Not sure what's causing it but here's the specs on the car:

1967 Coronet, smog 440, Summit 6401 cam w/matching springs (installed at 106), Holley SD intake, Edelbrock 800 Thunder Series (AVS style), Carter mechanical fuel pump, 5/16 line from tank, 3/8 rubber line from pump to carb, generic see through filter. I checked the fuel pump push rod last fall when I installed the pump and it was in spec. Doesn't run hot, needle comes to just under the temperature bar on the factory gauge (175 F t-stat). Use 91 octane supposedly non ethanol

Symptoms:

When I first start the car cold, the fuel filter fills up like it should. After driving it around for a bit (operating temp) the fuel filter is almost empty. You can see fuel entering the filter but it's very erratic, not a steady pumping like I though it would, just kind of dribbling in erratically. It doesn't seem to cause it issues running for the most part, I can go through the gears with my foot to the floor and it doesn't cut out, until today. I took it to work and on the way home was in some heavier traffic, didn't overheat by any means, but was hotter than if I was on the highway. Got on a side street and pinned it and after about 3500 rpm or so it started breaking up.
Got onto the highway and the temp came back down to normal and had another chance to
romp on it and it was back to normal, went through all the gears fine. I pull up home after a leisurely 15 min drive down the highway at 65 mph and quickly jump out and check the filter and it's full of fuel. I go in the garage (leave the car idling) and open the overhead door , come back and check (maybe a minute) and the filters now empty and gas is erratically spurting into the filter.

Do I have a pump problem or a fuel percolating problem?

Might be the sock in the tank plugged up

Is your filter mounted sideways or upright? If it is sideways, air will get trapped in high spots until pushed out.

It's mounted sideways so that could be an issue.

I thought about a plugged sock in the fuel tank but it seems to be heat related, so I'm not sure

Take the sock off the sending unit and throw it away. Check back w/ us again after you've done that.

FWIW...I've run filters sideways and never had an issue. You have a delivery problem somewhere in the system.

Sounds like that pump is getting hot and its
vapor locking when it spurts

Would a filter/vapor separator like they used on the Hemi/440 HP cars help?

you have 5/16 line to pump, then 3/8 to carb. that is backwards.
the pump is starving. need to run 3/8 from tank to pump. the pump cannot fill 3/8 line. any pump always has larger inlet and smaller outlet. keeps cavitation from developing. also, remove the sock off inlet. if you do replace tank outlet,cut end at a 45 degree angle. it will help with flow.

I agree that the lines are backwards and I'll likely do a 3/8 at some point but I'm pretty sure it's not the problem, a 5/16 line is starving a fuel pump at idle but not when I'm flooring it? We're talking a 350 hp 440 at best, and plenty of guys running much more HP than I am with a 5/16 line. More I think about it the more I blame it on heat. Went out this morning and cranked it over (didn't start it) and it immediately filled the filter with fuel. Maybe a heat shield between the headers and pump

I've seen several NHRA record holding Stockers back before NHRA outlaw the plastic inline fuel filters have the same issues your seeing when warmed up, they held records At idle the needles and seats are almost closed, at WOT there not

Since you have a clear filter watch which way the
fuel goes when you shut down... if it goes back to
the pump the pump has a bad valves in it.. the outlet
valve shouldnt allow it back in unless its slightly open
.. if its going forward its going into the carb.. that
happens a lot on heat soak (cooling down).. the fuel boils
in the carb and drips into the intake
EDIT
A gauge on it would help you

First a story... I run a '78 brand F PU with original spec engine. 180 net HP on a good day. I replaced the 35 YO tank pickup with what was available... 5/16" vs 3/8" OE. After that it would lay down at heavy throttle on a hot day. A fuel pressure gauge confirmed near zero fuel pressure at WOT when hot. Fine at idle or light load. The short length of 5/16" on the pickup caused restriction and vapor lock with today's boil prone EFI gas. I fixed it with an electric pump... on the rear frame rail where it is cool.

I think you need to upgrade to 3/8" feed from tank to pump or add an electric pump. Or both. The bypass type filter could help also with the 3/8" system.

I run a Mr Gasket mini pump on the Ford. A Carter would be better in your situation I think with a warmer engine.

Hmmm, maybe it is time to add a 3/8 line.....

I'd like to keep the 5/16 line as a return if I used a vapor separator type filter (or if I ever go to an electric pump and use a regulator). Only thing is it appears that the factory tank doesn't have a spot for a return like later Mopars. If I got a 3/8 sender, what would be the best way of plumbing the 5/16 back in? Drill a hole in the sending unit and weld/brazen it in?

