Engine/Transmission (1998.5 - 2002) Fuel Pressure driving me crazy...

[chazj]

I have replaced my Autometer 25# FP gauge with a completely new Autometer 30# set-up . Replaced metal in-line filter (the Fleetgard filter replaced 7000 miles ago) Until yesterday (for 3 weeks running) the truck idled at 18#, under normal operating conditions it stayed above 15# and no less than 10# WOT.



Yesterday it started to behave irratically . Pressure jumped to 20-22 #'s. Today while it would still 1) idle at 18#'s 2) Would drop to around 10# from a easy normal start, stay in that zone for awhile & lower if I pushed it ever so slightly. Never went below 7#'s tho. 3) Going down hill and with my foot off the throttle it would stay in the 10-13# range. However if I pushed in the clutch... it would immediately jump back to 18 #'s. Let the clutch back out... pressure dropped again to the previous level.



The Fass Pump sounds the same as it always had ( I have a noisy one )... and the truck's throttle response was completely normal. If I wasn't looking at a FP gauge... I would have not suspected anything was wrong!



I replaced the Autometer Sending Unit for my 15# guage at the beginning of 2011. It also worked correctly for about 6 weeks before it started to behave in an erratic manner. It was at that time I decided to purchase a completely new 30# gauge set-up from Autometer. Unfortunately it looks to have not solved the problem. Am not sure I believe co-incidences... .



Any help or suggestions in where to look next most appreciated.

 

[dpallas]

Curios... . where did you mount the sending unit for the F/P ? directly to the filter housing or remote? I'm getting ready to install a pressure gauge (electric) same one I think your using Auto Meter 30lb. and was thinking about mounting the sending unit off the engine where there are no shakes.

 

[chazj]

Curios... . where did you mount the sending unit for the F/P ? directly to the filter housing or remote? I'm getting ready to install a pressure gauge (electric) same one I think your using Auto Meter 30lb. and was thinking about mounting the sending unit off the engine where there are no shakes.

It's up by the firewall.



The 15# set-up was install in June 06. Everything worked well until Jan of 2011... then 15# gauge started to act up. Replaced sending unit... & it worked well for about 6 weeks... then erratic again. Decided to replace the entire set-up w/ 30# kit & was good until recently... #@$%!

 

[dpallas]

Okay thanks, will be doing install this week

 

[seafish]

I think that I have read somewhere that even with the Electric FP gauges it is important to use a fuel line snubber... has somehting to do with the very high pressure fuel pulses that the CR pumps on these engines produce.

Before you install the new one and waste it, you should do a TDR search for "snubber" and see what comes up... . like this--

https://www.turbodieselregister.com...378-snubber-electric-fuel-pressure-gauge.html

 

[chazj]

I think that I have read somewhere that even with the Electric FP gauges it is important to use a fuel line snubber... has somehting to do with the very high pressure fuel pulses that the CR pumps on these engines produce.



Before you install the new one and waste it, you should do a TDR search for "snubber" and see what comes up... . like this--



https://www.turbodieselregister.com...378-snubber-electric-fuel-pressure-gauge.html

I wish the answer were this simple Seafish... Have always had a snubber installed since Day 1.

 

[Hillflyer]

Electronic Gremlins

Sounds like you have defective sending unit or an electrical short. electronic gremlins, the worst kind of entertainment I can think of. You might want to get out the multimeter and test a few wires and resistance across the unit.

 

[chazj]

Sounds like you have defective sending unit or an electrical short. electronic gremlins, the worst kind of entertainment I can think of. You might want to get out the multimeter and test a few wires and resistance across the unit.

Had it checked out yesterday with mechanical gauge. PSI never went below 12#'s WOT under load. Because the sending unit and gauge are brand new, I'm leaning towards a wiring issue of some sort... bad ground etc as all the connections look to be in perfect shape.

