Fuel Delivery Troubleshooting

[harrydunn]

Drove the GMC around a bit this afternoon and it ran well. Well plugs are leaking causing some loading up but that's another story. Shut it off for 30 minutes, cranked it back up and it was not running well. As soon as I gave it gas to take off down the road it fell on its face and not just a minor hesitation. Its idle continued to deteriorate for about 20 seconds and then it died. It's never done that before. Could not restart it. Had plenty of gas in the tank. I verified that I was not getting any gas to the carb. Cracked the fuel line loose at the carb and cranked the truck. No gas. Poured fuel down the carb and cranked it time after time for at least 30 minutes. It would start and run for 2-3 seconds and then die after it had burned the gas that I poued down it--still no gas coming out of the line. Finally, another 30 minutes later, fuel began pouring out of the fuel line when I cranked it. I buttoned everything up and it began to run fine. Stuck her in the garage to figure out what could be going on.

It's extremely hot here today so I thought vapor lock but I could not even get any gas to the carb. Can the fuel line to the carb get vapor lock as well? Is there a fuel sock on the gas tank sending unit that could have been clogged and after an hour of sitting the setiment perhaps dropped to the botton of the tank?

Also, what epoxy, etc. will I have the best luck with sealing the well plugs? They were done a long time ago but probably need to be done again.

Thanks.

--Mark

[GASoline71]

Do you have the evap/venting system hooked up (1971 and 1972 trucks)? If not... do you have a vented gas cap?

Gary

[harrydunn]

Originally Posted by GASoline71

Do you have the evap/venting system hooked up (1971 and 1972 trucks)? If not... do you have a vented gas cap?

Gary

Hey Gary,

Thanks for the reply.

Hmmm...not sure exactly. The gas cap does not look like it would be the venting type. As far as the evap/venting system, are you referring to the two lines coming from the actual gas tank? One for delivery, one for return?

Thanks.

[MagmaJct]

I think Gary may be referring to a gas cap that's vented one way, that is, it allows air into the tank to replace the gas that's being pumped out. If there's no venting it would be like trying to drink from a glass soda bottle without letting air in, you'll stop getting soda!

However, I am not sure of the specific details on how the evaporative system is vented in these trucks, so I will share my experience.

I had symptoms of vapor lock as you are experiencing now. I live in the desert, and it would "lock up" during the hot days here. But eventually it would do this at night when it's much cooler, (in the 90s!)

Then, it wouldn't start at all, I diagnosed the problem as a bad fuel pump. When I replaced the pump about a year ago, I haven't had a SINGLE no-start condition since, regardless of the temperature. I think another poster on this board had the same symptoms, and it ended up being the pump.

I can't say that it is your pump, but that would be the direction I'd look.

[GASoline71]

The '71 and '72 Chevy/GMC trucks (maybe even '70) have an EEC (Evaporative Emmission Control) built into the tanks. They are vented externally and have the vents in the drivers side door jamb. If you don't have the system functioning and don't use a vented gas cap... then it will be just as described above by MagmaJct. The trucks with the EEC system did not have vented gas caps from the factory.

I have mine disconnected in my '72 GMC and had to use a vented gas cap.

Gary

[harrydunn]

I'll check out the EEC. It's run like a champ though since I picked it up in December '10 and I haven't touched a thing except for a carb rebuild in January. Fuel pump is only a year or two old but could be bad I suppose. I just got back from a 30-minute cruise this evening and she ran great so I started thinking a bit...

It was 97 this afternoon when I took it for a drive. Since owning it, I've never driven it when it is that hot out and then tried to start it again so soon. I'm leaning towards vapor lock.

I need to replace my sending unit anyways to get my fuel gauge working. If I order a sending unit from LMC, etc. will they come with a new sock? It doesn't show one in their catalog. Luckily, I've got all the time in world to tinker with this truck so I actaully somewhat enjoy figuring out these riddles.

Thanks guys.

[harrydunn]

Looking through my LMC catalog and do not see a pump for my year with three fittings. Isn't the third line coming off of my pump and heading towards the back of the truck a return line to the fuel tank to prevent (help prevent) vapor lock?

[kcsa75]

You should be able to pick up a fuel pump at any parts store. I had to replace the three-line fuel pump on my 72 Chevy this spring. Was about $20 at CarQuest.

[harrydunn]

Good deal. Thanks.

[harrydunn]

New pump on and everything in the ignition system replaced except the coil. Has been running well until last night when it was very hot out again. About 25 minutes into my cruise, it began to struggle, lose power, and was not running on all cylinders. I think it's got to be vapor lock. Thoughts?

[chilihed]

Don't mean to hijack, but I am having the same problem. I got an insulating sleeve for the fuel line from the pump to the carb, but haven't had the chance to see if it worked or not.

[MagmaJct]

So the new pump has the three fittings? Did you ever find out if there's a restriction in the line? A restriction in the suction line will make the pump work harder to pull the fuel, lowering it's boiling point.

(The same effect is realized although reversed with a sealed cooling system. The boiling point of the water is increased due to increasing pressure.)

[harrydunn]

Yep, three fittings on the pump. To test for a restriction, are you simply referring to removing the tank line from the pump to see if I get gas? When I changed the pump last week, the tank line was flowing extremely freely. My clothes quickly became very flammable.

[harrydunn]

Well, back again folks. Did not get a chance to drive the truck much over winter but it is acting up again just like before with our hot weather here. I'm going to slap a spacer under the carb to see if that helps. What size spacer do you suggest for partial vapor lock?

Also, I've read that some are putting an electric fuel pump back by the tank. Are they by-passying the mechanical fuel pump on the block or are they using that too in conjunction with the elec. pump?

Thanks.

[ole dollor]

when it quits running, have you took the gas cap off to see if it has suction or pressure....might give that a try and see what it does.....

[GASoline71]

I would check the gas cap first and see if it's just a $15 part.

Gary

[harrydunn]

I'll give that a whirl and see what I get. Not my daily driver so I've got some time to sort it out. Frustrating to not have much confidence cruising around town on hot days at stop lights.

[harrydunn]

A little rusty underneath, but I do believe this is a venting gas cap?

--Mark

[harrydunn]

Found the correct cap at AutoZone and will try that as well as replacing the short rubber line between the tank hard line and the pump hard line. Everything else on the fuel and ignition systems is new except for the coil.

[rico7769]

Same issue from buildup in the short rubber line from the hard gas line that connects the tank line to the fuel pump hard line. Replaced and rolled over with no problem. I would take out the sending unit and check it too. Sediment builds up in these tanks and the sending units corrode like crazy after a 40 year gas bath. My problems culminated with a broken carb due to crap from the fuel tank. It's nt a bad idea to replace the whole fuel system up to the carb. Money well spent in the long run. From tank to rubber to fuel pump to the carb will prob run you about $250-$325.....or the price of a new carb..
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