Re: Mercruiser 485 I/O overheats all the time
Howdy Dave!
I'm no mechanic, and certainly Don and the boys give sage advice, but I do know a little about these engines.
There's a coupla' other issues with these engines that should be addressed if you want to run them with minimum grief; one in particular is specifically related to the cooling system. Your problem however, might be related to something else.
Your engine is a trumped-up "470" Merc. The 470 was produced from 1976 to 1989 and Merc pumped out a whack of em'. It was the only marine inboard/sterndrive block (aluminum with cast/sleeves) that Merc designed and built themselves; all others were automotive based blocks. The cast iron head is from a '460'-Ford. If memory serves, it was also known as the 485, 488, 165, 170, 3.7LX, 190, as well as whole nother' class of names invented by owners and mechanics alike. Some love-em'... some hate-m'...
They crank out serious horsepower for a small engine. Say what they will, I'd bet the 4-barrel version probably boasted the best power-to-weight ratio of any I/O in history.
When Merc started producing them in 76', they included closed cooling as standard equipment.
Problem #1. - the 3-inch diameter heat-exchanger was barely adequate to cool the engine under normal operating conditions, let alone when it was working hard. With an aluminum block and a cast iron head - overheating these things equals - head-gaskets popping like balloons at Billy's birthday party.
Merc knew about the problem but (if memory serves) they took a long, long time to address it. Which they finally did by replacing the 3" heat-exchanger with a 4" diameter jobby that pretty much solved that problem. You want the 4-inch heat exchanger!
Aside from the regular things that Don and the boys mentioned, impeller, manifolds etc...you may also want to do the alternator conversion on your engine.
Problem #2 ...Yep... the water-cooled voltage regulator/stator setup on these engines wass another one of Merc's famous brain-farts... Fortunately, it's not a big deal to bolt-on a regular 1-wire marine alternator to these things.
You should also keep an eye on your camshaft water-pump seals.
Problem #3... My guess is, when the '470' engineers at Merc were all sittin' around after work one Friday afternoon (drinkin' beer, smokin' crack, droppin'-acid and listening to old Black Sabath records...) one of em' had an epiphany and came up with the cam-shaft seal idea. The seals tend to leak after varying length of service and can let coolant into the engine block. Not good. Fortunately, there's good info out there on how to replace these things yourself, if your reasonably good with a wrench. It can be a cheap fix.
You also may want to do the Petronix ignition conversion, although some guy's prefer the simplicity and security of breaker points.
Do a search on the net for Breezeworks and if the "470" Forum is still going, join it. There's a whack of '470' guys on there who are very knowlegable about these things; I learned a lot there.
As I mentioned in a previous post, I recall a couple of guys on that forum who did some upgrades to their 470's (port/polish the head, roller-rocker's, ignition upgrade etc,) and probably pushed these sucker's to 225/hp or more. Imagine that from a package that weighs in at about 650/lbs.
Good luck with your mill!
Cheers,
Terry
