Fuel Starvation / Engine dies

[6wt]

C10, 1984, 350 crate engine, basic eldebrock carb / electric choke, 700r4 trans, clear fuel filter in-line just before carb & after fuel pump.  Engine and all accessories replaced updated back in April 2015.  Fuel lines are 3/8" and new.  Fuel pump is new.  Filter is new.  Radiator is new.

Problem:  Engine starts and runs fine... until running on freeway for approx. 1hr, then stop/start city traffic.  At that point, the engine starts stalling; ie sounds as if bogging down.  Engine may die.  Engine temp is normal at 185-190.  Look under hood.  Fuel filter is empty or almost empty with small splashes of fuel coming into the filter visible.  Temp outside is 85-90degrees with 70-80percent humidity.

Let the truck sit for couple of hours, then starts and runs fine again.  Visually check fuel filter and it is full of fuel.  Check again next morning on startup; everything looks fine and truck runs good.

The gas cap is relatively new from Jan 2015.  I removed pollution control devices back in April, but left the charcoal canister at the radiator and vent hoses to/from the fuel pump intact.

Any ideas?  Any help would be greatly appreciated.

Thanks!

[zieg85]

I would check for a kink in the fuel line around the gas tank connections.

[bd]

Fuel level in the filter is not a reliable indicator for diagnostic purposes.  It's worth noting, but cannot be relied upon to indicate a problem.

In addition to Zieg's recommendation, loosen or remove the fuel cap when the symptoms occur - see if there is a "whooosh" sound from pressure balancing and whether the symptoms disappear.  Does the fuel pump have a third (1/4") hose that returns to the tank?  Are you sure that hose is not restricted?

You may need to plumb in a vacuum gauge between the tank and fuel pump, and a pressure gauge between the pump and the carburetor.  A mechanical fuel pump should develop >20" Hg vacuum and 5-7 PSI pressure, while cranking with its hoses disconnected from the vehicle and connected to a gauge.  In service (hoses connected to the vehicle), there should be virtually zero vacuum and 5-7 PSI pressure developed.  The pump should deliver >1 quart of fuel at zero pressure in ~20 seconds of cranking.

[Gwtt]

if fuel pressure is good and the tank isn't under a vacuum, the lockup converter maybe sticking causing it to stall coming to a stop. sometime the engine has enough power to overcome the converter clutch but you will also damage it. If it originally came with a 700R i would assume it has a relay for the clutch and that could be shorted to power (bad relay)

[6wt]

Thank you for the vacuum / pressure description.  I removed the gas cap yesterday after the symptom developed again; no whoosh and no change in the amount of fuel in the filter.

As I get my vacuum / pressure test set up.  May I get a brief description of the function of the 3rd hose fitting on the fuel pump?  Thanks if my understanding is correct, it is some type of fume recovery system?

What happens if there is a leak or loose connection in the "3rd hose circuit?"

Thanks!

[bd]

The "3^rd hose" is a 1/4" return line to the fuel tank.  The return line allows continuous loop fuel circulation through the 3/8" supply line to help cool the line and the pump and minimize vapor lock.  The 1/4" return barb on the pump is manufactured with a restricted orifice to manage return flow.  Return lines are not used on all vehicles, but are worthwhile in hot climates or applications in which the engine idles excessively or the vehicle hauls heavy loads that result in above normal heat production with decreased air circulation. 

If the return line is plugged, continuous loop fuel circulation stops, and the vehicle becomes more susceptible to vapor lock (a good candidate for the muggy 90° degree weather you reported).  If the return line leaks there will be evidence of liquid fuel around, below and rearward of the leak.

Single or dual fuel tanks?

[Captain Swampy]

Sounds like vapor lock. Your fuel lines are to close to the exhaust, engine, or transmission. Your turning the liquid gas to vapor, and the pump only pumps liquid. After several hours everything cools down and works again.

[6wt]

Back with another update.

Closer inspection yielded two hose fittings that were not completely tight, and three suspect hoses (a 1/4", 5/16", and 3/8") on the frame under the truck where the lines make a right turn to go to the fuel tank; replaced the hoses and hose clamps.  Also, replaced the fuel pump.  Now time for another road test.

Why didn't I just purchase a pressure/vacuum gauge and test?  Well, I am currently located 25 miles from the closest town and away from my shop... only a basic set of tools and no shop    Actually making repairs in parking lots with a parts store next door... ugh.

What I have determined is the problem develops after approx. 30 minutes of highway driving then stop/go in city traffic; ie. enough heat is being generated to make the problem materialize.  Then, a vicious cycle of stop, make a fix, start up, looks great, until start driving to bring the temp back up to make the problem appear again.  So, the prevailing thought is there is a small micro leak/crack in the fuel circuit that is closed until temps rises enough to create expansion for it to open.

Replaced the fuel pump and made it 30 miles home.  Now for a road test today to see what happens.  Best of luck to me.

[Irish_Alley]

update?