53 willys

Busted!! I think I will run an Isolator Diesel is really stinky if it does leak and it would be my luck to be the guy who had some dumb leak..

Fisherguy

Oh, yeah, right!

Maybe you can explain hydraulics to me then¿

P.J

Uh, mmm, yeah.

Tell that to my last two properly functioning fuel pressure gauges.

This is a VERY common practice, hook up the line and be done with it, over time more than likely the air will be displaced with fuel.

There cannot be any scientific proof that a air compresses (on a gauge) any different than liquid.

Jfaulkner

I'm not trying to start an argument here but what kind of scientific proof do you want. Air compresses, liquid does not! It doesn't matter if it's in a tank, cylinder or a gauge. If you don't believe this put air in your brake lines on your truck and go for a drive.

P.J

How is it again that ALL 18 wheelers come to a screeching halt everyday????

Hmmmmmmmm.......


Air.

Put it this way, I was told here to not worry about weather or not there was air in the line to the gauge (I don't) and mine along with hundreds others work just fine.

banshee

Guys, there's a difference between an open system and a closed/sealed system and how the property differences of air and lquids affect them. It is correct that air will compress (change volume) with a change in pressure (assuming constant temp) and liquids do not (it's actually a very small fraction of 1 percent, but effectively zero). This difference becomes aparent in a sealed hydraulic system where the volume of fluid between a master cylinder and slave cylinder/piston is fixed. Examples of these systems are the brake system on a passenger car, or the line that runs from an isolator to a gauge. If these sealed systems are filled with air (or any other gas) then the volume of air will compress with the application of a slight reduction in system volume when the master cylinder/diaphragm applies. Effectively what you get is a large reduction in pressure at the slave cylinder and therefore less force when applied through a piston. What this will give you is very slight apply pressure on a brake caliper or a low reading on a pressure gauge. Another way to look at is, assuming the same piston size on either end, a 1 inch movement in the master piston will not result in a one inch movement in the slave piston. In some cases the trapped volume can be small enough that this compression doesn't affect the readings much, so the larger the volume the worse the effect. An open hydraulic system is different in that there is a constant supply of volume (liquid or gas) applying the pressure. An example of this would be a truck's air brake system, or a fuel pressure gauge setup where the fuel line is run directly from a pump source to the gauge. In these setups the fluid in the line (air or liquid) does not matter because there is not a fixed/trapped volume of fluid. It will take more gas (mass of gas) to fill a given length of line (volume) as the pressure is increased, but you have effectively an infinite mass of the gas at that pressure from outside the system (called a control volume) to allow that to happen. Effectively there is a infinitely of volume at the apply pressure, so whatever compression of gas you get is just filled in with more gas. It just comes down a difference in how the systems are applied and used, but most of you guys are arguing different sides of the same point. To sum up, for a sealed system with an isolator I would fill at least the diaphragm volume with liquid, but it is best to fill the entire line as much as possible. For a line run directly to the gauge, hitch it up and go about your way.

John

kipo

been running an isolater from dieselmanor.com for over a year and a half with no problems. make sure you use good thread sealant and fill the tube to the the guage with a needle and syringe. my isolator is mounted (welded) to the bracket that holds the large power steering lines that run up toward the brake resevoir. don't know how much it vibrates but haven't had one issue.

P.J

Banshee,

Can you comment on a closed system, the end of the line being a gauge?

I don't want to give false info here, I usually am willing to try what I read, then offer my results (as I have tried them for myself) back here to other members.

In short, back a few years I read it would be fine to just hook up my gauge, crack open a needle valve and go on with life.

I (first hand) found this to be good advice. Gauge read 15 lbs about 3 seconds after cracking the valve, continuted to read that (at idle) for many, many months after. I sold the truck, got a new one and did the same exact thing, same parts, with the same results.

If indeed the reading I am getting is low, I guess I can live with that, better than being inaccurate to the high side.

Any thoughts?

P.J

D'oh!


Guess it would help if I read the LAST line of you post, huh?

Ignore me.

XLR8R

The only thing I would add to John's explanation is the difference in hysteresis between 100% liquid and a mix of liquid/gas filling the line to the pressure gauge... both will be equally accurate during steady-state conditions, but the liquid-filled line will be more responsive than the "mixed" line - the air in the line (even if in suspension/aereated) will act as a "spring" due to it's compressibility, resulting in a dampening or buffering of the gauge needle's movement.

I've noticed that members' gauges have different response rates and accuracy; my cheapo mechanical gauge/direct line system (which I purged of air) mounted on the cowl reads quite accurately and immediately to 1/4 psi delta pressures.

Of course, what John said holds true for mechanical oil pressure gauges, etc.

XLR8R

p.s. Banshee - you have the makings of an excellent fluidics engineer!

truckjunkie

What he said - I haven't had any issues with my FP/Isolator setup to date - almost two years now...

CRXsi

I voted Yes as well. I have had my gauge and isolator from Diesel Manor on now for over a year. No issues and I too don't want to have to clean up the inside of my expensive truck if a gauge fails or if a line leaks. You may not be sorry but if you had one and something happened you would be very happy you did have one.

SSminnow

Why does it say you can not run the Electric Auto Meter Fuel Pressure gauge on a Dodge, but you can on a Duramax or PSD??

I must be missing something

banshee

PJ - You got it!

XLR8R - Thanks! You bring up an interesting and valid point regarding the excess air in a sealed system acting as a spring or sponge which softens the response. In a brake system this sponginess is an undesireable thing because you lose pedal feel, but in these fuel pressure gauges I think it is a good idea to leave a little bit. In my experience, filling the gauge side of the isolator and the line leading to the gauge 100% with fluid can cause gauge noise/rattling over time. Every boost gauge I have ever had has ended up rattling, and fuel pressure gauges will do it over time, too. The vibration can in fact get bad enough that it will cause the entire nylon line to vibrate/rattle with the pulsing in the fuel supply line. While the solution to the rattling problem is a snubber in the line (needle valve to stop the pulsing), I have found that a short column of air does the same thing.

One other tidbit I found on my gauge... With just air on the gauge side of the isolator my gauge would read just 7 psi at idle when it should have read 25 psi with my fuel pump setup (as verified by a direct hookup mechanical gauge). Filling the isolator only with fluid brought that number up to the correct value. Upon changing my fuel system to the MITUSA and adding a rising rate regulator I now run somewhat higher fuel pressures at WOT. The same isolator only filled gauge setup would cap its reading at about 50 psi because it was compressing the air in the nylon line too much. I filled the line from the isolator to the gauge about 3/4 full and it now reads perfectly.. .30 psi at idle and 95 psi at WOT

SSMinnow... The dodge lift pumps put out some serious pulses in the fuel (think of pressure as a bunch of really fast hammer hits vice one solid push) line where the pressure is normally read. This hammering literally beats the electric sending units apart over time by ripping/degrading the diaphragm inside. While some guys have had good luck over time, I personally have never had one last very long. On my current truck I went mechanical with the isolator and haven't had a failure or leak yet. The Duramax doesn't have a lift pump (not sure about the newest models), and the Fords run a Walbro pump similar to what Glacier Diesel sells which pulse a lot less.