VMax water pressure 'fix'

[Hollywood1053]

It seems that Yamaha VMax's don't have good water pressure, especially at idle.
Mine registers 0 at idle and maybe 12 at 4000RPM.
I've seen the following recommended 'fix' on at least 2 occasions on other forums.
Does anyone have any first hand experience with this modification?
Seems too simple to be true...8-10lbs at idle??

"The fact that you are not getting any kind of water pressure (0) at idle tells me that the water pump is probably not priming and you are sucking air past the impeller housing. The 150 Yamaha VMAX in notorious for this problem. If you take your lower unit off, you will probably find that the plastic housing that contains the rubber impeller is warped which allows air to enter between the housing and the metal plate that it seats against. The result is no vacuum and no water pressure. Even a brand new plastic housing can be warped which is a manufacture defect(we found this to be true on a new one that I bought). To make this seat better against the metal plate and avoid sucking air, you have to take the housing and belt sand the top part of the housing to make it completely flat. Once this is done the housing will fit perfectly to the plate and now you will have good water pressure even at idle around 8 to 10 and at 4400 rpm it should be 20 or above."

 

[iammarcuse]

Re: VMax water pressure 'fix'

The telltale stream at idle shoud be strong enough that it has straight stream. You will still only see 2-3 psi. If it arcs or dribbles like a prostate patient it is not strong enough.
It is possible the housing is warped. Have a good look, use a flashlight. If it doesn't look like it is warped it probably isn't. (I would replace it, not sand it if it is).
I had the same problem as you and discovered the grommet was missing at the top of the housing where the water tube enters. W/O the grommet I was losing water pressure.
If all that is fine check the poppet valve to make sure it is sealing and not full of gunk. Best to get a new gasket and poppet grommet and just replace it.

 

[rodbolt]

Re: VMax water pressure 'fix'

I wonder how many motors both 2.6L and 3.1L as well as a 3.3L version were made ? .
each has its own points some common some not some ran more pressure at idle than others,
some even used a different insert cup in the pump housing specifically for some of the Vmax models.
biggest reason I see for water pressure loss,melted housings from a dry start. any sign of melt means the housing is now scrap.
bad seal on the pressure control valve grommet.
missing or stuck open thermostats.
and in salt water the rubber dam just beneath the aft edge of the pump wear plate is corroded and pusing the plate up allowing an air leak.
most typical after a pump change when the installer failed to remove,clean and correctly reinstall that dam and the metal holder.