Put a Pricol 0-30psi fuel gauge in when new and it's been fluctuating, especially in hot/warm weather, and pulling hard. The engine runs perfectly without a miss. I have a FASS installed and it produces 15psi steady. I'm suspecting the "isolator", the thingy that's filled with antifreeze. I'm about ready to remove the isolator and run the line directly to the gauge or just bite the bullet and change to an electric gauge. Do these "isolators" have a reputation for failure? Kind of makes me wonder when the pressure jumps around but I'm relying on the FASS to give steady pressure. Thanks. This is my first post here...

Put a Pricol 0-30psi fuel gauge in when new and it's been fluctuating, especially in hot/warm weather, and pulling hard. The engine runs perfectly without a miss. I have a FASS installed and it produces 15psi steady. I'm suspecting the "isolator", the thingy that's filled with antifreeze. I'm about ready to remove the isolator and run the line directly to the gauge or just bite the bullet and change to an electric gauge. Do these "isolators" have a reputation for failure? Kind of makes me wonder when the pressure jumps around but I'm relying on the FASS to give steady pressure. Thanks. This is my first post here...

Michael- Thanks for your reply. It is as I suspected. I figure I'll direct plumb as you have. Presently I have my fuel pressure gauge coming off of the stock fuel filter assembly via a rubber grease gun hose to the isolator. I am trying to figure out how you have your gauge plumbed? I must admit that I am not nearly an expert on the Cummins (I'm better with Model A Fords). It looks like you have your fuel line directly to the VP44 with the needle valve assembly plumbed into a brass tee. Do you think that I could use some new fuel line as you recommended to come right off of the pressure side of the isolator and go right to the gauge, removing the isolator all together? I would not have the needle valve as you suggest which could be part of my original problem. I guess I could re-plumb my fuel line and bypass the stock fuel filter assembly which is what it appears that you did.Thanks again.Lee

I've got a old school Vulcan big line kit in between the stock fuel filter and the VP44. In that line is a push loc tee with a 1/8" NPT test port. Then the rest comes out... Elbow to the needle valve the the brass tee with the fuel pressure gauge line and low pressure switch. Needle valve is required for both electric or mechanical gauge without a isolator. The isolator becomes a damping device in its self trying to translate diesel fuel pressure into anti-freeze pressure. Typical no on complains about buzzing noises while using a isolator... It will be very evident when the needle valve is too open the the gauge will buzz in the cab. The Polyon line is really cheap and you can buy 10 feet of that for less than 6 bucks at a NAPA. Just remember to get new farrules... As for the needle valve ...

Im running Mike's settup on my 98 and it has worked flawlessly for over a year. My Di Pricol gauge has never given me a hint of trouble either.

I don't see much to fail in an Isolator, rubber diaphragm.The two most important steps.1. Make sure the diaphragm is pushed all the way to the Fuel side (end). I used Q-Tips so as not to damage.2. Make sure the connections on the gauge side are tight, or you will get a heart attack as your FP goes to ZERO. Not really just the gauge quits working when the diaphragm reverses.3. Make sure you bleed the fuel line to the isolator, mine has a hex screw.I used a Vulcan 30" universal fuel line that comes off of the test port on the VP$$ and takes a natural path up between the battery and power box to the isolator mounted using the strap clamp attached to a Fender bolt by the Hood hinge.

2 main issues with isolators... 1. Lose coolant and drops low.2. Inaccurate readings. After had a few call me at different states of problems typical what happens after they plumb directly to fuel the number always changes (goes higher!) Usually I tell them to re-test without the isolator and 99% of the time the pressure was fine it was just the isolator either lost coolant or damaged internally some how. But only 2% of the people actually go back through the trouble of reseting the diaphram and reloading and priming with coolant. But once again owners choice. The other issue with isolators is cost... A needle valve is only $8 bucks at a NAPA store isolators range from $40 to $60 bucks.

I've been running the needle valve for about 1 1/2 years and over 30,000 miles without any problems other than once when I bumped it open a hair more so my gauge shook until I realized what has happened. No problems other than that and only cost like $5.

I would post pictures but can't yet. I really don't like the idea of fuel in the cab, just not a good idea. I have an ISSPRO gauge with large line to the isolator and the standard 1/8" tubing to the gauge in the A pillar, that and to boos line are inside 1/4" split loom tubing for protection from chaffing.I made sure the gauge side of the isolator was full of antifreeze, as close to zero air as possible, my gauge is rock solid.I came off of the schraeder valve on my 01 VP$$ with a Vulcan universal 30" fuel line to the isolator.

