8.1 Fuel Pressure

Tacoma290

Active Member
Oct 5, 2006
497
Tacoma, WA
Boat Info
340 Sundancer 2007
Engines
Twin 8.1 V-drives
One of the engines would randomly stop running at idle. Changed the IAC and foam filter. No difference. Today I put a pressure gauge on the fuel rail, and here is what I observe: At startup, the pressure is steady at about 30 psi. Engine sounds fine. After a few minutes, the pressure suddenly starts to flutter. Then it flutters down to zero over about 2 minutes and the engine dies.
Is the next step to take apart the cool fuel module, or just look at the pressure regulator? What else should I be looking at?

Previous owner already changed out the cool fuel module. I changed the fuel filters last month.
 
The more I think about it, might not be regulator, since the pressure I measured was on the Schraeder. Will pull the cool fuel pump assy.
 
Mine did the exact same thing, bad cool fuel module. I changed the entire unit instead of changing out parts inside.
 
Good suggestion; Bummer for me. I already pulled the module, so I could look at the pumps. I am home now, so that seems like the next step, then take back to the boat and plug into the dingy fuel tank.
Research says that I should have 40 psi at the rail not 30.
The possibilities are: restriction on the inlet piping, failed regulator that closes too much (does not seem likely), or a failed low pressure pump with a failing high pressure pump.
Assuming that there are not restrictions, the options to fix are to either replace the cool fuel module entirely, or re-build. Anyone have any experience with re-building? Where would the pumps be sourced from?
 
Wonder if it was a complete module with the pumps or bare housing? If complete Merc unit I doubt that's your problem.
 
I pulled the regulator. The upper o-ring has some stories to tell
Photo1-4-23-2023.jpg


Here is the housing, which is missing a little paint:
Photo2-4-23-2023.jpg


The good news is that the strainer at the regulator is clean:
Photo3-4-23-2023.jpg

So perhaps I should clean this up, then rig it to the dinghy tank, and measure pressure.
 
So I connected the cool fuel assembly pump to a jerry can at the same level as the pump. It would not prime. The engine was able to start and run on the fuel that was in the pump, and I was able to verify the pressure regulator works (30 psi with vacuum hose on, 38 psi with vacuum hose off). But I should have 40 psi with the vacuum hose on. By that time, the fuel in the cool fuel was used, and the pressure at the rail was 0 and the engine died. I reconnected to the boat fuel tanks (I had left the solenoid valve closed, so the line was should have been still primed). Same thing: would not prime. I cycled the engine switch on and off a number of times. I could hear a pump run, but no pressure built up.
Is there a trick or a process to priming the cool fuel pump assembly?
Is this proving out that the low pressure pump was bad?
 
" At startup, the pressure is steady at about 30 psi. Engine sounds fine. After a few minutes, the pressure suddenly starts to flutter. Then it flutters down to zero over about 2 minutes and the engine dies."

Had similar issues but didnt measure pressure. Was a loose connection on the low pressure fuelpump. It would prime, but couldnt keep up due to low voltage
 
I rigged the cool fuel with a dinghy fuel tank 3 feet above the engine, and the temporary line had the priming bulb. After priming the suction line, I keyed on the engine. The fuel pump ran and pressure started to build to 30 psi (with vacuum line attached). Engine ran for a few seconds, then the pressure gauge fluttered down to 0 psi, and the engine died. Pump was running the whole time. So with the intake fuel piping on the boat ruled out, looks like the pump module is the culprit. The module in my boat is a Gen 3, model 8M0064825. The question is whether this module is re-buildable since the current version is 8M0047215.
 
Sounds like you just need the pumps, if there is no corrosion in the housing. If the cover isn't anodized you can just blast the paint off the cover if that was an issue
 
Yes, I am working with S & J to get the pumps.
Assuming that gets pressure back, next step will be cleaning the injectors.
 
Yes, I am working with S & J to get the pumps.
Assuming that gets pressure back, next step will be cleaning the injectors.
I have a detailed post on here for pulling the injectors on the 8.1 if you want some pointers.
 
Can you please post a link
 

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