No gas to carb

[KTraun]

Help! I replaced sending units, gaskets and lock rings, all soft fuel lines in the back. I broke the tank selector valve in the process so I figured I would bypass it for now and hooked the drivers side tank lines directly to lines that go to front. Ran for a minute on what I assume was leftover fuel in lines but now it won’t run. Not picking up any new fuel. Will run if I pour gas into carb! What did I do wrong? Does there need to be something else in line in order to bypass the valve? One mechanical fuel pump on side of engine…’79 K20 with the 350 so tanks only have sending units. It has the three line sending units and all three lines are different sizes. GM wouldn’t be so crazy as to switch the size of the line mid way? The forward is 5/8, there is a return from engine to tank that is 1/2, and a return from selector that is 3/8

 

[CheemsK1500]

Before you dig deeper, you should double check the mechanical fuel pump function. It's possible the fuel is just stopping at your carb. I've ran into this issue myself.

1.Unhook the fuel line feeding the carb, and put something over the end of it (plastic bag, condom, etc.) Make sure to wrap the edges of whatever device you use to keep the fuel from seeping out the sides.

2. Next turn the ignition key just long enough for the engine to turn over a few times (no more than a few seconds). If your makeshift fuel trap that you made in the previous step has fuel in it, than you have confirmed that your fuel pump works and the fuel is stopping at the carb. The solution to this issue could range from a simple inline filter change and cleaning to a complete carb rebuild/replacement.

If your makeshift fuel catcher remains dry, than your issue could be one of the following:

1. Bad fuel line plumbing (you can test this out by running your fuel pump feed line into an external gas tank.)

2. Your fuel system is sucking air somewhere. This could be a pain to find.

3. Your fuel pump is installed incorrectly. An incorrectly installed pump simply won't pump. A hack saw blade holding up the falling pin during installation of the fuel pump can keep this situation from happening.

 

[AyWoSch Motors]

Do you have the push rod for the fuel pump?

 

[1987 GMC Jimmy]

The fuel trap test is an interesting take. Probably more expedient than than feeding out of a clean fuel can, which is what I would do. If it runs that way, you know everything upstream of the pump is fine, and you need to look further back.

 

[KTraun]

Thank you! Gonna try working on it this afternoon. The truck ran fine before I started all this and I never touched the fuel pump but probably best to have a starting point and just start eliminating possibilities. Is it possible to have introduced some sort of vapor lock that is causing the problem?

 

[CheemsK1500]

Thank you! Gonna try working on it this afternoon. The truck ran fine before I started all this and I never touched the fuel pump but probably best to have a starting point and just start eliminating possibilities. Is it possible to have introduced some sort of vapor lock that is causing the problem?

It's possible that you could've vapor locked it when you introduced air to the system by opening your fuel lines. If you aren't sucking air anywhere in the system with everything put together, dumping fuel straight into the carb and letting the engine run until your fuel stream comes back, should eliminate a vapor lock. Though it's still entirely possible that your fuel delivery system is fine and your carb is just rejecting it for some reason. That's why you should first try the test I mentioned earlier.

 

[Designo614]

Is it possible to have introduced some sort of vapor lock that is causing the problem?

Vapor lock is caused by the fuel in the lines turning turn to vapor because of excessive heat, essentially boiling it. This happens when you may have the line too close to a header, perhaps; but that's not what's happening here, because you're not getting fuel under any circumstances. An engine that's experiencing vapor lock will at least run until it gets too hot.

I agree that you need to check the pump, both in and out. Run the out line into a bucket and turn it over a few times. If you get nothing, install a temporary feed line from a gas can and try it again. This test isolates the pump as the issue. Also, when you disconnect the incoming line to the pump, it should drip some leftover gas from the line; if it's dry, that's another indicator that the issue is further back towards the tank.

If that's the case, I wonder if the new sending unit you installed is hanging up in the tank somehow, such that the feed line/sock isn't submerged. A plugged tank vent can limit fuel flow also, because the liquid can't leave if not replaced by air; think about how a gallon jug acts when you try to pour it out too quickly, as it gurgles and gulps air to replace the liquid. In this case your tank would probably be oilcanning/imploding. Is there a rush of air into the tank when you remove the cap?

 

[75gmck25]

You didn’t mention replacing any rubber fuel lines. If you moved them around a lot in the process of working on the tank switch you might have bent a rubber line enough to crack/break the rubber and create a hole. Usually it will just leak, but it might be a big enough hole so the pump is just sucking air.

If you replaced the sending unit, are you sure you got the lines hooked up to the right ports? My truck has a 3 port sending unit and the 5/16” and 3/8” lines are very close in size. You need to be hooked to the 3/8” line.

 

[KTraun]

I did replace all the rubber lines. I will double check the correctness of how I hooked them up but I’m pretty sure the biggest went to the selector valve area, the middle sized went to a return line coming straight from engine area and the smallest followed the biggest as a return from the selector valve area

 

[KTraun]

Update…so it would seem I measured the fuel lines incorrectly so whe I moved the tank into place it linked the rubber fuel lines so bad fuel couldn’t flow. Ran out of time putting it all back together again but the lines stayed in place much better this time and I have high hopes for a running truck tomorrow afternoon! Thank you all for your help!

 

[KTraun]

Kinked, not linked!