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MCM 470 coolant lossoverheating

G

gmcinnes

Member
"Sorry I posted this to the wr

"Sorry I posted this to the wrong thread ealier. I have 1978 Merc MCM 470 that we have just put in the water this year and the closed cooling system is losing coolant and starting to overheat but we cannot tell where it is going, not in the bilge. Engine is running fine and pulled 3 plugs and they are burning nicely. also no water in the oil. How can we test the 3" heat exchanger yes, still a 3" in there, to see if there is a crack in the unit and allowing the antifreeze to get out there. Compression test was good. There is also signs of steam coming out the back of the boat at the engine. Where else would the coolant be going? We appreciatre the help. as we are only on this Island (Pender Island, B.C.) for 2 more weeks."
 
"What about the fourth plug? Y

"What about the fourth plug? You say you pulled 3 of them. Steam and loss of coolant with no water in oil is usually from a bad head gasket. Your heat exchanger doesn't get hot enough to produce steam. So it is being produced somewhere and that would be the cylinder.

There probably isn't much for services on that island but if you can get a cooling system pressure gage, that might tell you a lot. Put it on your rad cap opening and pump it up to 15 lbs. If it holds your good and probably just an air pocket in the system that hasn't been filled up yet. If it doesn't than you may tell where the coolant is going with a little pressure behind it. Pull you spark plugs whan you pressure test."
 
"Hello Puget Sounder.

Were

"Hello Puget Sounder.

Were you going to the Gulf Islands this summer? We talked a few months ago? Yes we have now taken all the Spark Plugs out and they all look very good, burning a little rich but OK otherwise. We are trying to get a pressure tester. Yes steam is coming out the back of the boat as soon as we start it. We thought it might be the heat exchanger leak but there is no antifreeze out the back. What about the Exhaust manifold, could the green antifreeze be leaking out from there? There is a dried white milky substance below the exhaust manifold. and the riser out to dump the sea water. Problem is not to much help here on the Island and we are getting ready to go back to Sherwood Park, Alberta. Boo Hoo."
 
"Yes we cruised the Gulf Islan

"Yes we cruised the Gulf Islands, awesome place. Started out in Sidney, Maple bay, Telegraph Harbour, Silva Bay (my fav), Montegue and Ganges for 2 nights a piece. My wife has relatives in three of those places so we didn't have to sleep on the boat the whole time. No wonder it's a boater's paradise.

Can you get a compression tester as well? That will tell you if the problem is a cylinder or not. I don't think it could be the ex mani as it is raw water cooled. The steam tells me it's a cyl problem but a test will tell otherwise. I would bet against a cracked block since you say your oil is clean. Is there any black residue in your coolant reservoir? If your getting coolant in your cylinder, chances are you're getting burnt residue in your coolant as well. Just trying to rack my brain on all the things to check for. Good luck. The Gulf Islands aren't such a bad place to be broke down. If you can get to Ganges or Sidney you will have better access to tools and marine shops."
 
"Don't forget this is a cl

"Don't forget this is a closed cooling system and coolant runs through the exhaust manifold. Frustrating without all the tools to check these things. Looks like I might have to take it back to Alberta to do some more repairs, going to change the exchanger from the 3" to 4", upgrade the ignition. Thanks for all the help.. We'll keep pluggin away. Cheers"
 
You're right. How is your

You're right. How is your temps on the exhaust mani when it starts to steam? Is it very hot to touch or too hot? (Careful) If not I still think it's coming from the cylinders.

Curious did you trailer the boat to the island? How are you going to get back to the mainland? The straights of Georgia can be pretty unforgiving even in summer. We met a lot of vancouverites stranded at Silva Bay because the winds were kicking up for three days in a row and it was too rough to cross. Not a bad place to be stuck but the boss may not be happy.
 
