4.3 normal temp at idle, cold at high speed.

[lonewolf41]

Just upgraded my 1998 4.3GL (4.3GLPBYC) to a GS. While I had the top end apart, I thought it would be a good idea to replace the thermostat since I had it off anyway and it was probably the original. The boat ran fine last year, BTW, but i figured an easy maintenance item to do now to help protect my boat in the future. This year, while sitting at idle (on muffs or in water) the temp stays in the 160-170 range...might get close to 180, but comes back down. However, when on the water at high speed (3000 RPM for example) the temp goes down close to 120 and stay there until I stop. The temp then climbs back up to 160.

First of all, this seems abnormal. Assume it has something to do with the thermostat, but not sure what. Secondly, do I need to fix it?

TIA,
-Keith

 

[TyeeMan]

Probably your raw water pump. When was the last time you changed it? Even if it was a couple years ago I'd change it, and do yourself a favor and use a real Volvo impeller.
I had this same thing happen on another boat, it had a Mercruiser 4 banger. At idle in the water, even early season 45 degree water, the temp would creep up to 160+, and then once under way the temp would come right down to 120.
Pulled the impeller and the fins had deffinately taken a set. Keep in mind that when I got that boat it had a new out drive with new water pump and just a couple hours on it, , , but it sat for about 3 years.

Installed new impeller, probablem gone. Ran at a seady 120 degrees no matter what.

 

[dypcdiver]

Remove the thermostat housing and check the bypass port does not have any debris blocking it. A flexible piece of wire will do it. I had the same problem for 2 seasons until I was advised on here to check the bypass, I got 2 vanes from the raw water pump out of it, I could not see them but the wire got them out. Seemingly the extra pressure due to the blockage opens the thermostat.

 

[lonewolf41]

Forgot to mention that I put a new impeller in it. I used one of the run-dry types, but it was installed 4 days ago. I can try a regular one I guess.

Boat systems can be odd ducks, but it does seem a little strange that an impeller would be better at idle and worse under high RPM...at least in my case getting too cold. Even if the impeller was pushing too much water, it seems like the thermostat should regulate it to the correct temp.

Thanks for the feedback!

 

[TyeeMan]

I would venture a guess that your normal operating temp is 120 degrees. My 95 4.3 GS is 120 degrees all day long, as did the Merc I had before this. Not too many boats run warmer than 120. Maybe an engine with a closed cooling system might run up to 140, not sure about efi engines.
At low engine rpm like idle the water pump is a volume pump, so you need nice strong vanes on the impeller to push out against the walls of the water pump housing. When engine speed increases the water pump becomes more of a centrifigal pump, so if the vanes are a little shot it's not that big of a deal.

You are correct in that the thermostat should regulate temperatare. But if your t-stat is rated to open at 120 or even 140 degrees if the water pump isn't pumping enough water at idle the t-stat will go wide open and you'll start to over heat. Now get the engine r's up, pump more water, the engine temp drops and now the t-stat closes up to regulate engine temp at 120 or 140, , , agian, whatever temp is stamped on the t-stat.

 

[HalfFish5087]

Just upgraded my 1998 4.3GL (4.3GLPBYC) to a GS. While I had the top end apart, I thought it would be a good idea to replace the thermostat since I had it off anyway and it was probably the original. The boat ran fine last year, BTW, but i figured an easy maintenance item to do now to help protect my boat in the future. This year, while sitting at idle (on muffs or in water) the temp stays in the 160-170 range...might get close to 180, but comes back down. However, when on the water at high speed (3000 RPM for example) the temp goes down close to 120 and stay there until I stop. The temp then climbs back up to 160.

First of all, this seems abnormal. Assume it has something to do with the thermostat, but not sure what. Secondly, do I need to fix it?

TIA,
-Keith

What temp thermostat did you put in?

I'd have to respectfully disagree with TyeeMan, every manual I've read and inboard I've run have operating temps of 160 to 165*F

 

[lonewolf41]

I put a sierra part number 18-3677 which is supposed to be a 160 degree thermostat...and it does stay around 160 at idle so it seems like it is correct.

I did not see anything in the housing when I had it out...that included changing the thermostat...so I had it pretty well torn down, but I will take off the hoses and see if anything shows up. The old impeller was not missing any vanes if I remember correctly. In fact, I thought about reusing it, but sanity took over and I replaced it.

Thanks for the feedback.

 

[skydiveD30571]

What temp thermostat did you put in?
I'd have to respectfully disagree with TyeeMan, every manual I've read and inboard I've run have operating temps of 160 to 165*F

Yep, 160 starts to open, fully open at 175 give or take (for a raw water cooled engine).

 

[dypcdiver]

As I said above, a visual inspection of the thermostat housing will not reveal anything, you really do need to force something like a wire right through the bypass port.
I removed my thermostat housing several times until I used the wire, engine now runs at 160/165 deg F all day.
I also have 3 almost new thermostats on the shelf along with several gaskets and "O" rings, all accumulated whilst seeking the cause.

 

[redneck joe]

you did actually put the tstat in? Without it it might act like that?

 

[lonewolf41]

you did actually put the tstat in? Without it it might act like that?

LOL, I actually did. I remember making sure I had the o-ring seated well so that it and the thermostat neither one would fall out and cause issues. Never underestimate the obvious though, so thanks!!

 

[TyeeMan]

I have to correct myself. Not sure what I was thinking, , stupid human trick. It jogged my memory when I jumped in my boat tonight to do some work. Yes 160 degrees would be a normal operating temp. Why your engine would run cooler at cruising speed and warmer at idle is a bit of a mystery.
Could it be possible that something is stuck in your tstat preventing it from closing?
Like redneck joe said, , what you are seeing would be typical of either an absent tstat or a tstat that is stuck open.

You could test your tstat by placing it in a pan of water. Start with tap temperature water and put the heat to it. Use a candy thermometer or some kind of instant read thermometer. Your tstat should start to open at approx 160 (or whatever is stamped on the tstat) and be fully open by 170-175 as previously stated in a post.
Elevation where you live will have a some degree in your results but nothing too much.

Sorry for the previous bad info. . ..