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Overheating Mercruiser 350 inboard/outboard!

rjan0626

New member
Hi, I am a new boat owner and I just purchased a 1986 Century Mirada. It sounded great when I ran it in the driveway on a pair of muffs, I took it out on the lake with 5 people and couldn't get it to plane out. After doing some reading, I took it out by myself and I got it to plane out by putting the trim tabs down once I gained some speed. I ran it for about 30 min, keeping an eye on all the gauges, listening for anything out of the ordinary, all seemed good. Took it out the next weekend with 5 people again and couldn't get it to plane out on the first try. I moved to people to the cabin and tried again, it plained out. I moved them back to the rear as we were plained out and continued running full throttle for about 20 min, then I noticed it starting to slow down slightly, I backed off the throttle almost completely and it shut down! The temp gauge was nearly all the way to the hottest point, I didn't take note of the temp. I checked the oil dipstick while I was on the water and it was just above ADD. I tried to restart when the temp. got down to about 3/4 and it would try but wouldn't start. Got towed back and now its sitting in the slip on a hoist. Im going down this weekend to do some troubleshooting, about a three hour trip, so I want to know what would be best/smartest things to check out in what order.
Do you think the poor planing could have anything to do do from a problem with the cooling system? Its freshwater cooled.
From what I have gathered, it originally game with a mercruiser 260hp, it was replaced with a mercruiser 350 in 08.

Transom rebuilt at marina few years ago. Motor replaced with Indmar 5.7 vortec.



Last year: new plugs, wires, fuel pump, cap, rotor, ignition control module, ignition pickup and distributor shaft and belts.


New this year: water pump in lower unit and thermostat, new/used prop 15x15 he thinks..

Ill try and post the pics I have

Thanks in advance,
Ryan


 

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This is the reason you must have a on water test run before purchase.

Dont see a heat exchanger so it looks like a raw water cooled system
1 replace the impeller in the drive.
2 remove elbows and inspect for corrosion .elbows appear to be 3 inch centerrise type.
 
I think your boat is over propped, most likely an attempt by the previous owner to get more speed out of it by increasing the pitch. How many RPM @full throttle??? It's been my opinion and of others, that running for prolonged periods of time at wide open throttle greatly shortens engine life. How big is your boat? I have the same engine on my 23 ft deep V that weights 4000# and am running a 15x17 prop ( 1.61: 1 reduction) .
 
Sandkicker - im not quite sure what my rpms were when I was planed out, forgot to look, when I wasent planed out I was at 3000-3500. Its a 1986 25' Century Mirada
Thanks
 
So it turns out the batteries were both shot, I don't understand how this could have happened, but I bought two new ones and it started right up. I ran it with the muffs on for a minute on idle and the temp gauge didn't move from 165. I did find the smaller exhaust bellow was broken. The risers looked clean inside and the oil looked fine, a little low, but not milky.
 
Remove the battery isolator. They are worthless. They suck down almost two amps. I'd argue it's better to have both batteries in parallel and charge them up as a single large bank than to have two weenie batteries separated by a selector switch or an isolator (both bad in my opinion)
The modern way of doing the charging on a two bank system is with an automatic combiner and a voltage sensing diode. This eliminates any need for manual switching and guarantees that both banks get charged. If you need emergency starting battery, the automatic combiner will parallel both banks allowing you to start the engine if the starting battery dies.
Your boat probably needs to go on a diet in addition to being over propped. Unload everything not bolted to the boat. Put back aboard the required safety gear (pfds, signalling, anchor etc) Toilet needs, engine supplies and a couple fenders, lines. I'll bet that what's left over is a significant amount of weight. Grill, scuba tanks, pool toys, it all adds up.
It's a pretty good sized boat the have only a single 350. 25 feet is about the breakover length where you can fit two engines. Maybe not two 350s but a pair of inline 6 would wake that thing up. Did it have twin engine option?
 
as far as not planing, start with a compression test of the engine. And see if you can find out what that replacement engine is from - I see center bolt valve covers. That's not original to a 1986 I don't think. You could have a truck engine with a wimpy cam in it.
 
