Fuel pressure problems

LNicholson

LNicholson

New member
Joined
Jan 10, 2009
Messages
19
It's my first post here; I have been hanging out and reading for awhile.

Symptom - Low fuel pressure (5 pounds at idle, less while engine is under load), 12 psi with pump running and engine off.

Repair attempts:
-Yet another new lift pump on frame rail near tank. (Original lift pump for 97,000 miles, Carter 4601HP for over 100K miles, Holley Black for one year)

-Backflushed all diesel lines from injection pump to the fuel tank to ensure they were not plugged.

-New fuel filters. 100 micron before lift pump, and stock fuel filter in canister.

-New overflow valve on VP44.

I had a Holley Black, when it developed this problem (along with a leak from the weep hole) I bought a 35 gph Airtex to get me by and serve as a "toolbox spare" and ordered a 160 gph Aeromotive (it has not arrived yet).

The Airtex is currently installed. All of the factory lift pump wiring is bypassed and I run the fuel pump from a dash switch/fuse/relay setup wired directly into the batteries.

The fuel gauge is the digital gauge in the Attitude monitor. The pressure sender is located between the factory fuel filter and the injection pump. I have a hard time believing the sender is the problem, because with just the lift pump running (not the engine) it holds 12 pounds of pressure.

3/8 fuel line (that has worked perfectly for 140,000 miles) from the tank to the injection pump. Tapped the factory filter canister for 3/8 fittings both in and out, and found a metric adapter for the injection pump to take 3/8 fuel line. The only banjo fitting from the tank to the injection pump is the banjo fitting for the overflow valve.

I am out of ideas. All help will be appreciated.
 
Pick up a cheap mechanical gauge, check pressure with it. Electric sending units have been known to go wonky before.
 
Cheap mechanical gage installed, confirmed that I have 5 pounds at idle. The electrical sender checks out.
 
Ok, now we can get somewhere since the reading is known to be good.

First off, is the airtex pump internally regulated? I don't know what specific model you have, but from looking on their website, many of the pumps in that GPH range are limited to about 9PSI, matter of fact, the only one I see that is rated to supply more than that is made for diesel, and it's only rated at 10-14PSI. Reading that, the first thing I think of with all that you have already done is question the internal regulator of the pump, if it has one.

Secondly, have you verified that the pump is getting a full 12VDC? One bad connection can cause a voltage drop with corresponding lack of performance from the pump, while you are checking, take a good look at the ground connections as well, they are super important on DC.

If you were holding good pressure before now with this same pump, and now it's not, and you can verify all of the above, replace the pump and see what happens, you already have another one coming, take it easy on the truck and you should be OK. With a VP it's not really the pressure of the fuel so much as the volume that's really important. 7 PSI and the VP will live just fine, IF there is enough fuel being supplied.
 
Thanks for the replies Billysgoat,

I have 10 gauge wiring direct from the battery to the pump (through a fuse and relay) on both the power and the ground side. 14.2 volts at the pump.

The Airtex pump is new - but the Holley Black was replaced with the Airtex when it began to exhibit the same low pressure symptoms the Airtex now has. The Holly was also leaking, which is why I ordered the Aeromotive pump....before the Holley started to show low pressure.

In all honesty I am beginning to think about dropping the tank to take a look at the fuel module...and building a larger "draw straw" type of setup to go in the fuel module while I have it apart.
 
Possible fix

I pulled the tank today; the basket had come apart where the lower basket is screwed to the upper basket through some sheetmetal. The screws had vibrated right through the sheetmetal. The bottom basket was setting crooked & I suspect the "bottom float" that lets fuel into the bottom of the float basket was hanging up so fuel couldn't flow into the basket properly.

I gutted it out the basket & installed a homebuilt .5 inch drawstraw. I made the straw from 1/2 inch soft copper line and went through the top of the module using a unused plug in the top of it. The copper is connected to the fuel outlet using a compression fitting. I flared the bottom of the drawstraw and ground some notches to allow fuel to be sucked into the straw. The straw is now just under 1/8 inch from the bottom of the tank, centered in the fuel module. Repaired the fuel basket with some screws and washers.

The Aeromotive pump arrived Wednesday, it had the same problem with just a bit more pressure than the Airtex had.

No gunk in the tank at all, surprising for over 230,000 miles. The tank is going back into the truck tomorrow morning, we will see how it works....fingers crossed!:pop:
 
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Problem solved, pulls the tank down to empty without sucking air. A solid 16 pounds on the highway, will pull down to 11 pounds on "kill level 6" on the Juice.
 
