merc 470 cooling issue

[nola mike]

Sorry in advance. Since I know this info is on here somewhere--i'm on a blackberry and am having a tough time with searching...
Anyway, merc 170. Noticed a bit ago that overflow level was low on coolant. Refilled last week, and after some fairly hard running noticed that the overflow was again low, and coolant had overflowed through the reservoir cap. Coolant level in the reservoir was fine. No overheating. I did a compression test about 2 mos ago, and all 4 were 165 psi. Today I did a coolant pressure test, and it held at 17 psi after tightening a couple of hose clamps. Cap was a little low at 10 psi, but I doubt that's the prob. Refilled everything, ran on muffs for 15 min and noticed a bit of white smoke coming from gasket between engine and exhaust manifold @ #4 cylinder. Now, I know that these engines leak into #4 because of a failed...something. But I don't remember what. Gasket between riser and manifold maybe? Anyway, could this be the cause of my symptoms, or could this still be something more serious like a headgasket or warped/cracked head?

 

[nola mike]

Re: merc 470 cooling issue

Hmmm...now that I look closer, I'm seeing maybe some dried coolant coming from between the manifold/riser @ #4?

 

[stonyloam]

Re: merc 470 cooling issue

I know that these engines leak into #4 because of a failed...something.

Head gasket.

 

[Rocky_Road]

Re: merc 470 cooling issue

Head gasket.

Agreed.

Usually due to overheating...did yours get too hot?

If you end up replacing your gasket, spring for the Mercruiser part...proven to be the best.

Keep us posted!

 

[nola mike]

Re: merc 470 cooling issue

No, it hasn't overheated since I've owned it, and still isn't overheating. And HG isn't what I'm thinking of I don't think. It's something in the exaust system that allows coolant to get into the exhaust, go backwards into #4 and lock up the engine eventually. Again, what about this suggests a HG? Oil level is unchanged, and oil is black. Coolant level falls till overflow is empty (going thru res cap), but remains full in res and doesn't keep falling. Compression and coolant pressure test ok...

 

[stonyloam]

Re: merc 470 cooling issue

Mine never overheated either, but needed head gasket (and a valve job) just the same. I was also losing just a little coolant, not much but it was going somewhere. When I pulled the head, the gasket looked fine however my machinist pointed out black stains where the coolant had leaked between he head and the gasket into the cylinders. The coolant had eroded the valve seats in mine, had to replace all 4 exhaust and 1 intake seat. It may be that over time (22 years in my case) the normal heating and cooling cycles will result in a head gasket failure. Good luck.

 

[nola mike]

Re: merc 470 cooling issue

ok, so after pressure testing everything, refilling, and taking her on a cruise today...all was well. there was a little white smoke coming out of the #4 exhaust manifold to head gasket, but that quickly stopped and didn't recur. i went about 10 miles, including a couple of miles at about 4200 rpm/45 mph. coolant needle never made it over 160, no overflow, no coolant loss. ok, i'll take it. i still want to inspect the mysterious part of the exhaust that i'm thinking of. anybody know what it is? i think it may be the manifold/riser gasket that can leak, allowing coolant into the #4 cylinder? does that sound right?

 

[stonyloam]

Re: merc 470 cooling issue

I am not 100% sure but I think the only way coolant can get into #4 is from a leaking head gasket. The only other gasket is between the reservoir and the exhaust manifold, and I don't see any way for that to enter #4. I had very little coolant loss with mine, but was an obvious head gasket leak into #3 & 4. Sounds like you are running pretty well now, maybe just keep a close eye on it for the summer and over the winter put in a new head gasket and have the valves done. 20+ year old motor is probably ready for it. The new Mercruiser gaskets are engineered to seal better than the old ones. In the spring you should be good to go for years of fun. Sorry for rambling.

 

[nola mike]

Re: merc 470 cooling issue

I'm almost certain that you can have an exhaust leak that can do this, though maybe only with the engine off. I's killing me that I can't think of this. You have coolant flowing thru the riser, right (maybe elsewhere)? So if there's a gasket leak, it could make its way into the exhaust, and thus back to the engine (again, I guess this would only happen with engine off).

 

[stonyloam]

Re: merc 470 cooling issue

I think you are right, if the reservoir to exhaust manifold gasket were to leak it could fill the manifold and leak into the cylinders (probably unlikely). That should be an easy fix. You know, if the engine is running OK and your coolant loss is not too bad, you might consider just using it this summer and plan on replacing the head gasket (valve job while you are at it) this fall after you put it away. It is not that bad of a job, especially if you have a couple of months to do it, then no worries and a sweet running engine next summer.

 

[sdoomaha]

Re: merc 470 cooling issue

Save yourself some headaches...rather than guessing, get out some wrenches. #1, pull the exhaust manifold if you are convinced that is it....takes 30 minutes and you can easily change the exhaust manifold gasket. If that doesn't fix your problem, then #2 pull the head.

I fought a coolant leak for a month before it finally got bad enough I had to do something. Head gasket cracked and leaking into the #1 cylinder. Never any overheat, never any hydro lock...just leaked enough to burn it through and eventually result in a low coolant level.

Someone smarter than I may tell you different, but after tearing mine apart looking for my leak....it would be pretty difficult for coolant to get through a crack in the exhaust manifold and get back to the cylinders...especialy while running unless it was a very substantial crack...which would lose more coolant than you are experiencing.

Good Luck.....