Been reading up on the problematic OEM lift pump & aftermarket solutions. I have to say I'm a bit confused as to why the aftermarket pumps have a relief valve. I can understand that there needs to be a return to the tank from the lift pump to dump the air that has been removed from the fuel. But doesn't the OEM fuel system have a pressure relief valve that maintains supply pressure, and bleeds the excess into the fuel return line at the injection pump? Also doesn't the excess fuel used to cool the IP? If the pressure relief valve on the LP opens up, what happens to the required extra flow to the IP? Is the OEM pressure relief/return system overwhelmed by the aftermarket pump's flow so it cannot control pressure? So to compensate the aftermarket LP pressure relief is set at a slightly higher value than the OEM system's pressure relief valve, acting like a secondary pressure relief? That set-up may allow all the flow the OEM system can handle but prevent an over pressurization of the IP - just guessing here. I saw a YouTube video Michael made showing a slow flow of fuel running into a bucket out of an open hose. Does that represent the 70% of supply flowing back to the tank? Thanks for your time,Joe in St Louis

Been reading up on the problematic OEM lift pump & aftermarket solutions. I have to say I'm a bit confused as to why the aftermarket pumps have a relief valve. I can understand that there needs to be a return to the tank from the lift pump to dump the air that has been removed from the fuel. But doesn't the OEM fuel system have a pressure relief valve that maintains supply pressure, and bleeds the excess into the fuel return line at the injection pump? Yes

Also doesn't the excess fuel used to cool the IP? Yes

If the pressure relief valve on the LP opens up, what happens to the required extra flow to the IP? The lift pump can pump a lot more than the VP44 can use or even relieve through the overflow valve, that is why the lift pump has it's own pressure relief so that it doesn't build too much pressure into the system.

Is the OEM pressure relief/return system overwhelmed by the aftermarket pump's flow so it cannot control pressure? You got it!

So to compensate the aftermarket LP pressure relief is set at a slightly higher value than the OEM system's pressure relief valve, acting like a secondary pressure relief? Yep

That set-up may allow all the flow the OEM system can handle but prevent an over pressurization of the IP - just guessing here. Yep

I saw a YouTube video Michael made showing a slow flow of fuel running into a bucket out of an open hose. Does that represent the 70% of supply flowing back to the tank? That is showing the fuel going through the overflow valve. The lift pumps everyone use can pump 100-150GPH and that shows that the VP's overflow cannot handle it, which is why there is the relief on the LP

Thanks for your time,

Joe in St Louis

Answered them all in whatever color that is. Dark blue.

ISX nailed it on the head, but would like to add that their is a return line on the injectors. i believe that the "70%" of fuel returned to the tank is from the combination of both returns.in mopar1973mans movie, the first shot is without the truck running. it shows that even at reasonable pressures there is little flow thru the VP44. when the truck is running he is getting a lot fuel thru the relief valve...and there is still fuel being returned from the injectors

ISX nailed it on the head, but would like to add that their is a return line on the injectors. i believe that the "70%" of fuel returned to the tank is from the combination of both returns. in mopar1973mans movie, the first shot is without the truck running. it shows that even at reasonable pressures there is little flow thru the VP44. when the truck is running he is getting a lot fuel thru the relief valve...and there is still fuel being returned to the tank from the injectors

On my 12V, the only fuel being returned is the fuel going out of the overflow valve on the p7100. The return lines on the injectors T into the fuel filter. I don't know if your's does the same or not. I know the 24V has a return for the injectors on the back of the head but I never traced it out.

