My '99 has a FASS fuel delivery system. I have always had a constant pressure reading of 18 regardless of rpm or load. In the past few days though, I have noticed that at idle or low speed the gauge needle has started to bounce between 18 and 24. The gauge smooths back down to a constant 18 at cruising speed. Is it telling me that it's time to change the fuel filter, or is it likely some other issue. The existing filter and water separator have clocked 10,000 miles. Any input would greatly appreciated. Thanks

 

just out of curiosity, does your voltmeter guage bounce around too? when my alternator went out, my lift pump wasn't getting enough power and the guage was acting funny too.

 

To tell you the truth - I haven't checked both gauges at the same time, but I will the next time the fuel gauge needle starts to bounce. Good suggestion.

 

so how are you getting your fuel pressure reading. mechanical? electrical? if mechanical how? check those connections, if you have a needle valve or fuel baffle of any type that helps reduce fuel pressures and pulse that go to your fuel gauge those could be going bad if its a rubber diaphram style. if its mechanical and you just run a fuel pick up line straight to the fuel gauge well that's bad and that could cause a gauge to go bad.

if electrical gauge set up. check all connections and power going to gauge,
and like the last poster suggested. What was your normal fuel pressure before all of this. you never stated that. when i see way higher than normal numbers to me this is never about restrictions. ie dirty. I would think dirty or really dirty filter would be a slow showing of a lowering psi.

 

Thanks for responding DIESELWRKS. My gauge is electrical (Glow Shift) and as I said in the 2nd sentence of my inquiry, my gauge has always read 18 psi, but is now bouncing between 18 and 24 intermittently. The sending unit is threaded into a port on the FASS pump housing. I will check all electrical connections as advised, especially the one at the sending unit. By the way, my voltmeter remains steady and unchanged when the fuel gauge does its bouncing act.

I really don't know what effect a clogged filter would have on my fuel pressure gauge. My thought process was that a pump forces fluid at a constant rate. If there is a restriction beyond the pump, like putting your finger partially over the end of a garden hose, then volume is decreased but psi is increased. My gauge says it reads psi, but again, I really don't know and that's why I'm asking. Thanks again for the advise and I'll check those connections.

 

ok, my gauge is hooked in after the filter (by the vp) so that when my filter is getting old it drops in pressure but still remains fairly consistant (not as constant as yours). being your gauge is tied into your lift pump, your gonna get different pressures then mine. I would say a new filter wouldn't hurt anything. if I'm ever in doubt about the filter I just change it, but that's just the way I am.

 

Thanks for your input dubc2323. FASS has set up their system so that they have a couple ports after the filters. One is for the fuel service line to the engine and another is auxiliary, like for installing a fuel pressure sensor. I will buy new filters if nothing changes after checking and cleaning the electrical connections. FASS filters are rather pricey and I didn't want to fork out the money and not have it fix the issue. About how often do you change your filter? I know there are several factors involved but just curious.

 

with a new filter my pressure gets 21 psi at an idle. when it drops down to about 18 at an idle I change it. it's roughly once a year, or 3 oil changes. did you bypass the factory filter?

 

When I was running a Nordskog electric fuel pressure gauge, the pressure would suddenly start fluctuating. The sending units start going bad. Switched to mechanical gauge - no more problems.