Engine/Transmission (1998.5 - 2002) Fuel pressure issues after VP44 removal

[leathermaneod]

Hey all,

I know there is a ton of threads out there about fuel pressure, but I hope my issue hasn't already been covered so here goes. I got my truck back after having the tappet cover and fuel guage sending unit replace and my fp guage has been acting strange ever since. The first thing I noticed was I don't get as big of a spike when I turn the key on and the pump runs for a second. It's only like 5 psi max. I'm not sure what it used to be but I'm pretty sure it was higher than that. Next thing I noticed was idle pressure, it used the be 19-20 now it's 17-18. Finally is my driving pressures, it used to not drop below 16 unless I really got on it, now it's down at 15 with light acceleration and down to 10 if I Step on it. When I first got the truck back I had the number 2 fuel line leaking at the block and I thought that was the cause so I had it replace. Now I have no leaks and a new Baldwin pf7977 fuel filter. I also have a new vp44 return check valve on the way from Geno's to try. The truck seems to start ok and it runs like a dream. Has anyone else ever experienced anything like this? I don't know what fuel filter the PO was using, could the 5 micron pf7977 be causing that much restriction? Or something have got messed up with the vp44 when they had it off? Or should I be looking at the fuel tank since they dropped it to replace the sending unit?
Thanks in advance!

 

[GregOreilly]

Fuel pressure sending unit may be bad, May be trash in it.

 

[leathermaneod]

its a mechanical gauge with an isolator so I don't think that's it. Thanks for the reply though

 

[hcole]

I have a mechanical gauge with isolator also. Mine does this same thing about twice a year. I have a hypodermic needle








I have the mechanical gauge with isolator also. Mine does this same thing about twice a year. I have a hypodermic needle I fill the line AFTER the isolator going to the gauge. I fill it with antifreeze and I'm good to go for another 6-8 months. The isolator doesn't displace that much volume and air will compress while liquid won't. Try it. Hope this helps.

 

[Chris Bartolomei]

leathermaneod, I believe I may have the same issue as you ... I haven't replaced the VP44 but am having similar fuel pressure issues I've been trying to diagnose for months now. Here's something to test to see if it's truly the same: Open up your fuel filter/water separator housing and lift the filter out. Let it drain, then place it back in. Start the truck and see if your fuel pressure comes back up to "normal" (19psi idle, ~15 WOT). It should stay up for a short while ... I was getting 13 miles before it would drop again (17-18 psi idle, 10 WOT). I have a tell-tale in my truck. I'm running my fuel pressure gauge with a needle valve instead of an isolator. It's quiet until the pressure drop occurs, then I get a buzzing sound in the gauge. I've done all sorts of stuff to remedy it and no luck: new fuel filters, new DDRP, relocated the DDRP to near the tank, 1/2" lines from DDRP to VP44, new filter canister housing/heater, got rid of banjo bolts, new return valve at the VP, new in-tank module... still the problem persists. Very repeatable. I have a youtube video of the symptoms: https://www.youtube.com/watch?v=YQE3ffbGV8s. If you fidured your issue out, please let me know - I would like to try the fix!

 

[hcole]

Chris, I don't know where you are monitoring your pressure, (mines at the banjo bolt on the VP 44) but when you take the filter out you're introducing air into the fuel system. Now, air will compress, fuel won't. The vibration in the gauge is picking up the individual pulses off the pump rotor vanes and it is spiking the gauge with those pulses. Your true pressure is probably 20 PSI. I'm trying to think how you could put a temporary buffer in the line so there would be a cushion or if you will an accumulator like hydraulic pumps have. If you can find a way to put something in the line after the fuel pump, then it will level out and show you 20 PSI.

 

[Chris Bartolomei]

hcole: I'm checking it at the same location on a needle valve on the fitting going into the VP44.
I'd like your opinion on the results of some of my testing:
I've run 2 pressure gauges at the same time to isolate the issue... one on the schraeder valve on the inlet to the stock fuel/water separator canister and the main one on the VP44 inlet. With a fresh filter install, they both read the same (~20psi). After driving about 13 miles, the VP gauge shows that pressure drop (and can get sucked down to near zero if I nail it) whereas the pressure gauge at the filter canister reads about 16... I pull the filter out and put it back in and the pressures read the same again... I'd like to believe the true pressure is that by the canister but I've nailed the throttle to test and I got a check engine light when the pressure hit 0. Before anyone else jumps in and says "filters" I've changed them a dozen times already... I've also changed out the whole canister body. Granted, with the 1/2" lines and no banjos now since I made that video (and a new DDRP) I'm getting 18-19 psi ... but I still see this pressure change after about 13 miles however not as pronounced (drops to about 10 WOT). Is the air introduced by removing and reinstalling the filter buffering the pulses from the VP44 and would it take that many miles to purge the air out that was introduced by lifting the filter out? I guess another good question is what should the pressure drop change be with a good fuel system from idle or "just driving" to heavy pedal at like 2300-2500 rpm? After replacing the filter, I get about a 5psi drop, after the buzzing starts, I can drop almost 10psi ... maybe even more.
Just for more info, I also changed out the in-tank basket to an '06 Liberty module, cleaned the tank (twice just because I had it down and to be double-sure), ran BioBor-JF in a couple tanks, and checked my fuel was good and fresh ... ran these tests hot and cold engine, low tank and full tank. Just the same thing time-and-time again.
It just seems weird that it can go 13 or so miles silent and strong then bzzzzzzzz and a pressure drop at the same instant. I'll believe you on the buzzing being from the VP44 .... but before the issue started I had the same gauge/needle valve setup for a few years with no sounds coming out of it... Can you think of anything else? air getting in somehow causing cavitation? would a leak in the return cause issues? I have no fuel leaks anywhere ... dry as a bone. Starts and runs like a champ, doesn't blow smoke on warm or cold starts at all. I'm going to check the oil and see if fuel leaked past the injectors somehow (I think my tank is pressurizing a bit but haven't verified if it is pressure or vacuum. I drive with the (vented) cap loose for now) Am I over-analyzing this ?!?! It scares the crap outta me regarding the sensitive nature of the VP44 and its cost... I don't want to fry it.
Thanks again for your input - I really appreciate any thoughts you (and anyone else) have.

 

[hcole]

Well, I still stand by what I said. You misunderstood what I said about the fuel pump. I meant the lift pump, not the VP44. The lift pump is the source of the buzzing coming from the pulsing of the vanes in the lift pump. I think the name for what you need is a water hammer arrestor. Go to Ebay and type it in. It will cushion the pressure spikes from the lift pump. I don't think you're hurting anything driving it except the fuel gauge will have a short life.

 

[Chris Bartolomei]

Gotchya! I did misunderstand... old(er) age does that I guess...lol.
And thank you again.