1988 Chevrolet K3500

[THEMRMCB]

Good morning all, I've got an '88 5.7L TBI in my shop that is kicking my butt. Everything seems to be in order but the truck leans out with basically any throttle input. It started out as a rough running/low power diag. Scantool data shows that the O2 sensor is going lean as soon as throttle is depressed. If you stay below 25% or so, it runs reasonably well, albeit pretty consistently lean. Checked fuel pressure and found it to be good. BLM goes high (172) but system just runs leaner and leaner as throttle is opened. Could see some pattern irregularities with the injectors so we threw a pair in early on. No change. MAP signal is good, TPS is good, ECT is good, RPM is good (all measured with scantool to verify good input to ECM. Finally connected labscope to injector pulse signal and found good solid signal on both injectors with no noise. However, it doesn't matter what conditions are present, the pulse width doesn't change. Frequency changes with engine speed (of course) but no change in injector on time. At that point, I thought maybe ECM trouble. Verified all powers and ground were present and then sourced a known good, used ECM and installed with no change. I feel like I am missing an input right in front of my face that would basically throw the ECM into "limp mode". That is what it is acting like. All of the inputs are there to cause the ECM to increase pulse width, it just won't do it. What am I missing? ANY ideas are welcome. Thanks, Joe

[kur4o]

Looks like injector issues. There was a similar thread with ghost symptoms and the issue got fixed on 2nd or 3rd injector change.

On another note it could be false lean condition. Blms goes higher and it reads lean, but does it really feels like running lean or pig rich. It could be missing cylinder that dumps unburned air in exhaust causing false lean condition and pcm compensating with maxed out blms.

[dave w]

Possibly the exhaust is restricted? Perhaps a catalytic converter is clogged or there is a crushed exhaust pipe? Maybe someone vandalized the exhaust by forcing a potato (or something equally restrictive) up the exhaust?

Possibly use an IR thermometer to measure the temperature on each exhaust port, to identify a misfiring cylinder?

Possibly use an AFR meter on the tailpipe to verify rich / lean. If ARF meter reads lean, add extra gas down the throttle body with the engine idling / throttle closed to verify extra fuel reads rich at the AFR meter.

[In-Tech]

Hello, are you checking the fuel pressure at the TB or back at the filter?

[THEMRMCB]

Hello, are you checking the fuel pressure at the TB or back at the filter?

Tied into the line coming into the TBI (post filter).

[THEMRMCB]

Looks like injector issues. There was a similar thread with ghost symptoms and the issue got fixed on 2nd or 3rd injector change.

On another note it could be false lean condition. Blms goes higher and it reads lean, but does it really feels like running lean or pig rich. It could be missing cylinder that dumps unburned air in exhaust causing false lean condition and pcm compensating with maxed out blms.

It's definitely leaning out. I had one of my techs power brake it and pull against the transmission. It fell with its head under. I gave it a squirt of fuel from a bottle and it came to life and almost ran me over. Definitely NOT a false lean.

[THEMRMCB]

Hello, are you checking the fuel pressure at the TB or back at the filter?

Tied into the line coming into the TBI (post filter).

[In-Tech]

Hiya, I asked because I was checking right after the filter and all was fine, I thought. Fought that biatch for about a month till I checked FP at the tbi with a long line into the cab so I could watch when it acted up(mine was intermittent). Turns out the original GM braided teflon line had separated internally and it would block most all the fuel whenever it decided to. Never leaked fuel.
I made a new line and all was fixed.

p.s. Also I checked to make sure the regulator wasn't stuck at full bypass idle flow mode and mine was fine, you might check that too.

[In-Tech]

If you can, throw up your .bin file or the broadcast code. If it's a file I don't have, I'd sure like to add to my archives.