2nd Gen Non-Engine/Transmission fuel pressure gauge vibration

[dieselvet]

I have Isspro gauges and the fuel pressure gauge is mechanical. Braided SS line goes from the fuel canister banjo bolt to the gauge mounted on the dash in a Rod's pod. (which is sweet by the way!!). Been using the gauges for about 14 months and 50k miles with no problems.



Took the truck to the dealer for a new lift pump (original pump lasted 99,600 miles, not too bad!), next week the APP sensor went out, so, back to the dealer at 99,900 miles. All fixed under warranty.



Now there is annoying vibration coming from the fuel pressure gauge. It is loudest at idle and gets quieter as you add pedal (but still annoying).



Why would a new LP or APP sensor do this? The only thing I can think of is my idle fuel pressure went from 6 psi to 15 psi with the new pump. The gauge did not vibrate when the original pump had idle pressures of 12 psi.



Any ideas on a fix? Mechanical isolator??? Never had ANY fuel in cab leakage issues, so I did not even consider putting in an isolator when I did the original gauge install.





Rich

 

[DLBurns]

My FP gauge was vibrating due to the pulses from the VP-44, I installed a needle valve in line before the isolator and it cured the problem!

 

[R-N-R]

Nice installation Buff!



A needle valve cured my gauge needle flutter.

 

[DLBurns]

Nice installation Buff!



A needle valve cured my gauge needle flutter.

I can only take credit for the needle valve install, the isolator was installed by Blair Pine.

 

[dieselvet]

Thanks, You jogged my memory, I remember reading somewhere about the needle valve install or it seems like you can close almost all the way down the shutoff valve on the hose to the gauge, to stop the vibration.



It still seems strange that this all started after the LP installation.



Regards



Rich

 

[wes reeves]

I've been fighting vibration issues with my Isspro mechanical fuel pressure guage since it was new. I originally installed it with a braded stainless hose from the banjo bolt on the VP 44 to the guage. It vibrated something awful and I thought a rubber hose might transmit less vibration so I replaced the stainless hose with a rubber one. That solved the problem for about 6,000 miles then the vibration noise slowly came back. I moved the hose from the banjo on the VP 44 to the banjo on the fuel filter canister and that seemed to solve my problem for about 500 miles then the noise started to come back. Now I've figured out that every time I disconnect the hose and empty the diesel out of it the noise goes away until the hose fills back up with diesel.



With the original vibration and the braided stainless line i could actually feel the guage vibrate. The vibration I have now, I can't feel at all. It just sounds like something is loose inside the guage. I've been thinking about buying a new guage to see if that's the problem, but I would like to know if anyone else has the same problem first and any possible solutions.

 

[rweis]

I had sort of the same problem with electronic pressure sensors and the VP. I monitor both pre ff and pre VP.



I use Westach solid state sensors. I thought that the solid state would eliminate any failures for several years. WRONG for the VP, RIGHT for the lp.



I tap into the fuel line just before the VP. I run AN6 ss line through the whole fuel system (except for the VP return line). I have a bypass regulator after my OEM lp to keep as even a pressure to the VP as possible (12#).



I had previously had 2 sensors on the VP line that failed. Both of the failed (and replaced by Westach for free) solid state sensors were on about 24" AN6 snubber lines. This time (third time) I decided to put ALOT (10 feet) of AN6 line (about 6 cubic inches of fluid) and wrap it in a non regular way so any pulses transmitted from the VP would have to go a LONG way through the fluid as well as make LOTS of turns. Of course the pressure registers the same regardless of the distance of the line. So far so good for about 6 months.



Before I would see a kick up in the fp on the VP side by about 3 - 5 # when slowing down from a fairly sustained high engine speed (interstate or hard acceleration for 15 seconds (pulling the RV) to idle (like a stop light). I think it showed VP pressures bleeding back internally into the VP input port and back down the fuel line until the spikes were smoothed by normal fuel flow. The spikes would only last about 3 - 5 seconds. That is a seperate indication from what I had read about what is believed to be "normal" pulsations from the VP.



