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Vapor lock issue

Dillyp

Dillyp

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Location
North Carolina
I am having a possible vapor lock issue going on with my 82 k5. It runs fine in cold weather, but as soon as the temp gets above 60* it starts acting up. It will run fine until I stop for a few minutes, then once I restart it, it will suck the lines dry and won't start until it cools back off. I have a thick thermo carb gasket, it's got a new ac Delco fuel pump, I have replaced all the rubber in the fuel lines and blew the lines out, I have unhooked the vent from the charcoal cannister, wrapped the line from pump to carb in thermo tape, even drilled a small hole in the fuel cap to make sure it vents, yet it still does it. What gives?
 
Vapor locking at 60 degrees is more than odd by itself. I think with all you have done to the fuel system that has not improved the situation it’s probably not fuel related.

I would check to see if the ignition system has spark after it has stalled out when it warms up. If it don’t I’d be looking squarely at the ignition module inside the HEI distributor. They are known to quit when they get hot when they are failing.

Just a hunch.
 
Yes if it were happening at 80 or higher maybe esp with all you have done.
How do you know it has sucked the line dry? A dry fuel with functional pump, Indicates air getting in on the suction side.
How temperature would effect the fuel lines, maybe a crack in metal lines opens with sufficient heat, this is speculation.
@ZooMad75 advice is sound.
 
Wes Harden said:
Yes if it were happening at 80 or higher maybe esp with all you have done.
How do you know it has sucked the line dry? A dry fuel with functional pump, Indicates air getting in on the suction side.
How temperature would effect the fuel lines, maybe a crack in metal lines opens with sufficient heat, this is speculation.
@ZooMad75 advice is sound.
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I've thought about the ignition module, and thats likely my next step. But, when it does its bizarre thing, i can disconnect the line from pump to carb and its bone dry. When it first started I thought it was the fuel pump diaphram being slightly torn, but i installed a new parts-house pump, then an AC Delco one with the same problem.
 
Let me ask when does it run out of fuel at 45 mph plus under medium heavy load?
 
Wes Harden said:
Let me ask when does it run out of fuel at 45 mph plus under medium heavy load?
Click to expand...
It usually happens after i drive 25 minutes from work to my inlaws to pick up my kids. It sits for 15 minutes or so, then i go back down the road, it makes it to the exact same spot about 1/4 mile. It did do it once while going over a mountain, 25-30 mph moderate pull.

The engine temp stays a pretty consistant 190*, will climb up to 200-205 after sitting a few minutes, but goes right back to 190 once the coolant starts flowing. The time it died going up the mountain, the engine temp was at around 200*. Not scorching hot temps by any means.
 
Random thought, try getting fuel to siphon out of the tank on the delivery line? See if it slows down or stops after flowing a bit? Possibly the tank has too much water or crud in it, or the in-tank filter has failed.
 
Yeah you aren't getting vapor lock. The fuel line to the carb has no check valve to hold fuel in there when the engine stalls. What little that is in there dribbles back down to the pump or into the carb. No surprise to me if it stalls and no fuel is there when you pull the line at the carb.

Pretty easy to check for spark after it dies. Pull a plug wire and use a spare spark plug or a screwdriver in the wire and ground it to the engine while somebody cranks. It either has spark or it don't. Work forward from there.
 
ZooMad75 said:
Yeah you aren't getting vapor lock. The fuel line to the carb has no check valve to hold fuel in there when the engine stalls. What little that is in there dribbles back down to the pump or into the carb. No surprise to me if it stalls and no fuel is there when you pull the line at the carb.

Pretty easy to check for spark after it dies. Pull a plug wire and use a spare spark plug or a screwdriver in the wire and ground it to the engine while somebody cranks. It either has spark or it don't. Work forward from there.
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10-4 ill check it next time it does it. I probably have a couple hei modules laying around.
 
If you really feel like it's fuel pressure related install a cheap gauge close to the carb. You can get a clamp type inline fitting and a little gauge for under $40 and use it to see if you have pressure while cranking. A fuel pressure gauge can be a really valuable tool when diagnosing problems. Know that these little gauges can be susceptible to heat issues and such but sometimes just seeing any pressure (vs exact) can help a lot.


https://www.summitracing.com/parts/sum-800135?rrec=true
 
So it just now did it. Again. Took out a sparkplug, have spark. It's definitely fuel related. Could my pump be getting heat soaked? I've never had an issue like this. Ran ponitacs and chevys for years and have never encountered such.
 
