Dark engine water--exhaust leakage?-gctid391703

[Guest]

Well, in this week's overheat chase I went to the raw water flow test as prescribed by Mercruiser (7.5 qt @ 15 seconds)

I thought I'd found the culprit when I discovered a piece of old plastic pump housing lodged in the impeller last week, but alas, that made no difference in my high rpm overheat problem.

Anyway, checked raw pump output at 1000 rpm, hit the 7.5 qt mark consistently. ( Boat is in the slip btw).

Then I decided to check the final flow to manifold from thermo housing, hit the same flow--all good.

Out of curiosity, I then revved up to 2200 rpm to observe water flow at higher rpm---flow was great, but noticed the water was now a very dark grey! At low rpm was clear sea water. I am thinking I'm possibly getting exhaust gas in the circulating water at a high rpm??

Manifolds off again now, flow through them looks good, flappers still in place....etc!!!

My next test is for high rpm on the water investigation to check for collapsing hoses, but this dark water has me concerned!

[Guest]

eh, im not a marine mechanic, but just a thought. Blown head gaskets on closed loop cooling system will over pressurize the cooling system with combustion gases from the cylinders, causing overheat issues, can run fine at idle, white smoke from burning coolant ect... cylinder leak down test or a combustion gas in cooling system test might be beneficial. Again, not a marine mechanic, but something to definitely check if you haven't already. You can get a test kit for combustion gas for about 40 bucks.

[Chief_Alen]

Ski good call, i hope to see more reply's from you.

[2850Bounty]

skidooseadoo wrote:
eh, im not a marine mechanic, but just a thought. Blown head gaskets on closed loop cooling system will over pressurize the cooling system with combustion gases from the cylinders, causing overheat issues, can run fine at idle, white smoke from burning coolant ect... cylinder leak down test or a combustion gas in cooling system test might be beneficial. Again, not a marine mechanic, but something to definitely check if you havenÔÇÖt already. You can get a test kit for combustion gas for about 40 bucks.

MikeMag wrote:


Then I decided to check the final flow to manifold from thermo housing, hit the same flow--all good.

This would suggest an "open system"...... (i.e., raw water cooled)... not a Closed Cooling system.

I'm somewhat stumped on the dark water issue.

Normally the first spent cooling water (after sitting for a while) will be dark due to iron oxide. But after running for a while, I don't quite understand the dark spent cooling water.

Now.... apparently Merc has had an issue with some of their bronze pump bodies. Doug C. comments on this in his https://"http://www.baylinerownerscl...ts thread.</b>

Doug's engine is the SBC, and I don't know how similar the two Merc belt driven pumps are....., but it may be worth your time reading it.

.

[dktool]

Ski good call, i hope to see more reply's from you.

Except for the fact that he does not have a CLOSED SYSTEM

[Guest]

Yes, raw water, salt water.

The dark water at high rpm was surprising, though come to think of it, my last couple thermostat changes, I noticed the white plastic holder was covered in soot, and that was before my overheating issues. Backing up, these are newly installed heads (rebuilts from Perfprotech), compression is 120 across all cylinders and no water in oil. I've read of small fissures in the block that can leak exhaust gasses into circ water when rpms applied....?!

I've thought about getting up to the overheat and running some of the water going into the manifolds into a bucket, checking temp. I assume if the water coming out of the block is superheated, then my problem is engine related----head gasket/warp or crack in block?