Thermostat replacement

[OldSkool]

Seems like every season my 5.0 runs hot on initial start up. Last season I replaced the impeller and installed a 160 degree thermostat. Yesterday I started Old Skool for the first time this season and at spec 4800 RPM she went into the red just like last year.

Should I go to a 140 stat or stay with a 160? Last year she stayed just under the red at 4800 rpm with the 160. Still don't know why it needs changed again though.

 

[dvx216]

closed or open cooling system

 

[Lazy Daze]

Is this a newer engine? What year? If so, are the gauges still original - what about the temp sensor?

 

[fc3]

I thought Merc typically used 140º thermostats on raw water cooled engines.

 

[Jetmart]

Try running without the thermostat and see what the temps do.

 

[Lazy Daze]

I thought Merc typically used 140º thermostats on raw water cooled engines.

Not as a hard and fast rule, though. I don't recall all the "rules" :smt001 offhand, but from at least the mid-90's on, small blocks, V6's and 4cyl's used 160*. I'm sure there have been exceptions, too.

 

[OldSkool]

Sorry folks, I've been on the site for so long I though everyone knew Old Skool. Open water system, Original Ford 302 vintage 1976. I installed new exhaust manifold and risers 2 seasons ago but they were direct OEM replacements. I ran the engine with the outdrive in a tub so the water was picked out from the raw water pump and into the engine naturally not forced like when using muffs. (Yes I had a hose running in the tub. LOL)

The gauge appears to be working correctly. When I back down to 3500rpm the temp begins to drop right off. and hold at 3/4 green.

 

[Lazy Daze]

When you did the impeller, what was the housing and/or wear plate condition? Even the smallest amount of scoring can prevent the impeller from the proper amount of water.

You mentioned "on initial setup"... does that mean after the first run of the season it's good for the rest of the season?

I don't know the exact spec, but take the inlet water hose off the t-stat and run the engine for about 30 seconds to a minute (start with a cold engine). Observe the flow of water coming out - it should be a very healthy flow. See how much it changes as you rev it a bit.

 

[CliffA]

Try running without the thermostat and see what the temps do.

+1.....this will tell you for sure if the t'stat is your problem....if the engine still runs hot without a t'stat you have a different root cause for the overheating....

cliff

 

[Bob & Sue]

I would think it is a GB kind of problem just gives up once a year.:grin::grin::smt021

I thought I read somewhere on that setup they did run a 140* tstat. Are you running it at 3500-4500 rpm in a barrel? After you make sure the impellor & t-stat are good and there's good water flow like mentioned by Dennis, you might want to launch it to see what it does under load.

When we ran our funny cars for warm up at 3000-4000 rpm they heated up fast, most of the time we'd run a mix of gas & alcohol to make it run cooler at break in plus it's a lot cheaper than nitro.

Good luck hope you can figure it out, I hate having a problem without finding the cause.

 

[moparlvr4406]

Try running without the thermostat and see what the temps do.

+1.....this will tell you for sure if the t'stat is your problem....if the engine still runs hot without a t'stat you have a different root cause for the overheating....

cliff

Negative !!
the lack of a stat will cause one to overheat also.. nothing slowing the flow to allow heat transfer

 

[CliffA]

Negative !!
the lack of a stat will cause one to overheat also.. nothing slowing the flow to allow heat transfer

not doubting you but boat engines must be different than car engines in this respect.....many times as a teenager messing around with cars we knew that when a t'stat stuck open the engine had a hard time reaching proper operating temp....i also removed the t'stat on a couple engines as a temporary measure when the t'stat stuck closed until i could get a new t'stat....the affect of removing the t'stat was the same as one that had stuck open...slow to reach proper operating temps....

maybe it has something to do with a car engine having a closed cooling system and most boat engines are sea water cooled....

cliff

 

[OldSkool]

Great comments by all I'll remove the water hose and see how healthy the flow is. I'm not a proponent of running without a stat. If I take the housing off I'll go ahead and install a 140. I bought a 160 before as it was what the Mustang 5.0 ran back in the day according to the pimple faces parts guy.

When I told him it was for my Sea Ray he about fell out. LOL

 

[Lazy Daze]

Other than losing efficiency (due to a colder engine), it's OK to run w/o the stat if you want to test that out. With the Ford, the cooling system different - it's under pressure, which allows for higher operating temps without boiling. Newer boats run higher stats to try and help with efficiency/emissions. IF everything else with the cooling system is healthy, my "guess" is that you really aren't overheating the engine - it's just that your gauge is calibrated for the 140* stat. A 160* stat tends to show about 170* on a normal gauge - which, again, may be just a bit high for the gauge... but not necessarily the engine.

Overheats (at the gauge) all the time, or just the first time out?

If you have an IR gun, what temps do you see around the temp sensor? When it's "overheating", what do the top of the elbows feel like (with your hand)? As long as you can relatively comfortably keep your hand there, you're fine - regardless of what the gauge says.

 

[OldSkool]

I'll get an IR gun from work and shoot it this weekend. My gauge is not metered in numbers just the old green then red zone. Not even a yellow. It has only done it once last year and I changed the stat. Temp went down but still hovers right at the red and a tab bit north of red. Could be that the 160 stat is seemingly causing an overheat. I'll drop to a 140 and see if she centers in the green again.

Thanks again.

 

[moparlvr4406]

not doubting you but boat engines must be different than car engines in this respect.....many times as a teenager messing around with cars we knew that when a t'stat stuck open the engine had a hard time reaching proper operating temp....i also removed the t'stat on a couple engines as a temporary measure when the t'stat stuck closed until i could get a new t'stat....the affect of removing the t'stat was the same as one that had stuck open...slow to reach proper operating temps....

maybe it has something to do with a car engine having a closed cooling system and most boat engines are sea water cooled....

cliff

more due to the rpm factor... most old v8s don't operate at 3000+ rpm for hours on end
the merc 260 in my 240had a bad overheat problem at speed until I realized no stat is bad

 

[Westie]

Negative !!
the lack of a stat will cause one to overheat also.. nothing slowing the flow to allow heat transfer

Interesting, so removing the thermostat changes the laws of thermodynamics somehow?

 

[moparlvr4406]

Interesting, so removing the thermostat changes the laws of thermodynamics somehow?

if the water moves through too fast it doesn't absorb and dissipate the engine heat as efficiently

 

[Air O'Nautical]

Interesting....I don't recommend running witout a stat..

But, I do know my old (1995) 7.4L will run cold without a stat.

 

[Westie]

if the water moves through too fast it doesn't absorb and dissipate the engine heat as efficiently

Wow you really believe that?
Now if it were moving fast enough that friction was causing it to heat up you may have a point, but the speed of the water in an engine is many orders of magnitude below that.