470 Overheating?

[luckyluke]

I purchased my boat (22' 1990 IMP Sportsman with 470) late last season and soon began worrying about the operating temperature as the gauge seemed to creep up pretty high.

The engine was rebuilt just prior to me purchasing the boat and it really runs nice.

I soon found out the 470 is notorious for many things including overheating.

I replaced the temp gauge and sending unit, replaced the thermostat (genuine Mercruiser), replaced the coolant and had the seawater pump impeller changed (new kit).

At between 1500 and 1800 RPM the temperature stays very constant (about 175 on gauge).

Up on step, at about 3200 RPM she starts to creep up and appears to be in the 190 to 195 range (on gauge).

When I bring her back down to around 1500 RPM she soon cools down to about 175.

I've never had her at WOT for more than a couple of minutes and I haven't tried running her at moderate speeds (3200 RPM) for more than about 20 minutes for fear of overheating.

I've discussed this with my mechanic and he suggested I pull the heat exchanger and clean it (maybe a soak solution to remove deposits). I think he's getting fed up with me, or 470's.

I guess my question is, how should I proceed? Run'er with some electrical tape over the gauge so I don't worry so much (not my style an the wife and kiddies wouldn't enjoy the joke if she pops) or how can I determine if in fact I have a problem with the cooling.

Any constructive feedback would be greatly appreciated.

Thanks,

 

[stonyloam]

Re: 470 Overheating?

Pull the front cover off of the heat exchanger to make sure there is nothing blocking the tubes (don't worry only raw water will come out). Pull the hose off of the water inlet on the exhaust elbow and put a garden hose on it to make sure that water flows freely with no restrictions. Learn how to change the impeller yourself, not hard to do.

 

[luckyluke]

Re: 470 Overheating?

Thanks for the tip. I've had the front cover off the heat exchanger before (to winterize). Isn't that a water "outlet" on the exhaust elbow? If I run water through a garden hose in there, I should see water coming out the front end cap, right (ie. reverse of normal flow)?
View attachment closed cooling diagram.pdf

Just want to be sure I "comperstand" correctly. Maybe now you see why my mechanic is getting fed up with me.

Thanks

 

[stonyloam]

Re: 470 Overheating?

Thanks for the tip. I've had the front cover off the heat exchanger before (to winterize). Isn't that a water "outlet" on the exhaust elbow? If I run water through a garden hose in there, I should see water coming out the front end cap, right

You don't have to remove the cap to winterize, simply remove the aft drain plug on the underside of the exchanger and let it drain. The outlet is the aftmost tube on the top of the heat exchanger. The hot raw water from the exchanger flows from there up through the power steering cooler (if you have one) into the inlet of the exhaust elbow. If you attach the garden hose to the inlet on the elbow the water will flow through the elbow and out of the outdrive. That will tell you if you have a restriction in the elbow water passage that reduces your water flow through the exchanger. If you stick the garden hose in the hose that attaches to the elbow the water will flow back through the exchanger.

 

[luckyluke]

Re: 470 Overheating?

Excellent, thanks. Will try garden hose in both inlet and outlet to check for restrictions.

On a side note, I removed the end cap because it's so much easier. The aft drain plug is very difficult to access, I took it off once to check for crud build up but it's in such a place as to encourage stripped nuts and busted knuckles.