I am going ask a stupid question. How much fuel is in the tank each time it has quit? Is the pickup in the tank broke off part way up?
 
ktmoutfront said:
I am going ask a stupid question. How much fuel is in the tank each time it has quit? Is the pickup in the tank broke off part way up?
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I always fill up when the needle hit 1/8th, which is about 8-9 gallons in the tank. I just had the tank out (ugh what a job) and replaced the sending unit and rhe rubber lines. Tank was clean, no debris/white lacquer. Luckily this happened right at my house, so I could at least work/diagnose instead of sitting on the side of the road.

Upon inspection, the rubber supply line from frame to pump was kinked almost closed. Now that the hood has been up and it has cooled off the hose has returned to its normal bend. The rubber has to make almost 180* turn there, so next thing I'll try is putting a curved metal line between two short straight rubber lines.

Don't know if this is the cause, odd that it would only happen at 60* or warmer, but definitely worth noting.
 
If you need a curved rubber hose check Amazon for a Gates "Sidewinder" hose, it's a single piece with 4 different angle bends in it (~$20).
 
Dillyp said:
Upon inspection, the rubber supply line from frame to pump was kinked almost closed. Now that the hood has been up and it has cooled off the hose has returned to its normal bend. The rubber has to make almost 180* turn there, so next thing I'll try is putting a curved metal line between two short straight rubber lines.
Click to expand...
So it sounds like once it's warm the pump is sucking the hose closed. Kinda like some cheap lower radiator hoses. I'd try to lessen the angle of the hose or get some fuel injection hose. And see if that helps.
 
I never had problems with my 74K5 back all the way to the later 80’s until this garbage ethanol fuel showed up did the issues show up with carbs.

Assuming you have ruled out the ignition failing at temps. Coils can fail when hot then work after they cool a bit.

I’m not sold on wrapping metal fuel lines but that’s just me.

You can remove the vent valve from the gas cap rather than drilling a hole.

Your symptoms are similar to what I’ve had. Go to Sonic’ on a hot day. Sit there for 15- 20 minutes or so. Hard to restart then then stumbles after short distance. Was always worse under heavy throttle. Doesn’t have to be super hot out either but the hotter the worse.

Ditto on the fuel pressure gauge if you like and/or a see through fuel filter temporarily up near the carb. I did both. You will see the gauge drop off and the fuel in the filter go down on a hot day sitting in the driveway after driving to full Temp and heat saturation.
Putting a water hose on the fuel pump and you will see the filter fill back up and the pressure come back after a few minutes.
Heat related will show itself.

I went as far as to install an electric fuel pressure gauge on top of the dash in the cab and low and behold whenever I had issues the fuel pressure dropped to zero in real time.

How many lines on the fuel pump, 2 or 3 ?
More on that later.

Get yourself a WIX 33041. Probably have to order it. You can use your evap vent line from the tank to canister as a return for testing
Fuel return to the tank is where I’m going with this. That’s why I ask about the fuel pump lines.

That’s all for now.

30C6F45B-9EC5-414D-957F-EAAFA3EFFEE6.jpeg
 
So I was having fuel delivery problems with a q jet and and mechanical pump. long story. Ended up with a holley red label electric pump and holley return to tank regulator. Has been several thousand miles. No fuel issues, other than an unrelated failed 20 year old carb float.
My symptoms different, I never completely ran out of fuel. At cruise speeds under lite to moderate load in high gear 3rd or over drive truck buck and stumble and have no power.
I blame ethanol, symptoms started in 2011(right when Kommiefornia went corn) I blamed the 700r4 I put in the truck, and parked it. 2018 I had the trans looked at, a band had failed, was fixed. Same symptoms. tried numerous mechanical fuel pumps, including an 80g per hour one none of them could feed the carb enough fuel at cruise.

something is restricting the flow if your pump can suck the delivery hose into a kink. Or it just very crappy hose.
 
I used Gates fuel hose throughout. Guess even the best brand isn't good enough anymore...
 
Dillyp said:
I used Gates fuel hose throughout. Guess even the best brand isn't good enough anymore...
Click to expand...
Yes, this can be true.
Maybe swap that piece out with a new one and see what happens. I don't think that you can reverse the "memory " it has at the kink.
And you may be able find a length that lessens the bend some.
 
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