Possible fuel system problem?

[OrngK5]

Hey, coming to the gurus again for some help... I finally got a new tank installed on my 1978 K5, but the other day it was stalling out at stop lights and sometimes dying during driving. The first time it died at a stop light it started up fine, but every time after it got progressively harder and harder to start eventually I had to drop it off at work. The final time it died out it backfired and I figured it was time to stop.

I assume vapor lock? I live in CA, it was a fair amount of stop and go driving and the air temp was around 98, so I assume it was extremely hot on the pavement too... The fuel filter is fine, the tank is new, there is only one fuel pump and the engine a 400 with a carburetor.

Cliffs:
1978 K5 400cib carb'd
Died repeatedly in stop go driving
CA driver, 98 - 100+ degree weather
New tank, clean fuel filter, fluid levels fine
Possible vapor lock?

Thanks in advance!

 

[diesel4me]

Could be vapor lock...test the fuel pressure first though,if the pump is weak it'll aggravate vapor lock problems..fuel pimps on A/C equipped vehicles often had a "return" line that diverted air and vapor bubbles back to the gas tank,if you only have two hoses on your fuel pump it might help to go with the three hose style ,but youll need to add a return line if your truck doesn't have one factory--most did,,in CA,I'd say ALL probably did..


If you have a carb,stock Q-jets and aftermarket ones like an Edelbrock need a thick base gasket about 1/4:" thick to insulate the fuel bowl from manifold heat,otherwise on hot days it might boil the gas in the bowl and cause this kind of problem..sometimes using a carb spacer helps..

Its possible the ignition module may be failing at high temps too..you'll have to narrow down why it wont re-start,if its got spark or not,before condemming the fuel
system..I once had a similar problem as yours and I did everything I just listed and replaced the fuel pump,added a carb spacer--it turned out to be the ROTOR in the distrubutor was arced right through, and was grounding out on the disributor shaft!

 

[OrngK5]

Could be vapor lock...test the fuel pressure first though,if the pump is weak it'll aggravate vapor lock problems..fuel pimps on A/C equipped vehicles often had a "return" line that diverted air and vapor bubbles back to the gas tank,if you only have two hoses on your fuel pump it might help to go with the three hose style ,but youll need to add a return line if your truck doesn't have one factory--most did,,in CA,I'd say ALL probably did..


If you have a carb,stock Q-jets and aftermarket ones like an Edelbrock need a thick base gasket about 1/4:" thick to insulate the fuel bowl from manifold heat,otherwise on hot days it might boil the gas in the bowl and cause this kind of problem..sometimes using a carb spacer helps..

Its possible the ignition module may be failing at high temps too..you'll have to narrow down why it wont re-start,if its got spark or not,before condemming the fuel
system..I once had a similar problem as yours and I did everything I just listed and replaced the fuel pump,added a carb spacer--it turned out to be the ROTOR in the distrubutor was arced right through, and was grounding out on the disributor shaft!

Hey thanks for the reply! It has 3 lines, one to the fuel filter, two heading back towards the rear of the truck. It has the stock carburetor, so I'll look into a spacer. As for restarting, it turns over, and idles and dies again which is leading me to a fuel delivery problem. I'll take a look at the rotor too, but I did a tune up kit and a new rotor cap when I first got it back in October of 08'...

The same problem happened today, it struggled to turn over, it would idle but then die out again. If it's the original pump, but new tank, is there a chance the pump may be bad after 30 years?

 

[Kenny78]

I hate to say throw parts at it, but an AC Delco 3 line mechanical pump is $30 around here. Couple gaskets and inspect the pushrod against a new one for good measure. That is good preventive maintainence to me. Good Luck with the problem.

 

[Thunder]

I would check the distributor module . If it is failing it will cause your exact problems when they get hot. Also the magnetic pickup coil can be going bad.

 

[OrngK5]

I would check the distributor module . If it is failing it will cause your exact problems when they get hot. Also the magnetic pickup coil can be going bad.

What should I be looking for when deciding if that part has gone bad? The engine has ~26k miles on it, but the parts are pretty much 30 years old since it's a 1978. Even though these parts rarely were used, could age and lack of use cause degradation?

