Pulling hair out over 502 MPI Mag overheating, looked at everything, please help

Hi all, first post here. I have a 1998 502 MPI w/420 hours on it, freshwater use only, rawwater cooled, and it has an issue with intermittant high temps. The boat will run around 165 for the first part of the day, but I've noticed especially at higher power settings the temp creeps up to 200. The alarm did not sound. (Doesn't happen at idle or slow speed) I once took the boat on a 30 mile river trip- it was fine all day until the return leg- went through some chop and the high temp came back. When dropped the power back it kinda drops, but not all the way back to 165- more like down to 185. I've swapped the seawater pump impeller/pump body multiple times, backflushed all hoses to include oil cooler, drained block and manifolds, replaced the transom to pump inlet line (correct reinforced line), replaced temp sending unit and guage, had computer history checked (no stored codes, but max temp recorded 203!), and checked risers. One side riser is pretty darn warm, and the water exiting the thru-hull tip is hot too. I pulled that riser- the passages look clean. Not even rust coated, more like rust colored if that makes sense. Do you think I should just go ahead and swap the manifolds/risers just to eliminate that as a source? I'm out of ideas, looking for any suggestions, thanks guys.

Re: Pulling hair out over 502 MPI Mag overheating, looked at everything, please help

have you checked the water tube at the transom housing.corrosion has been known to crush the hose restricting flow.
It would be better to have a thru-hull pickup installed though.

Re: Pulling hair out over 502 MPI Mag overheating, looked at everything, please help

I see no mention of the thermostat.

Re: Pulling hair out over 502 MPI Mag overheating, looked at everything, please help

Whoops, forgot to mention that, sorry. I'm on about my third thermostat. Not sure why I replaced so many, just hoping for any resolution. When I swapped the transom to pump hose, I did feel around in that nipple for any distortion, and it seems okay from what I can tell. I know what you're talking about- the Bravo fever. Would that cause the starboard riser to run hotter than port? BTW, my thermostat housing is the type WITHOUT checkballs, if that makes a difference.

Re: Pulling hair out over 502 MPI Mag overheating, looked at everything, please help

Has it ever destroyed a raw water pump impeller? If so there could be bits of it causing a restriction somewhere.
Check the hose between the drive and the transom plate.

Re: Pulling hair out over 502 MPI Mag overheating, looked at everything, please help

Well, I may have actually found the problem, so here's what I got for archives sake. I found a loose and poor sealing vacuum hose for the fuel pressure regulator. It was barely connected, and would not seal due to being swollen and dryrotted. This is the one that connects to the intake plenum, and commands the fuel pressure to increase with higher throttle settings. I think what may have been happening is since the pressure regulator was more or less disconnected, and at higher throttle settings the engine would lean out and heat up. I'm thinking it was intermittant because it would probably seal enough to let the engine run correctly until vibrating loose, or heating up, expanding, and leaking even more. Since the 1998 Mercruiser EFI does not have an oxygen sensor, the ECM would not be able to flag a lean burn condition other than to retard the timing when prompted by the knock sensor to do so. I took the boat out today, and really romped on it, tried a number of trim and power settings, the temp wouldn't go above 178. I'm cautiously optimistic that this was it, but happy to have found that loose line either way.