'87 150 Mercury XR2 Overheating at High RPM's

I am having overheating issues with a 150 XR2 above 3500 rpms. It runs about 170 degrees at 3500, but anywhere above, it gets hotter as the rpms increase. At wot, it will set the alarm in about 3 minutes (gauge reads 250). Things I have done to correct this issue (with no positive results).

1. Replace water pump (tried two different pumps)
2. New t-stats
3. New poppet valve
4. New head gaskets - milled heads (checked for any obstructions while down)
5. New center exhaust plate divider gaskets.
6. New powerhead to drive shaft adapter plate gasket (did not replace the drive shaft adapter plate to leg gasket (any thoughts?)
7. Good pee stream. Installed water pressure gauge. 6 psi at idle, 18-20 psi at wot.
8. Installed temp gauge on port side. Gauge on port side reads HOT when alarm on starboard side sets. Both heads are getting hot. I have shut down immediately after the alarm sets, then turned the ignition back on, it still reads HOT. Gauge and alarm seem to be working properly.
9. Installed jack plate. Let leg down in further in water for a test (eleminated any potential water pickup problems)
10. Checked timing
11. Checked for correct carb jet size

Any thoughts?????? gary

Re: '87 150 Mercury XR2 Overheating at High RPM's

The exhaust and water passages are right next to each other, a gasket failure there, on either side of the adapter plate will allow higher pressure water to bleed of into the exhaust, thus reducing cooling when it is most needed, at or near WOT.

18 - 20 psi @ WOT seems a bit too high, as if the poppet is not opening. Double check that the poppet SEAT is firmly i place and not moving out along with the valve, thus not opening the flow path. You should be able to see the small pressure drop as the poppet opens.

Is this a new problem or did you get the boat with this problem?

Re: '87 150 Mercury XR2 Overheating at High RPM's

Thanks so much for the quick response!! I have actually been dealing with this same issue for about 5 years! It is an ongoing project. When I bought the boat, the overheating was an issue, but I thought, how hard could that be to fix? Well here I am.

When I replaced the poppet valve, I also replaced the rubber seal grommet. I've had it in and out a few times. Haven't seen anything wrong as far as I can tell. I did at one time completely remove the poppet valve and took it for a test run. With no poppet valve, it still ran hot at high rpms. No change at all.... (still had very good pee stream at wot)

gary

Re: '87 150 Mercury XR2 Overheating at High RPM's

Pressure is within bounds. Should be cooling.

Perhaps the answer is on the other side of the equation.

What's your timing. That engine has an advance module on it that could be over advancing the timing. Try a run with it disconnected.

Check your plugs. Could be running a bit lean at WOT. The way to check it is to run WOT as long as you can, then shut down suddenly (make arangements so it is safe, and you can do the next step.) pull the plugs and examine them. The insulators should be roughly a shopping bag brown. If some or all of them are too clean, yer too lean. BTW over oiling can cause this.

What's the part number on your carbs, and what's the jet sizes, especially main and vent?

hope it helps
John

Re: '87 150 Mercury XR2 Overheating at High RPM's

Just had a thought. Check the dump hose on the thermostats, between the tattle tale T and the adapter plate. If it's plugged, that would allow a very strong tattle tale and still not accomplish the extra cooling the t-stats are supposed to provide.

hope it helps
John