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HELP ME! fix a TBI 350

MAX89K5

MAX89K5

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Hey y'all.

My work truck is a 92 C2500 with a tbi 350 and 4l80e. It currently has 92k miles.

I've owned it for over a year now and I've had a consistent problem the entire time.

It loves to shut off at idle- as if the key has been switched off. If the key is not turned to the off position before attempting to restart, it will not start until the key is turned back. If the key is turned back, it will start normally. 98% of the time it only does this in gear sitting at a red light, occasionally it will do it in park or cruising.

The truck will go a months at a time without a problem, and then it will be plagued with this issue for a few days. And then it's perfect like nothing ever happened...

While it is in this "no start" condition it has: injector pulse, spark, and fuel pressure, but the injectors are not spraying fuel.

Before I got to this stage of diagnosis, I replaced every sensor and electrical component on the engine besides the pickup coil and knock sensor. I've also replaced the ECM and ignition switch, rebuilt throttle body, replaced fuel pump and filter, and cleaned grounds.

I am leaning towards bad injectors at this point because I don't know what else it could be. The truck runs like brand freaking new whenever it runs, so it is hard for me to believe it is the injectors...

Does anyone have a clue where to look next??IMG_20170525_184607_261.jpg
 
This kind of diagnosis is always a "shot in the dark". With the key having to be cycled off, and then on, it sounds like the fuel pump relay is having to be re-energized. Could be bad relay or electrical problem in the relay system.
 
My only concern there is: why would the fuel pump relay cause it to shut off whenever the the fuel pump is getting its power from oil pressure sender while the engine is running? It does throw a code 54 whenever it is in this no start condition- low voltage to fuel pump relay.

I guess I am fixing to pull the whole engine harness out and trace some wires....
 
MAX89K5 said:
My only concern there is: why would the fuel pump relay cause it to shut off whenever the the fuel pump is getting its power from oil pressure sender while the engine is running? It does throw a code 54 whenever it is in this no start condition- low voltage to fuel pump relay.

I guess I am fixing to pull the whole engine harness out and trace some wires....
Click to expand...

The oil pressure sender is just a back-up fuel pump relay if the main fuel pump relay (located on the right side of the firewall) should go bad. If your fuel pump is running off the oil pressure sending unit as a relay, then there is definitely a problem with your fuel pump relay system.
 
I see. I do know that with the truck running, I can unplug the fuel pump relay and it will continue to run. I can also do the same with the oil sender- unplug it with the fuel pump relay plugged in, and it will continue to run.
 
You reckon it could still be the pickup coil even though I have spark and injector pulse with it in a no start condition?

I seriously had a new distributor ready to go in last night but I started digging a little deeper and found that I still had spark and pulse.... I'll probably stick it in there tonight and see..
 
Pickup coil delivering spark tells the ecm if it should pulse the injectors. They are not separate systems like with a carb.
 
Have tou tried cracking the incoming fuel line to verify the fuel is supplied?
 
Yes. I have even tested the pressure with a gauge...

I just dropped a distributor in it since I already had one and set the base timing. Running really well, but we'll see what it does after driving for a while.
 
bp71k5 said:
Pickup coil, pickup coil, pickup coil.
Click to expand...

I did not know it was the pick-up coil that sent signal to the ECM for fuel injector pulse. I have seen people post that the ignition system in general does this, but this is the first time I seen somebody post that it is specifically the pick-up coil that does it. It definitely seems like the problem is something is turning the fuel system off when the truck was not receiving some type of signal or power to the fuel pump.
 
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Well that didn't last long..... lol

Seems to be doing it more frequently whenever there is a large load on the engine. i.e. in gear, a.c. on, wheels cut all the way to lock. It just happened just like that, and would not restart until I cycled the key.

Idk wtf is going on!

If it were wiring, I would think that engine load would have no affect on it??
 
Im going to agree with you on that. Load related is usually heat or fatigue
 
We really need to narrow this down just a little more. We need to find out for absolute certain if we have normal fuel pressure when it quits. One thing that happens when the engine is idling under a load, is a drop in oil pressure.
If the bearings are worn its possible that the pressure is dropping far enough to open the oil pressure switch that keeps the fuel pump running. Plus, the switch might be dropping out at too high a pressure.
Switching the key on and off sounds like a lockup problem with the ECU. My thinking, is that something is glitching it, causing it to kill the fuel pump relay after its been running a while. The engine keeps running on the oil pressure switch until a high load idle condition which opens the switch.
Then cranking does not create enough pressure to turn the pump back on.
Cycling the key resets the computer, letting it turn the relay back on.

You might try temporarily bypassing the oil pressure switch to see if it cures the problem. Don't leave it that way for long, or you may run down the battery, since I suspect it will continue to run with the key off.
Plus in a wreck which kills the engine and ruptures a fuel line, would let gas pump out everywhere.
 
Should I make an inline fitting and run a temporary fuel pressure gauge inside the cab to monitor fuel pressure?

If I unplug the oil pressure sensor and run it, then the fuel pump will run off the relay correct? Thus eliminating the possibility of the oil sensor cutting fuel pump power from low oil pressure?

It's been through 2 ECMs in the diagnosing process, but both with the same PROM. Could it be an issue?
 
MAX89K5 said:
Should I make an inline fitting and run a temporary fuel pressure gauge inside the cab to monitor fuel pressure?

If I unplug the oil pressure sensor and run it, then the fuel pump will run off the relay correct? Thus eliminating the possibility of the oil sensor cutting fuel pump power from low oil pressure?

It's been through 2 ECMs in the diagnosing process, but both with the same PROM. Could it be an issue?
Click to expand...

Highly unlikely it's the ecm or PROM. Way to many other things to double check first. Not impossible though. Do you have electric fans that might be turning on when the thing shuts down?
 
I doubt its the computer either. Those things are dead reliable. When you start cranking, the computer turns on the relay. When the oil pressure gets high enough, that switch provides an alternate supply for the fuel pump.
Unplugging the oil switch is the wrong way to go. As long as the relay is turned on, the oil pressure switch does nothing. Even if the pressure went too low, the computer would keep the relay closed.

My thought, is that something is causing the relay to turn off after a few minutes of driving, and the truck keeps running on the pressure switch until the pressure drops too low at idle. If the problem is occurring nice and often, I suggest shorting across the oil pressure switch to keep the pump running whether the relay is turned on or not. If the problem does not occur, then something is probably killing the relay.
As I mentioned, this is a temporary test only. If it fixes the problem, then we need to look at the computer. I still doubt its bad, but a bad or corroded ground or power lead could cause it to crash. It has to see the pulses from the ignition module, or it will turn off the relay.
If shorting across the switch does the trick, then I suspect that it is not seeing the correct pulses, or has a power problem. Once it loses the pulses, it will not turn the relay back on until you turn the key off and then back on. Sound familiar?
 
Knock sensor..... Try it. You seem to be having the same thing I had in my 88 burb. Chased it for 2 years and changed every single component you listed plus a few. Still did it. Never changed the knock sensor and wished I had ever since. It was the only component I never changed..... If it's the only one you haven't changed, TRY it lol. If it shorts it can pull down the voltage that your ecm outputs for various things
 
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