Mercruiser 7.4 Running Cool - Temp Gauge Fluctuates 100 to 170

[Jgyatesjr@aol.com]

My 7.4 (Sundancer 260 with Bravo III) is not running hot - it is running cool! Temperature gauge fluctuates from 100 to 160 but mostly in the 100 to 130 range(at idle) and if I run up to 2000 rpm for a 10 min the alarm goes off (beeps every five seconds of so).

Prior to this problem temp always set right at 170.

Changed thermostat but did not help. Checked housing with infrared thermometer gun and temp gauge appears to be right on. Checked risers/manifolds and nothing very hot.

I am thinking that the thermostat housing is defective. Engine water pump?

What does the alarm mean?

Any thoughts?

John
Old Town Alexandria

 

[Don S]

Re: Mercruiser 7.4 Running Cool - Temp Gauge Fluctuates 100 to 170

Does your engine have a year or better yet a Serial number?

 

[Jgyatesjr@aol.com]

Re: Mercruiser 7.4 Running Cool - Temp Gauge Fluctuates 100 to 170

Serial number is : 0L362605

 

[jerryjerry05]

Re: Mercruiser 7.4 Running Cool - Temp Gauge Fluctuates 100 to 170

Could be the sensor is bad.Corroded/dirty sensors can do this.A bad connection.A bad ground.Is it pumping water good?J

 

[Jgyatesjr@aol.com]

Re: Mercruiser 7.4 Running Cool - Temp Gauge Fluctuates 100 to 170

I think I put the t-stat in wrong. How does the round rubber gasket go? Above the t-stat o,r does the t-stat fit in the groove in the gasket? When I put it the groove, it will not seat properly.

I am not talking about the gasket between the t-stat housing and manifold.
tks

 

[robjen1019]

Re: Mercruiser 7.4 Running Cool - Temp Gauge Fluctuates 100 to 170

I was going to say thermostat

 

[Jgyatesjr@aol.com]

Re: Mercruiser 7.4 Running Cool - Temp Gauge Fluctuates 100 to 170

I am having a problem getting the t-stat to seat properly in the housing. Does the round rubber gasket go on the t-stat or, in front of it - see pictures below:

tks

 

[Jgyatesjr@aol.com]

Re: Mercruiser 7.4 Running Cool - Temp Gauge Fluctuates 100 to 170

Oops! The dealer gave me the wrong gasket! The correct one is shown on the left below.

T-stat fits great now! The saga continues.

 

[Jgyatesjr@aol.com]

Re: Mercruiser 7.4 Running Cool - Temp Gauge Fluctuates 100 to 170

I replaced the t-stat (correctly) and the t-stat housing and the sender in the t-stat housing and I still have the problem (gauge fluctuates from 100 to 170). I did test the t-stat to be sure it opens at 170. I also get an alarm under certain conditions. I plan to replace the other sender on the manifold just below the t-stat housing. It has 2 wires coming to it. I guess it could be the gauge. How does one ck the gauge?

john

 

[Jgyatesjr@aol.com]

Re: Mercruiser 7.4 Running Cool - Temp Gauge Fluctuates 100 to 170

Finally solved my problem. I replaced the t-stat, then the t-stat housing, then the sea water pump and still had the problem. Finally, I changed the circulating pump. Turns out that the circulating pump failed. See picture below:

The rotor broke down the middle!

So problem of fluctuating temp gauge solved. The circulating pump was replaced 5 years ago by the previous owner but has no maker numbers on it. Obviously a cheap replacement part though he paid $161 for it!

 

[HT32BSX115]

Re: Mercruiser 7.4 Running Cool - Temp Gauge Fluctuates 100 to 170

Good thing you checked the pump!!

They do fail!

Yesterday, I changed a circulating pump on a 98 ford Taurus that had a problem with the heater not working very well. (the engine DID NOT overheat or even run hot!!)

The circulating pump had a stamped steel impeller.

The impeller looked like a little saw blade!!! It had NO impeller "vanes" on it at all! they were completely corroded away! I don't see how it could have worked at all!! but it did.

You were actually getting enough raw water flow via the raw water pump to keep the engine from over heating.

Just make sure you don't put an automotive pump back on it....


Cheers,


Rick

 

[ziggy]

Re: Mercruiser 7.4 Running Cool - Temp Gauge Fluctuates 100 to 170

thanks for the follow up.

i think that's the first circulation pump failure i've read about. pretty interesting way it failed too..
good diagnosis..