90 HP Thermostat

[Crappie Bob]

I did that again yesterday at the lake. I was thinking maybe my water hose was adding to the pressure, but I guess it wasn't. The gauge stops dead on the peg when off, Water Pressure Gauge.jpgas soon as it cranks it idles at 13, give it any throttle at all and it maxes out the gauge. The engine idle is 800 rpm. I removed the thermostat and poppet valve and see no defect there, stat opens and closes in heated water in a pot. And when I originally did the work I was very careful and stuffed shop towels in all the openings so not to collect any gasket debris, and yes I did remove it all before assembly I also removed the pressure tube from the block and blew into it thinking maybe the gauge went bad and was swinging to easy, as hard as I can go it only went to 1-1/2, so I don't think it's the gauge. I am tempted to take out the stat and poppet valve and crank it to see what happens. I noticed the pee stream seems exceptionally strong also, almost pressure wash the boat with it. Just really stumped, been running Mercs of various sizes for over 40 years and never seen anything like this.

[EuropeanAM]

Here's two methods you can use to confirm:

1. Use ANY "pressure" gauge that can read in the 0 - 30 psi range.

2. Unhook hose with engine running and see how high the water will "shoot" from the hose (open end pointed straight up). 13 psi would be a pretty considerable distance.

IF pressure is really that high, you probably do have a passage blocked in the adapter plate/tuner.

[Crappie Bob]

Different gauge with the same results. Removed the stat and it idled at 3 PSI, revved a little and it went to about 10, I didn't rev too high. Put the stat back in and the same thing, double the pressure. Water from open line will go a good 20 foot in the air. I guess I will just run it like it is till I get a gasket set in, but it still baffles me how anything could be in it as careful as I was, oh well, never claimed to be a boat mechanic...

[Crappie Bob]

I just ordered a gasket to remove the plate and inspect under it, does it get gasket sealer or just torque it dry?

[EuropeanAM]

Powerhead and adapter plate gaskets recommended.

Something as small as a fingernail clipping can potentially block this particular opening.... most common items I've seen are oak-twirly-bird seeds and gasket scrapings.

[JOE54]

dry

[Crappie Bob]

So Don your saying I have to pull the power head off again? I had it all off pretty recently when I changed all the motor mounts. I guess if I must I must

[EuropeanAM]

I had to make a tech pull one TWICE for this same problem, some years ago. Any stray material can plug that passage.

[Crappie Bob]

Well hopefully my gasket will be here next week, already had the bottom gasket from previous repairs.

I went to the lake today and ran it, as soon as you get up on plane and some good RPM's the pressure drops back a bit. It starts at about 15 PSI when you go to plane off and then drops back to about 13...As soon as you let of the throttle and the boat settles down you can watch the gauge fall back to about 5 PSI, and then a second later jumps right back the 13, and I am guessing that is when the poppet valve closes back up.

Just for sheets and giggles when I got home today I took the thermostat completely out and put that washer in that came in the kit when I bought it. Here are the results
Idling about 800 RPM20220707_144249.jpg Reved up about 3000 RPM20220707_144304.jpg


Damn thing has me baffled. With the washer it doesn't peg the needle. I'm guessing the thermostat isn't opening...
When the gasket comes I will pull it all apart, and hopefully it's not a repeat of when I put new motor mounts in and every part I took off 2 bolts broke. I didn't have the stuff to proper drill and tap so I just replaced everything, took a load off my wallet

[EuropeanAM]

I think what you're going to find is that the water pressure bypass passage (which dumps water into the exhaust at the tuner) is blocked with a piece of debris (organic or perhaps even gasket material). Using the "washer" reduces the overall pressure much like the "bypass passage" is supposed to.

[Crappie Bob]

20220710_171430.jpg20220710_171439.jpg20220710_171419.jpg

Nothing anywhere in these, even the little tiny hole under the bottom cylinder in the back is open.
I will be pulling the power head off tomorrow to inspect the bottom plate.

[Crappie Bob]

Got the rest tore down today, where else should I look for contaminants? One pic won't load but there is nothing in it either.
I'm at my wits end

20220712_123107.jpg20220712_123033.jpg20220712_123047.jpg20220712_123057.jpg20220712_123144.jpg20220712_123206.jpg

[EuropeanAM]

Check for blockage in the openings marked up in the below image (note that the CENTER one is the one most likely to cause problems). Hole should be open all the way through.

[Crappie Bob]

These are the only holes in the plate. I dug down in the lower sump part where your 2nd arrow is and there is no hole, never was. I poked really hard to be sure.

20220712_171955.jpg20220712_172008.jpg

[EuropeanAM]

That middle one is the most common area to cause elevated water pressure (and can be blocked or restricted with even the smallest debris). It's open in your Post#34 pictures.

[Crappie Bob]

So, I am back to my original question. How did changing the thermostat and poppet valve double my water pressure?
I guess I'll just put it back together and install a temp gauge and drive it like I stole it till it blows and then go by another one.

[EuropeanAM]

Did you have to scrape any portions of gasket while you were doing that? I once had a tech that disassembled one of these to clear the very passage we discussed- only to leave tiny shavings of gasket in the passages, which lodged in the passage yet again.

[Crappie Bob]

Well, yes I did, but I am positive there is nothing there just like the first time. The only reason I tore it down again knowing it was clean is because I don't know what else to do. I have been doing mechanical work for years, like 55 on various types of engines from small gas to large diesels. I am meticulous about my work, some say to nit-picky but I was taught Dad if it's worth doing it's worth doing right, the first time. I do know no one is perfect but this thing is just nuts. Like I said it was perfect until I replaced the thermostat and poppet valve. What I did do was put the old stat back in and it ran high pressure like the new one does, it didn't before. And with the washer it reads a little lower than normal but stable, idles about 2.5 and at 3000 rpm about 8'ish. I know that is low but at least it's not idling at 13 psi. When I ran it at the lake, as I got on plane the pressure would actually drop some like it was normal again but as soon as you let off to stop while watching the gauge it would drop back about 5 psi and a few seconds it would jump right back up, like in the seconds it takes for the poppet valve to close.

The original poppet valve seat was not tight in the plate, it was letting water around it. It was actually riding back and forth on the poppet stem. The new one I seated in, so it doesn't move making the poppet work as designed.

Oh well, I'm putting it back together today and ordering a temp gauge kit to install by the pressure gauge so I can monitor both.

Thanks for trying to help, and letting me bend your ear, I will not bother you on "this" subject again, unless I figure it out then I'll let you in on the secret

[EuropeanAM]

Only other thing I can think of would be that the poppet seat isn't properly set/seated fully, or the spring somehow was mixed up with a higher-tension spring.

[Crappie Bob]

I popped the seat in with a socket and a small hammer, and the spring is the same as the old one. I even swapped it out to see. I got it back together and cranked it and it still has the same high pressure and now I have a leak I didn't have before, gaskets this time seemed really thin, probably Chinese. Looks like the front of the block and exhaust plate, trickles a bit so I'm running it, maybe some dirty water will stop it up Again thanks for your help.....