Engine Going Cold After Coming Off Plane
 

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

http://www.mercruiserparts.com/images/COMMON/20115.png

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 !

The plastic hold down spacer is a pain to get out but the trick is to use a small 2 finger gear puller and hook it to the t-stat, It will pull up both at the same time.

8 out 10 times it saves the plastic hold down spacer, # 6 in your parts list.

I am gonna rip it apart again and check it out.

The only thing I forgot to mention is before I get back to the dock (7min idle) the temp is back to 160* and everything is normal again before the boat is docked. But it happens everytime I use the boat......... could almost set my watch to it. Some have told me not to worry about it but since it has been happening I seem to get more condensation in the breathers and under the oil fill caps. It never did it before so something is wrong. The port side motor doesn't do it.

Thanks anymore input would be appreciated

These are what are called Gremlins!!!-- Don't ask where they come from but as sure as God made little green apples they always appear. I have the voltage meter on mine that requires (every year) to take the connector wires off the post- scuff them up a little and then re-attach -- or else the volt meter will peg at 18 volts.

Ground issues are in my opinion the cause of most of these problems.

all the best

3poinstar

I will echo what SB and BUP have said. I am fairly confident this is not an electrical issue but a mechanical one involving the thermostat, housing and or plastic insert. Also check for an errant piece of debris that may be in the housing perhaps missed before.

Replaced the Thermo housing and thermostat with all new senders ............. still does the same thing. I even changed the sea water pump and made sure the circ pump was working. Everything checks out, any other ideas? I am getting milk in my breathers (not on the dipstick and barley under the fill cap) which I think is a condensation issue....... possibly caused by the water temp dropping under 100 after coming off plane??

Any help is appreciated

Did you ever mention what app you have (motor) and year model along with is this a stock set up. Is your oil cooler stock set up,


What t- stat did you buy and installed ? Also did you install the correct spacer to hold down the t-stat in the housing / they have 2 different ones FYI. talking about # 6 in the parts that SB posted.

When you bought your parts did you use your engine serial # to get the correct parts for your app ? Do you have the correct t-stat housing installed per your app ? Did you install the T-stat correctly and used a new corked gasket or what ever is called for per your app to seal the t-stat in the housing ?

next just wondering what Exhaust system do you have ? If Mercruiser stock do you have the correct riser gaskets installed ?

It's a Mark 4 540. Oil cooler is stock merc 502 cooler. It's not a new build I have been running it for years.......just a new problem. I just swapped the housings from the port side to the starboard side. I am running EMI exhaust.