So far so good. I don't get any abnormal fp readings regardless of what the engine speed functions have been or what the time frame has been. I don't know if the length of the line and turns to the sensor makes the difference, or if the volume of the fuel in the line makes the difference. I do know something is different. I used the 10 ' line because I had it. I was thinking of making a fuel accumulator vessel in the VP feed line and mounting the VP fp sensor on that. I don't think the back pressure from the VP is a concern to the VP since 12# - 14# is the recommended by Bosch, but the fp gauges are a WHOLE lot more steady.



A secondary benefit is it is super easy to bleed air from the fuel line on ff change cause the VP fp sensor connector is right up by the brake reservoir. Change the ff, crack the VP sensor, tighten the fitting when the lp bump blows fuel through the loose fitting.



Bob Weis

 

[WestTN]

You may have had a small air bubble trapped on the gauge side of your isolator. Higher pressure means a smaller bubble and it may now be unadequate to absorb the pulsations.

 

[dieselvet]

OK I figured it out (with your help!!)



The thing that confused me was why did all this start right after a new LP??



The answer is the mechanic opened the needle valve I had closed down when I did the install. I cranked the needle valve back down to almost closed and no more vibrations.



Gettin old is heck, I should have figured it out before I took the dash apart!!



Thanks, Again



Rich

 

[BPeck]

VP44 pressure pulses explained

I've been lurking a while and have read many threads addressing the VP44 pressure pulses or fuel pressure spikes. These spikes have been attributed to both the Carter P4601HP lift pump (LP) and the Bosch VP44. They cause problems such as early failure of fuel pressure gauges and fatigue/failure of the LP spillover ball valve (apparently the spring fatigues or breaks), which can lead to starving the VP44 injection pump for fuel thus affecting it's lifespan.



These pressure spikes are most noticeable at idle. Here's a typical post: "there is annoying vibration coming from the fuel pressure-gauge. It is loudest at idle and gets quieter as you add pedal. "



Like many others, I installed a mechanical pressure gauge (US Gauge, no snubber, not glycerine filled, a cheapie... ) on the the output side of the fuel filter on my '99, and the pressure spikes caused the Bourdon tube in this gauge fatigued and ruptured after about 3 weeks, dumping fuel at about 1 gpm; fortunately it ruptured in the final 1/4 mile of my 30 mile one-way commute, so I didn't lose much fuel. But I've fixed the problem for good now!



Here's my theory on these pulses: they are "water hammer" pressure pulses returning upstream from the VP44 as the vanes in its four-vane pump section pass the fuel intake port at low RPM; this stop/starts fuel flow in the supply line, leading to shockwaves propagating upstream through the filter housing and getting absorbed by the little bitty ball and spring spillover valve in the Carter LP. All you need to do is install an accumulator (hydraulic shock absorber) between the LP and the VP44 to absorb/dissipate the "water hammer" pressure spikes, and you should see much longer service life from the lift pump.



Theory development:



The Carter 4601 "lift pump" (LP) is a constant volume rotary vane type (four vanes) and develops 16 psi as limited by a spill-over bypass valve (a ball-check return to intake) to regulate pressure gain to 16 psi. The four vanes spin continuously at a ~ 5000 rpm (20,000 pulses/min, 333 Hz), producing about 80 gph under no load.



The Bosch VP44 (high pressure injection pump) also has a four vane rotary pump in its first stage, but much larger and more robust, reportedly making about 300 psi, most of which is returned to the tank via spillover valve presumably set to around 300 psi (similar to but much heavier duty than the mechanism in the Carter LP). The VP44 is rotating at 1/2 engine rpm, or about 400 rpm at idle (where the pressure spike/vibration problem is most evident), thus with four vanes it has 4*400=1600 pulses per minute, or 26 pulses per second (26 Hz).