I would post pictures but can't yet. I really don't like the idea of fuel in the cab, just not a good idea. I have an ISSPRO gauge with large line to the isolator and the standard 1/8" tubing to the gauge in the A pillar, that and to boos line are inside 1/4" split loom tubing for protection from chaffing. I made sure the gauge side of the isolator was full of antifreeze, as close to zero air as possible, my gauge is rock solid. I came off of the schraeder valve on my 01 VP$$ with a Vulcan universal 30" fuel line to the isolator.

Mine is plumbed with -4 AN line so I'm not too worried about it.

Thanks to all for the replies. I am not that worried about direct plumb of fuel to the gauge and in the cab, I know many don't like that idea however. I'll likely do a direct plumb with the needle valve as soon as the temps go up a bit and it's not so darn cold out in the shop...maybe about April or May:pray:If that doesn't work then I guess I'll start looking for an electronic gauge

Thanks to all for the replies. I am not that worried about direct plumb of fuel to the gauge and in the cab, I know many don't like that idea however. I'll likely do a direct plumb with the needle valve as soon as the temps go up a bit and it's not so darn cold out in the shop...maybe about April or May:pray: If that doesn't work then I guess I'll start looking for an electronic gauge

Or just a new mechanical gauge?

Put a Pricol 0-30psi fuel gauge in when new and it's been fluctuating, especially in hot/warm weather, and pulling hard. The engine runs perfectly without a miss. I have a FASS installed and it produces 15psi steady. I'm suspecting the "isolator", the thingy that's filled with antifreeze. I'm about ready to remove the isolator and run the line directly to the gauge or just bite the bullet and change to an electric gauge. Do these "isolators" have a reputation for failure? Kind of makes me wonder when the pressure jumps around but I'm relying on the FASS to give steady pressure. Thanks. This is my first post here...

I would recommend running the line directly to the gauge and lose the isolator. I have IssPro gauges and it works perfectly, no bounce at all. Just make sure the valve is barely open and when turning the engine over the needle should rise slowly to the proper pressure. Mopar1973Man has a video showing how it works on his website thats how I know! Good Luck Arcwelder

I did the same thing as mopar1973man, different gauge but otherwise same performance, was less that $5 at Napa although that could be because my ex's boyfriend works there,

Thanks to all for the help. I have dumped the isolator and plumbed in a needle valve as described above. The valve was installed in the former place of the isolator which is at the end of the rubber grease gun hose which is attaced to the downstream side of the stock fuel filter housing. I have a FASS and am getting 15 psi at idle and down to 12 or 13 psi when I push on the "go pedal" that is pretty much as usual.Now for the question: I have noticed that the fuel pressure gauge will all of a sudden drop to ZERO for just a moment and then back up to 15 psi while running at say 60 mph. I also noticed that the needle tends to drop if I get on the throttle and then back off quickly...then it will drop. Sometimes it will drop to ZERO while just running at 60mph on level ground in 5th gear. The engine runs perfectly and doesn't miss a beat. This up and down thing has me wondering and also worried if you know what I mean.Could the gauge just be bad or dying or could there be air bubble(s) in the fuel line running to the back of the gauge? I haven't replaced the stock fuel filter for a time as I figured the FASS filter would take care of the fuel. Also the FASS fuel filter is not very old, a recent replacement. After installing the needle valve and a length of 1/8" tubing long enough to reach the gauge on the A-Pillar, I did cycle the FASS enough to flush the air out of the fuel line before I attached it to the back of the gauge. This has me scratching my head....Did I screw-up? :wtf:Lee

Anyone have any ideas? Thanks

How much fuel is in the tank? If the tank is below 1/2 a tank its still possible on hard launches to suck a air bubble into the system and the pressure to fall and them pop back up. Try filling the tank if you can and try again. I bet it goes a way.

Tank is full. I usually keep it over 3/4 full at all times. This way it's cheaper when I fill it up...:broke:Still a mystery.....Thanks for your $0.02. Any other thoughts?

It got to be a suction line problem. If the suction line is drawing in excesive amount of air it will flood the pump with to much and then lose prime and take a second to get prime again.