Fixing vapor lock now that I barely made it home, experience with 3 port filters?

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dkraven

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Took the truck on its longest drive since owning and sorting it, 60 miles including some freeway time, and on the way back the power started to drop out in surges and I had to limp off the freeway and the rest of the way home. Surging kept getting worse and right before I got home a backfire through the carb blew off the air cleaner lid. Fun times. Two days later I took it for a drive once it was cooled off and it's driving fine, so I'm putting my money on vapor lock. Especially since when sorting the truck I got frustrated with a piece of crap metal fuel line from the pump to the carb and used vinyl instead and it ends up touching the intake. Nice setup for boiling fuel. So, obvious solution out of the way, I'm going to put a metal fuel line back in or find a way to route the vinyl fuel line to not touch anything. I know that's the first step. But I've been reading about using 3 port filters to bleed off fuel back to the tank and think it wouldn't hurt to do that. But I have a question!

My truck doesn't have a return line, but the sending unit has the metal port for one. I stuck a rubber cap on it back when I put the new fuel tank in. I'm assuming if I install a 3 port filter I just route a return line back to that metal return line port in the sending unit? Any tips on installing that return line? Any problems with using vinyl for the whole thing?
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RustyPile

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IF, (that's a big if) you have the proper fuel pump, it will have that 3rd return fitting. It has an orifice built-in to meter the proper amount of return flow. Return fuel lines were designed and applied before the advent of squarebodies. Look closely, unless someone has removed it, there should be a return line running parallel to the main feed line. If it's gone, simply use brake line to fabricate a new one. It's not a good ideal to use "vinyl", plastic, rubber, or copper for a fuel line that long.

The stock carburetor was a Quadrajet with a fuel filter at the inlet fitting. If the carburetor has been changed to a different design, instead of adding more rubber fuel lines for a fuel filter on the top of the engine, a better location is at the fuel pump between it and the delivery line.. Readily accessible and away from engine heat.
 

dkraven

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IF, (that's a big if) you have the proper fuel pump, it will have that 3rd return fitting. It has an orifice built-in to meter the proper amount of return flow. Return fuel lines were designed and applied before the advent of squarebodies. Look closely, unless someone has removed it, there should be a return line running parallel to the main feed line. If it's gone, simply use brake line to fabricate a new one. It's not a good ideal to use "vinyl", plastic, rubber, or copper for a fuel line that long.

The stock carburetor was a Quadrajet with a fuel filter at the inlet fitting. If the carburetor has been changed to a different design, instead of adding more rubber fuel lines for a fuel filter on the top of the engine, a better location is at the fuel pump between it and the delivery line.. Readily accessible and away from engine heat.
The quadrajet has a filter in its inlet. I was more looking for a way to reduce vapor lock by keeping the fuel cool by sending it back to the tank rather than just sitting in the line behind the carb. I'll have to look at my mechanical fuel pump. I installed it months ago and don't remember a 3rd port on it. I'll get under the truck and look for that parallel line, it may very well be there under the dust!
 

Old Guy Bill

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I believe there’s a little confusion here…
The sending unit assembly at the tank can have one, two, or three ports, depending on the year of vehicle, or what someone has installed in the past.
These ports are 3/8” for fuel supply, 5/16” for fuel vapor recovery, 1/4” port for fuel return from the pump.
The mechanical fuel pump on the engine can have one, or two ports.
3/8” for fuel supply, and 1/4” for fuel return.
 

dkraven

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My fuel pump is only a 2 line so would need to replace it with a 3 or add a filter with a return line. The sending unit has 3 ports. I think I do want a return line, any difference whether I replace the 2 line fuel pump with a 3 or add a filter after the pump with a 3rd port?
 

75gmck25

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I’ve never see a filter with a return port, so I don’t know how well they work. However, the mechanical fuel pump with a return line was used on many GM vehicles, and the pumps are not expensive. I would go with the OEM solution.

Rubber gas line will work to add the line, but you want to use low pressure EFI line (SAE J30R7 spec) because it will hold up to Ethanol gasoline and not disintegrate over time. It’s expensive to buy it per foot at local shops, so you might have to buy it online to find a decent price.
 

legend57

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Before you start changing things, you may want to add a fuel pressure gauge near the inlet of the carb to monitor. Check it occasionally during a drive and see if it changes. I had a similar issue, but it ended up being sediment in the tank that would slowly gather on the filter sock in the tank. After driving a while, it would start to act like how you describe. When I shut the engine off and waited, the sediment would fall back to the bottom of the tank and it would drive fine again for a while. Very frustrating to diagnose. Happened a long time ago when I knew even less than I do now, but I'll never forget that one.
Vapor lock should not be occurring under light condition driving (not towing, not going up a grade, not excessively hot ambient temps). If the line is stock, it shouldn't be coming too close to the manifold.
You could also have sediment in the float bowls of the carb that float around while driving and find an orifice to clog, and then settle back down when the engine is off.
I'd go through the system from tank to carb to make sure everything is 100% clean and clear.
 

