Ok I have a wierd fuel pressure issue and am stumped.

My problem is my fuel pressure will drop from a solid 15 to just about anything below that but will come back. It may drop just a bit or it may drop down to 2psi. It generally will fall slowly over a period of time and may stay there for minutes or hours. Then next thing you know it will be back up to a solid 15psi. How far it drops is inconsistent. It has nothing to do with load on the engine. And for the sake of this discussion lets assume the pump is good as it has done it with 3 different pumps. I have a gauge plumbed post filter and I have added a temporary gauge prefilter. Mostly to determine whether my primary gauge was at fault but have left it on for now. Both gauges read approximately the same. Truck runs great even when it is showing very low pressure. So while it may have low pressure I am thinking it is getting plenty of flow. Starts fine everytime.

I had a RASP mechanical pump on it with the stock carter as a backup that worked great. I put in an aftermarket cam that has the fuel pump lobe on it. I have since converted to a cummins 12 valve style mechanical lift pump (have used 2 different pumps and it still does it). It did this with the rasp pump as well which is one of the reasons I changed the pump and to get rid of a whole bunch of plumbing.

I have no leaks that I can find. All lines appear to be in good shape. The only stock line left is the 5 foot long or so steel line along the frame rail from the tank to the engine bay. My in tank fuel module cannister had a torn screen in it and I thought maybe the fuel pickup was sucking up that torn screen flap causing a restriction so I replaced the module with a modified Liberty module from Vulcan performance with the drawstraw installed. Which did away with the Dorman style fittings on the top of tank as well. I also have the fuel pump regulated with a simple bypass type regulator set at 16psi that I used with the rasp setup and continue to use with the 12 valve mechanical pump conversion (the 12 valve pump will produce upwards of 40psi at idle). It will do this with the bypass installed or not so it is not the bypass not closing. I have also swapped out the checkvalve on the side of the vp44 and it made no difference. Also the rollover valve seems to be operating fine and to be sure I have left the fuel filler cap off to double check that the tank is venting.

So in a nutshell the gauges are reading right, the bypass is not it, the pump is not it, the fuel pickup is not it, it is not leaking, it is not the fuel filter or housing as it drops both pre and post filter. The tank is vented. My conclusions are one of two things. One there is an inconsistent restriction inside fuel the supply line but I find that doubtful as it runs too good to be really running low on fuel and I have never found anything wierd in my filter or tank that would suggest something like that. Secondly that the vp44 is letting the fuel free flow through it. IIRC there is a pressure relief valve inside the vp44. Could that have gone bad? I have never heard of this happening before but that means nothing. Just stumped at what to do next.

Any ideas would be welcome. Does anyone know how the vp44 flows fuel or have a diagram of the inside of one. I know I cant change parts in the vp but if I knew what was in there and how it flowed it might help. I thought about trying to test the return flow but many times after it is doing it when I shutoff the truck then restart it later it is fine at least for a while. Not to mention it has been really cold here and not looking forward to doing it oustide laying on frozen concrete. Again it doesnt do it all the time just sometime. Though it more often than not has lower pressure than it should.

 

"Truck runs great even when it is showing very low pressure. So while it may have low pressure I am thinking it is getting plenty of flow. Starts fine everytime."
don't look at the gauge so much

 

I could probably live with the pressure and not worry much about it if it was just a little low but sometimes it creeps down into the below 5psi and even below 2psi never knowing when it will go back to normal. While it still runs good there is something wrong and am too often out in the middle of nowhere with a very long walk ahead of me if it does crap out.

 

I have had the same problem, twice, just at the start of winterdriving. I do not know why this works it just does. You go to a truck stop and buy some fuel additive to take the water out of your fuel. The big rigs do it. They will be selling it by the gallon jug this time of year and the gallon will treat something like 300 gallons. Your fuel pressure will go back to normal with the first tankful. I keep the rest and add it occasoinally through the winter.

 

there is a pressure relief valve (nut looking thing) next to the feed line on the VP, its basically a small pin hole I cant see how it could dump a whole bunch of pressure, but stranger things have happened, it seems to me you've eliminated just about everything in the fuel system, and for all the money it sounds like you have spent, the relief valve is only 20-30$(I think)
I have one from my old VP you can have if you want I'll send it to you

 

Man them little gremlins are wearing you out.
I can understand the fuel pressure varations with my POS $10.00 setup just fine. Try purging any air captured in top of filter by loosing cover till leaking. It fuel foams out, there was captured air.

 

Nick G Thanks for the offer. The valve you speak of got replaced as well not too long ago, When I replaced it I was really hoping that would do it but alas no joy. That particular valve is a check valve according to cummins.

I am having a hard time believing it is air as it still runs good even when really low pressure, not to mention I cant seem to figure out where the air would be coming from as there are no leaks, it holds pressure pretty good after I shut it off, and it starts fine. However I will give it a shot next time it does it.

As far as water in the fuel I run an additive. I have had the tank off and empty as well as the fuel filter assembly off there doesnt seem to be any sign of water in the system. Not sure how water would change the pressure like it does.

Thanks all for the ideas keep them coming

 

What kind of additive are you running? When this happened to me I added Howes and it cleared it right up, twice now. I have a vulcan draw straw still sitting in my shop because after using the howes I did not have to install it. I had a theory that I was not getting enough fuel through the screen in the fuel pick up so that is why I bought the draw straw. Hey, you've tried everything else what can it hurt to try it. The big boy 18 wheelers use it. If it works I would like to hear back. Everybody thinks I have lost my marbles when I mention this cure. Of course I did not start with as many marbles as most, so losing a few is a big deal. I do know why this works, any theories out there?

 

Ah, now see there, thats what I am talking about. On the previous thread I meant to say "I do not know why this works". Man, its tough to get these computers to say what I want. And I also noticed that you already installed a draw straw! Wow, these kind of glitches really get me thinking.

 

pump supply voltage might be the culprit, when my grids cycle the psi drops 6-7 lbs. The alternators on these trucks r controlled by the pcm and when certain conditions r met output is cutback. the heater grids draw a lot of amps too.

 

I am using a mechanical pump so no electricity, thanks for the thought though

 

I have had a primitive water system similar to our fuel systems in my "rustic" house in the hills of TN for 30 yrs. Anytime the system loses prime, it is caused by an air leak - always, even if I can't find it (easily). Any time I have had low pressure out of the pump, supply was restricted as in blocked at the intake. Perhaps you should temporarily add another tank (say 5 gal can - connect before lift pump at metal to rubber joint) & check fuel pressure. We have similar fuel problems with 70s era Datsun Z cars. Pressure will be good then drop over time, shut it off or wait & pressure comes back up. Invariably, it is caused by trash blocking the pick up screen.

 

I was thinking a restriction as well. However I have dropped the tank, nothing in it as it is clean, I replaced the fuel module inside the tank as the filter screen was torn and I thought the suction was maybe sucking up that torn flap of screen causing a restriction but no joy on that. The only thing left for me to check as far as restrictions would be the line from the tank to the pump. I have changed out the rubber portion of the line quite a while back. Worth checking. However it will now have to wait until it warms up a little been below zero and single digits for the last couple days.