Temperature getting too high Merc 4.3L V6 205 HP '86

Engine: Mercruiser 205 V6 4.3L from 1986 on a Wellcraft Elite 185

When going underway on a plane the temperature maintains at about 142 where it is supposed to be.

When I come down to an idle and eventually nuetral the temperature begins to creep up to and around 160.

Do we think this is thermostat? Enging water pump impeller? Water pump itself?

I replaced the impeller and housing and gaskets in the outdrive last year. Not sure if that would contribute?

But it is interesting in how it is fine under way and then when the speed comes down the temp creeps up....Doesn't happen every time either and can be resolved by putting the throttle back down and getting on plane.

Any thoughts, experiences, and suggestions welcome!

Thanks again boaters!

Re: Temperature getting too high Merc 4.3L V6 205 HP '86

sounds like a bad impeller to me. I would pull it and check it before I would do anything else. Did you happen to run it dry at all? Even for a few seconds.

Re: Temperature getting too high Merc 4.3L V6 205 HP '86

FishHog said:

sounds like a bad impeller to me. I would pull it and check it before I would do anything else. Did you happen to run it dry at all? Even for a few seconds.

Click to expand...

Nope never ran her dry at all. And I was hoping to not have to remover the outdrive again!

But I do appreciate the insight!

Re: Temperature getting too high Merc 4.3L V6 205 HP '86

JUST had this problem with our boat. We replaced the impeller and it works just fine now! The impeller was pretty shredded. I would start with that (housing, gaskets, impeller) and see how it does with that. I have no idea but I think since it was at a high speed the water was able to pass through the shredded impeller a little easier but at lower speeds it just couldnt make it. Thats just my opinion, in no way an expert on boats lol. But yes we had the excact same problem and just replaced the impeller and such and now it sits just below 140 the entire time after warm up Good luck!

Re: Temperature getting too high Merc 4.3L V6 205 HP '86

a 143* t stat starts of open at 143* and becomes full open at +20*. so imho, your t stat is in it's operating range and working correctly.
that said. new impellers are good. alpha1 impellers imho are fragil. i replace mine yearly for peace of mind. every year. the impeller shows signs of use too. usually have crease that's starting to crack on the inside of the curl of the impeller blades. if ya abused the impeller at all, it'll considerably shorten it's life span...http://www.boatfix.com/merc/Bullet/87/87_14.pdf

Re: Temperature getting too high Merc 4.3L V6 205 HP '86

The 86 said:

Nope never ran her dry at all. And I was hoping to not have to remover the outdrive again!

But I do appreciate the insight!

Click to expand...

Ayuh,... There's Maintenance to be done in there, Annually,...

I'm with Ziggy, I change mine 'bout every season...

You can change the T-stat, just to be sure,...
Could be a restriction in the P/S Cooler, or even the manifold/ Risers...

If the Impeller has ever been trashed, the pieces get hung up in the P/S cooler...
The manifolds/ riser get rusty 'round where they meet at the gasket, though in sweetwater, that's low on the list...

Re: Temperature getting too high Merc 4.3L V6 205 HP '86

Ok, sounds like impeller. Yeah prior to last year it had been around 3 years since its last replacement. So thats why I was skeptical. So some have mentioned that after replacement it would be ruined after use???? Is there any special way to ensure that it is installed more correctly than I had or you had done it?

Looks like another outdrive pull! Ugh

Bond-o said:

Ayuh,... There's Maintenance to be done in there, Annually,...

I'm with Ziggy, I change mine 'bout every season...

You can change the T-stat, just to be sure,...
Could be a restriction in the P/S Cooler, or even the manifold/ Risers...

If the Impeller has ever been trashed, the pieces get hung up in the P/S cooler...
The manifolds/ riser get rusty 'round where they meet at the gasket, though in sweetwater, that's low on the list...

Click to expand...

Re: Temperature getting too high Merc 4.3L V6 205 HP '86

UPDATE!!!

OK, so I took the outdrive apart, and, YUP! The impeller was destroyed. So! Replaced the impeller and the housing and gaskets and put the outdrive back together. Cleaned out the intake vent and then took off the hoses to the thermostat and cleaned them out of impeller debris too.

Launched the boat. And?!?!?! Same issue. Cools on plane and overheats at idle.

Spoke with a few mechanics. It is currently being worked on - focusing on the copper tube that connects the upper and lower housing. that tube came out when I serparated the two housings. That tube is supposed to be sealed in the upper. So i asked the mechanic to start there and see what he thinks.

I will keep you posted!