 

[Troubleshooter]

Just out of curiosity, when you "bump start" the key (to get the FASS pumping without actually starting the engine) does the fuel pressure remain steady, or does it give you the same erratic readings as you are getting with the engine running?

 

[rweis]

I take a slightly different approach.



Rather than try to balance the fuel GOING to the VP44, I bypass fuel COMMING OUT of the Vp44 back to the tank with a mechanical bypass valve. Then I feed the VP44 with more fuel than it can possibly swallow from a rotary gear pump (RASP) that does not create any pump cycle pressure oscillations.



I also took out the OEM return lines and replaced them with 3/8" open fuel lines (with the mechanical bypass valve in the line). When I start the engine I see the fuel psi rise to 15 psi (bypass setting) and stay there regardless of engine speed. Ther are no pump oscillations in the rotary gear pump, and the bypass valve modulates the return flow so the VP44 ALWAYS sees 15 psi.



Bob Weis

 

[chazj]

Just out of curiosity, when you "bump start" the key (to get the FASS pumping without actually starting the engine) does the fuel pressure remain steady, or does it give you the same erratic readings as you are getting with the engine running?

When I bump the starter, the Fass kicks-in and holds a Steady 18#'s until the pump turns-off. The it drops back to zero.



Took me 6 tries to "just bump the starter" & get a pressure reading. Dang truck kept starting on me with just the slightest bump of the starter...

 

[Hillflyer]

If you had good grounding and resistance across the sending unit, and no wires rubbing or kinked (shorts) it probably lies within the fuel system itself. I believe the pump allows the fuel to flow back to tank when it doesn't run, so fp going back to zero doesn't really tell you much. I would be looking for damage on your fuel lines (pin holes) and check the pressure relief valve on the VP it may need replaced. Your hitting 18 psi , It goes off at 14 psi. I would turn down your Fass to run 12-14 psi and run it again to see if the erratic gauge fluctuations go away. I was under the impression that the VP44 is designed for about 12psi and above those pressures damage may be happening to the unit?

 

[Hillflyer]

One other simple thought, have you checked the fuel filters lately or changed them out?

 

[chazj]

One other simple thought, have you checked the fuel filters lately or changed them out?

Filters were checked ( just changed 9K ago ) and were still very clean!

Mechanical fuel gauge was connected at injection pump (in place of existing line)... truck driven City-Hwy-WOT conditions... never went below 12 lbs PSI.



Autometer set-up completely New (30# sending unit, Gauge). All connections CLean. Looks as though the gauge maybe around 5 Lbs. off (LOW) under load conditions. Gremlins working OT I guess... .

 

[rweis]

There is a thread called "pump, lines and what not" that DTT tested a Vp44 and fuel pressure from 10 - 25 psi. I think if I remember correctly that about 12 - 20 was the fuel range and damage to the diaphram did not occur until about 22 - 24 psi.



I run mine at 15 psi and have for almost 10 years. I run a mechanical gear pump (RASP) and bypass vlaves after the VP44.



Oh, one more thing: I filter the fuel as it comes out of the tank for water seperation with a RACOR 10um then through the fuel system, then through a 2um filter about 8" before the VP44 to catch anything that might have sluffed off the inside of the fuel lines on the way up to the Vp44. As I recall the vp44 can handle 10um but no larger.



Are you sure that the fuel is totally clean as it goes into the VP44? No water, no particles so the vp44 is not passing the fuel erratically?



Something else to consider, the return line back to the tank after the engine T where the injector drain joins the vp44 bypass comes out from the T in a line that is about 1/16" and that may be clogged.



Bob Weis

 

[Hillflyer]

It is amazing the mechanical gauge would be that far off from the electronic, but typical past experience would say the electronic gauge would be more accurate. I know of no one who does comparison testing anymore. . . manufacturers tend not to like that.

If the last few audio electronic pieces of equipment I've bought are any indication of the gauge industry, 'the race to the bottom' in quality is gaining steam.