"Ok, sorry PS, so I lost my d

"Ok, sorry PS, so I lost my dial up internet here on Pender Island for 5 days. Yes we trailered it to Pender island. Yes exhaust manifold is extremely hot not able to touch. I had it running in the yard with a garden hose hooked and the steam was coming directly out of the center of the prop. Gone back home now Work on it this fall. Where do I start, coolant pressure test, compression test what else, need all the help I can get. Steep learning curve, but none the less, Learning. Thanks
Cheers"
 
"Hi Mark,

sorry, do you mea

"Hi Mark,

sorry, do you mean phone number? Won't be home in Sherwood Park until Sunday. I can send you an e-mail. Thanks man, Cheers, Gary"
 
"I hope all in all you had a g

"I hope all in all you had a great vacation. The gulf islands are a boater's paradise and I wish all the members here on the board could have a chance to cruise around in paradise!

First thing I would do is a compression test. It's fairly quick and doesn't cost anything if you have or can borrow a tester. That will tell you a lot. If you have a low cylinder or two side by side you can bet on a bad head gasket.

Then a pressure test of your cooling system would be next. Hopefully you can tell where the coolant is going with a little pressure behind it. Pull all your spark plugs. You will probably want to put more anti-freeze or some kind of dye in your closed system to do this. Environmentally, dye is probably better. While you are pressurizing the closed system, pull the end cap off the exchanger and see if you have coolant/dye coming out. That will tell you if your exchanger has a leak from the closed to raw water side.

After that I would search the numerous posts on this board about checking the rest of the cooling system such as exhaust manis, elbows, and such. It will be your winter project."
 
"Hey, Puget Sounder, Did a co

"Hey, Puget Sounder, Did a comprerssion test today, nbr. 1 @110 lbs nbr. 2 @ 110 lbs #3 @ 120 lbs #4 @ 130. In May of this year it #1 135, #2 @ 125, #3 @140, #4@ 145. Ok, so I left the spark plugs out, filled up the coolant system with Anti-freeze put the coolant tester on to 15 pounds and within seconds the # 3 cylinder was pouring out anti-freeze. Looks like you were bang on, Head gasket. hey.I take it the valves should be done while the head is off? Now when the engine is running is this antifreeze being burnt and going into the Exhaust Manifold, could this be why the Exhaust manifold was getting so hot, coming out the prop as steam? Compression test not necessarily sign of a blown head gasket? Exhaust Manifold looks original, 1978, Is it time to replace it? I am going to replace the Heat Exchanger to the upgraded 4" from the 3".
Looks like lot of work coming up but somehow I feel better today. Sorry for rambling on. Appreciate your help. Hope this helps other as well.
Cheers"
 
Perform a wet compression test

Perform a wet compression test to see if the rings are OK. You may just want to rebuild the engine.
 
"Yes the steam is from coolant

"Yes the steam is from coolant being sucked into the cylinder on the down stroke and then being burned instantly on the ignition stroke through the hole in the gasket from cyl to water jacket. Usually not enough to cause hydrolock but enough to cause steam. Pull the head off and see what you got, might as well rebuild it completely. You can check out the cylinders then. And yes replace that exhaust manifold if in doubt about it's condition. You can check it out as well when it's out of the boat but if you've been running in salt water it very well will need replacing. Also replace other items as well like lifters and other things that can wear out over time."
 
"Ok, tomorrow I will perform a

"Ok, tomorrow I will perform a warmed up engine, Dry & Wet test. My test today was engine cold. I guess it should always be operating temperature, true?
Cheers.}"
 
Doing a compression test cold

Doing a compression test cold is OK. Been that way for years. Merc sent out a SB when the OEM redefined the procedure.
 
"I always do a warm test. When

"I always do a warm test. Whenever I buy a boat I do a test run on the lake first check over all runability and to warm things up then do a comp test as well as check for water in the oil, oil in the coolant, any visible leaks, etc. The way I figure it, your boat is warm when you are running it you might as well test it that way.

You might find that a blocked exhaust manifold might have over heated and caused the gasket to break. I would be suprised if the head warped but anything is possible. These things are one huge chunk of cast iron!"
 
"Ok , so I was going to do a w

"Ok , so I was going to do a wet test but just too much coolant(anti-freeze) in # 3 & 4 cyl.
I just want to start to tear it down, exhaust manifold, intake and head...but I would like to know what might have caused the head gasket to blow, What might I look for as I take it apart.
Cheers."
 