Randy and Easton

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We had similar issue and found the vent line to our 2 port sending unit was cut just under lower radiator hose. We could not find a charcoal canister at the time so we made a similar system using charcoal and a fuel filter to
Vent the tank. Made it run better when it was warm eliminated the gas smell the truck had all the time.

Randy and E
 

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Ricko1966

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Do you have the original carbureator and steel line pump to carb?
 

Trucksareforwork

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Unless a certified mechanic has a different view, here’s mine: Vapor lock is unlikely at Highway speed. Lots of airflow across the engine and fuel flow into the engine vs fuel being pulled and boiled in the lines sitting in traffic. Now if you were sitting in traffic on a hot day, then yes, maybe.

The backfire is a little baffling too. Vapor lock is fuel starvation. Maybe that was a lean backfire but I don’t think so.

Might be fuel pump itself. Could be an obstruction in the line or some trash in the float bowls.

Does your truck have an HEI? Could have an ignition module going bad. They can change condition with heat. Maybe?


If you really think it’s fuel, I think I would try to replicate it with a fuel pressure gauge on the line first, which will rule out actual fuel delivery to the carb.
 

dkraven

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Thanks for all the ideas. Haven't had time to work on it because of the holidays, but I did drive it around town running errands without issue on Monday. There was none of the jerking/engine cutting out at all. But then I noticed something else, when I blip the throttle under the hood the upper radiator hose contracts under suction like there's hardly any fluid in it. This was after 20 minutes of driving. I'm wondering if the thermostat is stuck and my engine is running hotter than it should and maybe that's causing vapor lock? The fuel line from the pump to the carb right now is rubber and passes pretty close to the block. (why does he have a rubber fuel line? because the replacement metal one I got didn't fit and rather than bend it I just wanted to get the truck running for now, a new metal line is in the future).

I'm reasonably sure the line from the tank to the pump is fine as I replaced the tank, sending unit, and fuel pump in the past 6 months. Maybe the sock came off in the tank and was clogging up the tube is the only thing I can think of there. For right now I haven't been able to reproduce the problem, but replacing the thermostat is cheap and I haven't replaced the coolant since I got the truck last year, so might as well flush it while I'm at it. I also got some of that foil wrap for the line from the pump to the carb and am going to reroute the rubber line around so it's not so close to the block.

@Ricko1966 It's the original quadrajet rebuilt with a Cliff kit a couple of months ago. Original steel line from pump to carb got a pinhole in it and the new line I got was being a pita so I threw a rubber line in for now.

@Trucksareforwork I was wondering about ignition. I installed a new module from one of those distributor rebuild kits, and put plenty of heatsink goop on it, but I've read the components in those kits can be crap. For right now I have to see if it happens again.

Double checked and I've got no return line to the tank, even though this year truck supposedly came with it, so installing a 3 port filter or fuel pump would require making a return line. Not going there just yet. A fuel pressure gauge is a good idea because that'll give me good info if it happens again. As an aside, one of the things I want to do is get more measurement on the truck all around, install an AFR gauge, be able to see what's going on.

Will update once I get more.
 
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MrHorsepowerLSx

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As side note, if you install a return line don't hook it up to the tank vent, hook it to the smaller 1/4" nipple coming off the tank. You could always check which line you need by pulling the sending unit and inspecting or by blowing air with your mouth into the tank with a piece of hose. The return line will go easily into the tank but the vent nipple should stop you from being able to blow air into the tank.
 

dkraven

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Update, not a huge one. Still dealing with a fuel starvation issue, haven’t had a lot of time to do work but have some new information.

The first time I had the problem I’d driven the truck 30 miles, sometimes at freeway speeds, then let it sit for 2 hours and tried to drive it back. It was most of the way back that the truck started jerking back and forth, kept getting worse, and finally wouldn't stay running under any acceleration.

Next day truck started fine, I drove it to do some errands, no fuel issues. Short drives were fine. Then this week on two occasions I drove it for about 20 minutes, parked it for a bit, then when driving again the fuel starvation issue didn’t start right away, it reared its head when demanding power from the engine, and then the truck required a long sit before starting again.

I know there are in depth diagnostic things I can do, check fuel pressure, hand check the line from the tank to the pump, etc, but I’m thinking the truck’s behavior should be a big clue. Shouldn’t vapor lock be an issue right away when restarting the truck after it’s heat soaked? What could cause it to only do this after a long drive, a short sit, and then driving again with any power?
 

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I don't know that you have a fuel issue. Especially with the behavior you're getting from the truck, in January, in Oregon.
I'm leaning towards Ignition Control Module... (maybe because I just dealt with that being my issue). My issues started out very similar, and I'm wondering if when you can't start it, if you were to tap on the distributor or distributor cap, if would maybe start then. Don't ask me how I know that one.
 

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