stalling engine

[fiveseven]

I have a 390 in my truck. Im trying to figure out why my truck stalls at hywy speed around 55.I can usually restart after a few.   I replaced most of the normal things like pump, filter, rerouted fuel line, I replaced the fuel cap with a vented one and it seems like it fixed it for awhile.Then last week it stalled twice. No Im really baffled.Does anyone have any ideals.

[cjshaker]

How does it quit? Like someone flipped a switch, or does it lose power first? Any popping, backfiring or bucking? How long is "a few", 5 minutes, 10 minutes, a half hour? Does it have spark after it quits? Those might help narrow it down.

[1964Fastback]

People have mentioned a shop rag in the tank getting sucked into the outlet randomly.  Any way you can see in there with a cheap HF borescope?  Probably easier to see with the fuel level low.

Pat

[fiveseven]

The motor just cuts out no back fire and stepping on gas pedal dosnt do any thing. I have to coast to stop then wait for around 5-10 minutes  to restart. I know this isnt vapor lock cause it only happens at around 50 mph. The tank is clean and line is clear.

[gregaba]

I would drop the fuel tank and check the sock on the pick up tube.
I don't know how old your truck is but I have had them plug up on me before. Sometimes they will do what yours is doing if they are partialy pluged.
Greg

[runthatjunk]

Well, if you're running a carb it's as simple as Fuel, air and spark.  I would imagine the air is fine so are you losing gas or spark when this occurs?  Spark could definitely go away all at once, a carb usually will give just a bit of sign as the gas gets to the bottom of the float bowl, but not necessarily.  You'll have to investigate when it happens while everything is hot, Pack some gloves!

[WerbyFord]

Could be an ignition short too.
I've had a tach short out & kill the spark. If you have a tach try disconnecting it from the coil.

Also check the wiring to/from the coil for any old, brittle wiring with cracks that might cause intermittent short (BTDT).
As we all know, Lucas wiring gives trouble after 10 years, Chevy after 30 years, but even FoMoCo wiring gets old after 50-60 years.

Coil or condenser could give out when hot but as noted those tend to give warning, as does low fuel.

If it feels like you just shut the key off, that may well be what happened.

[WConley]

Could be an ignition short too.
I've had a tach short out & kill the spark. If you have a tach try disconnecting it from the coil.

Also check the wiring to/from the coil for any old, brittle wiring with cracks that might cause intermittent short (BTDT).
As we all know, Lucas wiring gives trouble after 10 years, Chevy after 30 years, but even FoMoCo wiring gets old after 50-60 years.

Coil or condenser could give out when hot but as noted those tend to give warning, as does low fuel.

If it feels like you just shut the key off, that may well be what happened.

+ 1.  Sounds electrical.  If it was fuel you'd get bogging and sputtering, plus a long protracted re-start as the fuel bowl filled up.

[427John]

I agree with the last 2 posts,depending on what ignition you have it could be a coil that gets hot after a while and and dies then after it cools down a little it starts working again.If you have a Duraspark ignition some of the earlier modules would also do the same thing,die all of a sudden and then after cooling for a while start working again.

[cjshaker]

I've had pick-ups on MSD distributors act like that, and also coils. If you have a points ignition, even a condenser can do it. I agree it sounds electrical. A short typically won't suddenly work after a cool off period, so I'd look towards something electronic, like the above.

[fiveseven]

The holley 750 is new and tuned good,new  filter, pump,and hei dist, , tank is clean and line is clear, I was having this problem last year someone told me to replace the fuel cap with a vented one and boom it seem to work. didnt have the problem for 6 months. Then last week it happen twice.. I notice that the cap when taking it off it is under alot of pressure. Is this normal.

[jayb]

Is it pressure building in the tank, or vacuum?  Either way, it's not normal and its possible that the vents in your gas cap are clogged.

[1964Fastback]

What Jay said.  Look at the vented gas cap closely.  Is it possible to modify it a bit, like removing or shortening the spring, or maybe removing the gasket?  Worst case, you could drill a 1/16" or 1/8" hole clear through it.

Pat

[My427stang]

Whether it is pressure or vacuum, neither should be there with a vented cap.  What year truck and how does the vent work?  I am with the others, if you are hearing a "whoosh" when you open in, something is plugged.

67-70 should be vented cap, late in the 71-72 they went with charcoal canisters under hood all the way (I think) through 76.  Either way, you have to replace the fuel you suck out with air somehow

[machoneman]

On some vented caps, the gasket actually blocks the vent holes on certain fill pipes. Duct tape the filler completely, poke a 1/8"- 1/4" hole in the tape and drive it. Then you'll know...........