Thermostat Replacement Procedure Merc 3.0

Searay87

Petty Officer 2nd Class
Joined
Sep 10, 2009
Messages
106
So far this forum has been great for me, a novice boater. Thanks!

My 87 Mercrusier 3.0 L 130 HP with an Alpha One Drive is running cool at about 100F. From what I've been able to read on-line it should run around 160F.

I'm looking to troubleshoot this myself but like I said I'm a novice and would like to have a procedure if possible or any advice and potential pitfalls to avoid. I've checked some of the other threads and so far I haven't been able to find a procedure. Any help would be much appreciated.
 

bigdan1

Petty Officer 3rd Class
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Aug 9, 2010
Messages
95
Re: Thermostat Replacement Procedure Merc 3.0

change the thermostat

easy .....remove the 1" hose that attaches to the housing
remove the 2 bolts that hold the housing

clean both surface ( to remove old gasket)

install new thermostat + new gasket

reinstall 2 bolts that hold the housing
reinstall 1" hose

et voila !
 

BAproject

Petty Officer 1st Class
Joined
Sep 2, 2009
Messages
276
Re: Thermostat Replacement Procedure Merc 3.0

yes it's fairly simple. Dow hat BigDan1 said.
The only thing that may be tough is removing the brass collar that holds the thermostat in place (this is the set-up on the 4.3's of that age).

You can tap the thermostat out from the bottom side forcing the cuff out enough to grab with needle nose pliers or vice grips. Be careful no to deform the brass collar or you will need to replace it.

Get new gaskets and reinstall.

It's interesting that you are runnign that cool....almost like you don;t have a thermostat in there or your temp gauge is off. Might want to get a IR temp gun and shoot it at the base of the t-stat housing and see if the temperature matches what your gauge is reading. They cost $29 at autozone
 

marlboro180

Lieutenant Junior Grade
Joined
Jun 23, 2009
Messages
1,164
Re: Thermostat Replacement Procedure Merc 3.0

Searay87,

Yup as they ^^^ said, it is really easy, even for a novice. I like to test thermostats when I take them out, and before I put them in.....Assuming yours has one in place, take it out and put it in a pot of water on the stove.

Heat up the water , and monitor the temperature with a meat thermometer. As it approaches the correct temperature ( around 160, if marked as such) it will open up when hot and close as it cools. If it fails either way ( opening or closing) , it is bad and get a new one.

100* sounds like someone had an overheating issue and took the stat out to help solve the problem, but that is just my guess !;) That is too cool for that motor . Wrench on!
 

bowler

Petty Officer 1st Class
Joined
Nov 9, 2009
Messages
281
Re: Thermostat Replacement Procedure Merc 3.0

Hi Searay87,

I had a similar situation with my engine ('96 3.0 LX). In my case it was just a bad stat. It ran around 110 for a few weeks and then it got to the stage where the needle wouldn't budge at all. This concerned me and low and behold I discovered they should run much higher. I switched the stat (easy as pie) and now the needle hits 150 and does not budge. It is also way quicker to heat up after starting. Before the switch I had to wait for AGES before I could put it in neutral without it cutting out, that is not longer an issue. Get stuck in...

D.
 

Searay87

Petty Officer 2nd Class
Joined
Sep 10, 2009
Messages
106
Re: Thermostat Replacement Procedure Merc 3.0

Thank you for all of the great advice. I had a look at this job tonight after I replaced my old alternator. I was able to purchase a new thermostat locally but could not get the gasket. I loosened the 1" hose clamp and tried to gently pull off the hose on the thermstat housing and it wouldn't budge. I was afraid I would tear it.

With only a good 3-4 weeks left in my season I'll do this at home in my driveway and have her ready for next season.

Thanks again.

Bob
 

BAproject

Petty Officer 1st Class
Joined
Sep 2, 2009
Messages
276
Re: Thermostat Replacement Procedure Merc 3.0

it's easier to remove the hose if you twist it first, breaking the seal will make the hose come right off
 

Searay87

Petty Officer 2nd Class
Joined
Sep 10, 2009
Messages
106
Re: Thermostat Replacement Procedure Merc 3.0

Thanks for the tip BAP. Was speaking to a mechanic at the local marine shop and he suggested working the end of a small flat head screwdriver beween the hose and the housing and then twisting it off. If I damage the hose the cost is only $11.
 

bowler

Petty Officer 1st Class
Joined
Nov 9, 2009
Messages
281
Re: Thermostat Replacement Procedure Merc 3.0

I used the screwdriver method with no problems.

No gasket with the new thermostat??
 

Searay87

Petty Officer 2nd Class
Joined
Sep 10, 2009
Messages
106
Re: Thermostat Replacement Procedure Merc 3.0

I just wanted to follow-up on this thread and say thank you to those who gave me such great advice. I finally got around to replacing the thermostat today (see attached photo). I'm no expert but it looks it failed in the open position. I also replaced the temperature sending unit at the same time. Temp gauge now reads between 160-165F. I'm not sure what temperature my engine was operating at with a faulty thermostat and temp sending unit. Now I'm good to go for next season!
 

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