Testing Fuel Pressure (under load?)
 

I have a 2000 454 that is not turning up as fast as it should. Only has 160 hours on it. It runs strong right up to 4200 rpms, no miss, no fuss, seems smooth. Just seems like it should have more. It is a 23 ft bowrider with a 21p mirage + prop. The boat is new to me, so maybe I am just expecting too much. If the pump were not supplying the propper volume would it still be smooth?
I am awaiting the software to turn my lap top into a scanner to see what the sensors are reading.

At any rate, I would like to test the fuel pressure. Since it only may be weak at WOT, I assume that is the only valid way to test it.

Is it as simple as just hooking up the gauge, having my wife nail it, and read it? I really only ask, because extra fuel plumbing on a boat under pressure makes me nervous.

Alrighty then.
I don't think I need to do it under load. I hooked it up in my driveway, and got 43 psi (spec) with the engine off.
But then I started it, and only got 36 at 1000 rpm.

I am looking at a fuel pump here, eh?

Am I wrong?

There will be a high pressure filter some where, change it and checking your pick up in the tank for obstruction would be a good start.
Jim

Thanks MM.
Just doing a search here, and found something that may pertain also.
I measured with out doing anything to the vacuum line going to the regulator. My seloc manual makes no mention of this, but some folks here state maybe I should have dissconnected and pluged the line? Makes sense, the pressure would drop under idle vacuum. I have a *real* merc manual on order.
If this is in fact the case with the vacuum line, seloc left out (or I missed) an important detail!

What motor do you have? There are different fuel systems. How many hours on motor?
Jim

Double post!

2000 7.4 mpi, 160 hours.
Turning 4200 rpm and 45 mph.
The boat is new to me, it may just be " it is what it is". Based on others with same boat/prop/ altitude etc, I think it should be turning a few more rpms/ mph. Who knows? Maybe it is just the tach, etc.. Seems to run smooth enough, just trying to make sure everything is as it should be.

Its better to be near the top of the recommended rpm range. Not knowing whether you have a problem or not some basic things to check would be compression, riser gaskets, pick up restriction, and Hi pressure filter. If everything checks out try a different and/or smaller pitch prop for comparison.
Jim

O.K., just so I can eliminate this off the list, what exactly should I see when I hook up in the lake?
At WOT, I need to see 43 lbs, correct? Or is measuring this a silly pursuit, as the only spec listed is 43 lbs @ 1000 rpm. (With, apparently, the vacuum line disconnected)

:cool:

All you need to do is hook up the fuel pressure gage and run it under load and if it starts dropping, then it will be clogged filter or hi pressure regulator on the cool fuel. Just went through this and the pressure was 42 at key on and idle, But dropped to 35 under load and it was not enough fuel. Long story short another shop had put on everthing but the hi pressure reg. on the cool fuel...Problem solved. GOOD LUCK

Hopefully going out tonight and see what it reads.

I know MErc had an issue with fuel pumps not delivering enough volume (with a service letter that excludes my serial numbers). Hoping it will be 42! That looks like a neck breaker to get at.

When the service manual states the pressure to 43 psi it is in reference to the manifold air pressure (or vacum), that's why you see 43 with ignition on before starting.
As soon as the engine runs there should be a 43 psi difference over the injector, i.e. between fuel rail and intake vacum.
To ensure the pump delivers full pressure when running, disconnect the vacum hose to the fule pressure regulator and confirm the pressure is still 43 psi .

Ran it last evening, and it was 43 psi wide open and running hard.

I guess I can scratch fuel off the list.

Thanks for all the help.

Next up is swapping the mirage + 21 p for a rev four 19. Bring the rpm up from 4250 to 4500 or so.

Had motor that ran like hell to 4000 & then stay there. Turned out to be coil. Motor then ran up to 6k as it should.