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Strange bobbing of the fuel pressure gauge?


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I used to monitor the fuel pressure with an Dakota Digital electric gauge.  After 8 years of service the ULSD ate the pressure sender so I replaced the gauge with an ISSPRO mechanical and isolator.  Same arrangement just different gauge.
Its been on there for about 2 years now but even though it reads quite accurately, it does something odd.  At idle the gauge needle is solid around 20 psi and when cruising along down the freeway the gauge is solid at around 18 psi.  Then all of a sudden the needle will "bob" down a couple psi and quickly return.  Its a more fluid action as this is a mechanical gauge but is also very random and kinda hard to watch for too.  But.....if I'm pulling a trailer and have reason to be hard on the throttle then the "bob" of the needle can become more regular as I'm holding steady heavy on the throttle.  Still, its only a "bob" of about 2-3 psi but its a strange action and the only way I can describe the way it moves would be like watching a fishing pole bobbin dip under water when a small fish is tasting the bait.

In saying all that, I have yet to take the tester gauge out for a drive and see if the test gauge does the same thing but I figured I'd ask around first and see if anyone has experienced the same.  My tester gauge drips and I need to fix it before I'm holding it in the cab.....

I really cant imagine why or what the bobbing action could be other than maybe the Raptor 100 (older better unit) has a regulator which has trouble regulating a smooth amount of fuel pressure when the fuel volume demand increases. :think:

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They symptoms have been there for the entire time I've had the ISSPRO mechanical gauge on there.  If it is related to the Raptors regulator then I dont think the DD electric gauge would have been able to read the fluctuation that fast which is why I never noticed it before the mechanical gauge.

 

I thought I remember reading that the Raptor fuel pumps did weird things like this but I cant find anything regarding.....

Edited by KATOOM
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I have noticed that my ISSPRO mechanical gauge will do the same exact thing. Most of the time I can relate the bob to a bump in the road that I had hit. Like you said, it is a fluid motion and will come right back up after a split second. I am also running a raptor 100. Mine is about a year and 6 months old. Not sure but I think it must be one of the earlier models. Otherwise I would have had problems with it by now.

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I used to monitor the fuel pressure with an Dakota Digital electric gauge.  After 8 years of service the ULSD ate the pressure sender

 

Usually the water hammer pulses eats the senders not the ULSD. Like I got a Cummins low pressure switch and water hammer ate it in about 2 years. Now with the 2nd low pressure switch I've got it behind the needle valve with the gauge and both live a very happy life now and no issues for many years.

 

I replaced the gauge with an ISSPRO mechanical and isolator.

 

I would start by removing the isolator and hooking directly and see if there is a isolator issue.

 

Also remember the water hammer pulses are being sent to the isolator so the diaphragm is vibrating internally so it does wear out as well from the pulses. This why I'm asking to direct connect and see if its a translation problem between the isolator and gauge.

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I  think the  mechanical  gauges  are  going to  show  'road rage'...    you   have  jiggling  within a line,   fluid  motion  can't  be  too far behind....     another   thing  I've  never seen  mentioned is   fuel tank  sloshing.    when  the  fuel  sloshes  past  the  pickup...  there would be   a   tendency for   a  little  'spike'  in  overall   lift pump efficiency..  and  vice versa.     probably  way more noticeable  when  tank is   pretty low...

Perhaps  the  Isspro  is   sensitive enough  that  you guys  are  'seeing'  the  fluctuations  of   any or all of above?

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I have noticed that my ISSPRO mechanical gauge will do the same exact thing. Most of the time I can relate the bob to a bump in the road that I had hit. Like you said, it is a fluid motion and will come right back up after a split second. I am also running a raptor 100. Mine is about a year and 6 months old. Not sure but I think it must be one of the earlier models. Otherwise I would have had problems with it by now.

 

 

Mine does the same thing and is related to sharp turns, hard stops, hard take offs and bumps. I think it's the lack of baffles in the tank so you get a slight fuel vacuum with excessive fuel slosh.

 

It occurs more when the tank is  down to 3/8ths of a tank or less.

Edited by JAG1
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Usually the water hammer pulses eats the senders not the ULSD. Like I got a Cummins low pressure switch and water hammer ate it in about 2 years. Now with the 2nd low pressure switch I've got it behind the needle valve with the gauge and both live a very happy life now and no issues for many years.

 

 

I would start by removing the isolator and hooking directly and see if there is a isolator issue.

 

Also remember the water hammer pulses are being sent to the isolator so the diaphragm is vibrating internally so it does wear out as well from the pulses. This why I'm asking to direct connect and see if its a translation problem between the isolator and gauge.

 

You're right.  The pulses are what generally takes out the senders but when I installed the DD gauge it was one of the ONLY electrical gauge manufactures which was proving reliable against the pulses.  And I installed a snubber in the line too.  Other electrical gauge manufacturers weren't lasting very long at all.  So after 8 years of good service, the sender did finally quit and when I talked to DD they said that those older senders I had couldn't stand up to the new recipe in the ULSD which causes the fuel to wick internally inside the sender.  The new updated senders were solid state.....and very expensive.

 

And yes, it may be the isolator.  I have a test port pre the isolator so I think its high time I make some test runs with the fuel pressure test gauge held in the cab while driving.  That way I can isolate the isolator and gauge.  When I get a chance to do this, I'll post back.

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Mine does the same thing and is related to sharp turns, hard stops, hard take offs and bumps. I think it's the lack of baffles in the tank so you get a slight fuel vacuum with excessive fuel slosh.

 

It occurs more when the tank is  down to 3/8ths of a tank or less.

