Tips on checking fuel pressure?

boater2065

boater2065

Active Member
Jul 30, 2018
222
Everett, WA
Boat Info
Searay
Engines
merc
Hi everyone, i'm looking for any tips on how to check the fuel pressure on my 5.7 EFI. It's something i have never done before but would like to start doing as part of a maintenance routine, if possible. Also, i have reason to believe the fuel pressure may be too high as there is an obvious smell of fuel in the exhaust and fuel slicks in the water when the engine runs (this was part of another thread i had, where the mechanics ended up having the injectors cleaned/repaired, however the issue never fully went away and now seems to be getting bad again.)

I've watched videos on youtube of people checking fuel pressure on car engines and it seems easy enough if the engine has a schrader valve. So far i havn't found one of those valves on my boat engine, maybe that is by design. Anyhow, if anyone has tips on how this is done, and if it's something an amateur can safely do, i'd appreciate it. Otherwise i may have a mechanic do this for me so i can get a baseline and also see if there is a fuel pressure issue
 
@scoflaw Awesome, i will look harder. Regarding the fuel pressure gauge, do i have to use the one recommended in the manual for the engine, or is this something that is universal of sorts? The Merc recommended gauge kit is upwards of $600. Would love to find something that will work and not hurt the pockets as much.
 
After some research, it might be important to note that my engine has the TBI system. Does this change anything in regards to where that valve is located? From the diagrams i see, the fuel line runs directly into the TB with no obvious signs of a "fuel rail".
 
Been a while, but I believe you have to T into the line with a TBI (in Merc terms it's "EFI", not the newer "MPI"). No Schraeder.

Any fuel pressure gauge will be fine. But fyi, this isn't usually somethig you do for maintenance. You do it there's a problem. "Read" your spark plugs (google if needed) - see what's going on in there.
 
scoflaw said:
Been a while since Iworked on that rig, The tester can be borrowed from Autozone for free.
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Lazy Daze said:
Been a while, but I believe you have to T into the line with a TBI (in Merc terms it's "EFI", not the newer "MPI"). No Schraeder.

Any fuel pressure gauge will be fine. But fyi, this isn't usually somethig you do for maintenance. You do it there's a problem. "Read" your spark plugs (google if needed) - see what's going on in there.
Click to expand...
Both of these guys are correct and from your other thread I would make it be your first step.
As far the tool itself is concerned, Dennis is correct that you connect a T in line between the throttle body and the supply line. Just make sure you get a kit with the correct adapters for the older systems.
 
Be sure to use the gage with a bleed off like the ones shown. I bought a cheap harbor freight one with out the bleed off and it made a mess when I disconnected it. Follow advice from these guys (tech and E)
 
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boatrboy said:
Be sure to use the gage with a bleed off like the ones shown. I bought a cheap harbor freight one with out the bleed off and it made a mess when I disconneded it. Follow advice from these guys (tech and E)
Click to expand...
Yep, not only that but the other positive with the "bleed" off is that most have a hose also so you can not only direct the the fuel away safely but also purge/bleed the hose to the gauge when testing for a more accurate reading.
The good shit:https://shop.snapon.com/product/EEFI-Series-Adaptor-Sets/Master-Fuel-Pressure-Kit/EEFI500A
 
Wow, thank you for all the input. Looks like i have some more research to do but this helps get me going in the right direction. I'll report back or follow up with more questions if i have them.
 
I'll need to inspect my fuel line and throttle body inlet to see if the adapter @Espos4 linked will work

A couple early questions i have are:

Is this adapter something i will leave in place, or only put on when i need to check the pressure?

With the engine off, can i safely remove the fuel line from the throttle body without too much spillage? Can I assume if the engine is cold, there is no fuel pressure?
 
For the adapter, it shouldn't be left in. Unknown if it might restrict fuel flow and/or be another source of fuel leaks.

As for your second question, you'll probably still have pressure in the line when you open the fittings. If you can easily access the electrical connections to the pump (or its relay), you can disconnect its power, start the engine, and let it die for fuel starvation (should only take a couple of seconds). Then you're home free.
 
Thanks @Nater Potater

Regarding the second question, i have seen a youtube video of what you are describing and shouldn't have any issues doing that. When i get some time to go to the boat, i will do this as step 1 and then get some things ordered.

I have been doing some reading about causes of high fuel pressure and the most common causes seem to be a bad fuel pressure regulator or a clogged/kinked fuel return line. The only nuance i'm seeing about the fuel regulator is that apparently this engine has two of them. One inside the throttle body with a spring and diaphragm (this one is NLA from Merc or anyone else for that matter) and the other one sits on the fuel cooler. Further digging, and it sounds like possibly the one on the fuel cooler is the one that would need replacing and not the one in the TB unit. If anyone has more info surrounding that, i'd love to hear it.
 
After some more reading and video searching, i found a video of the same engine and fuel system (TBI), and the mechanics were checking it for some fuel issues. They pulled the Fuel Pressure sensor that screws into the TB unit and put their fuel pressure gauge into that hole to check for pressure. That seems like it may be easier and less messy than fulling off the fuel line. Any thoughts?
 
Update:

I purchased the pressure gauge kit referenced by @techmitch above because it mentions connections to a GM TBI. I removed the fuel line from the TB unit and connected an adapter from the kit into the TB side. It seemed to have a nice tight connection with no wiggle. When connecting the fuel line side to adapter from the kit, i couldn't get as tight of a fit as i could on the other side. I tried a few washers/gaskets from the kit, but there was still some wiggle happening. I figured if i engaged the fuel pump at this point i would have had fuel leaking and i didn't want to risk it. I put everything back the way it was and checked for leaks, all good. I'll need to keep looking around the internet for other adapters so i can check the fuel pressure.

I did check the fuel return line to make sure it wasn't kinked, and that looked fine.

While i hunt down an adapter that will work, are there any reasons other than high fuel pressure that would cause the symptoms i am having (Fuel smell in exhaust, fuel streaks and blots in the water, heavy carbonated plugs)? Regarding the plugs, most are lightly black and smell of fuel, while there is one or two that are way worse, much blacker and heavier smell of fuel.

As usual, appreciate any advice or tips you may have for me.
 
At 20 years old I would replace the FPR just for the hell of it and how about the temp sensor that gives the ECM info? That would be suspect also.
 
I replaced the temp sensor a couple years ago as my first attempt to fix this issue. I will look into replacing the pressure regulator. Just wish it was a bit easier to get to
 
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