84 720 dies randomly

[aaahotdog]

1984 720 dies randomly only at time of when engine is just starting to warm up and thermostat is nearing opening.  Basically the truck at this point is starving for fuel.  All of the gas is out of carb.  I can turn the key and hear the fuel pump running, it has new fuel pump and filters.  All I have to do to restart is just put a squirt of gas down the carb thoat, it starts right up and drives fine for rest of the night.  Other thing noticed is sometimes when it is starting to struggle at this temp, I can rev it up and it starts running just fine again.  Every time it does die, the choke is open like it should be.  Suspected a vacuum leak, replaced all vacuum lines.  A vacuum gauge does show the vacuum is a little unsteady.  But the vacuum is correct.  Since it only happens at the same temp each time (randomly, not every time) I'm guessing something temp related, but not sure what.  It is always around time when thermostat opens, once pass this point, engine purrs.

 

Any ideas??

[datzenmike]

You say you can hear the pump running but the carburetor is empty?? You're looking at the round glass on the front of the carburetor and it's not showing any gas?? Well that's a problem.

 

When it stalls next time, pull the fuel line off the carburetor and direct into a suitable container and turn the key on. Does gas come out?

 

 

[aaahotdog]

Yes glass is empty.  I haven't pulled the fuel line off of carb, but I suspected the fuel pump initially, and replaced it.  New pump and filter do the same as old.  Isn't there a solenoid on carb to stop fuel flow?  Again, seems something related to engine warmup.  Not sure what.

[datzenmike]

On the passenger side, above the kick panel just forward of the door is a fuel pump 'shut off' relay, it's almost out of sight. It needs several inputs to keep the pump working and is problematic.  Rather than piss around you can remove it from operation to see if this fixes the problem.

 

 

This little bugger..

 

 

Unplug and make a short jumper wire and connect the Blue (L) terminal to the White/Black (WB). All the jumper does is eliminate the relay and allow the pump to work. If the truck starts working normally again then the relay may be at fault. This is not a fix just a diagnostic test.

 

 

[aaahotdog]

Yup.  I am aware of the relay.  That is not the issue though as the fuel pump is running.   I had that fail once.  Totally different symptoms.  At that point, fuel pump had no power.  Fuel pump has power, no fuel in the carb.  I just put a teaspoon of gas in carb, truck starts right up and fuel pump fills carb immediately.  Runs fine past that point.  Doesn't make sense because a sticky float carb would remain starved.  One guy thinks it might be purge valve/charcoal canister as he suspects opens or closes as the engine approaches operation temp.  Truck starts easily though.   I am thinking about bypassing the purge valve just to test his theory, what do you think?  

[datzenmike]

 

 

 

 

 

Pump on but no gas in carburetor. Has to be one of the following....

 

1/ Pull the fuel hose off the carb and aim into a suitable container and turn the ignition on. Got Gas?????

 

Yes... float sticking closed inside the carb.

No ... blockage between pump and the carb?

 

 

2/ Pull outlet hose off the electric pump and aim into suitable container. Got gas????

 

Yes... line is definitely blocked between pump and carburetor.

No.... Pump sounds like it's working but is not delivering gas. Got to 3/

 

3/ Have you checked the fuel filter between tank and electric pump? You said you replaced the electric pump but have no idea what pump you got. The stock one has a twist off bottom with another filter inside.