I have stock original 1997 502 mpi mags with about 350 hours on them that have always ran flawless. I just started having a problem with my port engine the end of last year and it is doing the same thing this year. If I am running on plane anywhere from say 2800 to w.o.t. and slow down to come off plane my port engines temp gauge will drop down to the lowest point on the gauge which is 100 degrees and that engine will automatically go to a higher idle speed just like if the engine was never ran or started cold. Then in a short period of time in a couple of minutes the temp. gauge climbs steadily back to its normal operating temperature. I have replaced the thermostat thinking it might be sticking open but it still does the same thing. Is there some sensor telling my engine its running colder than it actually is? Thanks in advance for any help.

yes and no... the gage and sensor on the dash doesn't tell the ecu anything... but they run off the same harness ground... ill bet the ecus water temp sensor loses its ground as well and lies to the ecu... ide look at all the plugs and connections. bad ground in my opinion

Thanks, but how does the temp sensor loses its ground only under the conditions I described? Also the slower I slow the boat down to an off plane condition my problem happens less or not quite as drastic temp drop.

who knows ? thats the thing about intermittent electrical problems... you have to touch everything , one at a time ... disconnect/clean/reconnect with a vo meter until its all done. and you may find nothing big at all... but if your observation is factually correct, i.e. the gage READS cold but its not and the injection THINKS the motor is cold but it isn't, then the only circuit that those two elements share is the harness ground . one to the gage and the other to the water temp switch for the ecu ( not the sender for the gage.... )

i admit that it seems a odd situation but electrical stuff ( if thats what it is) will do that to you. the only thing worse is not using a disciplined and methodical approach for fixing it.

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Just wondering if you ever figured out what the cause of this was.

I have the exact same problem, boat runs great on plane at 160* soon as I come off plane the temp goes to 80* ? I have confirmed the temp with an infrared temp gun. I am at a loss trying to figure this out. I have changed the Thermostat and it has good water pressure not sure where to look next?

Sounds like a problem with the plastic spacer/flow director that sits under the t-stat. Either missing a piece off it or not sealing the t-stat against the top of the t-stat housing well...letting too much water that should get heated, go right out the exhaust.

edit in to show parts and orientation of what I am saying, plus BUP below:
http://www.mercruiserparts.com/Show_...sp?dnbr=804807 99&ivar=images/COMMON/20115.png&inbr=4854&bnbr=130&bdesc=Thermostat+Hous ing+and+Hoses

A couple of things here. The is a round cork gasket that sits up against the T-stat to help seal it in the housing. A lot of times it is missed or doubled up. The old one has to come out when checking or changing the t-stat and a new one replacing the old one. Also it is wise to clean the or scrape the housing out of crud build up and especially clean out the area the t-stat rests against so it does sit flat and seals properly. Also what SB said about the plastic hold down needs to be in good shape (not cracked and in place).

next depending on the t-stat gasket, if it has the metal buttons or calls for that gaskets, it has to be use and no sealant should be applied to the gasket. The reason the metal buttons or some call them rivets in the gasket is to ground the housing so the sender(s) in the t-stat housing read correctly.

SB man your fast I changed up my post click back to it and U already hit liked.

I came back to edit mine and I saw yours. Good info.

I see people f*k up the t-stat, water (sleeve) flow director, and gaskets all the time.

BTW: I upped the compression and cammed my keyboard. So hell yeah, it is fast !