Mercruiser 4.3 thermostat location?

[MalcolmV8]

Hey Guys,

I have a 91 Mercruiser 4.3 V6 and the issue I was having is the temp gauge shows cold all the time unless you idle for a period of time then the temp will come up. If you get on the gas it swings to cold in about 3 seconds flat.

I pulled off this big water manifold of sorts with various hoses coming out of it off the intake manifold and there was the thermostat. I removed it and exchanged it for a new one. Cleaned up all gasket surfaces and put it all back together.

Then tonight I was looking at some exploded view diagrams and it seems like the thermostat does not sit directly in the lower intake manifold of the engine. Instead you are supposed to bolt that water manifold to the intake manifold and then slide the thermostat down into that water manifold followed by the plastic spacer piece and then put the plate on top. That's if I'm looking at the exploded view diagram correctly.

Did my boat have the thermostat in the wrong place?

Thanks
Malcolm

 

[Don S]

Re: Mercruiser 4.3 thermostat location?

It's in the thermostat housing. There are a couple different ones, post a picture of yours.

 

[MalcolmV8]

Re: Mercruiser 4.3 thermostat location?

It's in the thermostat housing. There are a couple different ones, post a picture of yours.

Thanks for confirmation. I started realizing that after googling PDF after PDF showing the same thing. This is the best pic I have from earlier today when I was wondering what these plastic balls are. Look like some sort of one way valve for some reason. Possibly to stop exhaust back pressure? Looks like the hoses that attach there go to the exhaust manifolds.

Well I just ran out there, midnight and all, with a flash light and tore it all apart while silicon was still not fully cured from early evening, cleaned it and redid it the correct way. I'll take it out to the lake after work tomorrow and see how it works now. I also noticed the tstat that was in there, in the wrong place, was a 160F unit and the Mercruiser OEM unit I got is a 140F.

 

[Bondo]

Re: Mercruiser 4.3 thermostat location?

Ayuh,... Tighten up the check-balls a little bit,....

If they ain't closin', you'll get an Overheat at Idle,....

kinda like yer seein',...

 

[MalcolmV8]

Re: Mercruiser 4.3 thermostat location?

Ayuh,... Tighten up the check-balls a little bit,....

If they ain't closin', you'll get an Overheat at Idle,....

kinda like yer seein',...

Check-balls are as tight as they go. The nut is spun up on all the thread there is on that rod. The only way to make them tighter is to get longer springs. However it may be fine. My thermostat was in the lower intake of the engine instead of in the water manifold housing above so cold lake water was passing directly on the temp sensor continuously make the gauge read cold all the time. When it would sit and idle the water from the impeller slows way down and the warmer water from the thermostat got a chance to touch the temp sensor allowing it to show normal readings. Not over heating, but just normal. Then as soon as the engine would rev the impeller would pump up lake water with some volume and almost immediately you'd see the temp gauge drop cold since it was flowing over the top of the thermostat directly onto the temp sensor.

Now that I have the thermostat in the correct place with the temp sensor now below it showing actual engine temps I'll re-evaluate how it all works.

 

[Bondo]

Re: Mercruiser 4.3 thermostat location?

Check-balls are as tight as they go. The nut is spun up on all the thread there is on that rod. The only way to make them tighter is to get longer springs. However it may be fine. My thermostat was in the lower intake of the engine instead of in the water manifold housing above so cold lake water was passing directly on the temp sensor continuously make the gauge read cold all the time. When it would sit and idle the water from the impeller slows way down and the warmer water from the thermostat got a chance to touch the temp sensor allowing it to show normal readings. Not over heating, but just normal. Then as soon as the engine would rev the impeller would pump up lake water with some volume and almost immediately you'd see the temp gauge drop cold since it was flowing over the top of the thermostat directly onto the temp sensor.

Now that I have the thermostat in the correct place with the temp sensor now below it showing actual engine temps I'll re-evaluate how it all works.

Ayuh,.... I understand, I read yer other post,...

Even with the t-stat in it's proper place, yer Gonna have localized overheatin',...

The balls Have to seal,... Stretch out the springs that are there,....

or get the kit, 'n replace 'em,...

 

[MalcolmV8]

Re: Mercruiser 4.3 thermostat location?

Ayuh,.... I understand, I read yer other post,...

Even with the t-stat in it's proper place, yer Gonna have localized overheatin',...

The balls Have to seal,... Stretch out the springs that are there,....

Gotcha. OK will stretch springs this evening before I take it out for a test run.

 

[Volphin]

Re: Mercruiser 4.3 thermostat location?

I'm pretty sure the 160 degree stat is for closed cooling system and the 140 is for raw water cooling. Looks like a raw water cooled setup in your pic...

 

[MalcolmV8]

Re: Mercruiser 4.3 thermostat location?

I'm pretty sure the 160 degree stat is for closed cooling system and the 140 is for raw water cooling. Looks like a raw water cooled setup in your pic...

Best I can tell from following hoses last night it's a raw water cooling system. The corrosion in the various parts supports that too.

 

[Don S]

Re: Mercruiser 4.3 thermostat location?

I doubt the check balls are the cause of the cold running.

You do have a raw water cooling system and you need a 143? for that engine. If you boat in fresh water, it won't hurt a thing to run a 160? thermostat.

 

[MalcolmV8]

Re: Mercruiser 4.3 thermostat location?

I doubt the check balls are the cause of the cold running.

Agreed. I'll tighten them up to avoid a possible over heating issue at idle is all.

You do have a raw water cooling system and you need a 143? for that engine. If you boat in fresh water, it won't hurt a thing to run a 160? thermostat.

View attachment 200165

In that diagram you attached. My thermostat was installed below gasket marked #8. I now have it in the correct position per that diagram.

 

[Don S]

Re: Mercruiser 4.3 thermostat location?

In that diagram you attached. My thermostat was installed below gasket marked #8. I now have it in the correct position per that diagram.

Then whoever installed it, put it in the wrong place. Put it up where it belongs, and be sure items 7 and 9 are also there.
Then your cooling system might work.

 

[MalcolmV8]

Re: Mercruiser 4.3 thermostat location?

Then whoever installed it, put it in the wrong place. Put it up where it belongs, and be sure items 7 and 9 are also there.
Then your cooling system might work.

Yup exactly. I changed it all out last night once I realized what was going on. I replaced 7 (oring) with a brand new one and item 9 t-stat of course was missing from that location but is now there (a brand new one). Everything is now installed per that diagram. Providing this light rain goes away and it's hot and sunny by end of day I'll take it out for a test.