Engine/Transmission (1998.5 - 2002) Fuel Pressure Isolator?

[DLausche]

Yet another FP thread here. My mechanical DiPricol FP gauge started acting up this past summer. It would read zero for quite a while after starting and then slowly creep up to about 10 or 11 max. After a while it would never move off of zero. Has been like that for a month or two now. I've tested with another gauge on fuel canister and had 16 psi so I left the gauge to fix when I had time. Several people recommended to check the isolator on the gauge side and refill it if needed. Seemed logical to me as air in there should cause the symptoms described above. So, checked the isolator last night and sure enough, just air in there and no anti-freeze. Filled it up, got all the air out as much as possible and put it back together. I trigger the LP with engine off and get 11psi. Scratch head cause I think it should be higher than that, but at least the gauge moved off of zero. Turn pump off & gauge is stuck at 5psi. OK, bleed some excess liquid out of line at gauge and gauge goes down to zero. Trigger LP again with engine off and have 11psi. Turn pump off and gauge is stuck at 5psi again. Bleed a bit more liquid out and gauge goes right to zero again. Put it back together and go in for the night.



This morning on the way to work, it was reading 21psi going down the side road. I'm thinking cool, that is decent pressure and better than I had hoped for. Keeping my eye on the gauge on the trip to work (~20 miles). Pressure keeps creeping very slowly up and up and up. After 10 miles or so the gauge is pegged at 30psi. Get to work and shut down. Gauge is stuck at 30psi.



A Holley Black pump won't put out 30 psi, will it? I did stretch the pump spring pretty good when I put it in a few weeks ago. The way the gauge sticks leads me to believe the diaphragm in the isolator is stiff and not releasing the pressure on the gauge side. But, that doesn't explain why I am seeing 30 psi on my gauge. Or is it possible the pump is putting out 30psi and the isolator diaphragm is so stiff that it just took a while to get to full reading? Do I need a new isolator and/or a gauge?



-Deon

 

[EFretwell]

mine would do the same thing, just peg itself with the stock LP, so I bled more coolant out of the guage side of the isolator, now it reads where it should. I figured that the truck would warm up and the heat was causing the excess pressure, I would turn off the truck and the pressure would drop from thirty to about 12 psi, then the next day it would be at zero again.

 

[DLausche]

mine would do the same thing, just peg itself with the stock LP, so I bled more coolant out of the guage side of the isolator, now it reads where it should. I figured that the truck would warm up and the heat was causing the excess pressure, I would turn off the truck and the pressure would drop from thirty to about 12 psi, then the next day it would be at zero again.

The heat expansion is a logical explanation. I just went outside to check the temp of the isolator. It is cool to the touch. Truck has been off for 4 hours now. Just for the heck of it I opened the vent screw on the fuel side of the isolator. This is where I bleed air out of the system after filter changes, etc. A little bit of fuel came out and the gauge dropped to about 12psi (didn't start engine or LP). Now I'm even more confused I didn't think a rotary vane style of pump like the Holley would hold pressure on the output side.

 

[SSabick]

Do a search for using a needle valve on a fuel pressure gauge. The isolators are as undependable as the OEM lift pumps. Just run with out it. There are many people running mechanical gauges without the isolator. I screwed up, and didn't do my research about my problem, and I just bought a electronic gauge. With my FASS system, the gauge just reads pegged, because it only goes up to 15 psi. I couldn't tell you what psi I am really at, and it annoys me to no end. I am probably going back to the mechanical gauge again as soon as I buy the needle valve, and other parts.

 

[DLausche]

Well, I went to bleed some anti freeze from the gauge side tonight. Loosened the fitting and a good deal of fluid came out. Gauge went down and is stuck at 11psi with no line or fitting on the back at all. I think this is pretty conclusive proof that the gauge is bad. Will replace that next. If the isolator is also bad then I'm going to just drain the anti freeze and cut a hole in the isolator diaphragm and run the fuel right to the gauge. I do have a needle valve on the line at the top of the fuel canister.



-Deon

 

[mhenon]

Well, I went to bleed some anti freeze from the gauge side tonight. Loosened the fitting and a good deal of fluid came out. Gauge went down and is stuck at 11psi with no line or fitting on the back at all. I think this is pretty conclusive proof that the gauge is bad. Will replace that next. If the isolator is also bad then I'm going to just drain the anti freeze and cut a hole in the isolator diaphragm and run the fuel right to the gauge. I do have a needle valve on the line at the top of the fuel canister.



-Deon

My gauge was acting goofy as hell for awhile (especially during hot summer temp's >100F) but seems to have settled down.

I never thought of puncturing the diaphram in the isolator. Is there any downside to doing that?

Mike

 

[DLausche]

I never thought of puncturing the diaphram in the isolator. Is there any downside to doing that?

Mike

It would be just like plumbing it without an isolator. For me it would be quickest fix since the isolator is there already. I also like the vent screw on the isolator to bleed air from the fuel system after a filter change. Downside would be fuel going right to the gauge which probably voids gauge warranty. But many do it this way. Another is if you have a leak and get diesel in the cab - probably hard to get rid of smell. I'm not too worried about that.



