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1997 7.4GiPLKDCE: Let's talk fuel pressure readings (or lack thereof...)

C

carboncow

Contributing Member
As mentioned in another post I'm chasing some bugs that might be fuel related, as seen here: https://www.marineengine.com/boat-f...or-regulator-part-3854517.481358/#post-776430

Mechanic buddy said to consider fuel regulator but obviously we need to rule out some things before throwing parts at it. Was told low side should be about 6-12lbs and high side should be 38-42lbs.

Needed to get a fuel pressure gauge on the low and high side and didn't want to wait a week until I'm back at a boat...buddy ran to Harbor Freight and got their finest set :oops: and got some readings today...or lack thereof...

1. Rail side pressure = 0lbs at idle and 2500rpms
2. Low side (reservoir valve) pressure = 0lbs at idle and 2500rpms
3. Gauge verified working by putting on a tire and got 32lbs as expected.
4. Replaced sharader valve cores with new. 0lbs both.
5. Removed Schrader valve cores and took reading that way. 0lbs both.
6. Engines starts fine but shaking idle like a miss. engine runs fine under load to approx. 3600rpm and cannot go any more. DP-S (OEM) so it's most likely propped correctly and they are in great shape.
7. Rail doesn't seem to hold pressure nor pressurize when key is "on" but not running.
8. If I push in the Schrader valves while engine is running :eek: it dribbles out both high and low. I would expect 40lbs to shoot out pretty good.

Can an engine run that well with but a few lbs of pressure? or am I getting incorrect readings?

Assuming all this is correct, where do I start? Low pressure pump since the low side is showing poor pressure too?

IMG_4045.JPGIMG_4052.JPG
 
Low side 5 +/- 3 psi
High side 39 +/- 3 psi

Not seeing any pressure check the antisiphone valve on top the tank. It's a barbed fitting that the rubber fuel line connects to
 

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AllDodge said:
Low side 5 +/- 3 psi
High side 39 +/- 3 psi

Not seeing any pressure check the antisiphone valve on top the tank. It's a barbed fitting that the rubber fuel line connects to
Click to expand...
Thanks We ran down to look and see no such thing on the pickup and the hose goes into the abyss that would require surgery. Are they always at the pickup? Would a psycho builder put one in a non accessible area requiring floor surgery to get too? I guess we could test an aux tank to test the theory.

We also ran a bore scope along the pickup line. Best we could chase it was two feet before it turned someplace weird. This tank sends the lines forward (for the fill hose) and then they have todo a ridiculous 180 somewhere to get back to filters.

I understand their importance for safety so would assume they must be right at tank and not downstream such as at the filters...which they are not.

Attached photo of our fill hose and pickup on a 1997 Pursuit Denali 2460

IMG_4062.JPGIMG_4063.JPG
 
Well it's a USCG requirement unless the filter is above the top of the tank.

Either way ok.
Try this, disconnect fuel line going into the filter and attach a piece of fuel to it and put the other end inside a gas can. See if pressure can be built up this way. If it can then it's in the fuel line or tank
 
AllDodge said:
Well it's a USCG requirement unless the filter is above the top of the tank.

Either way ok.
Try this, disconnect fuel line going into the filter and attach a piece of fuel to it and put the other end inside a gas can. See if pressure can be built up this way. If it can then it's in the fuel line or tank
Click to expand...
Sounds like a plan...we'll start at the source. Now to find some fuel line at 5:30P on a Sunday....
 
OK, we found some clear vinyl 1/2" ID and put it in a jug with a 1/2 gallon of fuel. Bypassed the first (of two) filters and it pulled fuel just fine. Same 0lb readings on low and high side after letting it idle for 20 minutes. Nothing seemed to improve in any way.

Would you consider low pressure pump at this point?

NOTE: Neither pumps make any noise like they are working hard.
 
I'm starting to think the fuel pressure gauge your using is bad.

Main reason, is how is the motor even able to run with zero psi on the fuel rail. Don't see how it will even start much less run even at an idle
 
That was my initial thought too. Now I know liquid vs. gas is very different but I took it up to our place and put it on my car tire and it came up 32lbs as expected. I also thought that the shredder valve nipple/point wasn't getting pushed in on the low and high side so I removed the core completely to reduce that possibility. Still nothing...very confused.

AS I may have mentioned before when I push in the Schrader valve after turning off the engine there is just a drip or two that come out of both the high and low side.
 
AllDodge said:
Remove the fuel rail schrader valve and see if there is gunk behind it
Click to expand...
I may have mentioned before but the cores were swapped with new ones and then completely removed at one point (so no possible obstruction or pins not being pushed in by linseed) with same results white testing, but...

An old timer brought down an old set of HVAC gauges this morning...said I could have them to ruin, says they have been used on fuel rails in the past. Put them on the car tire and got 32lbs. Then put them on the high side and got 40 lbs on the first start of the engine! With that said they started out at only 10lbs and took about 5-7 seconds to get to 40lbs. So now I'm assuming you are correct and my new Harbor Freight gauge set is bad, but...

We turned off the engine and put HVAC gauges on the low pressure side and no reading. That's when my buddy said your low pressure pump is whining pretty good and didn't do so on the first start of the day. We then put the HVAC back on the high side and started and no reading...again. Took yellow line off and getting no flow up to the gauges like we did the first time.

So unless the HVAC gauges were ruined in some way on the first attempt (we doubt) the consensus is low side pump worked on the first ice cold start of the day. Then on subsequent starts it's not doing as well and it's been noted a very noticeable (but quite) whine from it while the high pressure side produces no decearnable noise.

This is a weird one!
 
Agree weird, haven't seen one doing this but hey that's what is happening

Looks like you have proved it's either the pump or regulator. If you can take the Reg return line and install a valve. Turn key on a few times are bleed air out of rail. See if pressure will go above 50 psi. Don't let it get above 50 or so because that can damage the Reg.

If it can not get the pressure up then it's the pump
 
My mechanic friend who started me troubleshooting said last week he believes the culprit is the regulator that started this whole conversation, so glad to read it getting included back int he discussion a bit.

Not fully sure what you are saying about "install a valve". Are you saying bypass the regulator in some way?

Also, (as you may know) this regulator is under the manifold and hardliner into the system unlike some I've seen (newer Volvos) which seem to be just push onto a flexible line. Point being fabricating something to take its place or bypass could be a small undertaking. Just want to make sure I'm understanding the testing protocol you mentioned.
 
Most use a return line after Reg to feed excess fuel back to the filter or in your case the HP pump chamber. Merc calls it a Vapor Separation Tank (VST) and forget what VP calls it. A valve "might" be able to be placed on the line that comes back to the tank

Could just pull HP pump and bench test it. Would need a line to supply fuel and a gauge on the output side to see if it will output at least 60 psi
 
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