"Overheating" --> Now Fuel Delivery Issue --> Dirty Tank?

fast 99 said:

Vapor lock more times than not is caused by low fuel pressure.

Think of it like a pressure cooker. Boiling point increases with pressure. Today's fuel is not designed for carbs, exacerbating any low fuel pressure issues.

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Yes and no. Vapor lock happens before the pump... where there is a vacuum being pulled by the pump. The vacuum being pulled by the pump pulls the boiling point down very low and causes vapor bubbles in the line that the pump can't move. A three line pump will constantly move a large volume of fuel through the system, which usually is enough to keep the fuel from warming up enough to vaporize in the line. It can still happen though if you have a restriction in the pickup tube that causes a higher vacuum to be puled in the line before the pump. Modern EFI engines don't have this problem because every bit of the fuel system outside the fuel tank is pressurized to 3 Bar or more of pressure, which raises the boiling point high enough for it to not matter. Also EFI pumps push while they are submerged in fuel, so there is nowhere for a vapor lock to occur as long as there is fuel in the tank.

Using that analogy on a 2-line system wouldn't a new pump drawing fuel to a lower vacuum cause more vapor lock problems not less? Everything else concur.

Thanks all! This got a much bigger response than I expected.

I'm running a mechanical fuel pump from a dual tank system.
with a recent oil change after the problem started, the oil did not smell like gas
I have a fuel pressure regulator set to 3 or 4 psi - I'll bump it up a bit and see what happens.
Fuel lines are mostly old or "original" I'll have to trace them and see what condition they are in.

There is/was a fair amount of crap in the fuel filter, but there always was a bit in there. I really don't want to drop the tanks and clean them out. This problem did happen after I started using the driver's side tank after almost exclusively using the passenger tank with no problems - might switch back and see what happens.

Bruce Wingate said:

This problem did happen after I started using the driver's side tank after almost exclusively using the passenger tank with no problems - might switch back and see what happens.

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Might be the key

Bruce Wingate said:

Thanks all! This got a much bigger response than I expected.

I'm running a mechanical fuel pump from a dual tank system.
with a recent oil change after the problem started, the oil did not smell like gas
I have a fuel pressure regulator set to 3 or 4 psi - I'll bump it up a bit and see what happens.
Fuel lines are mostly old or "original" I'll have to trace them and see what condition they are in.

There is/was a fair amount of crap in the fuel filter, but there always was a bit in there. I really don't want to drop the tanks and clean them out. This problem did happen after I started using the driver's side tank after almost exclusively using the passenger tank with no problems - might switch back and see what happens.

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Still need to know if it's a 3 port pump or a 2 port pump.

Ricko1966 said:

Still need to know if it's a 3 port pump or a 2 port pump.

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I didn't look yet, but it's the mechanical pump. I thought they were all two ports : one in and one out

Post removed.
Wrote a post and realized it made no sense: Vapor created between fuel pump and carb would be simply vented into the carb, allowing fresh cool gas to be pumped.

My ‘75 K25 has a 3 port pump, so they were used quite early. Return line goes back to the 6 port solenoid under the truck, and both supply and return are switched. Circulating fuel back through the return lines cools it down.

Buy several feet of SAE J30R7 (low pressure) or SAE30R9 (high pressure) EFI fuel line, and start gradually replacing hoses. The old rubber hoses start to break up internally and crack on the outside when used with ethanol. Then you get plugged filters, and lines that are sucking air through cracks that are hard to see.
The EFI fuel line is usually a lot cheaper if you buy it from Amazon or other online sources.

My 76 C10 is a two line setup, my friends 75 K5 was a 3 line setup. My 79 C10 is also a three line setup.

Not sure why the 75 would have a 3 line and my 76 a 2 line. My 76 and 79 also have the air cleaners with the large opening and fresh air snorkel, whereas the blazer had the small opening air cleaner with no fresh air tube. Weird.

Bruce Wingate said:

I didn't look yet, but it's the mechanical pump. I thought they were all two ports : one in and one out

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Just checked - two line pump

legopnuematic said:

My 76 C10 is a two line setup, my friends 75 K5 was a 3 line setup. My 79 C10 is also a three line setup.

Not sure why the 75 would have a 3 line and my 76 a 2 line. My 76 and 79 also have the air cleaners with the large opening and fresh air snorkel, whereas the blazer had the small opening air cleaner with no fresh air tube. Weird.

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Emissions back then were not the same between states and GVW changed requirement. Options might change it as well, such as high altitude. I have an 81 K20 LD 3/4 ton with a cat and an 85 K20 HD without a cat, same engines.

Bruce Wingate said:

Just checked - two line pump

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Okay there are some options the 3 port bypassed a portion of fuel back to the tank,to prevent vapor lock,fresh, cool fuel always in the lines. GM also did this with a 3 port fuel filter 1 in 2 out to bypass fuel,big block camaros/novas , GTOs. I've gotta go reread your post brb. 20 minutes seems awfully quick to vapor lock,not saying it's not,but seems more likely a vent problem or trash in the tank problem. Run without a gas cap see if the problem goes away. Post back. What happens with trash in the tank is some gets pulled up against the filter sock, leaving less area,then some gets pulled into these areas until it's completely covered. Turn off the truck no more vacuum from the pump,no more vacuum to hold the debris it falls away from the sock.

Ricko1966 said:

20 minutes seems awfully quick to vapor lock,not saying it's not,but seems more likely a vent problem or trash in the tank problem. Run without a gas cap see if the problem goes away. Post back.

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I forgot about this, a tank vent issue could very easily be the culprit. Do like he said, leave the cap loose/removed and see if it fixes it. Or, drive until it fails, pull the cap, and see if it starts back immediately.

This happened to a buddy with a Mustang, every day at the same spot in his drive to work, the engine would stall out. 5-10 minutes of sitting, and he could re-fire it and drive the rest of the way. Ended up being a clogged 1/4" vent line, blew it out and he was back in business.

Added a temp sensor and hooked it up to the dash gauge. Started the truck up and ran it in the driveway for a while. The gauge rose to about 210 and stayed right about there. Didn't want to risk running and getting stuck.

When running on the driver's tank, the fuel in the filter is really low - the sightglass is almost empty. It might fill up here and there, but never more than half-way. Taking the gas cap off did not affect it at all. Switched over to the passenger tank and the sightglass fills all the way up after a minute or two to get the suction going (that tank is almost empty).

I've got a boroscope for my phone somewhere - I'm going to take a look in the tank and see what I can see. I'll report back later. I may also siphon out the tank with a larger diameter hose and see what comes out.

Sounds like you might be sucking air, as stated up thread, a fuel line with under suction may be cracked or compromised, but being under vacuum it won’t leak.

Maybe get some 3/8” fuel hose, disconnect the drivers side tank feed from the switching valve and draw from a gas can on the ground, if results change the problem lies from the switching valve to sending unit on drivers tank.