These guys can explain it better than I can: http://www.pureflowairdog.com/how.html

Yes, both Airdog & FASS systems look like the real deal, & lots of testimonials out there. Might be the way I go too. First things first - fuel pressure gauge (parts in the mail).Another couple questions if I may, specific to my '99.Looks like the installation of an aftermarket lift pump system with filter & water separator makes the OEM fuel filter obsolete. I've seen examples of installations that bypass the OEM filter & route the discharge from the lift pump directly to the VP44. So what about the fuel heater; isn't it in the OEM filter assy.? Or is that a water-in-fuel sensor who's wires are passing through the grommet in the top of the filter assy (I have seepage there)? Is the heater/water sensor unnecessary (St Louis area, seldom below 0 deg F.)? What about the ECM; is it looking for an input from the heater and/or water sensor? If there is an open in that circuit will it turn on the "check engine light"?Thanks for your patience - appreciate the help!Joe in St Louis

You can route the fuel line through or around the OEM fuel filter it will work either way. Many can get away without their fuel jelling bypassing the OEM fuel filter which incorporates a fuel heater element. I personally routed my Air Dog and 1/2 inch fuel hose lines through the OEM fuel filter cannister deliberately to obtain the benefit of the fuel heater. The ECM gets no input from the fuel heater or OEM fuel cannister although the WIF sensor does illuminate a light on the dash. At least that is my understanding of it. If you live in a climate that encounters cold weather, enough to possibly jell or freeze the fuel, I strongly recommend routing the fuel lines through the OEM fuel filter.

I have the parts in stock to keep your stock fuel canister if you don't want to by pass it

:cool:Raptor 100 is a solid pump. Easy to install with the line kit provided. I also got the big line kit to go from fuel filter to VP. Very good product for the money.

Marking answer in RED...

Yes, both Airdog & FASS systems look like the real deal, & lots of testimonials out there. Might be the way I go too. First things first - fuel pressure gauge (parts in the mail).

Another couple questions if I may, specific to my '99.

Looks like the installation of an aftermarket lift pump system with filter & water separator makes the OEM fuel filter obsolete. I've seen examples of installations that bypass the OEM filter & route the discharge from the lift pump directly to the VP44. So what about the fuel heater; isn't it in the OEM filter assy.?

AirDog and Full FASS has filets already and does away with the sotck filter can. But now Raptor you must keep the stock filter can beacuse the Raptor or the FASS 95 don't have filters.

Or is that a water-in-fuel sensor who's wires are passing through the grommet in the top of the filter assy (I have seepage there)?

One of the members here has a write up on how to repair that problem and has a whole bag of the O-rings needed to do the job.

Is the heater/water sensor unnecessary (St Louis area, seldom below 0 deg F.)?

As for fuel heater usage it only needed if you have un-winterized fuel. As for the water sensor it will be up to you to check you water seperator regularly.

What about the ECM; is it looking for an input from the heater and/or water sensor?

No and Yes. No the ECM has no control on the fuel heater... Yes the ECM looks for input from the Water sensor. But that the thing the water sensor plug gets wet then it trips the light so you must put a plastic bag over the plug and zip tie it on.

If there is an open in that circuit will it turn on the "check engine light"?

No. Error code are produced... Except for if the water sensor plug stays wet to long then you might get a code.

Thanks for your patience - appreciate the help!

Joe in St Louis

Hopefully that helped.

Great info Mike.

Just my worth. I installed the FASS some time ago and plumbed it through the OEM fuel filter. Guess my thought was that the filter assembly was good and the problem has always been the crappy OEM lift-pump. I figured that an added filter might not be such a bad thing. I still change the OEM filter but not as often. So far so good.

MoparMan wrote...

One of the members here has a write up on how to repair that problem and has a whole bag of the O-rings needed to do the job.

Yo Mike, it was me who started the thread back in November. I also did the write up on fixing the fuel filter leak & have the bag-o-rings if anyone needs a couple.

As for the fuel lift pump, I installed a gauge at filter discharge port & found my pressures low, around 5 psi @ 60 mph.

I went the cheap route & installed an Airtex fuel pump (~$125 with discount coupons). Fuel pressures have been good with minimum reading while underway at 13 - 14 psi. Two observations;

1) The fuel pressure gauge operates like an economy (vacuum) gauge on a gasser. As the engine load increases the fuel pressure decreases.

