2005 VP 5.0Gxi 2500-3000rpm max. Low pressure Pump 0 psi

320Busdriver

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Recently over the course of just a few days the motor began to lose ability to reach rated max rpm under load. Now we can only achieve between 2500-3000 rpms at FT. I tested the fuel pressure at the rail and found 50 psi at idle and throughout the range up to the reduced max rpm.
The gauge when hooked up to the Low pressure pump connector registered zero under all conditions tested. We tested the harness to see if Voltage was applied to the pump when key turned to run and found it to be normal.

******My question is would a failed low pressure pump exhibit itself in this way with reduced rpm under load. Is the high pressure pump simply running out of sufficient volume of fuel to attain more HP? The motor starts normally and idles smoothly and if I take it out of gear I can attain higher rpm.*****

I am considering removing the fuel pump assembly and either sending it out/swapping for a reconditioned unit(mancavemechanic.com) or disassembling it myself and R&R it with a OEM low pressure pump. I don’t believe I have a gross paint flaking issue as I have no evidence of paint fouling the screens on the HP pump or on the filter mating surface..

thanks for your thoughts
 

bruceb58

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Personally, if it was me, I would just spring for a whole new assembly. I know they are ridiculously expensive but you will have a complete brand new unit.
 

ripjmk

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Are you certain the low pressure pump Schrader valve was open when you tested it? I know an adaptor is required for my tester to connect to the LP pump, and that adaptor has to have the pin depressor in it to open the Schrader valve. Of course if it doesn't you will always read zero!
 

320Busdriver

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Are you certain the low pressure pump Schrader valve was open when you tested it? I know an adaptor is required for my tester to connect to the LP pump, and that adaptor has to have the pin depressor in it to open the Schrader valve. Of course if it doesn't you will always read zero!
Yes, because it requires the adapter I wondered if the adapter itself might be faulty. .The test kit was new and rented from an auto parts store and I connected it twice. I know there was some leakage when I connected it the first time though. I’ve talked to 3 different service managers and none of them could say for certain if a failed LP pump would exhibit this symptom. One is sure it’s a distributer cap/rotor and the other says I need to pull plugs and do compression tests. Frustrating
 

ripjmk

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Did you disconnect the HP pump and turn the ignition on and listen for the LP pump to prime? If so is it turning? I am having a hard time believing an LP pump could produce zero psi and still feed the HP pump! What was the range of the pressure gauge on the tester during the LP test? Would 4psi have been easy to see?
Could you test the LP pump with the HP disconnected and see what you get?
 

ripjmk

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Are you aware of the fuel cell paint peeling inside the cell of early MPI engines? Have you checked for this? If not I'd recommend you do. Paint or debris in the cell relief valve back to suction (controls LP discharge pressure) could produce zero LP pressure.
 

BRICH1260

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If you find that the LP pump is bad these days you can just replace that pump. There are many aftermarket pumps available online from ebay and Amazon as well as entire fuel cell assemblys for a fraction of the cost of OEM. This option might get you through the season, in the off season you could have your OEM rebuilt for a back up in the future.
 

320Busdriver

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Did you disconnect the HP pump and turn the ignition on and listen for the LP pump to prime? If so is it turning? I am having a hard time believing an LP pump could produce zero psi and still feed the HP pump! What was the range of the pressure gauge on the tester during the LP test? Would 4psi have been easy to see?
Could you test the LP pump with the HP disconnected and see what you get?
We did not attempt to isolate the LP pump in the way you described, but prob should have. If the LP pressure test was valid then we have to assume the HP pump is able to provide enough throughput to suction feed. Don’t know if it’s possible for it to do that. The fuel pressure gauge would easily show 4psi. I expected 10, the gauge is graduated in 2psi increments. I may go back and try other suggested tests to confirm.
 

