1989 90 hp new thermostats not opening- why not?

Dennisanoka

Petty Officer 1st Class
Joined
Jun 10, 2007
Messages
252
My old stats were acting intermittent so I replaced them but the new stats don't seem to be opening now. I put them in a pan of hot water to make sure they work and they do open @ about 150 degrees or so.

If I start the engine, it warms up to 140-150 but the stats don't seem to open. If I wait long enough the alarm will sound however if I pull one of the hoses off the stat housing water starts to flow from the head and they both cool down.

All 4 plugs look normal so I don't believe I have an internal water leak. The engine ran great at wot but then if I throttled down to idle the temp rose quickly.

I've seen some hints in threads about modifying the pressure relief valves or possibly the stat itself but I could not find any details. I will look forward to your input.

Thank you-
 

Dennisanoka

Petty Officer 1st Class
Joined
Jun 10, 2007
Messages
252
Re: 1989 90 hp new thermostats not opening- why not?

The only difference I can see between the old t-stat and the new ones is a notch on the sealing side of the stat but I don't understand how it would effect flow in any way, since it does not go thru the stat housing.
 

Dennisanoka

Petty Officer 1st Class
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Jun 10, 2007
Messages
252
Re: 1989 90 hp new thermostats not opening- why not?

Well I solved my own problem. Under better light conditions I can now see that the notch is cut deep enough which creates a small pin hole that would allow water to pass thru and get to the sensing side of the t-stat. I'm sure it will solve my problem.

By the way getting these in and out is not that bad. I use a box wrench that has thin walls and has a slight angle that gives clearance enough to get a 1/8 turn or so each time. It may take 10 minutes to get them out and 15 to get them back in.
 

emdsapmgr

Supreme Mariner
Joined
Dec 9, 2005
Messages
11,551
Re: 1989 90 hp new thermostats not opening- why not?

There is a valve body made of some type of hitech plastic in the thermostat housing. Depending on which year, it either has a single oval hole or dual pinholes in that body. Make sure they are open/clear. If the hole(s) plug up with dirt, debris, the engine will overheat, even with new stats. The factory put those holes there to permit hot water to get to the back of the stat, so it will heat up and open at the right temp. It's the bubble back models which are so difficult to access/replace the stat housing.
 

Fed

Commander
Joined
Apr 1, 2010
Messages
2,457
Re: 1989 90 hp new thermostats not opening- why not?

I thought the sensing side of the thermostat was upstream and the small holes were to allow for some flow to avoid having a pocket of cold water, are you sure you have them installed the right way around?

Having said that I don't know if it's physically possible to install the backwards?
 

seahorse5

Rear Admiral
Joined
Jan 24, 2002
Messages
4,698
Re: 1989 90 hp new thermostats not opening- why not?

You are on the right track as there is an airlock situation and the water cannot get to the thermostat element to get them to open.

The tiny hole in the plastic thermostat housing plate gets plugged up very easily. Just use a 3 cornered file and make a small notch in each white plastic pressure relief valve edge and that will allow the air to bleed out and the water to follow behind.
 
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