My port 454 Magnum engine (1990, 650 hours) has developed a cooling system issue. I personally have only had this boat in the water a few times late last season (new to me boat), and then a brief trial run this weekend. The last time out last Fall I noticed the port engine running hot. My starboard engine runs maybe 160 degrees average. The port engine, after warming and running, climbs well above 200 degrees, and varies some with different speeds. Ironically, the oil pressure is perfect even when the temp is up. Upon startup, both engines show nearly identical temps, so I'm thinking the gauge is good. Also, while running, as the port engine temp stays high, I (maybe) detect a faint ping or knock, sign of overheating (okay, detecting a faint sound with two mags running may be a bit psychological.)

Last Fall, while winterizing, the port engine took in fluid much slower than the starboard engine. I pulled the impeller housing, everything was great, impeller still had lube and was brand new. I did put a new one in anyway. Also, I changed the thermostat, which appeared to be the original. Also, all the hoses that were disconnected were all clear and in good shape, intake holes on the outdrive are clean and clear. With the recent Spring test run, no resolve in the temperature issue.

When I test drove the boat, and the first two times out, did not have the cooling issue.. I also have a very good total history of the boat and maintenance records.

What to check next? Thanks in advance.

I know you checked all the hoses, just make sure you check the transom inlet hose those can get clogged, and check the oil cooler may have some debris in there. good luck

Well, I backflushed the transom inlet, clear. Pulled the circulation pump, all good there. About the only way to access the oil cooler is remove the exhaust on that side of the engine (it's located port side down underneath), and with twins, ain't no other way to get there except via robot. Ordered a new circulation pump since I had it off anyway, and just in case it was sucking air or something. Anyone know the marina labor hours for an oil cooler swap?!?! "Sigh......"

Trust me i'm no mechanic, but with the new circulation on did you see a difference or test it yet? i know its not really a fix but you could run it with out the thermostat to see if any change. I'm just thinking here,you could pull the transom hose and see if you could stick a small hose down the tube attached to your shop vac you may get something. i know when i pulled mine i found nothing ended up putting a new cooler back in pain in the A$$.

Good luck

New circulation pump, no change. Ran the boat last weekend, easy at first, then all out. Comparing all the characteristics of both engines, I'm beginning to think the gauge is lying to me. No alarm, no overheating symptoms, same temps (to the touch) on both engines. Got a new temp sender, will install. After that, if no change, will swap gauges.

Get yourself a infrared temp. Sensor from harbor freight and start shooting hoses, thermostat housing, blocks, risers etc on both engines rather than relying on temp. Sensors and gauges for diagnosis.

I went through this on two boats... my 242 Formula and my buddy's Cigarette Bullet. This is a classic case of "bravo-itis." The transom plate corrodes around the hose inlet inboard restricting the incoming water to the sea pump. She'll run okay while on the hose with no load, but overheats (or depending on how bad... heats up more than expected) when the engine is under load.

On the Cigarette we installed thru-transom pickups, and the problem was cured. On my Formula the same solution pointed me toward a bad transom (and lots of repairs... all worth it).

Based on the year of your boat and the hours, this is almost surely the problem. I replaced everything chasing that problem. Finally, I pulled the outlet hose off the transom, and found that it was pinched almost shut (Formula).

The permanent fix (unless you want to convert to thru-transom pickups) is here:

http://www.amarket.com/BRAVOitis.htm

The engine will more than likely have to come out for this fix. My transom plate needed this fix because it was too corroded to seal properly with just the hose and plastic fitting (would have sucked air).

I hope this isn't the problem. Pull the outlet off to see if your pinky will go into the hole. It should be wide open. If not, that's your issue. If you do one, you're going to want to do the other.

It was the temp. sender. Changed it out before going to Erie this weekend, put a few hours on it....ran same temp as other engine the whole time.
Oh well.....thanks for the advice folks.

Good find, and good follow up.