Nashville

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We did not attempt to isolate the LP pump in the way you described, but prob should have. If the LP pressure test was valid then we have to assume the HP pump is able to provide enough throughput to suction feed. Don’t know if it’s possible for it to do that. The fuel pressure gauge would easily show 4psi. I expected 10, the gauge is graduated in 2psi increments. I may go back and try other suggested tests to confirm.
Spec. is 4 to 12 LP pump, and 50 to 60 HP pump for a 5.0GXi. Suggest taking readings again. If LP pump is indeed zero, yes, the engine is fuel starved and the HP will be hot to the touch and whining when under load (in gear). Needs more fuel than the HP pump can supply solo. Happy flying.
 

Senior B

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What you describe will happen if the LP pump is bad. The HP pump by itself can't bring in enough volume. You can do a simple test with a multimeter to check the LP pump. You mentioned that you have checked for and had 12V at the connector to the pump. Now disconnect the harness to the LP pump, place your meter in Resistance mode and put the meter leads to the pump connections. You should read very low ohms like less than 4. If its open or very high then your LP pump is done.
 

320Busdriver

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Update…isolated both fuel pumps separately and found both to be operating when key turned to run position. Also used a screwdriver to my ear and find both pumps operating with motor at idle. Still unable to get fuel pressure to read at low pump Schrader valve, but depressed valve with engine running and fuel squirts out. Replaced distributer cap and rotor and found lots of corrosion on cap contacts so expected to have found the fix. Unfortunately upon water test no change to HP delivery. Max rpm about 3000 at WOT. I think I must be down 1 or two cylinders..
Spark plugs are ordered and will replace next week.
On another note I inquired at skipper buds parts dept about replacing the LP pump and when he checked for my engine it cross referenced an entire pump assy which retails for 425.00. So if need be plug and play fuel cell cost is reasonable vs tearing down old assy and replacing components.
 

QBhoy

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I think we most likely it might be the Diz. Or fuel is crappy, or the check valve or a boat of other restrictions in the supply. But if your valve is spraying fuel out when pressed in, you should get some sort of reading of fuel pressure surely ?
 

ripjmk

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Before you change out the fuel cell, screw just the adaptor onto the LP Schrader valve and get someone to quickly turn the ignition on-off and see if it squirts fuel! As has been said, if squirts when you depress the valve you should get some sort of pressure.
 

320Busdriver

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What plugs did you get for her ? If they aren’t the iridiums, they just don’t run well at all. Or not at all.
I was given Champions double platinum. Not sure if they are truly right, would NGK be better? I will probably do the wires too.
Plugs removed are acdelco R44L TS6
Installed are Champion RS12PYP
 
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QBhoy

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I was given Champions double platinum. Not sure if they are truly right, would NGK be better? I will probably do the wires too.
Plugs removed are acdelco R44L TS6
Installed are Champion RS12PYP
Hi. Almost certain that none of those two mentioned are suitable. NGK 5599 are what you’re after or at the very least, the ac delco 41-993. No doubt that the iridium NGK run best in them. The ac delco 993 are platinum and don’t work so well at all. The ones you have I’m not sure what they are for to be honest.
 

Nashville

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I was given Champions double platinum. Not sure if they are truly right, would NGK be better? I will probably do the wires too.
Plugs removed are acdelco R44L TS6
Installed are Champion RS12PYP
Morning.

What was installed is a standard plug with the proper 0.060-inch gap (R44LTS6).

If my information is correct on the following plug, what is now installed is platinum (longer life than standard) with an improper 0.050-inch gap (RS12PYP).

For the 5.0GXi series, the correct plug kit is 21467472 (3858996). Crosses to NGK TR55IX, ACD 41-993, etc. Iridium (longest life). Follow what @QBhoy posted in No. 17.
 

QBhoy

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you would be surprised how well MR44LTS work too in these engines too also BPR6EFS
Hi BT. The standard bpr6fs are totally wrong for these mpi engines. Horrendous in fact. They were never for these engines. The manual calls for minimum platinum and ideally iridium. Not un common for people to put these in by mistake though. Thinking they were just the same spec as a carb or efi.
 
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