Bouncing water pressure at gauge and tell tale

[RickCamilleri]

1996 merc 200 efi
0g318842

Had the boat out in 4 foot waves on lake ontario the other day. First trip out.

Noticed the water pressure gauge bouncing at about 2500rpm and the pee stream go from strong to stop to strong etc. Let off the throttle(hotfoot), would bounce a bit then settle around 2-3.

Removed lower, changed impeller even tho the one I replaced wasn't old at all. Replaced all gaskets. (Found one that was torn)

Removed t-stats and tested in boiling water. They opened.

Removed poppet housing and found a small chunk of debris and removed. Replaced poppet/spring and diaghram. Replaced one of the gaskets with a spare I had on hand as the original had a tear in it.

Did a tub test today and its still doing the same thing.

At my wits end. Not sure what else it could be.

Water pressure guage would bounce at 2k and pee stream would get weak that it'd just sputter and mist. Once off the throttle(hotfoot) it would settle back down to 2-3 after a few seconds but not right away

I'm attaching a video and dumb me forgot to record the pee stream haha I've read other post regarding this and still can't come up with anything

https://youtube.com/shorts/eqtyLBgQqfo?feature=share

[leonreno]

First I would suggest poppet but you changed it, best would be a head gasket or exhaust/divider gasket problem where exhaust or compression gaskets are leaking into the water jacket. However I would think it would bounce faster than that.

[JOE54]

put in lake and run it make note of gage readings while underway

[EuropeanAM]

I personally suspect that was a water supply problem (as Joe suggested, try it in the water.... I saw grass above the dash panel in your video).

[RickCamilleri]

So I've seemed to have rectified the problem.

Removed lower and t-stats. I had a suspicion that there was a partial blockage.

After removing tstats, I blew back from the t-stats to the main water intake (copper tube). Some debris came out and I could only feel a bit of air coming from that main tube. Repeated that process with my thumb over the tube end and releasing. The air pressure at the main pick up tube got a bit more distinct!

Also noticed (and this was one of those moments where you swear at yourself for an hour) some water build up around the outside of the water pump housing. Start cursing at myself. As I dont have an in-lb torque wrench I cinched down the brass housing nuts a little more.

Filled the tub, ran the engine for approx 10 mins at idle, water pressure was steady at 3psi, rpm around 800. Engaged the hotfoot(throttle) for the next 10 mins or so up to 1.5, 2k, 2.5k and briefly at 3k, water pressure pretty constant at around 5 psi, with a pretty steady pee stream. It would sputter on me once I released the throttle but the cylinder heads/block were cool to the touch (at idle it'd be pretty hot to touch, back to steady pee stream)

Can you guess what my next purchase will be? If you guessed an in-lb torque wrench you'd be right!! I was always afraid of over torquing as I've inadvertently broken off the screw once before. Once bitten...type thing

I do believe there was a bit of a blockage which contributed, but my own error was the major cause of it

Embarrassed, but got it fixed!

[RickCamilleri]

That's where I noticed the problem initially, on the lake.
Rpm gauge worked as it should

[JOE54]