 

That is very very rare for me to experience unless I'm driving like I stole it. Other than that I would say you have excessive clearance on the pickup tube. There should be about 1/16 to 1/8 of inch from the tank bottom to the tip of the pickup tube. Then the tube should be cut flat and not at a angle. Really common for old school draw straws and improper installment.

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the only reason  I  said  sloshing  may  cause  pulses...  is  from  my experience  pulling  a  1300 gallon   sprayer tank  in the field.     zero baffles.      Pump  inlet  was   at the  front bottom of  tank...  and  as  the  water  'hit'  the  front of  the  tank... I could see  quite a  spike  in  'after pump'  pressure    I'd normally  run  25 psi,   and it'd spike  upwards  to 30.        Just   a  thought  that  may  kinda  apply  here.      Mike  made  a  good  point  on the  'too high'  of   straw...      I've also wondered about   whirlpools   forming  @  straw,  and  momentarily  drawing  a  gulp of air.

 

I've  also wondered about   the  air  entrapped     yah,  I know there are   air removal  systems/filters  that  send  foam and bubbles  back to tank...  but!      Think about   the total volume  being  supplied to  lift   pump!   If  suddenly  'X'  amount is returned  to tank (via  the  foam and air return)  .  then    the  total  amount  (volume)   offered  for   downstream  use  will change.     I look at it  as   'head' available.      I really  don't think  there is  a thing  wrong here,    it's  just   hydrodynamics at work.

We  can't expect   a  rock solid  pressure side  when   supply side  can  swing  either way.

Edited by rancherman
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Mine does the same thing and is related to sharp turns, hard stops, hard take offs and bumps. I think it's the lack of baffles in the tank so you get a slight fuel vacuum with excessive fuel slosh.

 

It occurs more when the tank is  down to 3/8ths of a tank or less.

That is what I though it was but it will do it on a completely full tank just the same as an almost empty one. Also, I don't think I have a straw problem because I have run down to less than 2 gallons left in the tank. I figured that if I did have an issue in there I wouldn't be able to run the tank almost dry like that. Just kind of weird I think.

Side note, I am running my mechanical gauge directly with a needle valve tapped into the line right before the VP. No snubber or isolator.

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I dont think the fuel pressure gauge bobbing has anything to do with sloshing fuel because it doesn't matter if my tank is full to the top or not.....it still does it based on throttle position.  The way the needle bobbing acts, I think its related to the fuel pump regulator ball skipping around.  I know that the air separation fuel pump regulators recirculate back to the fuel tank as where the pumps like the Raptor dont but that said, there still is fuel passing through the overflow valve all the time so you'd think it wouldn't be a problem for those pumps without recirculating regulators.

Does anyone have the bobbing who have an Airdog or FASS pump?

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 there still is fuel passing through the overflow valve all the time so you'd think it wouldn't be a problem for those pumps without recirculating regulators.

Does anyone have the bobbing who have an Airdog or FASS pump?

Bingo.

When  there  is   2   regulators  within  a  system-   lets  face it.   the   overflow  act as  one too,     there  must  be   some   'fighting'  back and  forth  or  'lag'  within  the  hydrodynamic pressure  'wave'.      

Now... add  the  fact  that  the  engine  is   taking  some out of  this loop   at  different rates....     plus  all the  above  mentioned  possibilities  thrown in..

OH,  btw,    you  ever  check  to see  what  kind of   vacuum you have on your  fuel tank  after  a  long  run?

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OH,  btw,    you  ever  check  to see  what  kind of   vacuum you have on your  fuel tank  after  a  long  run?

 

There you go back to the cavitation because now you'll be primed for low pressure on one side of the pump and high pressure on the other plus the recirculation of the fuel within the pump. It was a known issue even back with the stock Carter pumps. A guy named Gary over on CF create a video showing the cavitation problem with even a stock pump. But now with a slight vacuum on the fuel tank I can imagine it gets worse.

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Good points guys but no, there's no vacuum in the tank.  Matter of fact I actually installed a filter on the vent just to assure that its never plugged up with junk.  Its clean.  And I would certainly consider cavitation except I would "think" that cavitation would be the at its highest probability during idle since thats when the pump is pumping the highest psi in relation to the amount of fuel used.  Why would the pump impeller experience cavitation when more fuel is being demanded?  Since the fuels volume and pressure is under constant regulation at all times, wouldn't more throttle actually decrease the effort placed on the pump?

 

On a side note, I've dealt with cavitation quite a bit since I have a high powered v-drive boat.  When you place hundreds of ponies on a propeller trying to shove a 3000 pound hull up out of the water from a stand still, its takes the right quality prop to not froth up in a mass of bubbles.

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 That  lift pump   is  going to  move  "X"   amount  no matter what: WOT, Idle, or  stalled engine! (at least until the pump shuts down)    What happens immediately   afterward  ( excess  is  either returned  immediately to tank via  lift pump bypass,  or   the overflow at vp..   the  rest  is  injected

The amount of  fuel per hour  going up the draw straw  is  pretty  constant,   regardless of  engine speed

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I'm not saying I'm right and you're wrong rancher but I'm not sure that would be an accurate explanation of the fuel pump.  The amount of volume from the fuel pump cant be more than the head pressure can supply.  Yet the amount of pressure from the fuel pump and in the lines will be directly related to the amount of fuel volume consumed by the engine in relation to the pressure generated.  If the engine has enough fueling then you can pull the fuel pressure in the lines down considerably.  That means that the pressure at the pump impeller should decrease as the fuel supply increases.

 

That is unless I have it backwards whereby the fuel volume running through the fuel pump is "X" amount during idle and cruise but when the fuel demand increases then so does the fuel volume passing through the pump and therefor the pump is pumping at or close to maximum pump volume which may cause impeller cavitation. :think:

 

Notice I use the head scratchy smile a lot.....

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