1994 Mercruiser 4.3L V6 Raw Water Overcooling

[electronG101]

Hello All -this is my first time on the forum. I have a 1994 Mercruiser 4.3L V6 raw-water cooled engine that I recently acquired and it is overcooling. I ran the engine for nearly 30 mins at over 3500rpm in the lake. The needle on the gauge was pegged all the way to the cold end and did not move at all.

I then tested the gauge by momentarily shorting out the tan signal wire to the block. The needle on the gauge instantly jumped up to full temperature.

Thinking the sensor was bad, I replaced it with a brand new sensor from West Marine. I also checked to make sure that the thermostat housing was getting a good ground, which it was. I performed another trial run in the lake, and the gauge was still pegged all the way on the cold end.

Finally, I replaced the 140F thermostat with a brand new one from West Marine. I then performed yet another test run, and received the same result.

As a note, I felt the top of the thermostat housing with my hand periodically during the trial runs, and it was probably 100-110 degrees -certainly not hot. All of the water cooling hoses were cold except for the hose that carries water into the water pump, which was warmer than the tstat housing, but not super warm/hot.

Despite the overcooling problem, the engine seems to run and perform very well with lots of power on tap.

Ideas? Thanks in advance for the help.

 

[thumpar]

Re: 1994 Mercruiser 4.3L V6 Raw Water Overcooling

I know there are different thermostat housing designs but does yours have the spacer in it if needs it?

 

[electronG101]

Re: 1994 Mercruiser 4.3L V6 Raw Water Overcooling

Hi Thumpar -Thanks for the reply. Yes, it does have a plastic spacer/sleeve that sits ontop of the thermostat inside the housing. The sleeve has two openings in it that allow the hot water to exit the cooling system when the thermostat opens up (ie. above 140F).

 

[electronG101]

Re: 1994 Mercruiser 4.3L V6 Raw Water Overcooling

Any other ideas?

 

[thumpar]

Re: 1994 Mercruiser 4.3L V6 Raw Water Overcooling

I might want to pick up an IR heat gauge and double check to make sure the built in gauge is correct. If the gauge is working double check the hose routing.

 

[GA_Boater]

Re: 1994 Mercruiser 4.3L V6 Raw Water Overcooling

Welcome aboard. Electron.

EDIT - wrong response. Sorry.

 

[britisher]

Re: 1994 Mercruiser 4.3L V6 Raw Water Overcooling

I am sure there will be someone along shortly more knowledgeable than me, but I seem to recall that the gasket on the thermostat housing has a metal side to it, which allows proper grounding. If this gasket is installed upside down, then the grounding doesn't take place and the proper temperature doesn't show.

 

[electronG101]

Re: 1994 Mercruiser 4.3L V6 Raw Water Overcooling

Britisher thanks for your response. I confirmed with an ohm-meter that the temp sending unit (sensor) is getting a good ground. Even though the gasket may not provide a good ground, the 2x bolts that connect the tstat housing to the intake manifold do provide a great ground.

Keep the ideas coming!

 

[dubs283]

Re: 1994 Mercruiser 4.3L V6 Raw Water Overcooling

ir temp gun to verify actual engine temp, otherwise its just speculation

 

[electronG101]

Re: 1994 Mercruiser 4.3L V6 Raw Water Overcooling

Per Dubs283's suggestion, I used a IR Gun (Fluke 62 Max), and found that the top of the thermostat housing was 128F after running the boat for 20mins at about 3000 rpm in the lake. I then aimed the gun directly at the outside of the sensor, and got a similar reading.

I also IR'd the water pump, which was 142F. It makes sense that the water pump would run a bit warmer than the tstat housing, since cold water passes straight through two of the tstat housing cavities directly to the exhaust heat exchangers, thus keeping the housing a bit cooler. The water pump seems to be a good location to get an idea of the water temp flowing through the engine. Bottom line, this tells me that the engine seems to be running at a fairly normal temperature.

Aside from the above test, I removed the temp sensor from the tstat housing, soaked it in hot water for a minute, then grounded the body and attached the tan signal wire. Voila, the temp gauge jumped to the mid 100s.

But, why doesn't it read a good value while in the stat housing? I know it has a good ground (ie. 0.3ohms from sensor body to block).

 

[Fun Times]

Re: 1994 Mercruiser 4.3L V6 Raw Water Overcooling

When you installed the black O-Ring first before the thermostat inside the thermostat housing, are you sure the O-Ring was positioned properly on the mounting flange of the housing? If the O-Ring was compromised during installation the cooling will act up like you described. It's worth a re-inspection and re-installation. Be sure you use the O-Ring and not the cork gasket inside the housing.

http://www.mercruiserparts.com/Show...r=120&bdesc=STANDARD+COOLING+SYSTEM(DESIGN+I)

 

[electronG101]

Re: 1994 Mercruiser 4.3L V6 Raw Water Overcooling

I just double checked the brand new O-ring, and it is seated well, and there doesn't appear to be any place that water could leak past it.

 

[electronG101]

Re: 1994 Mercruiser 4.3L V6 Raw Water Overcooling

One other observation -I noticed that the new sensor is much shorter than the original sensor. However, I purchased the part from West Marine, and it was the exact temp sensor replacement for a 1994 4.3L Mercruiser.

 

[electronG101]

Re: 1994 Mercruiser 4.3L V6 Raw Water Overcooling

Also, here is a picture of the hose routing with labels for hose function and hot/cold water flow direction (see attached).

 

[thumpar]

Re: 1994 Mercruiser 4.3L V6 Raw Water Overcooling

Also, here is a picture of the hose routing with labels for hose function and hot/cold water flow direction (see attached).

View attachment 232006

If you are raw water cooled you don't have a heat exchanger. Why do you have them labeled like that?

 

[electronG101]

Re: 1994 Mercruiser 4.3L V6 Raw Water Overcooling

If you are raw water cooled you don't have a heat exchanger. Why do you have them labeled like that?

Thanks for the correction. Then, what are the two hoses going to that I originally labeled as "output to heat exchanger" and "output to exhaust manifold"?

 

[dubs283]

Re: 1994 Mercruiser 4.3L V6 Raw Water Overcooling

exhaust risers (upper)

exhaust manifolds (lower)

 

[electronG101]

Re: 1994 Mercruiser 4.3L V6 Raw Water Overcooling

exhaust risers (upper)

exhaust manifolds (lower)

Got it. Labels have been updated accordingly (see attached).

Any other ideas?

 

[dubs283]

Re: 1994 Mercruiser 4.3L V6 Raw Water Overcooling

was the t-stat oem or aftermarket??

have you tested the t-stat in a pan of hot water??

 

[electronG101]

Re: 1994 Mercruiser 4.3L V6 Raw Water Overcooling

was the t-stat oem or aftermarket??

have you tested the t-stat in a pan of hot water??

Sierra replacement. I also tested it, and confirmed that it opened in a cup of warm water around 140F. Also inspected, and no leaks apparent.