exhaust manifolds rusting

[DrNate 426]

I'm trying to figure out why the bottom of my exhaust manifolds are rusting so bad after 6 years.  I have the half and half cooling system that uses coolant and raw water both.  There appears to be evidence of some leaking which I am going to check tomorrow on a fake a lake.  Always been run in freshwater only, doesn't make sense.  I even checked the zinc anode and it appears almost new, still 1.5 inches long.  Thoughts?

[h20king 794]

Only a guess but I would think you have a coolant leak which striped the coating of your manifolds. Once the coating was gone multiple times of putting the boat away with a wet bilge and lockers provided enough moisture to rust the manifolds. 

[DrNate 426]

I buy the coolant leak bc I had to add some today.  No wet bilge though.  I blame DEX-COOL

[RhuntIII 555]

From my research the Freshwater portion of the cooling system does not come in contact with the exhaust manifolds and The only place where the DEX-COOL and the raw water come in contact is in the heat exchanger. I'm am in no way a certified mechanic but I did stay at a Holiday Inn once.

@h20king, That does makes sense about the rust as there was plenty of evidence in the bilge and locker areas of long term moisture. 

 

[h20king 794]

Yes fresh water only through the manifolds. My thought is a leak somewhere up top allowed coolant to run down on manifolds and get cooked by the hot manifolds. 

[DrNate 426]

I buy that h20

[Shaka 146]

Replace them before they start leaking water into your block.

[DrNate 426]

On 2/22/2018 at 9:07 AM, Shaka said:

Replace them before they start leaking water into your block.

Is that really the likely expectation?  Would you replace them if they weren't leaking anywhere?  I haven't tested on lake yet at higher RPM's but on the fake a lake at idle and just above idle there were no leaks anywhere

[Shaka 146]

I had this issue last summer. My boat was was hydrolocking. Turns out water was getting into the cylinders from rusty risers. I caught it early enough that it did no damage other than frying a couple of starters. It could have been much worse though.

[DrNate 426]

4 hours ago, Shaka said:

I had this issue last summer. My boat was was hydrolocking. Turns out water was getting into the cylinders from rusty risers. I caught it early enough that it did no damage other than frying a couple of starters. It could have been much worse though.

 Yeah, that's scary.  Everything I read says that exhaust manifolds on a fresh water boat should last 20 years.  I guess not necesarily

[Mrwick 65]

Is the Ph level of your home lake water all screwed up?  I’ve heard of that causing problems with your exterior metals........

[DrNate 426]

Mr. Wick, I have no idea about the lake water PH but it seems like it might be aggressive.  I wanted to update this thread.  We finally got out on the water yesterday for a lake test.  At lower RPM's there are no leaks at all.  At higher RPM's after much running there is the tiniest of leaks in the riser gaskets, on both sides, where a drop of water will come out every 3-4 seconds, run down the exhaust manifold and turn into steam from the heat of the manifold.  Very little water reaches the bilge, in fact we ran all day and never bilged and at the end of the day, when removing the plug, no water came out.  So I ask again for you opinions because I've read where people have replaced the riser gaskets only to have more problems.  

I also wanted to add that I checked the coolant level of the degas bottle constantly all day and it never moved from the "max fill" line where I had filled coolant to the other day.  So I am pretty confident there is no coolant leak.

 

thoughts?

 

 

[Mrwick 65]

47 minutes ago, DrNate said:

Mr. Wick, I have no idea about the lake water PH but it seems like it might be aggressive.  I wanted to update this thread.  We finally got out on the water yesterday for a lake test.  At lower RPM's there are no leaks at all.  At higher RPM's after much running there is the tiniest of leaks in the riser gaskets, on both sides, where a drop of water will come out every 3-4 seconds, run down the exhaust manifold and turn into steam from the heat of the manifold.  Very little water reaches the bilge, in fact we ran all day and never bilged and at the end of the day, when removing the plug, no water came out.  So I ask again for you opinions because I've read where people have replaced the riser gaskets only to have more problems.  

I also wanted to add that I checked the coolant level of the degas bottle constantly all day and it never moved from the "max fill" line where I had filled coolant to the other day.  So I am pretty confident there is no coolant leak.

 

thoughts?

 

 

If it is only happening at higher rpms, it makes me wonder if you have a potential downstream obstruction causing back pressure, but that would be a odd problem......If your rust problem isn’t out of control, I would clean it up and install a new gasket.....that would be cheap stab at the problem......

[DrNate 426]

I also wanted to add that I checked the PH of the DEX-Cool coolant as per the owner's manual.  The PH was 8.5.  The owner's manual just says it should be "alkaline" but I also checked the PH of the DEX-Cool right out of the new plastic container and it was also 8.5 so I assume I am good.

[WakeDoc 471]

Have you checked the transmission cooler to make sure you do not have any impeller pieces?

[DrNate 426]

14 hours ago, WakeDoc said:

Have you checked the transmission cooler to make sure you do not have any impeller pieces?

No, but that is a good idea.  I also talked to another long time inboard owner yesterday that suggested that I try the easiest thing first:  Torque the bolts that hold the risers and exhuast manifolds together to make sure they're not just loos and loosing the seal at higher pressures.  Seems like a simple thing to do but don't want to over-torque them.  Anyone know what these should be?

Edited March 13, 2018 by DrNate

[WakeDoc 471]

6 hours ago, DrNate said:

Anyone know what these should be?

Just the right number of ft. lbs. or call PCM.