 

[Kenny78]

Next time it dies and wont restart, jump out and pull a pug wire. Stick an extra pug in the wire and hold against the block while cranking( easier with an assistant of course). If it fires blue and consistent rip off the air cleaner and manually work the throttle while looking down the front two bores. The accelerator pump shot should shoot out of the uppermost nozzles with each cycle of the throttle. These steps should help you narrow down whether is fuel or spark.

Good Luck

 

[dyeager535]

Vapor lock is highly unlikely. That is the very reason your truck has three fuel lines, they figured out that a return line kept the fuel from getting hot enough to vaporize.

What I've done to determine if it's a fuel issue is when it dies, don't try and start the vehicle at all. Do not allow the fuel pump to move fuel at all. Pop the air cleaner, and manipulate the throttle. Assuming most carbs are the same (and what I know from the Q-jet), you should see the fuel squirt out. If you don't, the float bowl is empty and the problem is most likely between the needle/seat in the carb, and the fuel pickup in the tank.

 

[Mike D]

The next time it acts up, open the hood and grab the fuel line near the carb. If you can't hold it the fuel probably is vapor locking ( boiling ). If the fuel line from the fuel pump to the carb is steel ( factory ) replace it with a stainless steel braded rubber fuel line ( AN-6 or 8 ) and route the flexable hose further away from the engine to minimize heat absorbtion. The stainless steel hoses aren't cheap but the braded stainless steel reflects most of the engine heat, the rubber hose within can withstand 100 PSI fuel, further insulates from heat and they look real cool.

 

[OrngK5]

Next time it dies and wont restart, jump out and pull a pug wire. Stick an extra pug in the wire and hold against the block while cranking( easier with an assistant of course). If it fires blue and consistent rip off the air cleaner and manually work the throttle while looking down the front two bores. The accelerator pump shot should shoot out of the uppermost nozzles with each cycle of the throttle. These steps should help you narrow down whether is fuel or spark.

Good Luck

By pug wire do you mean plug wire? I'd almost need to see this done in person before I try it myself

Vapor lock is highly unlikely. That is the very reason your truck has three fuel lines, they figured out that a return line kept the fuel from getting hot enough to vaporize.

What I've done to determine if it's a fuel issue is when it dies, don't try and start the vehicle at all. Do not allow the fuel pump to move fuel at all. Pop the air cleaner, and manipulate the throttle. Assuming most carbs are the same (and what I know from the Q-jet), you should see the fuel squirt out. If you don't, the float bowl is empty and the problem is most likely between the needle/seat in the carb, and the fuel pickup in the tank.

Unfortunately when it does die, it happens at a busy intersection while at a stop light or while I'm driving so I have to pull off into a parking lot quickly so I don't get hit ... I pulled the air cleaner off and looked down the carb and I didn't see any fuel shoot from the nozzles. It idles between 5-700 RPMs and runs at the middle to high range on the temperature gauge, but it doesn't reach the red.

The next time it acts up, open the hood and grab the fuel line near the carb. If you can't hold it the fuel probably is vapor locking ( boiling ). If the fuel line from the fuel pump to the carb is steel ( factory ) replace it with a stainless steel braded rubber fuel line ( AN-6 or 8 ) and route the flexable hose further away from the engine to minimize heat absorbtion. The stainless steel hoses aren't cheap but the braded stainless steel reflects most of the engine heat, the rubber hose within can withstand 100 PSI fuel, further insulates from heat and they look real cool.

The lines are steel, and run right on top of the engine block pretty much.

Again thanks all for your replies, I'm hesitant to drive in the heat right now because it's died on my numerous times and towing it back to the house isn't cheap not to mention embarrassing...

 

[Fordum]

Actually I'm trying to figure out where you get a fuel pimp......

 

[dyeager535]

If I had to guess and had no time for diagnosis, I'd try a fuel pump.

If I had time for further diagnosis, next time it dies I'd check the fuel line between the carb and pump for fuel, and to see if it was moving fuel as the engine cranked.

No fuel from the accelerator pump means either the accelerator pump is dead (highly unlikely with your issue) or that the float bowl is empty, which is either a stuck needle/seat/float, or the fuel pump isn't working right.

From my one experience with a failed mechanical fuel pump, I ran out of fuel a couple of times as a symptom, but it started back up after a couple of seconds. Later that same day it died and until I replaced the pump, it wouldn't start again.