Exhaust coming through thermostat housing when running dry

[Mark SS]

I aquired a 2008 SeaRay Sundeck with a 5.0 MPI Bravo 3. It sat on a lift for 5 years after only a few hundred hours after the Dealer installed a brand new long block. In the time it was on the lift, it was lowered into the water and started for 10-20 minutes every few months to which it would alarm and then we would shut it off. After I acquired it finally, I went thru and put new manifolds, seawater pump, thermostat housing with all the trimmings and a new water circulation pump in addition the chipping out and cleaning the risers.
Now when I went to start it finally after all the parts and labor, I removed the blue drain plug and started it. I had it in the water and also tried the muffs and was not getting any water out of the thermostat housing drain plug hole. I shut it down and removed the inlet supply hose and found the pump was not pulling any water....followed the hose and removed the fitting on the back and, yes....Bravoitis. Going to a thru hull fitting.
Here is my question: I was getting exhaust gas thru the removed thermostat plug hole when running. I was thinking that because there was resistance with the boat in the water and the exhaust was back pressuring back thru the risers into the cooling system. Am I correct in this thinking or do I have a warped head or bad head gasket?

 

[scoflaw]

I would think head gasket. Exhaust wouldn't have that kind of back pressure sitting in the water.

 

[Espos4]

If you had no water in the cooling system, I could see MAYBE the exhaust leaving the center of the elbow and flowing back into the water jacket around the elbows, into the hoses that are attached and pushing their way toward the thermostat housing.

If not, could be a head gasket leak.

 

[Mark SS]

Well I decided to pull the risers and manifolds and pulled the plugs and did a compression check of the cylinders...150-160 psi on all 8 cylinders which is what I was not expecting but it does only have <300 hrs on the new long block. The inside of the risers and back thru the water passages on the dry risers were all sooted up from the initial start of 10 minutes though. Thoughts?!?! It ALMOST confirms my thinking that some of the exhaust gas pressure was going backwards to the thermostat housing seeing as there is absolutely no water in the entire system.....

 

[scoflaw]

Seems like a long shot to me that the exhaust will travel through the small openings in the elbow gasket and thru the small hose at the bottom of the manifold to the t-housing. You'll find out soon enough.

 

[Thornton69]

Bravo's rely on raw water flow for cooling the upper case while on step. Are you installing a shower?

 

[Mark SS]

Bravo's rely on raw water flow for cooling the upper case while on step. Are you installing a shower?

I will be now that you mentioned it....I had thought about it before but now it is a for sure.

 

[Mark SS]

Seems like a long shot to me that the exhaust will travel through the small openings in the elbow gasket and thru the small hose at the bottom of the manifold to the t-housing. You'll find out soon enough.

But the exhaust at the riser could travel back to the thermostat housing through the L-shaped hose that comes from the bottom of the riser,. correct? It is the shortest route...just thinking about how it could get to the housing with the least distance traveled.

 

[scoflaw]

I would fix your water supply issue, if you have a blown gasket you'll know righit quick after you get it running