Mechanical Fuel Pump Question/Problem

Hi All,

I have a 1971 Charger with a mild 440 (about 400 hp). It has a Carter Street/Strip mechanical pump and a Holley 750 DP.

It started running poorly and I noticed it had 3 psi fuel pressure. I blew out the line to the tank with air and made sure the fuel filter was not plugged, fired it back up and had 9 psi.

Then I took a 0.67 mile, 25 mph trip around the block and had 7.5 psi when I got back.

Then I took a 2 mile, up to 55 mph trip around the bigger block, and when I got back I had 6 psi that dropped pretty quickly back down 3 psi as it idled, basically where I started. Not even enough pressure to fill the fuel bowls.

My first instinct was a pump problem, but now I am not so sure that is the case, but I am not sure what else it might be.

Any thoughts or ideas?

Thank you.

Greg

Big blocks use a push rod between the crankshaft & pump, the aftermarket push rods are infamous for wearing down & becoming too short.

I'd pull it & measure it.

Possible plugged tank vent? Another road test and loosen the gas cap, does pressure come back up?

Is this a new build or something that's been on the road for awhile?

Pressure isn't likely to come back if this were the case

i would look at tank vent/cab first. also make sure you don't have a fuel filter on the suction side of the pump.

If the PSI varies, can't believe it's the pump pushrod.
Might even be someone's practical joke: a rag in the tank, for example, will get sucked to the pickup tube.
I literally had a "buddy" pull that one on me about 30 years ago.
Car would run great for awhile - mile or two - than starve for fuel. Then restart, and another cycle...

Venting problem can do the same thing.

It could if the demand were low enough as with the shorter strokes it could produce the pressure until the demand ( flow) increases. I'd checkk the rod, if in spec, replace the pump

Tackle the easiest and most obvious first, the cap removal is a quick test.

Sounds like all the measurements are at idle, parked. Not in use going down the road?

Might have a tank full of crap choking off the suction sock.

From the OP's 1st post

So IMO, possibly a rod or pump

Line or tank has crud in it

i would certainly check the tank.
i had a newyorker that acted that way. it would run great for a distance, then struggle, then quit. blow back through the line to the tank, and it would act the same.
although i didn't do a pressure check, the symptoms were the same as the OP's.
crud in the tank was the cause, choking the pickup sock. blowing back through the line opened the sock enough to allow the car to travel a few miles, then quit.
i'm not saying there could be other issues, but tank crud could certainly be one.
just my experience with this type of problem. your "mileage" will vary.

I bought a duster years ago that had an issues with the tank/ sending unit line. It would literally stop up, and choke the car out from running. if you blew down in the tank, or from the fuel line back with an air hose it would run again until the clog showed back up. To see if that is your problem, remove the fuel line going to the fuel pump, and put it inside a bottle or another fuel can to see if that eliminates your issue.
Matt

Assuming it's even still there after blowing air back thru and into the tank ?

Wow, thanks for the responses everyone, I am going to work on it tonight some and will report back.

I actually forgot that I did run it straight out of a bottle of gas, and the pressure was still low, which is when I knee-jerk ordered a new pump, but I am going to try that again tonight to make sure I didn't mess something up (been known to happen...), along with loosening the gas cap.

Greg

Okay, getting a little more confused.

It appears the vent line is completely plugged, I could not blow air from the three-nipple breather back to the tank either by mouth or with my air compressor.

So I figured that was it and took a drive with the gas cap completely off, and while it seemed like it was better, it still did basically the same thing (psi drops with distance/time) although maybe not quite as badly.

So maybe it's two things wrong? Plugged vent line AND the pump is bad, maybe from trying to pull against the vacuum too long?

Or I guess it could still be the inlet sock, or the pushrod.

It also seems like it has a miss, I am not sure if that's fuel pressure related or not.

Any other advice? I am going to let it cool off and then try to run it off a gas can again.

UPDATE: I ran it off the gas can, it started out at about 3 psi and climbed to where it was hovering fairly steady around 6.75 psi at idle. At one point I shut it off and restarted it and it was down to 6 psi. I made sure the hose was not suctioned to the bottom of the can. It seems inconsistent and possibly low for a properly functioning pump (a Carter Street/Strip pump) doing nothing more than drawing from a gas can two feet from the pump.

Also, to add to the fun, cylinder #3 is apparently not firing/dead, that's the misfire. Hopefully it's just the plug and not something bigger.

My love/hate relationship with this car is careening into hate territory pretty quickly...

Thanks.

Greg

The gas can pretty much confirms Pump or pushrod providing the hose to the can was tight at the pump inlet. Could be a crapped up tank fouled the pump but could also be an old pump or ethanol related pump diaphragm issue. If your filter is clean doubt it's the tank . Pm me some pic's maybe I'll buy it

#3 being dead could be any number of things, from ignition to valvetrain, vacuum leak, valve spring, a rocker arm failure. get the fuel fixed and go from there

Update, I fixed the misfire issue, the gap on the plug was closed. It looked like damage, not something I did when I installed it, I think I would have noticed it sooner if that had been the case. Not sure what happened there.

With regard to the fuel pressure issue, I installed a new Holley pump, blew out the vent line (which I think was plugged), and blew out the feed line to the pump, fired it up and had 4 psi fuel pressure. But the bowls were full, so I decided to just take it for a drive and it ran like a beast. Better than it has in a long time.

I am seriously wondering if part of the problem is the fuel pressue gauge? I wonder if it's not reading right, possibly both low and erratic?. It doesn't seem like it should have run as well as it did with only 4 psi pressure. Actually, after my last run around the 2 mile block, it was showing just under 3 psi. But it drove great, and when I tried to restart it, it fired right up.

Anyway, I am calling it good for now, until I get data otherwise. My love/hate relationship with this car is inching back into the love zone, probably until the next time I drive it.

TJP- if you had been walking by my house on Tuesday and had a dollar with you, you'd probably already own it.

Thanks for all of the help everyone.

Greg

The pressure will drop when hot. If you have a liquid gauge it can even read 0 once it heats up.