The amount of fuel that passes through the VP44 is not determined by the lift pump at all. The internal vane pump in the VP both feeds the high pressure stage and circulates fuel as coolant/lubricant; it also limits throughput to its displacement volume which I estimated to be about 2. 5 cc per revolution, or about 0. 6 cc of inter-vane displacment. So at idle at 800 rpm the VP is spinning (at 1/2 engine speed) at 400 rpm, with 1600 vane swipes per minute, and about 1600*0. 6=960 cc/minute or about 0. 25 gpm (15 gph). At highway cruising speeds, at say 2000 engine rpm, the VP is running at 1000 rpm, with 4000 vane swipes per minute, and about 4000*0. 6=2400 cc/min = 0. 63 gpm (38 gph). So the Carter lift pump may be capable of pushing 80 gph, but the (stock) VP only accepts between 15 to 38 gph under idle to cruising conditions.



Because of its internal rotary vane pump, the VP44 fuel intake rate is variable: as a vane slides past the intake port flow from the lift pump starts filling the inter-vane volume with Q (flow) rising from zero to some peak rate determined by volute shape/eccentricity and rpm. However, inflow briefly stops as the next vane passes the intake port. So the fuel delivery rate changes from zero to some peak rate as the inter-vane area fills up, and this rate change (delta Q) is occurring at about 26 Hz at idle, and causes fuel in the delivering line to stop and start, causing the notorious "VP vibration" or upstream pulsation ('water hammer' effect), most noticeable under low rpm conditions.



A high quality, high durability accumulator installed in the fuel line just upstream from the VP44 would attenuate these shock waves. The best available small accumulator is the Hydropad pulsation dampener by

Flexicraft Industries (http://www.flexicraft.com/hydrop1.asp?catid=6), but their smallest unit (16 cubic inch) runs about $650. However, I installed the Watts 150A "water hammer arrester" with butyl membrane (http://www.watts-intelliflow.com/hammer.htm); this is plenty big enough, and costs $15 to $30 depending on where you find it. I have a a second 4601 pump back by the tank, so my two pumps in series make about 24-26 psi at the fuel filter, and the Watts 150A installed in the short rubber hose connector between the filter and the VP (on my '99) absorbs the vibrations. I'll let you know if I have any problems, but I think this setup will work for a long time. I have a diPricol FP gauge on the downstream side of the filter reading ~25 at idle, 23 at WOT.



--

Brian

Reno, Nevada

 

[R-N-R]

Welcome to TDR Brian!! Very cool write up, I look forward to reading more of your knowledge and insight.

 

[Walt-L]

Great write up! How many miles do you have on the 150 so far?

 

[Rman]

Brian do you experience any hard start conditions with this kind of fuel pressure? It's been pretty thoroughly covered here in the past that the VP44 injection pump doesn't care much for high pressures... . at least at start up. I'm pretty sure that the carter LP's are only at half cycle at startup so perhaps you aren't seeing any problems. Good explanation of the water hammer effect!

 

[rweis]

See previous post of mine.



I put a 10' section of line on my VP fp sensor after 2 repeatative failures. I also had already put a bypass regulator after my lp down by the frame set at 12 psi and bypassed to the fuel tank burp line.



With your theory, which sounds good at first glance, the pulsations would come down from the VP, have to make the 90* turn down the 10' (13 cubic inch) sensor line that is wrapped irregularly (figure 8, and oval), make all the turns, deteriorating the pulse as it makes the turns, further deteriorating the pulse with rubber hose wall absorbtion for 10'.



Then the rest of the pulse would have to go through the OEM filter which should slow it down, down to the bypass regulator which provides a path for the spikes to the tank burp line thus protecting the lp.



Maybe dumb luck,



Bob Weis



ps 50k and fine so far. VP fp is doing fine. I do see a short slight VP fp rise occassionally coming from high rpm to low rpm quickly. Dissipates within a second or two. LP fp stays steady and seldom ever moves from bypass regulator preset of 12 psi.