Vapor lock?

58 dodge pickup with a stock tank, 413, mechanical fuel pump. Ran great, had a problem with the fuel tank pressuring when hot and leaking gas out the pickup fitting. Vented the gas cap to resolve it, and now it runs fine when cold, but as soon as it warms up and is driven, the fuel filter runs bone dry. Checked for vacuum leaks on the line from tank to pump, found no loose fittings. Any ideas before I throw in an electric fuel pump?

A vented cap is designed to vent vacuum, how did you vent yours?

When it runs dry open the cap and listen for the sound of vacuum venting.

If that is the problem and electric pump won't fix it.

run a can of gas up front in the engine bay or front pass floorboard to the pump inlet with a 5/16 neoprene hose & take it back it back out. this'll tell you if the prob is rearward from the pump or if it is the pump or pushrod. EDIT What Supercuda said, check that out first (the venting). MORE EDIT leave the cap loose or even off (wont hurt you for a quick test) & (re)try it

To vent the cap, we cut two 1/4" notches out of the gasket. Didn't have this problem before we vented it this way. If the pump pushrod was bad, would it work intermittently? It seems to do OK until warmed up, then runs the filter dry, and won't pick it back up even after priming the carb.

venting the cap would not cause this. this is a sepperate issue happening at the same time (Mr Murphys handiwork) unless we are missing something in the repairs that has tied these 2 issues together. loosen the cap or remove it & take it out for a drive like before & see what happens. if it is real dusty you could duct tape a piece of porous cloth over the neck but unless it is a dust storm (just me) I would not sweat that. Then if it (still) acts up, plumb a gas can to pump inlet. Correct, a pushrod issue would not be intermittent. that is strange symptoms tho, that it wont pick it back up even after priming the carb (& thats priming the carb more than once I assume). I feel like it is just with reach but I cannot grasp it (but we will). EDIT even with the notches added I think the cap ain't venting. my other choice is bad pump, other possibilities are a clogged sock

It could also be the weather changing... getting hotter. My '47 PW has a running temp limit of about 85F with all stock style fuel system stock engine. Above that and the engine compartment including the lines that feed the fuel pump get hot enough after running for a while to vapor lock and she shuts down.

I fixed it with a booster electric pump on the rear frame rail. It operates on a toggle. When she starts stumbling, throw the switch and all is well. Also good for starts. I don't have to crank forever after she sits and I don't have to hose down the fuel lines and fuel pump with cool water for hot re-start.

Thanks Ashy, it may come to a booster pump. How did you plumb that?

I fought vapor lock for well over a year on my Charger. Same thing, when it was cool, it would run all day but let it sit and heat soak or take it out on a hot day and it would run dry and wouldn't restart until it was cooled off.

I refused to go the electric pump route and deal with some of the hassles that can come along with that. I added the factory vapor separator canister with a hemi reproduction mech fuel pump.

I have run this car on freezing cold all the way up to one day at 103* in slow traffic with A/C running for 30 minutes and it didn't die on me.

New pump was $50 (my choice - wanted to replace) and the vapor separator was $50 or so. I got the SS pump lines for $30 and called it a day.

I still feel a little power loss when the motor gets really hot like idling after a while on a HOT day but that's from the carb getting hot and fuel boiling a bit in the bowls. I added a heat shield a spacer to the carb which helps a bit on that. Electric pump won't help there though.

It ran fine unvented, damn mr. Murphy! The tank was boiled cleaned and installed less than a year ago, fuel line is 3/8 but has a few elbows that checked out to be tight. I think we may have to drop the tank, but fuel filter is not plugged and I don't think there is a sock on the pickup tube. Not sure why it would start vapor locking all of a sudden. Thinking about pulling the lines and wrapping them, also the exhaust where it runs next to the frame that is boxed and has the fuel line running through it.

what about putting a non vented cap back on (like it was before) & see if it straightens out! if running E10 could it be acting up with the hotter weather as Ahy said? emptying the tank (completely) & running a tank of non ethanol gas would answer that real quick & save you from pulling the sender to check the sock & if that is it then as you mentioned, insulating the lines

Only problem with returning it like it was is that with a full tank it would pressurize and leak..... is it normal in a vent line to have a check valve?

No check valve. To clarify I was thinking of just a 1 day return to a non vented cap to try & pin this down

I just went through this, my thread is probably on page 8 or something by now.

I had similar symptoms. Holley mechanical pump on a 440/493. The car ran great when cool and warm, but once it was fully up to operating temperature, the engine would sputter at WOT and the filter was empty with little spurts bubbling in. I changed to a Carter pump and plumbed in a fuel regulator. The regulator has a bypass that returns gas to the tank at anything over 6.5 psi. This worked for me. The filter stays full now.

During my efforts, I took a cheap Harbor Freight thermal infra-red gun to check temperatures. The fuel line from the regulator to the filter is now 18-20 degrees cooler than before. Previously, the fuel line ran from the pump to the filter.

I found it !

https://board.moparts.org/ubbthreads/ubbt...tml#Post2065508

Thanks, I'll check it out!

Might do that, if it does fix it, how do I fix the fuel tank pressure problem? Seems weird that it pressurized and leaked

Here is the electric pump setup on the PW. I used a brass check valve... automotive versions are available from Summit and others. I used a small diaphragm style electric pump I had laying around.