25 is pretty big for a single 350 alpha 1. My 350 Alpha 1 is in a 23.

Not being able to get up on plane with 5 people in it is classic over loaded, underpowered & over propped. Yes moving the weight to the bow helps big time.

your overheating could just be a symptom of being put under extreme stress with the excessive load.

So either you increase the power (by correct engine tune) or get a smaller prop, or both. ide borrow a smaller prop to try first.

I think the previous owner was looking for more power when he put the indmar vortec in it.

And looking at the list of parts he put in it seems like he was chasing down the same problem, gave up and sold it.

try the smaller pitch prop.

Yesterday i put a smaller pitch prop on my 25B cummins turbo diesel, it now gets on the plane quicker & has an increased top end speed as now the engine is not overloaded and is able to reach its rated 2900 rpm. Turbo exhaust gas temp has also dropped aswell. (I have 2 boats, petrol 23B & a diesel 25B)
 
..."borrow a smaller prop to try first."

Excellent advice! Lots of Mercruiser props around. Try a marina o ask to borrow one from local boaters. You want at least 2 inches less pitch.

Jeff
 
Thanks for all the input guys, im going back down to the lake tomorrow and I want to try a different prop. So if I have a 15 x 15 prop on now, am I looking for a 15 x 13 if I want to go down 2 inches?
 
Thanks for all the input guys, im going back down to the lake tomorrow and I want to try a different prop. So if I have a 15 x 15 prop on now, am I looking for a 15 x 13 if I want to go down 2 inches?

Ayuh,...... Good luck with that,..... 15" is the smallest pitch prop they make for our drives,......

If you find a 13" pitch prop, it's for a mid-sized outboard gear case motor,.....
It'll bolt onto the drive, but the barrel is to small, 'n it'll ventilate really badly,......
 
25' boat, 260 HP 350Merc, with a square 15" prop (15"x15") should plane with out an issue. Putting a smaller prop on is going to mask your real issue for a while.

I would do a leak down test and confirm the engine is healthy. I would also confirm the ignition system is working as it should.
 
There are a bunch of 25 footers with the 5.7 and a 14 x 19 prop running just fine.
So you did not do a on water test so you have no idea if it even ever ran good. First a compression test , pull the plugs and examine them, and a cylinder balance test. . For testing use a 14 x 19 or a 14 x 17 but be careful about over reving it .
If you find 2 cylinders that are low next to each other then you most likely have a blown head gasket.
Also check the spark plug wires for the correct firing order.
 
165 on a hose too hot if its at idle with enough water flow. If you are running in salt or brackish water w/o a heat exchanger you should have a 140 tstat.
 
So I got it back on the water this weekend and it was making a terrible knocking so I decided to just bite the bullet and take it to a boat mechanic and get it looked at and winterized . He said I have no compression in #4 :( Said it could be a $1500 fix or a new engine! Also said my u joints were full of water and spitting it back in the boat! Does anyone know what could have happened to #4 and is it something I would be able to fix myself from watching youtube videos? I am pretty handy but I have never taken an engine apart before.
 
Sorry, let the pros fix it. Youtube does not give warranty's.

Anyway, I would have the mechanic perform a leak down test on #4 to see where the leak is and leak down the remaining cylinders to find the overall health of the rest of the engine.

Why spend the money having them pull it down if the air is leaking past the rings?

If your Mechanic argues with you on this and tells you you do not need to do a leak down.....Find another mechanic.
 
That knocking sound MIGHT be a rocker messed up, but if it it's a rod knock the motor has to come out.

Good luck!

Jeff
 
Does anyone know what could have happened to #4 and is it something I would be able to fix myself from watching youtube videos? I am pretty handy but I have never taken an engine apart before.

Lots of things, rusted, stuck, bent valve ,bent push rod, broken rocker arm,
Can you repair it? Not knowing how good you are with tools, experience and instructions thats impossible to answer.

Water in the main below, very high bilge level, leaking bellow,
 
Well good news! I had the boat dewinterized and they rebuilt the lower unit and replaced the piston rings in #4 and shes running perfectly! Thanks for everyones help.
 
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