Fuel Pump Slowly Dies

[Doorkicker]

BLUF:
Engine starts, runs and then begins to stumble, I troubleshoot and see 0 psi on the fuel pressure gage. Once I check the lines, etc. and start the engine, the pressure is 5+ psi and then slowly drops to 0 psi again in about 4-6 minutes of idle time. The engine stumbles, runs lean (I have an AFR meter in the car), and it idles rough, etc.

Background:
New engine I built, <500-ish miles on it, mechanical fuel pump (High Volume Mechanical Fuel Pump - Black). New gas tank, new tank pickup and sending unit, new fuel lines, new filters. Simple fuel system, no return. Car has run great for several months now (other than the transmission giving me fits... it's been great)

Troubleshooting: (after each step, put everything back together. Ran fine, good PSI, then slowly drops off to zero psi)
* - Confirmed, full fuel tank. Brand new gas... filled up just the other day.
* - Removed and cleaned fuel filter. Was fine. Cleaned anyway.
* - Used compressed air to blow back (from pump to tank) the fuel line. No obstructions.
* - Removed pump to check it and the push rod. Push rod looked great. Checked the cam lobe with a bore scope, looked great. Pump seemed to operate ok, but at first it was "sticky", but seemed ok.
* - Tested gage, worked fine.
* - Confirmed all fuel lines were ok and not leaking.
* - I checked engine oil, several times, level consistent, no gas in oil.

Conclusion... I think it's the pump. Perhaps there's a pinhole in it that is slowly filling the other side of the diaphragm?.. but I also think I'm wrong. Something doesn't seem right about it having good pressure and then dropping off slowly to zero. And it's brand new.

I very much welcome your insights!

Here's some pics... always nice to see what you're working on

 

[OKDart]

I would disconnect the supply hose on the fuel pump, hook up another hose and run it to a gas can. If the problem still exists, bypass that AN hose to the carb and check again. But if it goes away, then it's something before the pump, maybe a collapsing hose, debris, etc.

 

[A70Runner]

Fuel Vent plugged?

 

[Don Frelier]

Pickup sock?

 

[dadsbee]

"Push rod looks good"... did ya measure it??

 

[DanielSlocum]

High vol pump has a stiffer spring which can and does wear out the little pushrod that works the pump. A hardened fuel pump pushrod may be your solution.
Ive been there. Dont want to go back. Good luck.

 

[twecomm]

Yep... I'd check the fuel pump push rod length. Think it should be just shy of 3.25"

 

[Doorkicker]

I would disconnect the supply hose on the fuel pump, hook up another hose and run it to a gas can. If the problem still exists, bypass that AN hose to the carb and check again. But if it goes away, then it's something before the pump, maybe a collapsing hose, debris, etc.

That's the ticket! I'll do that today.

 

[Doorkicker]

Fuel Vent plugged?

I don't think so. I thought about that too... and that would actually (arguably) make the most sense because I would be effectively pulling a vacuum in the tank, hence the slow drop off. Great thinking. But, in this case, I checked it... and ran a test with the gas cap off. But, fir giggles, I'm going to check the vent... something tells me I may have not installed correctly when I put in the new tank.

 

[Doorkicker]

Pickup sock?

My only reason for not thinking a sock is it's all brand new... tank, pickup, sending unit... even the gas

I will assure you though... if I don't find the culprit, that MF'er is coming out;)

 

[Doorkicker]

"Push rod looks good"... did ya measure it??

Actually... no! I just based my inspection on wear pattern, which there was none since it was so new. BUT... that doesn't mean it's the correct length or it did wear and I didn't see it. Nice catch. I will measure it.

 

[Doorkicker]

High vol pump has a stiffer spring which can and does wear out the little pushrod that works the pump. A hardened fuel pump pushrod may be your solution.
Ive been there. Dont want to go back. Good luck.

Thanks.

You know, I just went and checked 440 source, where I got the pushrod to check if it was hardened... it actually doesn't say on the site Fuel Pump Pushrod - New

hmmm

 

[Don Frelier]

My only reason for not thinking a sock is it's all brand new... tank, pickup, sending unit... even the gas

I will assure you though... if I don't find the culprit, that MF'er is coming out;)

I had one in a Diplomat where bits of the sound deadener pad got through the rusted top of the tank.
I never filled my tanks back then (broke) so it never really reaked of leaking gas.
Eventually enough fibers covered the sock and the car would stall after 30 minutes, then 25, then 15 etc...
I changed the pump and no change, tried everything coil, distributer, ECU.
Finally hooked up a small lawn mower tank right by the carb and it ran great.
Pulled the tank and the sock looked fine at first, but then when I looked more closely it was covered in the fibers from the mat. I put a used tank and cleaned off the sock and everything was normal.
I cut the old tank in half and still use it as a mortar box.

 

[Rebelrouser]

I build a lot of engines for muscle cars, most owners want the same thing, engine to look stock, have a little lope at idle and a little more power, so stock looking mechanical pump gets used. I have had terrible trouble buying a good one. On the dyno they hold pressure good low rpm but around 3,000 rpm the output drops off to nothing. 340 on the dyno right now stock pump from local parts store at the end of a 9 second pull to 5,500 rpm pump was only holding .5 lbs of pressure. Tried a couple more from the parts store same thing. got an Edelbrock chrome high volume it holds 5 lbs steady. I have also had issues with excessive pressure with some stock replacement pumps, it just seems they are selling us junk.

 

[Doorkicker]

I build a lot of engines for muscle cars, most owners want the same thing, engine to look stock, have a little lope at idle and a little more power, so stock looking mechanical pump gets used. I have had terrible trouble buying a good one. On the dyno they hold pressure good low rpm but around 3,000 rpm the output drops off to nothing. 340 on the dyno right now stock pump from local parts store at the end of a 9 second pull to 5,500 rpm pump was only holding .5 lbs of pressure. Tried a couple more from the parts store same thing. got an Edelbrock chrome high volume it holds 5 lbs steady. I have also had issues with excessive pressure with some stock replacement pumps, it just seems they are selling us junk.

Frankly, that's why all my cars - except this one - have electric fuel pumps. I just don't trust'em. Odd right? Most folks hate electric pumps... but I avoid cheap ones. I think the "low buck" mechanical pump got me here, but we'll see.

 

[OKDart]

The owner of the Charger in this thread used an electric pump which caused his car to burn up:
Check your fuel lines
Just remember, you're changing what used to be a suction line to a pressure line. Just need to make sure the hoses are up to snuff, and inspect the metal lines for damage/corrosion.