Issues with fuel pressure?

5shifter

5shifter

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Apr 3, 2008
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Here's the deal. Bought a FASS, I believe it is a 150/95, back in 2006, bearings went out on that model. It was covered under warranty at the time, so a new FASS was put on in 2008 or so. Always maintained a constant pressure on my gauge, but up until recently.

Before, it always "primed" the fuel system, and the gauge would jump up to 10 psi, before it went back down to zero before the ignition was turned on. Then I would start it, and the fuel pressure would maintain constant readings.

Now, when I turn the key on, you can hear the FASS turn on, but the gauge doesn't jump up to 10 psi like it used to. And then after starting, it takes about 10-15 seconds of idle time for it to go up to normal psi. If it is placed under load, the fuel pressure will go down(i.e. accelerating to get on the interstate, etc). Take the load off of it, fuel pressure goes back up.

So now what this thread is all about, is how would one go about testing the fuel pressure gauge (it's an electronic Westach model from Geno's) to see if it is faulty, or if the FASS is on the verge of going out again? With the Lightning VP44, I really don't want to have to replace another one due to disregarding these issues
 
This is one of the reasons I prefer mechanical fp gauges over electric. There is just to much money at risk on a vp truck to have a bad fp gauge. It could be the gauge or the sender. There also maybe dirt in the connections. I would check it with a mechanical gauge and compare numbers. Where is your sending unit mounted? Do you have a rubber hose between your sender and the injection pump? The rubber hose is used to absorb pulses and vibration that would kill a sending unit that is mounted directly to the ip.
 
The sending unit is is mounted on the front right corner attached by one of the three bolts holding the APPS to the block..right next to the cover.
And Yes, I have a rubber hose between IP and sending unit. There doesn't appear to be any cracks in it either.

So would you suggest getting a mechanical gauge and losing the electronic gauge altogether?
 
Mechanical vs electronic gauge is my preference only. I would trust a mechanical gauge before I would an electronic gauge since it doesnt need an electrical signal from a sending unit or a good ground. I would at least check it with a mechanical gauge to make sure your electric gauge is reading right. When the life of your ip is on the line you need a guarantee that you have good fuel pressure. To me the only way to do that is a mechanical gauge. Only down side to installing an in cab mechanical gauge is they are a little more of a pain since you have a hose going into the cab instead of a couple wires.
 
The best bet would be to check your FP with a mechanical gauge first before condemning any parts.

With that said, based on what you said your truck is doing, I do not think your gauge is at fault. If the gauge were bouncing around by itself, I would lean toward the gauge being at fault. However, if the FP is dropping consistently under load, I would definitely be leaning toward a bad lift pump.
 
Alright, thanks.

That's what I was thinking too. But I took the truck to a well known diesel shop back home(the same ones that did the warranty replacement on the original pump), and all the guy told me there was "It's not the FASS. If it was the FASS the truck wouldn't be running. It's probably a bad sending unit"

So how would be a good way to plumb up a mechanical gauge to check the fuel pressure coming from the FASS since it looks like I'm on my own with this one? Or maybe I should just say forget this shop and take it to one that will actually test it..like I asked them to....
 
There is a shraider valve that comes off the front of the VP that is perfect for testing. I think Source has a kit that will tap off that valve, but I can't seem to find it on their website.

The guy isn't totally off base with his FASS comments. When I lost the relay on my FASS, the truck would not run. The VP can pull fuel through many lift pumps, but it can't seem to pull it through the FASS. I don't think your FASS is completely dead though... So your results may vary.
 
5shifter said:
Alright, thanks.

That's what I was thinking too. But I took the truck to a well known diesel shop back home(the same ones that did the warranty replacement on the original pump), and all the guy told me there was "It's not the FASS. If it was the FASS the truck wouldn't be running. It's probably a bad sending unit"

So how would be a good way to plumb up a mechanical gauge to check the fuel pressure coming from the FASS since it looks like I'm on my own with this one? Or maybe I should just say forget this shop and take it to one that will actually test it..like I asked them to....
Click to expand...

Might be better to get a second opinion since the guy did not do what you asked. Where is your sending unit for your electric gauge? You should be able to connect a mechanical gauge in the same location if you cannot get to the schrader valve like MD said.
 
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The sending unit is right there in front with the wires leading to it...right in the way, but that's a different story.
So pretty much just connect a mechanical gauge in place of the electronic sending unit and it should tell me if the gauge I have now is bad or not...correct?
 
If you can make the connection, that should do it. I'm assuming there is a hose of some kind going up to that sending unit (can't tell in the picture). That would be the one.
 
Yeah it's on the underneath side, lighting was bad out there so I couldn't get a good shoot.

Alright, thanks I'll see what I can get done.
 
camch said:
what does the presure set at?
Click to expand...

20~22 psi from day one according to the gauge(since 2006). It would only drop slightly at wide open throttle.

Now it will eventually go up to the same reading, but it takes a little bit of idle time to get there. Like it will be at 0 at start up, then all of a sudden go up to 20~22 psi. And when I go to accelerate, the gauge will drop almost immediately if I do anything more than feathering the throttle.
 
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