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.

My electric Autometer UltraLite FP gauge does this on my truck also.  My sender is pre fuel filters, so I don't have a "snubber".  

 

I think is the "regulator" ball moving back and forth.  My regulator is in the return line going back to the tank.

 

Mine will also bob back and forth as the grid heater cycles in cold weather.

 

3rd gen truck here also.

For AirDog or FASS with 3 lines. The return line is the remaining fuel flow back to the tank so if your injecting 2.5 GPH the other 147.5 is flow back on the return lines (injection pump and fuel pump). Now Raptor and other like pumps with 2 lines (no return line) now those return the fuel internally. So again a 150 GPH pump will return the same 147.5 GPH internally and the return line of the VP44. This is where cavitation is possible.

 

The regulator is regulating "X" amount of pressure no matter if its on an Airdog or a Raptor but the amount of fuel being returned to the tank is not the same between those two pumps.  The Airdog has the advantage of a dedicated return line from the pump and the excess return fuel from the VP.  So maybe I'm not understanding what you mean here by what you said.

That's is the point the AirDog and FASS have a dedicated return line that returns the flow back to the tank. The Raptor on the other hand returns within the pump body. Raptors are prone to the caviation issues like the old stock Carter's because of the return within the body. If Raptor was a dedicated return pump then the problem most likely wouldn't exist.

 

As for my volume numbers they are just roughly speaking.

All  3  pumps   have  a   rotor  that   pumps  the  same gph  regardless of   engine load, speed...    only   voltage  change  will   cause  some  variance...   Now,   any  positive  displacement pump  must   either   'deliver' the  fluid....  or  stall out.     The  fuel   must  go  somewhere.

 

So, all  three   use  a  bypass.....AFTER  the  rotor..       FASS and   Airdog   send  the  unused  fuel back to tank..     Raptor   has  an internal   circuit that  sends  the  excess   back to the inlet side of  the  pump itself.   

I thought they were basically the same pump configuration but just on different bases which came with or without filters attached.  The pump isnt "separating" the air from the fuel, the second filter is.  Although I'm sure they've improved on the designs over the years.

Raptor has an internal circuit that  sends  the  excess back to the inlet side of  the pump itself.   

Same as the stock Carter lift pump which has the same problem for caviation because of recirculating fuel back to the inlet.

 

 

 

I guess I've never heard of the issue with Carter pump cavitation.  Thought I've heard them all too.....

Here you go...

http://www.cumminsforum.com/forum/98-5-02-powertrain/21962-cavitation-not-your-vp-44s-friend.html

 

Sorry to say the video is long gone...

I havent had time to drive it with the fuel pressure test gauge connected but I did get a chance to hold up my crappy phone to the fuel pressure gauge and record a little of what it does.  Note that the smoother action of the gauge is normal throttle variances but the quicker action is the "bobbing" down and back up of fuel pressure.  I said "down" because it never bob's upward.....only downward and then back to running psi.  Sorry ahead of time that the video is so poor quality.  At least it shows something.

https://www.youtube.com/watch?v=RccuEPhblAg&feature=youtu.be

Watched the video and my Raptor 100 does the same thing. If I get to a hill it will drop a couple when adding light pedal pressure. Yours does seem a we bit more sensitive.... Do you have 1/2 inch lines all the way, including 1/2'' draw straw?

I have JIC fittings and 1/2" lines from the tank to the VP.  The only fitting which is otherwise is the OEM supply line quick connect fitting on the fuel module.  I opted not to install the drawstraw since there was way too many issues being posted with the drawstraw.  I know they've supposedly solved most if not all of the drawstraw problems but I dont think I'll ever bother getting one.  My fuel psi is OK with the minimal fueling I have.

What are JIC fittings?

 

Like I said, yours seems fairly normal as mine will do the same thing only it does seems a bit more prone to pressure drop than mine.  I've noticed over time my Raptor 100 has  gotten more and more like yours.... like it's.a bit more tired than it used to be as miles pile up.

 

What are some of the old Draw straw problems? Mine is the first prototype?

That video is exactly what mine does too. Almost the exact same motion. Mine has also been doing it more often the older it gets. I just don't know if it is a sign that the pump is going under or not.

 

As for the draw straw, I though that most of the problems people were having were because of them being installed incorrectly. Either not cutting the bottom correctly or cutting it to short. If it installed correctly you should be able to run the tank dry with a straw. I myself am getting ready to put in a tank sump from beans diesel. Really nice product but a little spendy for what it is IMHO.

What are JIC fittings?

 

 

They are a type of threaded fitting. It is very commonly found in hydraulic systems and other pressurized fluid set ups. Here is a pic of a male-to-male JIC adapter.

 

Thank you

The pictures a great example of what a JIC thread and connection type is but the reason JIC fittings are superior to banjo fittings is because JIC fittings have wide open curved bends.  Banjo fittings work in a strange setup whereby having all the fuel flow into the line head and cram around the main bolt to find a hole which leads down a hollow center of the bolt and then into the source.  JIC fittings flow like any wide open flare fitting.

So anyone have any thoughts on why the pressure bobs like that? I wonder if it is a fuel pump issue? Or a gauge issue? Mine is plumbed direct to the gauge in the cab. The only thing that I have other than the open line is a needle valve.

 

On a side note, since my pressure has started bobbing like this I have also noticed that the raptor pump has also gotten a lot nosier. It used to be almost silent, but now it sounds like there arent any bearings left in it or something.

I had a VERY WIERD experience driving home from NoDak yesterday with my electronic Autometer FP gauge after getting back on the highway after a fuel up.

 

Pulled into the station, did everything I normally do when filling up.......................dump my 2 stroke oil in the tank, then swipe my card, wait for the card verification, grab the diesel line, pull my 2 stroke container out and insert diesel line and then dispense fuel.  However;  this pump sounded very odd.....................very AIRY sounding.  Anyhow, the pump clicked off, and I go to top off.................it wouldn't let me top off.  OK, I call it good, and we hit the road again.

 

After we hit the road, the fuel gauge is reading about 3psi lower than normal and is bobbing anywhere from 3psi above normal to 6-8psi below.  It acted this way for probably 50-60 miles before it settled to it's "normal" spot.  I really think I got a bunch of AIR in my diesel at that fill up and as the bubbles went thru the Walbro FP it was causing the huge fluctuations I was seeing.  My truck drove fine, and I couldn't "feel" anything in the way the truck drove while the gauge needle was dancing up and down.  My gauge is next in line after the pump................then my 2 filters..............then the CP3.  

 

That's the only thing I can attribute this to.  What y'all think???

Sounds like thats what happened.

Now that I've done the electronic gauges wit electric sender lets see how good I did and if I'll have any problems with my setup. I kept my needle valve, added the snubber as supplied with the ISSPro EV2 fuel pressure gauge and also remote mounted the sender to the fender. (The old vibration story. ) All my tubing is still 1/8 air brake line.

 

So far I've had zero issue with bouncing gauge reading and still right on the 16-17 PSI just like my old DiPricol gauge did for 10 years. But I've barely got the system setup so let some time go by and we'll see. But I'm also still leaning on the idea of cavitation with none returning pump vs. returning pump like my old AirDog.