I am banging my head now, had the exact same problem with a buddies truck and I forgot all about the pickup :bang :bang
 
Billysgoat said:
First off, is the airtex pump internally regulated? I don't know what specific model you have, but from looking on their website, many of the pumps in that GPH range are limited to about 9PSI, matter of fact, the only one I see that is rated to supply more than that is made for diesel, and it's only rated at 10-14PSI.
Click to expand...

Airtex makes a pump that is a stock replacement for the OEM Carter LP. I don't remember the model number, I'd have to look at the box, but it is internally regulated and holds about 17 psi at idle and normal cruise on my truck. Sometimes it gets closer to 20. However it is only good for maybe another 100-150 hp over stock. I bought mine at Carquest 2 years ago. I don't know if they still sell them or not. Uses the stock location and fittings.

Just FYI.
 
My mistake, the Aeromotive fuel pump is 140 gph @ 14 pounds. Red pump, 3/8 NPT. Aeromotive Pump

Sorry I did not take photos. I was so focused on fixing the problem that I did not even consider the camera. Oddly enough, after looking at multiple threads on "draw straw" installations, my fuel module was different from all of the modules in the draw straw threads.

One quick tip for those of you considering cooking up your own draw straw; use a chunk of clay on the bottom of your draw straw to check the distance from the bottom of the tank. In my reasoning, when the tank is empty the bottom of the plastic tank will be as high as it will ever be - when the tank is full and the bottom of the tank is bulging down, who cares, the draw straw will be well submerged.
 
Line drawing

The blue is the bottom of the fuel module, the purple outline is the factory pickup, which I removed. The green is the "bottom float" - removed also. The red is the bottom screen - removed too.

In the center of the fuel module is a rubber check valve, I removed it and reamed the hole to accept 1/2 tube. I also drilled 2 holes to allow returned fuel to drain onto the draw straw.

There are 4 legs molded into the fuel module that the bottom screen snaps over. I screwed the fuel module halves together so they would barely slide and firmly pushed the fuel module into the tank to set the module depth, then tightened the module halves. I then snaked the 1/2 copper into place, bent it so it would hook up to the compression fitting, measured carefully and cut it to length and flared the bottom.
 

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LNicholson said:
My mistake, the Aeromotive fuel pump is 140 gph @ 14 pounds. Red pump, 3/8 NPT. Aeromotive Pump

Sorry I did not take photos. I was so focused on fixing the problem that I did not even consider the camera. Oddly enough, after looking at multiple threads on "draw straw" installations, my fuel module was different from all of the modules in the draw straw threads.

One quick tip for those of you considering cooking up your own draw straw; use a chunk of clay on the bottom of your draw straw to check the distance from the bottom of the tank. In my reasoning, when the tank is empty the bottom of the plastic tank will be as high as it will ever be - when the tank is full and the bottom of the tank is bulging down, who cares, the draw straw will be well submerged.
Click to expand...


This is the exact same fuel pump I have on my truck. I havent had good luck with it not leaking. It has been back to aeromotive twice to be fixed. It started leaking yet again, but not as bad as the first 2 times. My buddy also had one one his dodge that started leaking. He was lucky. When he sent it back to aeromotive to be fixed they sent him an a1000 to replace it. I dont think these pumps like diesel to well. I just wanted to give you a heads up just in case yours starts leaking.
 
This is the exact same fuel pump I have on my truck. I havent had good luck with it not leaking. It has been back to aeromotive twice to be fixed. It started leaking yet again, but not as bad as the first 2 times. My buddy also had one one his dodge that started leaking. He was lucky. When he sent it back to aeromotive to be fixed they sent him an a1000 to replace it. I dont think these pumps like diesel to well. I just wanted to give you a heads up just in case yours starts leaking.
Click to expand...

Well that sucks :shake: .

I began to wonder when I called Aeromotive tech support after looking at their website and they told me a should have bought a A1000 and an external bypass regulator (to a tune of about $700). For that much money I could have bought and Airdog or FASS. They also mentioned that I might want to watch the pump this winter, because the operating temperatures in Nebraska are "outside of the pumps design". Grrrrrr.

I guess it is part of the price paid to try something different.

I would not think it would be that tough (or expensive) to put Viton seals in fuel pumps so they would be "universal fuel capable".
 
LNicholson said:
Well that sucks :shake: .

I began to wonder when I called Aeromotive tech support after looking at their website and they told me a should have bought a A1000 and an external bypass regulator (to a tune of about $700). For that much money I could have bought and Airdog or FASS. They also mentioned that I might want to watch the pump this winter, because the operating temperatures in Nebraska are "outside of the pumps design". Grrrrrr.

I guess it is part of the price paid to try something different.

I would not think it would be that tough (or expensive) to put Viton seals in fuel pumps so they would be "universal fuel capable".
Click to expand...

Now that you mention it. I think it was winter when mine started leaking. I know I didnt have very many miles on it.
 
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