View Full Version : 1985 Mercruiser 470 Overheating

12-14-2005, 09:20 AM
Had a problem with temp last year engine shutdown on high temp.
Replaced sender last week because temp gauge wasn't working - also took the guts out of the thermostat and put it back in.
Temp now is running 200 degrees. This is an i/o with fresh water syste using 50/50 antifreeze mix.
Any help on this problem will be greatly appreciated.
Thanks. Bob

Eddie from Oregon
12-14-2005, 11:10 AM
When's the last time you replaced the impeller in the raw water pump?

12-14-2005, 12:58 PM
My neighbor told me the man that owned the boat before me had overheating problems and that he had replaced the water pump in the outdrive. Bob

old user who can't get in
12-14-2005, 02:08 PM
"Sometime folks start the engine out of the water to show buyers that it starts and runs. this really mets the raw water pump fast.

I'd change the raw water pump, change thermostat (they're cheap with new gasket) and check for belt slipage (change it shiny) on the recirculating pump drive.

That gets the 95% of the causes of overheating and lets you start with a clean slat with a used boat that "had " overheating problems.


Eddie from Oregon
12-14-2005, 04:46 PM
"If I heard it once. "The guy that sold it to me said............" Like I said, check out the impeller and old user is right about the thermostat. You got your work cut out for you!"

12-14-2005, 05:42 PM
I would like to thank every one for their advice on my overheating problem.I have somewhere to start now. Has any one heard of the exhaust elbow getting plugged with rust and causing over heating? Thanks again Bob

12-14-2005, 08:45 PM
"The impeller is a good place to start. It's a wearable part anyway, so get in the habbit of replacing it every year or so, depending on how much you use the boat.
If there's no smoking gun there:
The exhaust riser and elbow are other likely culplets. Any restriction in the flow path will reduce the ability of the system to remove heat from the engine. The passages in the elbows/risers are pretty tight, and as they corrode, metal flakes off and can block the passages. If you can't determine how old the riser and elbow are, I'd yank 'em and at least take look. Also, these things should never be more than warm to the touch when the engine is at normal operating temperature. I'd guess from what you've already said, they're getting pretty darn hot.
Just to make sure the heat exchanger isn't blocked, I would pull the raw water goes into and goes out of hoses off the heat exchanger and run some water from the garden hose through it. Back flushing it wouldn't be a bad idea at that stage either... it'd be interesting to see what comes out.
Does that one have a power steering cooler? That's another potential blockage point in the raw water circuit.
Also, a good tool to have, recommended to me by other folks on this board (FASTJEFF, if I recall), is an infrared thermometer. Point and shoot at a component and you know how hot it is. Very helpful in troubleshooting cooling system problems."

12-15-2005, 06:06 AM
"Troy, Thanks for the good ideas.
I have some places to start now.
Yes this model has a power steering cooler."

12-17-2005, 12:01 PM
"had the same problem for years, tried almost everything with no fix untill i changed from a 3 inch heat exchanger to a 4 inch. no more problems. my infrared thermometer is now reading 165 to 180"

12-17-2005, 04:48 PM
"Just had a marine mech chech out my engine he pulled off heat exchanger hoses one at a time and had me run the engine,when he got done he said the heat exchanger was plugged up. Since it can't be rodded in place,I guess I'll have to pull it.Hope I can get it out. We used to wash water cooled air conditioner condensers with a product called scale x has anyone tried this?"

12-17-2005, 06:42 PM
"Never heard of that brand, but I have used Safe-D-Scale, and Rid-Lime. From the name it sounds like the same or very similar product. Those solvents work great on marine growth fowling. Best thing is, once they eat up the growth, the solution kind of self neutralizes so 1)it doesn't attack the pipes, tubes, and such, and 2)you don't have a big bucket of "methelethelbadstuff" to get rid of when you're done.. just some muddy water with a pH of about 7.
I've experienced some pretty severely fouled up seawater systems in ships, especially in warmer waters, and that stuff worked great... insides of pipes coated with zebra mussels, four hours of flushing that stuff through and the pipes looked brand new.
If you have the means, the best way to use it is to circulate it though the heat exchanger for a few hours with a pump. Soaking will work too, but it may take several soaks to get it clean.
Once you get it out, you should be able to pull the end caps off and take a look. It may be nothing more than blockage on the inlet side tube sheet.
However, if the engine was run too hot, and the seawater temperature in the heat exchanger got above 170F or so, there's a better than average chance there is hard scale build up inside the tubes.
Let us know how it looks when you get it out."