-Deon

 

[mhenon]

So I take it that the clear plastic looking line going to the gauge (the one filled with anti-freeze) needs to be tossed and replaced with a fuel line that's diesel compatable?

Mike

 

[DLausche]

So I take it that the clear plastic looking line going to the gauge (the one filled with anti-freeze) needs to be tossed and replaced with a fuel line that's diesel compatable?

Mike

Hmmm. Good thought. I always thought that would be ok with diesel but I should probably check first.

-Deon

 

[WTBurke]

Guys, I just finished doing the Vulcan update. I dropped my tank and did the "draw tube" mod along with all new JIC 08 & 06 fittings respectively.

Eric at Vulcan was very helpful and provided me with the floral carbon diaphragm isolator. The Hewitt industries "tube plunger" isolator plagued me for years with inconsistencies. I tried everything with no improvement. Now with the new diaphragm unit I have excellent response at the gage.



Deon, the small clear tubing used to provide liquid drive to your gauge should not be reactive to diesel fuel. I use antifreeze to drive my Hewitt gauge.



I've set my idle fuel PSI at 18. 5 PSI on my AirDog. While driving I see the modest changes from cruse to WOT. The thing I've not seen discussed is the "pop off valve" at the VP-44 is supposed to release at 15 PSI.

The reason I bring this up is I work in the pump industry. All the systems I work on have exact PSI drive levels. If I have a hydraulic unloader valve that pops off at a certain level then that is "what" it is!



I'm not trying to misdirect this thread but, GUYS... . if we feed 18 PSI to a VP-44 (not during start up) and we are driving down the road why do I see 18 PSI at light throttle? Why is this VP-44 pop off valve not working? I see many threads with guys stating PSI numbers that just don't make sense to me?



I know much of the fuel not used is circulated back to the tank and all is understood by the majority of the TDR group that we need this for cooling the VP-44. This is good practice and this is my reason for doing my fuel system after loosing a VP-44 at 75K miles. (I'm at 123K now) I like the idea of "over feeding" in volume for the cooling benefit... . but, I'm confused about the operation of this system. It goes against everything I understand about PSI regulation regarding devices that "limit" or "relieve" pressure?



The subject of proper telemetry and gauges with isolators is very important and 'understanding' what we are 'seeing' in a closed pressurized system with these instruments is key.



Sorry for the long question guys... . but I need a little help here.



BTW Eric at Vulcan ROCKS!



William

 

[OL RATLR]

Throw the Issolator away go mechanical why fool with something that is as much trouble as what you are monitoring? Mechanical gauge 4 1/2 yrs no problems. EV Series Isspro. :-{}

 

[WTBurke]

OL Ratlr... . the system I'm using is mechanical... . and the floral carbon diaphragm according to Eric at Vulcan is the same ISOLATOR that ISSPRO provides for their fuel pressure gauge.



I'm really hoping to have someone chime in with a fresh view of the 'pop off ' valve that we have on our VP-44 2nd Generation CTD's



ANYONE?



William

 

[NoSeeUm]

Throw the Issolator away go mechanical why fool with something that is as much trouble as what you are monitoring? Mechanical gauge 4 1/2 yrs no problems. EV Series Isspro. :-{}

I have been running an isolator for 3+ years no problems at all. Not sure of the brand, I'll check if anyone is interested.



Jim

 

[mhenon]

OL Ratlr... . the system I'm using is mechanical... . and the floral carbon diaphragm according to Eric at Vulcan is the same ISOLATOR that ISSPRO provides for their fuel pressure gauge.



I'm really hoping to have someone chime in with a fresh view of the 'pop off ' valve that we have on our VP-44 2nd Generation CTD's



ANYONE?



William

The VP44 is a complex beast and I certainly don't understand how it all works.

There is a ball & spring pressure relief valve at the outlet to the VP that is set to about 14psi. Will this regulate fuel pressure going into the VP? Doesn't seem to have any effect on inlet fuel pressure. . at least it doesn't in my system.

There is also a tiny bleed hole just before this relief valve to bleed off any air in the VP. Ever wonder how your fuel pressure drops off when you shut the engine down. It's that tiny bleed hole.

So it seems there's always some fuel being returned to the tank via this bleed port and the relief valve (if internal pressures are over 14ps).

Why this doesn't work to regulate fuel pressure at the inlet to the VP (or outlet of the LP) I'm really now sure. Maybe one of the VP guru's will jump in here and explain it. I can only guess there is a maze of small passages within the VP that allow pressure to be well above 14psi at the inlet. . but internally maintain it to that figure.

Mike

 

[Mark Craig]

DLausche,



Most likely your gauge is DOA, now the upside is it doesn't have diesel in it so if it's within the year warranty they'll fix you up under warranty, if it has diesel in it it's gonna get denied etc. If you have the square isolator they are pretty much bullett proof as they is really nothing to fail but the membrane and it's a material that really lives a long long time. If you have one of those cylinder isloators there's your problem, we sold those for almost a month and quit ASAP!!



Use anti freeze and water 50/50 mix and get out at least 95% of the air in the line and you're good to go.



Mark @ DPPI