I think the sending units all come with the return line elbow anymore. I'm in the process of installin one on the Challenger.

set a big can of gas on your pass side front floor & with a neoprene fuel line, run it forward up to the pump inlet & see if it still wants to act up. this'll take everything from the pump rearward out of the picture so you don't have to waste time where it ain't needed. As said, a restriction somewhere or pump issue. We ran a parts house "airtex" mech pump (class rules) on the 360 stock car & NO delivery issues at WOT at ~6200 RPM at the end of the straights & this was with 5/16 lines (fuel cell so no sock/upward bend in line to deal with tho). EDIT or something hot (exhaust) is too close to some part of the fuel system. MORE EDIT I'm gonna go out on a limb & say bad pump IF it ain't the sock

Those clear filters have caused more pain than any other filter made. I would mount a temporary fuel pressure gauge on the outside of your windshield and go drive the car before I did any changes. I am betting it is not a fuel pressure problem. You will have less pressure during the hot weather which is normal.

A not full fuel filter is not a problem to worry about at idle.

I used my moroso tire pressure gauge to check mine a few years ago after I took off my cowl mounted gauges.

And yes 5/16" discharge line is all you will ever need coming out of the fuel pump. 3/8" is silly and bad (because now the fuel pump has to pump a heavier load up hill) and adds more heat to the fuel because it is moving slower. I could power a 1000 HP motor with a 5/16 fuel line from the pump to your carbs. BUT the only fuel temp that really matters is the inlet side, not the discharge. Vapor lock occurs before the fuel pump, not up by the carbs.

I unscrewed my tire gauge from the hose/air chuck(it is the perfect gauge 0-15psi and it has been calibrated many times at NHRA races) and added my own hose and teed it in right at the carb and drove around for a few days. Use a few hose clamps and a brass tee.

My fuel pressure with a mechanical pump and a hemi fuel filter/vapor separator with fuel returning to my tank is 8-9 pounds at idle and 6-7 under light load. I have 2-3 pounds at wide open throttle and the car never stops pulling even with just 2 pounds. I have more pressure during the cool weather and less during the hot weather. I have driven it out at bonnelville salt flats wide open for miles at a time and my car never quit pulling, that is a real fuel delivery test and it was 96° one year. I have run my car 3 different years out there beating the crap out of it and it never ran out out of fuel with a stock mechanical pump.

And I have a eddie carb and it has been just fine with 9 pounds of pressure since 1986 when I got the carb. So I don't listen to the folks who say 9psi is too high because it ain't on my setup.

For 20 years I had cowl mounted gauges to look at, took them off so the cops look at me less now and didn't like the racy look on the street anymore.


I mounted them on a aluminum painted plate and used steel brake line to pass though the hood so not to mar the hood and sure was glad I did it that way now that I don't use the gauges no more and my hood is undamaged.

Thx for all the answers guys. I'm going to order a 3/8 tank to pump line and use the stock 5/16 as a return. I'll put on some factory fuel lines from the pump to carb (pretty sure they're 5/16) along with a pressure gauge and see what happens. I'll pull the pump as wells and take it apart to see if there's any problems.
Only thing that hurts is the reproduction factory style filter/separator is $60, and I've read guys have had problems in the past with them. Challenger1, what kind are you using? A factory/reproduction or an aftermarket?

Factory reproduction on both of my cars with a #56 restriction in the return line.

Why go to all that trouble before doing any diagnosis?

Don't use a hi po mechanical pump with 5/16" suction line, use a reg mechanical pump and you will be fine for years to come.

If you are going to replace the 5/16 suction line, then order the correct 1/4" return line while your at it. Because 5/16" will be too big for the factory return since it is 1/4" at the hemi VS and 1/4" back at the fuel tank.

I ran my car with 5/16" suction line the first time at bonneville with no problems and with a non return system.

Then I installed a hemi VS and new fuel lines a few years ago. Got the lines from Fine lines and they matched the old right on. I am sitting here trying to remember why I did that, I think to match my big block car that has the same setup. Oh yea there was some surface rust on my original 5/16 line and I replaced them before they leaked or sucked air.
I did replace my 5/16 with 3/8" fuel line for my sb car.
I bought 2 lines, 3/8 suction and 1/4" return and painted the vent line. Wanted no problems when I take the car out of town to drive it on vacations.


Bragging...here are fuel pumps I am used too running.

My moroso tire gauge next to a really good tire pressure gauge.

Electric wire is still there when I ran electric fuel pump on this car. Factory mechanical is way better because of the difficulty of feeding a electric pump out of a stock fuel tank. You can't reliably.

All factory original and works best even the vent system.

I have a stock low-po style pump I'll put back on and a vacum/pressure gauge I can use to check pressure. Did you just put a T at the inlet of the carb?

edit: didn't see your last post. Thx for the pics. Only problem with a 67 Coronet is they didn't have any type of return system from the factory (I think) and certainly didn't have that nifty manifold set up like your Challenger