2) As engine/ambient/under hood temps increase, so does the fuel pressure. On a cold engine this winter, Ill see the fuel pressure drop to 13 psi pulling away from a stop driving like I have an egg under my foot. As the engine warms up, like on my drive to work (18 miles, 13 on interstate), it will get close to 15 psi at 55 mph by the end of the drive, even higher now that the weather is warming. I just got back from a trip out to Salina (415 miles each way) and outside temps got to the mid-60s. I was running cruise control 67-68 mph (4.10 rear axle) & was seeing 15-16 psi at those speeds while climbing the Flint Hills. Was seeing close to 21 mpg. There was even a 120 mile stint pulling a 10K 5th wheel where I saw 15 psi at 60 mph on the flats. Idle fuel pressures once good & hot are 18-18.5 psi.

There are no mods on my truck other than an aftermarket air filter. I'm hoping the Airtex will keep up when I finally get around to mods.

Joe in St Louis

Can't really complain about those pressures. The reason the pressure goes up with temp is becuae the fuel warms up, and is easier to pump.

When does the fuel heater kick in? Also Mopar, the link about the write up concerning air in diesel fuel diesnt work for me. Been reading up on lift pumps and found out about air in diesle fuel is bad. I have a walbro 392 with about 130K on it and still running good. My bypass reg is going out, so I starte9 searching lift pumps to see what I should set the fp too.

When does the fuel heater kick in? Also Mopar, the link about the write up concerning air in diesel fuel diesnt work for me. Been reading up on lift pumps and found out about air in diesle fuel is bad. I have a walbro 392 with about 130K on it and still running good. My bypass reg is going out, so I starte9 searching lift pumps to see what I should set the fp too.

Fuel heater kicks in at 60*F and heats till 70*F or so. This is NOT ECM controlled...

Air in the diesel is bad because air is compressable. Just like your brakes on your truck if you have air in the brakes the pedal is mushy. Well now same thing on injection pumps. Injection pump is like you master cylinder and the air in the fuel is just like air in the brakes so it takes more force of the injection pump to pop the injectors hence the timing is LATE and the power is reduced.

Good fuel pressure is between 14-20 PSI. This will hold the overflow valve open and keep the vP44 electronics cool and the pump happy. Below 14 PSI the overflow valve starts closing and is completely closed by 10 PSI now you extra cooling and lubing is SHUT DOWN... Hence why the pressure gauge is marked in yellow 10-12 PSI...

Same sense of running these engines low in oil pressure the engine will still run fine at 10 PSI of oil pressure but don't expect the engine to last long. Just like the injection pump if the fuel pressure low then wear and damage will occur. Remember the only lubricant the VP44 sees is the fuel itself so if you low on pressure you loooking at doing damage.

When does the fuel heater kick in?

When the fuel temperature is below 45 ±8 degrees F,

the temperature sensor allows current to flow to the

heater element warming the fuel. When the fuel temperature

is above 75 ±8 degrees F, the sensor stops current

flow to the heater element.

Also, I don't think air is as a big of a deal as 2 companies want you to think it is. If it was then you would see OEM air separation on some level, on some enigne.. but I know of none.

Ok thanks. Makes sense on the air. I can see a big prblem with a lot of air/cloudy fuel from emulsified air in it. I did go ahead and turn the reg up as it started fluctuating on the highway. would bounce from 10 to 5 psi. I would jump on it real quick, just to basically puff smoke out and pressure would go back up. Its running about 13.5 psi now and doesnt drop really at wot either.

When the fuel temperature is below 45 ±8 degrees F,

the temperature sensor allows current to flow to the

heater element warming the fuel. When the fuel temperature

is above 75 ±8 degrees F, the sensor stops current

flow to the heater element.

Also, I don't think air is as a big of a deal as 2 companies want you to think it is. If it was then you would see OEM air separation on some level, on some enigne.. but I know of none.

Thanks... I was trying to work from memory...

Hi

This is article #2 dealing with the diesel fuel system. I hope you find it useful.

Rob Parelli

--- Update to the previous post...

I do apologize for posting an incorrect link. Use this one. Article number two.

Rob