"Look for irregularities in th

"Look for irregularities in the sealing areas. Corroded regions, too. If you find a suspect area, pay particular attention to that area of the head and block deck. A machinist's straight-edge & feler gauge will help find any warped regions.

Overheating gets many of them. pH being too low gets them too....probably more than gets admitted to. Other than than, they are reasonably durable."
 
"Definitely replace that exhau

"Definitely replace that exhaust..also be weary of moisture in the cylinders causing rings to stick..dont leave it sitting around for long. get that head off now and get some lube around them rings. of course this doesnt matter if your'e goin to strip the whole donk. helped a mate re build his 470 a few years back, same issue as yours, problem was head gasket but the water caused rings to stick and a simple head job turned into a full engine overhaul...seem to remember that donk is a ford V8 with 4 cylinders cut off? Mark."
 
Mark has a point. Pour some Ma

Mark has a point. Pour some Marvel's mystery oil in each spark plug hole if you're not getting around to fixing it for a while. In fact pour a lot of it because oil floats on top of water.
 
"Ok, so now I have it all torn

"Ok, so now I have it all torn down, exhaust manifold, intake manifold, and head and I cannot tell anywhere on the head gasket where it looks like it has blown or any sign of where the coolant, antifreeaze, has leaked in past the head gasket. Could the coolant have come from the exhaust manifold (maybe cracked inside)and into the combustion chamber via the valves. Still learning...
Cheers,"
 
Yes that is possible. The stea

Yes that is possible. The steam had to come from somewhere and most possible place would be insdie the cylinder. I'm suprised the engine didn't hydrolock with that much water but you got lucky. Pressure teat the manifold at a rad shop. If it's bad check on ebay for another one because they are pretty expensive.

You say you never got water in your oil?
 
"Just finished capping all inl

"Just finished capping all inlet and outlets on the Exhaust Manifold and filling the coolant reservoir on top with water, give it a couple of minutes and the exhaust chamber fills with water, Still no water in the oil. I must have been born on the wrong day because I wold have rather replaced the Head gasket at 50 bucks than the exhaust manifold at about $1000 dollars (new) if you can find one.
The cylinders and head look good, I'm surprised! I probabily will have the head done (valves). Problem is finally found. I have to think about this for a while. It was the not knowing that was a pain. Thanks all for your help. I might get to the Puget Sound one of these days.
Cheers"
 
Like I said check ebay. Go to

Like I said check ebay. Go to baot parts and type in "470" you will be amazed at how much stuff people are selling for this engine. I've seen some used manifolds from the mid west which are fresh water use only for a $150-$200. Ask the seller if he will guarranty it works and is not corrded out. I also saw some 4" heat exchangers on ebay as well.

I have priced out new ones and have kept my fingers crossed I won't have to replace the Ex Mani anytime soon. But since I use it in salt water every year it's only a matter of time. If I happen to come across anything online I'll send you a link.

Yes make a trip to the US San Juans they are more of the same beauty that the gulf islands are but everything is a lot closer together. And gas is way cheaper! Just have patience with the US Customs they pride themselves on being #%@*&$)(*@&! even to US citizens coming home.
 
"This has been an interesting

"This has been an interesting read. I'm having a similar problem (if not exactly the same) with my '87 3.7 LX (newer 470). I'll refill the coolant tank, run it an hour, then it'll be low on coolant in the tank but the overflow will be overflowing. It maintains temperature but runs hotter than normal (about 180ish compared to where it used to run, 150ish).

I thought it was the head gasket so I replaced it. Still have the problem. I replaced my manifold and riser (coolant tank) two years ago.

Thoughts?"
 
Ever change the impeller in th

Ever change the impeller in the OD or clean out the heat exchanger tubes?
 
Changed the impeller prior to

Changed the impeller prior to the season. Heat exchanger looks good on visual inspection (through the tube).
 
Yes I thought it was a head ga

Yes I thought it was a head gasket as well but when I took it all apart I just poured antifreeze into the coolant tank on top of the exhaust manifold it just started coming out the exhaust chambers. Have you pulled the plugs and done a coolant pressure test?
 
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