Vapor lock

[Matthewisproud]

So I have a stock 77k20 with no smog. It has dual tanks. A fairly ne fuel pump with no return line fitting. The truck performed well all winter. But the temps climbed to about 85 yesterday and the truck started to vaporlock. I have already installed a carb spacer with no luck. I have a clear glass filter. When I start the truck in the morning it is full. After 30 minutes of driving it is nearly empty. And after being in a stoplight the truck will die 10 seconds later. It takes about 30 minutes before I can restart the truck. I can also see bubbles travel up the fuel filter. None of the fuel lines feel too hot to me. However the heater lines lay across my fuel filter.

 

[1987 GMC Jimmy]

I have two suggestions. Ditch the glass filter as soon as possible. That's an engine fire waiting to happen. Second, I'd reroute the fuel line in the engine bay away from hot stuff like exhaust and heater hoses. If you're running a Quadrajet, I'd revert back to the pre-bent steel line, but if anything else, I'd just rerun the rubber away from the heat centers.

 

[donnieray]

Ive battled this problem for a while now. Bubbles in the filter, mostly empty filter, Edelbrock starving for fuel at random times. Things I did to my 86C and my 84K are new fuel pick up/sender units, thinking that the sock was clogged. It wasn't. Changed out rubber lines at tank, under cab and under hood. New filters, new pump on the C10. Rerouted the feed line leaving the pump to the fender well and over the valve cover. I even made heat shields and installed them where the exhaust pipes are close to the fuel lines. Both trucks are better but I still see a half full filter a lot. My next move will be an electric pump on the K. I want to see a full fuel filter. There is a lot of reading material on this subject here and on the web. Some call it percolating. Good luck.

 

[donnieray]

I have two suggestions. Ditch the glass filter as soon as possible. That's an engine fire waiting to happen. Second, I'd reroute the fuel line in the engine bay away from hot stuff like exhaust and heater hoses. If you're running a Quadrajet, I'd revert back to the pre-bent steel line, but if anything else, I'd just rerun the rubber away from the heat centers.

Good advice about the glass filter and rerouting, I use the clear plastic filters by the way and have them mounted pretty close to the carbs.

 

[1987 GMC Jimmy]

I think the plastic filter is a step up from the glass, and kept where you have it is one of the best spots, but plastic will still melt rather than shatter given the right conditions. You can't beat a canister type metal filter. You're right about fuel starvation. The best thing to do when you're unsure about the fuel system is to drop the tank, clean it up, replace the strainer sock (sending unit if needed, too), blow the lines out, and make sure the pump is working okay. Hell, a new Delphi pump is like sixteen dollars so I just threw a new one on when I went through my fuel system. Rubber line inspection is great, too. If any air is being pulled into the system through cracks in the lines, it won't run right.

 

[donnieray]

I never really thought about the filter melting. Damn. And a metal can will also keep me from seeing the bubbles. Then maybe I wont wear my hood hinges out looking at that filter every day!

 

[74 Shortbed]

Plastic are the worst, ends will melt off. This is the only way to go for a plain inline filter, just change them out religiously.

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[Matthewisproud]

Ok so I drove about 3 miles home from school today and it tried to die on me. It's about 95 degrees when I got home I popped the hood almost no fuel in filter so I got the hose and held it on the pump and the filter filled back up.

 

[Rusty Nail]

Nobody buys a new gas cap? Really?
There is a lot of science there.

Hey O.P.
Take your gas cap off tomorrow and see if today's problems duplicate tomorrow.
Try it!

And then you'll be all like "Leapin Lizards!"
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And then run that happy ass to O'Reilly's and buy a stock replacement STANT brand gas cap for your specific application.

Here's a pic of my new one, it says MOTO-RAD.
I think it cost $11.00.

40 years later the spring in the cap loses tension and you fail to vent. The pressure builds INSIDE the tank and forces air into the fuel line, resulting in your "half-filled" filter. Remove the cap under that symptom condition and it'll hiss like a goose.
Luckily the solution is both simple and cheap.

 

[Matthewisproud]

Ok so I decided to buy a new fuel pump just to check. Put it on and now the truck will not idle it will run fine once you get it going but it will idle rough until it dies

 

[donnieray]

The gas cap switch is good advice, although it wasn't my problem. When I was younger a friend of mine put a non vented cap on his Chevelle and the fuel pump collapsed the tank! And I only switched to the plastic filter when I suspected a starvation issue, I needed to see the flow. You may have more than one issue. Timing, leaking vacuum hoses, bad wires could be culprits of bad idle. As far as the bubbles in the filter...well good luck. I will add that on my C10 I have a fuel pressure gauge and weather the filter is full or half empty the pressure stays constant at 7psi.

 

[jetman]

Used to have lots of problems in the spring, most likely caused in part by winter blend fuel being used in hot spring weather. Added heat shields to fuel lines and installed factory cold air intake. No more probs to date.

 

[donnieray]

Used to have lots of problems in the spring, most likely caused in part by winter blend fuel being used in hot spring weather. Added heat shields to fuel lines and installed factory cold air intake. No more probs to date.

Good story, and I have after market breathers on all my trucks pulling engine bay air.