Engine/Transmission (1998.5 - 2002) From Carter to Walbro...

[Gary - K7GLD]

Yeah, another "Walbro" install - but with a few minor twists.



My OEM LP and added Carter pusher have been operating flawlessly for 50K miles - and no REAL need to change - but I do also have a Frantz sub-micron fuel filter installed, which adds significant restriction to fuel flow, especially under high engine loads such as mountain grades towing our 5th wheel.



SO, after hearing a year or more of enthusiastic claims about the Walbro pumps, I decided to try one myself. First, I made up and installed a homebrew regulator valve to bleed off the Walbro's excessive flow and PSI



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That is installed immediately in front of the VP-44 where my fuel PSI is also monitored, since it's the PSI my VP-44 sees that I am most interested in keeping as proper and constant as possible. I ran this setup with the OEM LP, relocated to the frame rail near the added pusher, for several weeks to make certain the regulator functioned properly - I got a steady 15 PSI under most normal operation - but higher loads would see PSI drop to 10 PSI - sometimes a bit lower. It was OK, but I wanted steadier and higher PSI, especially at higher engine loads - so it was time for the Walbro.



I removed the Carter 7 PSI pusher I had been using, and swapped in the Walbro - which also included the same added automatic fuel bypass valving I have used on both the OEM LP and pusher to avoid fuel flow failure if the pump fails:



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The OEM LP is still in the fuel line further up the line from the Walbro, but is disconnected electrically, and the fuel flow is routed around it by it's included automatic bypass valve. If the Walbro should fail, all I need do is disconnect the power lead from the Walbro, and connect the OEM LP - which would probably take only a minute or 2.



Result at this point is a typical 20 PSI as measured right at the VP-44 - dropping slightly to 18 under most driving situations. Heavy, WOT operation when towing our 5er up a steep grade drops PSI to about 14 - which is entirely adequate and acceptable in my opinion - undoubtedly the spread in PSI would be considerably less without the added Frantz restrictive fuel filter - but the combination of Walbro and Frantz gives me excellent fuel filtration, combined with excellent fuel flow and PSI.



OH, do the added bypass valves really work? YUP! Immediately after installing the Walbro, I started the truck to make a run into town - and my low PSI alarm and warning light activated. The engine was running fine, but PSI was at ZERO! I wiggled the power relay that controls the power to the fuel pump(s) - and the Walbro started up as it should, with normal PSI.



This happened a couple of more times before I got time to closely inspect the connections at the base of the relay, and found a loosened solder connection - which was quickly fixed.



Without that added bypass valve at the Walbro, the total fuel flow would have been shut off, and the engine would not have operated. As it was, It might not have operated well at any significant load, but WOULD at least get me to a safe place to provide an inspection and fix!

 

[BTowler]

Gary, I experience the same range with the factory filter, 20psi idle never below 14psi wot. It has more to do with fuel flow than the filter. The Walbro flows in the neighborhood of 90 gph, more than enough to supply the necessary fuel.

 

[Gary - K7GLD]

Gary, I experience the same range with the factory filter, 20psi idle never below 14psi wot. It has more to do with fuel flow than the filter. The Walbro flows in the neighborhood of 90 gph, more than enough to supply the necessary fuel.

Interesting - I've heard the internal passages in the OEM filter were pretty restrictive - could be they're more of an issue than I thought!

 

[BTowler]

I am planning on switching to the filter/seperator combo from Glacier and totally bypassing the factory filter. I will post the pressure readings after that (sometime after I recover from the transmission expense).

 

[Gary - K7GLD]

I am planning on switching to the filter/seperator combo from Glacier and totally bypassing the factory filter. I will post the pressure readings after that (sometime after I recover from the transmission expense).

At least keep me in mind if you do - I'd be interested in what you find...

 

[rweis]

My local auto parts shop builds extreme hp dragsters and knows a lot about fuel flow and psi's.



He convenced me that the whole OEM fuel and filter system was totally undersized. His general rule of thumb is AT LEAST 2x system capability to maximum fuel usage capability (ie at least 90 gph filters for a 45 gph VP44). There is always some degradation in fluid dynamics and 2x is usually sufficient to overcome the degradation.



I run a RACOR 690T (90 gph, 10um) right out of the tank and a CANTON #25-915 in-line housing with -12 O-ring inlet and outlet ports extreme racing filter (8um) just prior to the VP44.



I orgionally took my OEM ff off for temperature reasons. It turns out it needs to be changed for fuel supply reasons as well.



One thing I am reading about the pre VP44 bypass valves and the ability to regulate the psi very precisely, but the regulation range seems to be about 6 psi, and I think it is because of some flow restriction maybe in the OEM ff. There are a couple of members doing this and they are very good problem solvers and good system modifiers. However, I do a pre VP44 bypass valve regulation as well (just almost exactely like theirs BUT with much higher flow rate capability), but I think because I have at least 2x fuel flow to usage that I can regulate my VP44 psi much tighter to within 2 psi. I am NOT saying I have the silver bullet, but having 2x excess flow at good psi's gives you better ability to supply the VP44 and keep the supply pressure within a narrower range as the VP44 volume demanded fluxuates with rpm and load (towing). I tried to pattern off a comment by Brady (II) that the input psi to the VP44 Bosch test stand is a constant 13. 5 psi with varying flow demands as they calibrate the VP44's.



I would be really courious if Gary put 2 parallel bypasses (side by side) at the VP44 would his change in psi's (range) become about 1/2 of what they are with a single bypass. Each spring only responsible for 1/2 of the fuel flow instead of all of the fuel flow.



Just a side note. As the fuel tank fuel temperature rises I also see a 1 psi drop in the pre VP44 bypass regultor psi's. This seems to happen when the fuel tank fluid temperatures get into the 90* and above range. Thinner fluid?, warmer pre VP44 bypass springs?, warmer pre VP44 bypass valve body?, not sure. I just comment I have seen this consistently for about 2 months now.



Just some ideas and observations,



Bob Weis

 

[Gary - K7GLD]

I really think the variations in PSI I am seeing are related to the restriction at the Frantz fuel filter - possibly aided by similar restriction at the stock FF. I have 3/8 inch lines running everywhere but the section inside the tank - even the return line from my regulator is 3/8 back to the tank.



I'm not real keen on replacing the OEM FF, and wonder if anyone has isolated the restriction(s) it might present? If any restrictions are out of reasonable reach for correction, I might just live with it - but if there's potential for opening passageways up, I might give it a try.



Certainly, with the filtering ability of the Frantz, I could easily just bypass and eliminate the stock FF completely - but I sorta rely upon it to catch any stray paper fibers the Frantz element might shed when a new one is installed.



In all actuality, the NORMAL variation in PSI right at the VP-44 is typically only about 4 PSI from idle to typical full load towing up grades - I have to REALLY lean on it in a way I usually don't drive to force it down another couple of PSI - so probably am overly reacting as to obtaining a tighter PSI swing.