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1983 Mercruiser 470 (3.7L) overheating issue

G

gbartsr

New member
I took a 1993 Lund American with Mercruiser 470 (3.7L) in it. I finally got it running with new points and condenser, however when I took it out on the water it overheated within 5 minutes. There was water that blew out of the "radiator" cap, so would this mean the cap isn't holding the correct pressure, or thermostat is staying closed, or the water pump, or the impeller??? I was an auto mechanic for several years in the army and then afterwards, but this marine engine cooling system I'm not familiar with as of yet, I'm learning as I go. Any offers of help as to what I need to start with here would be great.

I posted before, but I don't think it worked. If it did work, then I'm sorry to post on same subject again.
 
Always start with the impeller and water pump kit. From there the water goes thru the rubber hose to the transom and then into the motors heat exchanger. The out from the heat exchanger goes to the riser.Heat exchanger is 3 inch but the 4 inch is a lot better
If you boiled the AF out ,that was hot. These motors do not take kindly to a overheat. It warps the head.
Refill with a 50/50 mix of regular AF, and never let the engine temp get above 180 max
 
Pull the plugs & check for moisture. Normally #1 & 4 will show signs of moisture if you have lost the head gasket. If no sign of moisture do a compression check. Like Bt Doctur said these engines do not like over heating.
 
Being I don't have a repair manual and this is the very first inboard boat I've every tinkered on, this is all new to me. I assume the impeller is in the lower unit and seawater pump is on the front of the motor??? I have not history on this boat being I took it in on a trade deal, but I was told the previous owner couldn't get it running. Once I put new points and condenser in, it started up. On the front of the motor on what I believe is the seawater pump has gasket sealer or maker on it, so I can only assume at some point it's been torn apart. Any help you can offer me here would be a blessing as I'm learning as I'm going.
 
Bt Doctur said:
http://www.boatfix.com/merc/Servmanl/6/6A3R3.PDF upper
http://www.boatfix.com/merc/Servmanl/6/6B3R3.PDF lower
Click to expand...

Thanks! I have another question on it though. I found that the previous owner had 1 of the 2 plugs out of the heat exchanger, so I picked up a couple of plugs and installed them. With my water muffs on, it seemed as if the impeller was working, but of course it could of just been the water pressure from the hose. That being said, I did take it back on the lake to see if the impeller was actually working and it almost seemed that it did while under idle, but once I brought the rpm's up it slowly overheated again and came out of the "radiator" cap again. I'm not sure what pressure the cap is, but do know that the previous owner had 2 caps in the boat, but only the one fits. I assume that you're going to say pull the lower unit and check the impeller regardless???? I don't want to pull it to find that it's good and then have to buy all new gaskets etc, being that I'm disabled and on a very tight income/budget. Also, are there any complete manuals that I can download for free???? The book manuals are about $30, but if I can get a free downloaded one, it would be a huge blessing. It's a 1983 470 Mercruiser in a Lund American boat. Please tell me what you can. Thanks in advance!!
 
Welcome to the world of boats.

as already advised one needs to go through the trouble shooting process systematically.

gaskets are cheap for that job as are water pumps.

how we're you able to test an impeller on the lake ? Not possible.

to check that it's pumping you need to run the drive in a big tub and pull the pipe to the heat exchanger in the engine bay and check the flow there and even that isn't an absolute.

the. Check the other side of the exchanger to the elbow reduced flow means a blocked heat exchanger.

the. Strip the exchanger and examine all tubes for bits of old rubber from old impellers.

if that fails to fix it and the flow from the leg to the elbow is 100% good then it's something more sinister.
 
I assumed that like an outboard you can feel the cold/cool water coming in by touching where it comes in, such as the heat exchanger. If that's not the case with inboards, than I stand corrected. As stated, inboards are new to me and I'm learning as I move forward with the headaches, lol. I will take any and all advise to improve my knowledge along the way. At this point the cooling system (inboard, not the lake water one) I have straight water in it rather than antifreeze until I get all the wrinkles ironed out, this wouldn't cause it to overheat, correct???
 
I'd be getting some 50/50 in there PDQ

cast iron head andcalloy block means non like metals corroding when no additives there to prevent it.

use only distilled water
 
I filled with distilled water, but didn't put antifreeze in for the diagnosing period only to see if everything worked first after I got the motor running. Using straight water as I did for the diagnosing period wouldn't be the cause for it to overheat though, correct???
 
No bit neat water has a lower boiling point.

these engines have unlike metal for head and block so I would get 60/40 Volvo penta green coolant in there ASAP to avoid corrosion.

always start at the water pump and work back to heat exchanger checking every water passage and the plastic water pocket under the upper unit while the lower is off.

back flush from the top hose on the exchanger out to the out drive and check for unrestricted flow
 
At this point what the boat owner is doing is fine for diagnostics. no need to spend extra cash for "volvo" antifreeze.........and when actually adding it any name brand will work. Best to purchase premixed so no water needs to be added!!!!!!

Any I/O whether raw water cooled or closed cooled (heat exchanger and anti-freeze) has four to six basic issues causing over heating.

1. lack of water coming in which would typically indicate a bad impeller or something blocking the incoming water.

2. lack of water going out, clogged elbows typically.

3. bad thermostat, not opening, not closing or plain old stuck.

4. pinched/collapsed water hose at gimbal to transom. see #1

5. Clogged heat exchanger

6 bad radiator cap leaking before normal pressure is reached. I believe it should be a 14 psi cap. Could be off a psi or two...I forget right now.

this engine should run a 160* thermostat and should run while under load int he water at ~ 170*-180*.


JMHO.
 
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