Hi Vacuum at Filter - Engine RPM drops

[DennisG01]

A while ago, with Boscoe's help (thank you!) I installed a simple vacuum gauge at the boat-mounted water/fuel separator filter head. It's screwed into the extra "out" port at the filter head. The reason was to check on the filter efficiency (clogged up).

It's been working great with typically less than 5PSI at idle.

Just recently, the OX66 250HP won't maintain WOT RPM - it will start to drop after about 15 seconds. It drops slowly/smoothly over the course of another 15 seconds or so.

I can maintain approximately 4,500 RPM just fine.

I noticed that the vacuum PSI at the gauge creeps up to about 15psi during this.

The gas is super clean in the boat - but I replaced the filter and the issue didn't change.

I tried running with the fuel cap off - no change.

I tried running with a portable tank hooked up to the filter head input - no change.

I'm stumped?

[99yam40]

you do not measure vacuum in PSI.
Inches of mercury maybe?

seems you have eliminated everything but the filter and housing itself.
could you have contaminated the new filter with what you had in your tank after you put the new on in there?

did you even notice what that gauge read normally when the motor was running OK at high RPM

[boscoe99]

Have you changed the filter?

[DennisG01]

Yeah, not sure why I said PSI!

"The gas is super clean in the boat - but I replaced the filter and the issue didn't change"

A month or two ago I had a LP pump starting to leak and while the RPM's would drop down, it was much more erratic and abrupt in nature when it did so.

I'm sure I did, but I can't recall any numbers from watching the vacuum gauge in the past when running WOT.

I'm leaning toward the filter head, as well - I guess I just never thought that these COULD clog up? I guess anything is possible.


Does anyone else have a vacuum gauge hooked up and can verify numbers at speed? Either way... 15" is a lot, right?

Am I correct that nothing on the engine side of the filter head can cause a vacuum at the filter head's "out" port?

[boscoe99]

The Racor gauge shows that 6" Hg is in the warning zone.

15" Hg is way too much.

Get a hose barb to hose barb fitting so as to bypass the filter assembly altogether. Retest and see what happens. If all is normal that indicates to me something within the filter or filter head assembly.

[boscoe99]

From Yamaha

"IMPORTANT: From idle to WOT RPM, the vacuum is not to exceed 4.0 in. /Hg for 2.6L engines up to
200hp.The vacuum is not to exceed 6.0 in./Hg for 3.1 and 3.3L engines from 200 ~ 300hp including the
V200 3.1L and new VZ200TLRD 3.3L engines."

[DennisG01]

Perfect - that's the exact info I needed, Boscoe. The boat is in Maine and I'm getting ready to return to PA in a few minutes. I'll tackle this again in a few weeks when I'm back up here.

It's been running close to 5" at idle for the last couple years. I could have sworn that it was more like 1" or 2" at idle for the first few years I had this setup... but I was questioning myself. The info you provided seems to reaffirm things.

Good idea with the barb.

I'll bring another filter head with me next time I come up here, as well.