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exbcmc

73 GMC 350 starving for gas

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HELP! Our 350 truck has always gone through periods of not wanting to run. We bought it new in 73 and it only has 88k miles. A Chevy dealership in about 75 said we needed a fuel pump....no I had just tried that. They insisted, put on the pump and called me to come get my truck. They called back in 5 minutes, "never mind it quit running." That time it turned out the dual tank system hoses were old and would collapse like a straw, preventing gas to pass. For about two years now the right side tank would not give gas to the engine, so I just used the left side. 3 months ago I had all the hoses changed, except the vent hoses on top. The other day she quit running on the fwy in the dark. Scary to say the least. So now if I spray starter fluid in the carb it fires right up...then dies. Anyone have some suggestions? Oh, it has a new gas filter and electronic ignition. I also thought vapor lock, but it has set for 24 hrs now and nothing.

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The tank solenoid switching valve's default mode is to pull only from the tank which came on the truck as standard equipment, which explains why it's only pulling from the one side.  That's also the "failure mode", as I recall.

 

ENSURE the switch on the instrument panel is the CORRECT SWITCH for the model year of the truck!!  Aftermarket switches have become "generic" in the later years and they are NOT all the same, although they might look like it.  The difference is on the plug-in terminals.  The specific terminal with the "locating tang" is different on some model years, which MUST match the plug-in on the chassis harness.  IF those don't match, things don't work correctly . . . which I know from selling the GM Parts switch (model year correct) after customers had been to the auto supply and gotten one.  Not just the placement of the tang, but the shape of the tang is important, too!!!  This even became apparent back in the earlier 1990s, that the GM-supplied replacement switch was usually the ONLY switch that would be correct for the vehicle AND WORK correctly.

 

The tank vent is a floating cap on a plastic tube inserted in the end of the tank vent hose.  It's the same type of vent that GM has used for many years and should not clog very easily, if at all.  The end of that hose lays on top of the tank, just rearward of where the gap between the bed and cab are.  Not sure if the fuel tank cap should be vented also, but probably not.

 

Later versions of the tank solenoid switch also switched the evaporative emissions to the charcoal canister.   Some of the later canisters had a pressure/vent valve in them, too, but the earlier ones just had straight plug-ons for the hoses from the tank and carb. 

 

There was also an earlier version for the 454s that was different from the 350s and 6-cylinders, too.

 

At this point in time, it's possible that ethanol has degraded the rubber fuel lines such that they are either collapsing under suction or are LEAKING.  If not already done, it probably needs to be done . . . tanks to carb.

 

I have a '68 Buick that would die or lose power, whether sitting at idle or running down the road.  If I caught it soon enough, I could pump the accel pedal and it might come back.  Fast pumping of the pedal was the only way to get it to restart, and hoped that it kept running as I put it into gear!  That issue was fixed when I finally put a new fuel pump on it.

 

Also, for general principles, do a complete check of the charging system.  Poor electrical supply can cause an engine to stop running, too.  This would also check to ensure that all of the engine/chassis ground straps are in place and functioning . . . as most were disconnected or left off over the years of people working on the vehicles.

 

If it runs reliably, but falls on its face when accelerating or under load, that's another issue.  PM me for details of how to fix that.

 

NTX5467

Edited by NTX5467 (see edit history)

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Is there not a vent requirement for the fuel tanks?  Did you try loosening up the gas cap and see if it would run?  Just a thought.

Yes, tried that...

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The tank solenoid switching valve's default mode is to pull only from the tank which came on the truck as standard equipment, which explains why it's only pulling from the one side.  That's also the "failure mode", as I recall.

 

ENSURE the switch on the instrument panel is the CORRECT SWITCH for the model year of the truck!!  Aftermarket switches have become "generic" in the later years and they are NOT all the same, although they might look like it.  The difference is on the plug-in terminals.  The specific terminal with the "locating tang" is different on some model years, which MUST match the plug-in on the chassis harness.  IF those don't match, things don't work correctly . . . which I know from selling the GM Parts switch (model year correct) after customers had been to the auto supply and gotten one.  Not just the placement of the tang, but the shape of the tang is important, too!!!  This even became apparent back in the earlier 1990s, that the GM-supplied replacement switch was usually the ONLY switch that would be correct for the vehicle AND WORK correctly.

 

The tank vent is a floating cap on a plastic tube inserted in the end of the tank vent hose.  It's the same type of vent that GM has used for many years and should not clog very easily, if at all.  The end of that hose lays on top of the tank, just rearward of where the gap between the bed and cab are.  Not sure if the fuel tank cap should be vented also, but probably not.

 

Later versions of the tank solenoid switch also switched the evaporative emissions to the charcoal canister.   Some of the later canisters had a pressure/vent valve in them, too, but the earlier ones just had straight plug-ons for the hoses from the tank and carb. 

 

There was also an earlier version for the 454s that was different from the 350s and 6-cylinders, too.

 

At this point in time, it's possible that ethanol has degraded the rubber fuel lines such that they are either collapsing under suction or are LEAKING.  If not already done, it probably needs to be done . . . tanks to carb.

 

I have a '68 Buick that would die or lose power, whether sitting at idle or running down the road.  If I caught it soon enough, I could pump the accel pedal and it might come back.  Fast pumping of the pedal was the only way to get it to restart, and hoped that it kept running as I put it into gear!  That issue was fixed when I finally put a new fuel pump on it.

 

Also, for general principles, do a complete check of the charging system.  Poor electrical supply can cause an engine to stop running, too.  This would also check to ensure that all of the engine/chassis ground straps are in place and functioning . . . as most were disconnected or left off over the years of people working on the vehicles.

 

If it runs reliably, but falls on its face when accelerating or under load, that's another issue.  PM me for details of how to fix that.

 

NTX5467

thank you!

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