87’ 350 tbi hot start issue

[kychevy]

Check the fuel pump relay jump it with a fused test lead. That truck has a redundant fuel pump circuit. Ecm is supposed to turn the fuel pump on for a couple of seconds to start then after that when the ecm senses a tach signal it keeps the pump running via the fuel pump relay. There is an oil pressure switch that is a redundant circuit to get you home if the fuel pump relay or its circuit fails. If the relay or its circuit is bad,the fuel pump will run,but only after you have enough oil pressure to close the oil pressure switch for the fuel pump. Oil pressure builds quicker colder.And a shot of either to start it would close the pressure switch immediately.

I bypassed the fuel pump relay with a jumper wire, couple weeks ago, while truck wouldn’t start and it started every time. Thought that meant the relay was bad so I replaced it

 

[Ricko1966]

I bypassed the fuel pump relay with a jumper wire, couple weeks ago, while truck wouldn’t start and it started every time. Thought that meant the relay was bad so I replaced it

Hmm so if you turned on a lamp but it didn't light,and you replaced the bulb and it didn't light. That means it's time to investigate further does the relay have power and signal? Do you know how a relay functions and what a relay needs to function? I drew you a pic. Typically but not always an automotive relay will be labeled #30 #87 #85 #86. #30 will go to a power source #87 will go to the device you are trying to power # 85 will be your elecromagnet power or ground # 86 will be the other lead for the electro magnet. You need to look at a wiring dighram and trace the inputs and out puts for the relay then you can determine,what you are missing. This is 100 percent from memory so double check me. I belive you will have a constant hot,that is in, for the pump,you will have a constant ground,thats 1 side of the magnet you alredy know which wire goes to the pump if you jumpered the relay. The remaining terminal is going 1to be hot from the ECM when you first turn the key and when the engine is running. P S. I do not remember if one of the ecm fuses on the fuse box will cut power to the relay,check the fuses and check the fuses for power with a meter. A good fuse with no power to it is still a dead circuit.

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[kychevy]

Hmm so if you turned on a lamp but it didn't light,and you replaced the bulb and it didn't light. That means it's time to investigate further does the relay have power and signal? Do you know how a relay functions and I'm what a relay needs to function? I drew you a pic. Typically but not always an automotive relay will be labeled #30 #87 #85 #86. #30 will go to a power source #87 will go to the divice you are trying to power # 85 will be your elecromagnet power or ground # 86 will be the other lead fir the electro magnet. IfYou need to look at a wiring dighram and trace the inputs and out puts for tge relay then you can determine,what you are m8ssing. This is 100 percent from memory so double check me. I belive you will have a constant hot,thats in for the pump,you will have a constant ground,thats 1 side of the magnet you alredy know which wire goes to the pump if you jumpered the relay. The remaining terminal is goingbto be hot from the ECM when you first turn the key and when the engine is running. P S. I do not remember 8f one of the ecm fuses on the fuse box will cut power to the relay,checkbthe fuses and check the fuses for power with a meter. A good fusecwith no power to it is still a dead circuit.

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Thank you. I’ll dig into it today.

 

[Edelbrock]

Thank you. I’ll dig into it today.

Cool. I'm curious to hear -watt- the -current- problem is.
Don't forget to -relay- the results to us.
Sometimes I have a -short- -fuse- when dealing with electrical problems, but its very -empowering- when I fix my own vehicle.

 

[waterpirate]

My 87 tbi has the exact same issue after motor swap. It was the timing. Did you bump hard enough to wiggle the distributor?
Eric

 

[Bill USN-1]

Shotgun trouble shooting is never good.
All that money and no results.
Some of you are thinking it thru with the fuel pump/OPSW circuit, good job.
The oil press sw is not needed at all. as mentioned, it just supports the relay once pressure comes up to relieve the load on the relay contacts. It's not a safety switch as some would put out early on.

Some other things i have ran into that give similar symptoms.
Weak distr pickup/corroded terminals/rust on star wheel(reluctor).
key switch giving 12v in key on and then losing it in the crank positions.
Sometimes will start as you let off the key back to the run position.

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