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73-87 Chevy _ GMC Trucks => Fuel Systems and Drivability => Topic started by: Wrought Ire on April 17, 2024, 08:22:00 PM
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Howdy guys, I have the STRANGEST problem going on with my '87 305 tbi not kicking on when I need to start it. Everything on it is fines. It's a pretty clean truck but while I was gone for a couple years in MT it apparently stopped running suddenly. Replaced fuel pump, replaced, fuses, replaced relay, replaced ignition module, replaced lines and hoses. Heres where things get weird.
When I key on theres no fuel pump prime, to start it cranks but again no pump, if I drip gas from a bottle it fires right up and runds until I stop giving it gas. Whe. The truck shuts OFF the fuel pump kicks on like its suppose to to reprise the system. So it works! Better yet when. I connect the test wire from the relay to the battery I csn manually kick on the fuel pump and it sounds lovely! Problem even with the test wire hooked to force the pump to run it won't start, it won't even pump fuel out of the injectors like it's not getting told to do so or something. Idk how these TBI systems work I just know it's an absolute nightmare that doesn't make any sense.
So why would it work every time besides when key on and run, and why when it's forced to run manually would the TBI not inject fuel?
I have NEVER needed to take a vehicle to another mechanic bc i couldnt figure it out and I'm so close to taking a big risk by paying someone else to run around it so annnny help would be greatly appreciated
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Dude...there are many peeps here that are over qualified to assist you.
Your challenge is to articulate your points and questions in a form that is easy to understand.
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If I am understanding you correctly it sounds like the truck will not run at all off the fuel pump, even when it SOUNDS like the pump is running. But you CAN make it run by pouring fuel down throttle body throat. This tells me your fuel pump is not actually pumping at all, even though you hear what sounds like it running. Based on this, next step is either verify pump can actually pump fuel or just replace it.
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Are you testing with the bypass lead off the fuel pump relay on the firewall to verify the pump works? for this part?
when. I connect the test wire from the relay to the battery I csn manually kick on the fuel pump and it sounds lovely!
If so is the relay working? There will never been any fuel coming out of the injectors unless the ECM triggers them to open.
Check this link for troubleshooting: http://www.73-87chevytrucks.com/techinfo/TroubleshootingTBISystems.pdf
Check these links also:
http://forum.73-87chevytrucks.com/smforum/index.php?topic=9072.0 - INJECTOR CIRCUIT DIAGNOSIS CHART A-4 (TBI)
http://forum.73-87chevytrucks.com/smforum/index.php?topic=37740.0 - FUEL PUMP RELAY CIRCUIT CHART A-5 (TBI)
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You're probably suffering from the same thing on my '89. I can hear it run when I apply B+, but it sounds like it's sloshing gas when I listen at the fill pipe.
The rubber hose section on the pump assembly is rotted out and the pump only produces 5 lbs of pressure, as the rest recycles back into the tank.
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Sounds like your ecm fuse is blown and you have leaking injector(s). Is the engine light coming on? Are you able to retrieve DTC's? The fuel pump has a backup function which will energize the fuel pump if there is oil pressure. There's no need to guess as the diagnostic strategy is black and white. Follow the strategy based diagnostic flowchart beginning with A3 provided A1 and A2 are not applicable to your issue. http://forum.73-87chevytrucks.com/smforum/index.php?board=51.0