How hot is "too hot" for Mercruiser 454 mag temp?

[valley_man0505]

I recently replaced a bad head gasket in my 7.4L Mercruiser 454 Mag EFI Ski (1995) with serial number 0F225581. Last weekend, I put it in the water and turned the key for the first time after this work was completed and it started right up. I let it idle at the dock for about 20 minutes, adjusted the timing (about 8-10* BTDC) and then pulled it out for an oil change. While idling, the temperature went up to about 180 and steadied there. After the oil change, I put it back in the water and did a slow (i.e. about 4-5 mph) lap or two around the lake (it's a small lake). This time, the temp went up to 200 and steadied there. This is obviously higher than it should be, but it held stable. The next day before I pulled it out of the water for the weekend, I did some more slow laps and temp went to 200 and stabilized again. I then tried a "hot lap" for the first time and it had it at full throttle for about 3-4 minutes. All the power appeared to be back after the head gasket replacement and the temp still held at 200 just like it had during the slow laps. It sounds fine at all speeds, oil pressure is within normal range...everything appears to run great now except for the higher than anticipated temperature.

Some other notes:

• I am running a small freshwater lake with water temps in the 80-85 degree range.
• After shutting the engine down, I was able to put my hand on all areas of the motor without burning myself or even feeling like I need to pull my hand away. It wasn't cool or anything like that, but it also wasn't scalding hot.
• I also replaced the temperature sending unit when reassembling everything. I literally only ran this boat for one day after buying it used before the head gasket blew. During that short run time, the temp gauge seemed to swing around quite a bit. The gauge is now climbs steadily and then stabilizes versus swinging all over like it did before. The upper range of the "swings" with the previous sending unit was around 180 during the one day I got to run at the start of summer before everything took a crap.
• I replaced both exhaust riser gaskets during the head gasket fix as well. There were no signs of blockages and no visual corrosion inside the exhaust manifolds to indicate an internal leak. The whole exhaust system actually looks pretty new and I think one of the 2 previous owners must have replaced it in the last few years.
• I DID NOT replace the thermostat, which I am kicking myself for. I had all the parts to do it, but we were so excited about getting everything back together after this had become a summer long project, that we missed this. I DO PLAN to change the thermostat this weekend as my first troubleshooting step.
• Also this weekend, I plan to use an IR temp gun to check surface temps to see if there are any hot spots.
• I also plan to check the water pump impeller first chance I get.
• I have not run a compression test since reassembly, but this is also on my short list of things to do this weekend.

Hopefully, I have a good path forward in regards to troubleshooting. My main question for everyone at this point is this: Do I have to be concerned that something will get damaged due to the elevated temperature while I continue to run this thing while troubleshooting over the next few weekends? Obviously, something isn't right, leading to the elevated temp, but similar automotive vehicle engines are DESIGNED to run at 210* and it is holding below that level at least.

Also, when shooting the IR gun, how much of a difference is there, typically, between the surface temperature and the internal temperature (specifically, the location of the temp sending unit right at the thermostat housing)?

Any other feedback/advice would also be appreciated.

 

[San_Diego_SeaRay]

• I also plan to check the water pump impeller first chance I get.

Earlier this year I was also struggling w. overheating on my 1998 carb'd 454. Was reaching temps slightly over 200. I changed the impeller and the impeller housing. Now I have a hard time getting it to get to temp. If I were you, that's where I'd start.

 

[Lou C]

200 degrees is definitely too hot for a raw water cooled system because they are not pressurized and you can get localized boiling in the hotter parts of the cooling system. The stat ratings vary from 140* to 160* depending on the model. If you have a 160 they will run from 160-175 or so.

I would first
take the stat out & test it
Then
Do a compression test & check for water in the cyls & combustion gas in the cooling water.
You said you replaced one HG why not the other?

 

[valley_man0505]

I would first
take the stat out & test it
Then
Do a compression test & check for water in the cyls & combustion gas in the cooling water.
You said you replaced one HG why not the other?

You said you replaced one HG why not the other?

Compression was fine in all cylinders except 5 and 7. Upon removal of the head on that side, a clear head gasket leak was found there explaining the issue. The other side wasn't touched since it wasn't worth the effort if the head from the problem side was shot since new heads for this model are hard to find...and expensive enough that it probably would've been "junked" at that point. Replacement of the other side may be a project for next spring when it comes out of winter storage.