good while running. Long and short fuel trim looks correct too. I would have already put a Qjet and HEI on it if it did not have a 4L80E

 

[suspect racing]

If #5 misfires, what is different on #5 than any of the others?

Which of those differences would be affected by open-loop/closed-loop operation?

I asked about plug wire routing, vacuum nipples on #5 runner, and some other stuff, and never got an answer.

Turns out is was a random misfire.