Quick Fuel Pressure Gauge Install Question

[DennisG01]

I searched over the Winter on this forum and found some excellent threads on installing a vacuum gauge at the fuel filter mount head. I installed it recently and just wanted to verify that I didn't do anything wrong...

I used this gauge: https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

I have one, spin-on fuel filter installed before the engine. The filter mount head, like most, has an unused (I have a single, OX66 250HP) "In" and "Out". I installed the gauge in the extra "Out" port.

I'm 500 miles away from the boat, right now, hence the reason I'm second guessing myself! After I installed it (I purged the gauge of air), the needle would move between 0 and 1 when I pumped the fuel bulb. At idle (elevated as I was out of the water... about 1,000RPM) the needle pretty much stayed at 0. I forgot to take a look while I was running the engine at speed.

Did I do anything wrong, or is what I wrote what would be expected?

Also, I looked through my service manual for specs on what is "allowed" in terms of vacuum, but couldn't find it. Maybe I missed it? I'm curious as to what is considered the "normal" range before a fuel filter should be replaced. Ultimately, I don't really care about a $10 filter - but I don't want to be wasteful, either. But the real reason I installed the gauge was to help with diagnosing issues, in the future.

[boscoe99]

One problem with the gauge you used is the lack of fidelity. By this I mean the gauge has a range of 0 to 30" Hg whereas the desired range is 0 to maybe 8" hg. It is difficult to see a change in value from say 1 to 1.5" Hg.

Anyway, you are where you are. With a clean filter and no anti-siphon valve in the tank I would expect the reading to only be about 1" Hg when the motor is running at a mid-range RPM. As the filter captures debris and the fuel flow restriction rises the vacuum will rise.

Yamaha provides a specification for maximum allowable fuel flow restriction but they are measuring at the inlet to the lift pump(s). There is other stuff in the system between your boat mounted fuel filter and the lift pump (hoses, primer ball, fuel filter, fittings and whatnot that will add to the increase in the fuel flow. The test is also done at a fuel flow value that will approximate the motor running at wide open throttle. Maximum fuel flow.

What you can do as a test is to have someone run the motor at cruise RPM. Say 4000. You will be monitoring the vacuum gauge. Let's say it is indicating 1.5" Hg. You can crimp the fuel hose leading to the fuel filter and see the vacuum rise. You are simulating the filter clogging with debris.

[fairdeal]

Boscoe has in the past provided some values for "what Yamaha considers restriction on the fuel supply - IIRC some low single digit of inches Hg.

I was recently noodling around with a vacuum gauge on my F225TXRD.

What I found, testing with a fairly recently installed 10 micron Racor filter element,

was _zero_ measurement on the vacuum gauge.

That is, even with the lift pump running continuously for ten seconds, with an unrestricted output,

there was so little "restriction" that the needle on the gauge (standard 0-30 in. Hg) did not budge.



I posted a couple of videos in this thread:

http://www.yamahaoutboardparts.com/f...r-th26846.html

[boscoe99]

Boscoe has in the past provided some values for "what Yamaha considers restriction on the fuel supply - IIRC some low single digit of inches Hg.

I was recently noodling around with a vacuum gauge on my F225TXRD.

What I found, testing with a fairly recently installed 10 micron Racor filter element,

was _zero_ measurement on the vacuum gauge.

That is, even with the lift pump running continuously for ten seconds, with an unrestricted output,

there was so little "restriction" that the needle on the gauge (standard 0-30 in. Hg) did not budge.

I posted a couple of videos in this thread:

http://www.yamahaoutboardparts.com/f...r-th26846.html

Racor states the restriction for a new clean filter, at a rate of 60 gallons per hour, to be about .6 psi. Or ~1.2" Hg. 60 GPH is a very high fuel flow rate. More than twice what almost any Yamaha motor can be expected to consume at wide open throttle.

At the fuel flow rate of a high HP Yamaha motor at a cruise RPM of 4000 I would guess the flow rate to be more or less 10 GPH. About one sixth of the maximum flow rating for the filter. In which case the pressure drop across the filter will be much much less than 1.2" Hg. Very damn hard to see on an analog gauge with a range of 0 to 30" Hg.

I used a digital absolute pressure gauge that measured to the tenth of an inch. With a clean filter it never indicated much more than an inch or so of fuel flow restriction at 4000 RPM. My filters got rusty and were replaced on that basis long before they got clogged enough to require replacement for that reason.

[DennisG01]

Excellent information, guys. Thank you.

For some reason I had it in my head that up to about 7" was normal/acceptable. Which is I why I didn't worry about the larger range of that gauge. Obviously I was wrong. Oh well, live and learn. I'll be back to this boat in about a month and I'll look at everything at a cruising RPM.

But for now, at least I know I did the install correctly - I was second guessing myself that it should have been installed on the input side of the filter head - but that didn't make logical sense to me... but based on the (incorrect) thought that I should be seeing about 7", it got wondering.

Thank you!