1987 V10 with carb and dual tanks will not stay running

[Shoote]

Still working on my 1987 V10. It is a big block carb with factory dual tanks and electric fuel pumps in each tank just like factory. (Both tanks and fuel pumps replaced 1.5 years ago) I had no fuel pressure, fuel guage and the tank selector switch had no power Replaced the tank selector switch in the cab and the fuel relay on the firewall. No change.

The fuel pressure issue began when I had a power steering line leak and I washed down the engine compartment. That is when the no fuel pressure issue started. I rechecked the grounds and fuses. This truck has a 30 and a 20 amp fuse that fit into the ignition fuse space on the fuse block. I pulled both fuses and the 30 amp fuse was bad. I replaced the 30 amp fuse and now the tank selector swich works and the fuel guage works. The truck will start and run for about 20-30 seconds then die from lack of fuel. If I continue to switch from tank to tank every 20 to 30 seconds I can keep the truck running. It will start right away when it dies from lack of fuel. It seems the fuel pressure builds when I first turn the key, but the fuel pump does not stay on after the engine starts. That is what it seems like. Anyone have ideas on what is wrong? I appreciate any help. Electrical is not one of my strengths.

 

[SquareRoot]

@Edelbrock will be a long shortly to provide some expert advice.

 

[Chevy 88]

Not sure if this is your issue, seeing as its throttle body - but you might be getting vapor lock in the fuel tanks. Fuel being removed from the tank creates a vacuum, and starves the engine of fuel. When the tank is not being used, air slowly makes it way into the tank to equalize the pressure. I would remove the gas caps and see if anything changes. I think it unlikely that both tanks would experience this problem at the same time, unless they share a vent or something like that - but its easy enough to check by pulling the caps. A quick 1 minute test should rule this out.

Seeing as this problem started with an electrical issue, the vapor lock might be way off base - but hey - the more ideas and things to try the better, right?

 

[Shoote]

This truck has a carb. I replaced the 350 tbi with a big block with a carb.

 

[Ricko1966]

How are you controlling the fuel pump relay? There should also be a redundant fuel pump power source. The way this worked factory was the ecm closed the fuel pump relay breifly and the pumps ran,to start the truck,then the ecm got signal from the ICM and controlled the injectors and fuel pump relay. There was a secondary circuit that was 12V to a normally open 2 or 3 prong oil pressure switch. When oil pressure came up the oil pressure switch closed and provided an alternate power source to the fuel pump,in case there was a relay problem you would still have power to the fuel pump. My guess is the ecm is no longer controlling the fuel pump relay,and the fuel pump oil pressure switch is bad or got disconnected. Jumper the fuel pump relay,or the fuel pump oil pressure switch,see if the problem goes away.

 

[Chevy 88]

Carbed? Ah, in that case - What fuel pump setup do you have? Electric or manual?


Another oddball thing is that crud is being picked up and blocking the fuel tube screens in the tanks, but when you stop the pump the crud falls away. Unlikely and then even more unlikely that it would happen to both tanks at the same time.

Again - another brainstorm idea, but I don't think that's the issue.

 

[SquareRoot]

Lol. Nailed that one.

 

[Ricko1966]

Lol. Nailed that one.

He didn't suggest putting a fan or a window unit in the gas tank,so we are making progress.

 

[Chevy 88]

Are both your tanks running on the same relay?

If that's the case, I would swap that relay with another from the fuse box.

If its the same relay for both tanks, then its possible that it could account for this, if the relay is overheating when in use.



Long shot, but its something to explore.

 

[Shoote]

Both tanks and fuel pumps were replaced 2000 miles ago. Electric fuel pumps in the tanks just like factory. Truck ran great after the engine swap for 1.5 years and about 1000 miles. When I washed down the leak in the power steering hose my problem began. I did find a 20 amp ignition fuse blown. Replaced it and then it would start and run for about 20 seconds. If I switch tanks before the truck stalls out from lack of fuel, it will keep running. I have to keep switching tanks to keep it running. I did replace the tank selector switch and fuel relay. Neither made a difference. Whatever sends power to the fuel pumps after start up is what seems to be the problem. I do not know what circuit or relay that is though.

 

[Shoote]

Is the red wire on the fuel relay the jumper for the fuel pumps? I noticed a solid gray wire at the top of the fuse box has no power. All other wires in the fuse box have power.

 

[Ricko1966]

Is the red wire on the fuel relay the jumper for the fuel pumps? I noticed a solid gray wire at the top of the fuse box has no power. All other wires in the fuse box have power.

The fuel pump relay is under the hood.A fuel pump relay will have 1 hot wire coming in,and a control circuit to close the relay. When the relay is closed it will have a power out to the pump.If you have a 12 volt constant hot,hot whether the truck is running or not, that is your 12 volt supply. It sounds like ECM is closing the relay briefly,just like it is supposed to for start up,after start up the ECM either needs to close the relay and keep it closed using a tach signal,which I'm guessing you aren't using a TBI distributor so your ecm wont get a tach dignal,so you have to have a working fuel pump oil pressure switch. I'll try to find a diaghram for you. I'd guess it got unplugged or shorted out while you were pressure washing. BIAB after I find a wiring diaghram. Okay looks like dark green is your relay control wire which Id bet is 0nly coming on to energize the relay for prime. The red wire is fuel pump prime should only by hot for a few seconds when you turn the key to run. Orange is your hot lead Tan and white is power out to the pump. Jumper Orange to tan white and Ill bet your truck runs. If you don't have an oil pressure switch for the pump get one and hook orange and tan/white to it like factory,that way the pump only runs when the engine has oil pressure,that is to prevent flooding and fires,not to protect your engine like so many believe.

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[Shoote]

Thank you, Ricko 1966. I will chase those wires and see what I find. I can take the oil pressure switch off of the original 350 tbi and put it on the 454.

 

[Sad Sack]

If I missed this information I apologize, I’m on my stupid arse phone, plus I’m not the sharpest knife in the drawer but this is just a question of more curiosity than anything but how much fuel pressure are the tank pumps supposed to put out as compared to what your carb can handle?

 

[Ricko1966]

If I missed this information I apologize, I’m on my stupid arse phone, plus I’m not the sharpest knife in the drawer but this is just a question of more curiosity than anything but how much fuel pressure are the tank pumps supposed to put out as compared to what your carb can handle?

TBI pumps unregulated would put out over 20 psi probably in the vicinity of 35psi. Definitely more than carbs can handle. I am assuming someone has installed a bypass regulator,or made a bypass.