Harves Marine
12-18-2005, 10:18 AM
Take it to a radiator shop and have them clean it out. Remove the end plate and plug on bottom.

12-18-2005, 07:29 PM
"Listen to Harves! Radiator shops clean up worse messes than that all the time, and they know how to avoid damaging the exchangers.


12-18-2005, 07:33 PM
What would a radiator shop charge?

12-18-2005, 07:37 PM
Probably less than 50 bucks. Cheap since new exchangers run 600- 700 bucks (and that's what one would need if it was ruined in backyard cleaning attempts).


12-18-2005, 09:26 PM
"Update: Spent most of the day on my heat exchanger, taking it off and putting it back on.
Ended up hacksawing one of the u-bolts off.
What I found when I took the end plates off was the inlet passage [this is a three pass heat exchanger] was 1/3 to 1/2 plugged up with chunks of rubber from an old impeller.
Since the previous owner was having the same problem as me I figure this rubber has been in there a long time.The inside of the tubes looked to be perfectly clear.
Will find out tomorrow when I try her out."

12-19-2005, 06:06 AM
Excellent! Check the power steering cooler too.

Harves Marine
12-19-2005, 03:01 PM
"I was going to mention the impeller pieces theory in the last post but you were so far along looking at the right things. I rebuild many drives and its amazing how broken impeller pieces migrate up into the cooling system. I had a customer that could not figure how at idle the engine ran fine but any rpm to it, it would overheat. When splitting outdrives for impellers I always remove the water pocket cover in the upper. I once found many pieces of a impeller in there, they were all trying to get out the same hole when the rpms were up, hence restricting water flow. Just another spot these pieces can drive you crazy."

12-19-2005, 06:49 PM
Ran this engine about three miles @ 3200rpm going out Fishing today. Definitely better but still running about 180 degrees. Ran about the same temp coming back.

12-19-2005, 07:15 PM
180 ain't bad. Really. The t-stat may be setting at that temp.


12-20-2005, 10:56 AM
"As posted apart I have recently installed the Smart Craft Monitor and although the instrument is going to be replaced because it gives a false alarm reading, the rest of the measurings work fine. RPM, oil pressure, fuel flow and used, engine temp. etc.

To my surprise the temp. reading on the SC screen shows 158° F while the old analogic instrument read 175° F.

Same thing happens with the revs - while the regular meter says 3000 the SC Monitor shows 2670 RPM
I believe the SC readings to be more reliable because the information is picked up directly from the computer that manages the fuel injection & other basic functions.

This is to say - don't trust too much the accuracy of analogic gages.

Maybe its a dumb advise but having your temp gage recalibrated might show that your actual engine temperature is below the readings you are now getting."

12-21-2005, 06:41 PM
"Good idea about those old gauges,might be a good idea to replace that old temp. gauge."

12-22-2005, 06:52 AM
"I wouldn't split hairs over a few degrees difference in indication. If you can check your installed gage against another means (like an infrared, or hell, tape a meat thermometer next to where the sending unit is), just to make sure what you're seeing is what you're getting, you'll at least know that you've fixed the problem(s) and the engine is running at or near the right temperature. From there, even if what you're seeing isn't dead nuts accurate, it's benchmark for 'normal'. You drive the boat, you keep an eye on your instruments, and you be able to tell when it's acting up again by way of deviation from 'normal'... what ever that number ends up being."

05-22-2006, 09:26 AM
"I currently have 815 hours on my 1983 Mercruiser 470, no knocking and it runs great. Starts right up and trouble free although due to the age once it heats up my oil pressure goes from almost 80 to 45. So I change the oil about every 20-30 hours. Runs at about 160 degrees at 3000 RPM, at 3500 RPM runs about 170 degrees. If I slow down it hits 180 temporarily until it cools or if I travel at 4000+ RPM for a length of time it will get up towards 180 degrees. I've tried several different types of cooling additives but nothing seems to work. So from what I hear, my temps seem to be about the norm. So far(knock on wood) the highest I hit was 180 degrees. I just bought a used 4inch heat exchanger I hope to put on soon. I had this idea of running hoses from the 3 inch into the 4 inch and using both heat echangers as it appears there is room down there but not sure if feasable."