 

[Troubleshooter]

When I bump the starter, the Fass kicks-in and holds a Steady 18#'s until the pump turns-off. The it drops back to zero.

What I find interesting here is that the problem seems to manifest itself ONLY while the engine is running. When it's just the FASS, he's not seeing any fluctuation, only after he starts up.



Makes me wonder whether the problem is being caused by vibration; either to the fuel lines, to the sensor and /or snubber, or perhaps a short in one of the wires or grounds. Might want to look into that as well as the other suggestions which have been made.

 

[CEdwards]

I take a slightly different approach.



Rather than try to balance the fuel GOING to the VP44, I bypass fuel COMMING OUT of the Vp44 back to the tank with a mechanical bypass valve. Then I feed the VP44 with more fuel than it can possibly swallow from a rotary gear pump (RASP) that does not create any pump cycle pressure oscillations.



I also took out the OEM return lines and replaced them with 3/8" open fuel lines (with the mechanical bypass valve in the line). When I start the engine I see the fuel psi rise to 15 psi (bypass setting) and stay there regardless of engine speed. Ther are no pump oscillations in the rotary gear pump, and the bypass valve modulates the return flow so the VP44 ALWAYS sees 15 psi.



Bob Weis

Wish I had read this before I just ordered a fass ddrp. I got sick of dealing with fuel problems and decided to replace everything from a draw straw to the filter. Wish I had done better research on the rasp first though, this sounds to me like a better setup. Bob what did you use for the bypass?

 

[CEdwards]

Chazj, I too have been chasing fuel pressure problems that are erratic. I have a mechanical gauge in cab. Ran good for years, one day dropped to 5psi or less and I put a new fuel pump on. Ran fine for an hour and since then has been erratic. Sometimes if I just turn the truck off and restart it the fuel pressure will be different for that day. One day I rigged the suction into my 50 gal. aux. tank on the back and fuel pressure was perfect. Any way my point is, just get rid of the original lines. There really is no way to nail down exactly where a leak may or may not be. It could change day to day. Just save yourself the trouble and replace everything. I have chased the problems round and round in both the 12 valve and 24 valve and for me it has always been in the factory lines. If it was the pump or the bypass you can narrow that down. Erratic points to lines. No 100% way to test beyond a shadow of a doubt. Just replace for peace of mind. Forgot to mention I saw where you tested with a mechanical gauge and never got below 12psi, but was that only one time driving. Most days driving with my problem I could stay above 10psi but the next day maybe only 5psi wot, etc. Just one test drive may not eliminate the gauge as being the culprit.

 

[rweis]

I think you are on the right track, that the fuel system is a SYSTEM from the tank to the VP44 back to the tank.

What we did was exactely what you are suggesting:

1. Drawstraw to get the fuel out of the tank,

2. Take all of dodge fuel lines out and replace with 3/8" ID lines.

*** be especially watchful about the line that goes from the engine T to the frame. You will not believe your eyes when you take that apart and see the 3/32" ID return line***

3. Provide way more fuel than the Vp44 can swallow and bypass the remainder.

4. Insure you can give the vp44 a steady 15# under all conditions (should not vary if you are bypassing the excess because the bypass will just not bypass as much)

5. Return the hot fuel back to a different place than you picked up the supply fuel so the tank volume can absorbe some of the heat from the vp44.

6. Cool the return fuel with passive fuel coolers if you think you want to do that.

7. Provide some sort of after engine shutdown cooling to the vp44 to keep the latent engine heat from heating the vp44 processor chip and killing it. (Also applies to the ECM)

8. Measure your fp where the vp44 cycles will not beat your guage to death. (As I recall one person hooked a 1 quart tank to the fuel input to the vp44 and put his guage on that so the 1 qt tank would buffer the vp44 cycles. I take mine AFTER the vp44 and BEFORE the fuel line return bypass valves (set at 15#) to prove the vp44 has 15# INSIDE of it and comming out rather than GOING to it (i. e. keep it full of fuel after the fact).



Bob Weis