'89 C3500 TBI 7.4 Injector Problem

[Tow Truck Man]

Hello everyone,

I hope that I can get some help with this problem. I have a '89 C3500 7.4 TPI I purchased a couple of weeks ago. When I got it it was running but needed some work. I changed the vacuum lines on the carb, put in new plug wires, new gas filter and a new battery and then tried to start it up again and have a problem.

Problem is that when cranking the ignition the injectors pulse fine but when it's started and the key is let back to on position the injectors stop spraying gas and thus it dies. If I spray starting fluid into the throttle body it'll stay running.

Any help would be greatly appreciated!

[rickpilgrim]

I don't know if the 7.4 is the same, but on the 5.7 when you are cranking the truck to start the injector wiring circuit is completed with the ignition switch and when it starts and you release the key the injector circuit is completed by the oil pressure switch. If you have an unplugged or defective oil sender it will do what yours does on the 5.7 .
It is also possible the ignition switch is out of position or was going bad and just happen to fail at this moment also. Check all your fuses and relay's first though, it could be that simple

[Tow Truck Man]

Well I replaced the ignition today and it's still doing the same thing. Anyone know what else I can do, or how to figure out what it is?

[rickpilgrim]

Oil pressure sender is also wired into the inj circuit, meaning no oil psi = no fuel injected. If the sender is bad it will start and then when you let go of the key quit as the ECM doesn't see oil psi. And if that doesn't help then it's scanner/0 scope time at our shop. Btw are all the grounds good and connections clean and tight on the wiring? Did you check the key on not running fuel psi? Or even the crankshaft position sensor? All of these things can cause this problem

[Tow Truck Man]

Where is it located on my motor, looks like it's behind the power steering resivor but I've seen multiple locations for it listed! This motor doesn't have a place to put a fuel psi gauge so I have to take off the fuel line to test it, should I do it next to the TBI?

[rickpilgrim]

Oil pressure sender above the oil filter near the bell housing and crankshaft position is on the right front of the motor at crankshaft level right behind the balancer in the timing chain cover

[BigBlocksRule]

You're dealing with TBI, not TPI - quite a difference! And you refer to vacuum lines on a carb - I assume you're talking about your Throttle Body Injection unit?

[Tow Truck Man]

Originally Posted by Tow Truck Man

Hello everyone,

I hope that I can get some help with this problem. I have a '89 C3500 7.4 TPI I purchased a couple of weeks ago. When I got it it was running but needed some work. I changed the vacuum lines on the carb, put in new plug wires, new gas filter and a new battery and then tried to start it up again and have a problem.

Problem is that when cranking the ignition the injectors pulse fine but when it's started and the key is let back to on position the injectors stop spraying gas and thus it dies. If I spray starting fluid into the throttle body it'll stay running.

Any help would be greatly appreciated!

You are correct it's a TBI very sorry