Condensation in exhaust manifolds...

[Lou C]

Ok got a question about condensation in exhaust manifolds. I came across this by accident because I had to remove the Barr Marine/OMC/Volvo exhaust I installed 4 years ago, for access to the steering actuator that needs to be replaced. I ran the engine on the water hose about 20 min, let it cool and then removed both manifolds with the elbows intact. I was surprised to find a bit of water in each exhaust port of the manifold, not in the cyl head, and nothing was rusted. What's got me wondering is this, for a long time I had heard, that salt water cooled engines should run a 140 stat. I looked high and low, never found one, till I found that Volvo used one on the 7.4 liter engines some time back and they used the same OMC style stat housing, so that stat fit fine even looks like its made by the same company as all the 160 stats I used in the past (same markings, patent etc). So the engine runs real cool now, between 135-145 vs with the 160, it ran from 160-175. I'm wondering if this is due to using the lower temp stat, because once the stat opens, the warm water from the stat housing exits the manifolds, but with the lower temp stat the engine is running approx 30 degrees colder. I know that probably 100% of OMC engines and 90% of Volvo engines all used 160 stats. The cooling systems used on all OMCs and Volvos (raw water cooling) all appear to be of the cold manifold design, so these do run cooler than a warm manifold set up where the manifold will be close to or even slightly above engine temp, since they don't get water till the stat opens and has been heated by the engine.
As far as salt build up in the engine, well when I had it apart 4 years ago to replace the heads, I did not see any evidence of this, but it was a nagging thought in the back of my mind.
Thoughts?
while I have the boat out for repair I put the 160 back in. I'm wondering if OMC and Volvo used the 160 stats due to their use of the cold manifold design. These idle at 95-100 and only get as hot as 125 after coming off plane, I recall reading somewhere there is a threshold temp below which condensation will occur, might be 120* or so....

 

[Scott Danforth]

remember, you have water flowing thru the exhaust manifolds all the time regardless if the thermostat opened or not

because you were on the hose, your motor never got up to temp so you would have condensation

or the slight reversion at idle that the 4.3 suffers

 

[Lou C]

I kind of figured that and I never checked the old one piece manifolds when running the 160 stat to see if would happen with them under the same conditions. I do know because of the # of times I changed the exhaust on this boat (3 changes in my ownership) I've never seen any rust in the cyl head exhaust ports or on the exhaust valves.
As far as reversion, I know that OMC and Volvo changed the shape of the original design of the 90* adapter pipe that connects the elbows to the Y pipe, to make it less likely.

EXHAUST & COOLING - 1991 Stern Drive 4.3 434APRRGD | Crowley Marine

View parts diagrams and shop online for EXHAUST & COOLING parts, 1991 Stern Drive 4.3 434APRRGD. Offering discount prices on OEM parts for over 50 years. FAST $4.95 shipping available.

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old design part #2

EXHAUST & COOLING - 1996 Stern Drive 4.3 434BPNCS | Crowley Marine

View parts diagrams and shop online for EXHAUST & COOLING parts, 1996 Stern Drive 4.3 434BPNCS. Offering discount prices on OEM parts for over 50 years. FAST $4.95 shipping available.

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new design part #2



When they came out with the Vortecs in '96 they added one way check valves to the elbows to prevent a vacuum from developing in the exhaust which contributes to reversion. I have never heard that the pre vortecs needed these but I guess someone could add them if they wanted to.

EXHAUST & COOLING - 1998 Stern Drive 4.3 434BPBYC | Crowley Marine

View parts diagrams and shop online for EXHAUST & COOLING parts, 1998 Stern Drive 4.3 434BPBYC. Offering discount prices on OEM parts for over 50 years. FAST $4.95 shipping available.

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one way check valve added to elbows, part #65
looks like they were added in '98, the '96s and 97s don't list them.

One thing still in the back of my mind, is a 140 stat really the way to go raw water cooling in salt? I wonder what was OMC's and Volvo's justification for not offering that as an option. I know Merc used these years back and then when they went to the EFI systems they appeared to switch to a 160 stat, even for raw water cooling. However, my brother has a 2020 Chapparal with a Merc 4.5, raw water cooling and it has a 140 stat and runs as cool as my boat did with the 140.

 

[Scott Danforth]

One thing still in the back of my mind, is a 140 stat really the way to go raw water cooling in salt? I wonder what was OMC's and Volvo's justification for not offering that as an option. I know Merc used these years back and then when they went to the EFI systems they appeared to switch to a 160 stat, even for raw water cooling. However, my brother has a 2020 Chapparal with a Merc 4.5, raw water cooling and it has a 140 stat and runs as cool as my boat did with the 140.

yes

raw water and carb was 140 ish

unless hx cooled, then 160 ish

EFI and raw water is 160 ish

EFI and hx cooled is 180 ish

cant explain why the 4.5 would have a 140 T-stat the emissions must be on the raged edge.

 

[Lou C]

that was surprising to me as well, given that these have the latest emissions with the cat converters.
I looked for the evidence of salt or calcium deposits in the block or old cyl heads when changed them and didn't see any of that. Even when I took off the Barr manifolds, the exit passages in the elbows are totally clean. And this is salt water not Fla salt water but Long Island salt water. Here's a pic of the old cyl heads....is that white stuff in the water passage on each side what people are talking about? Those were the sides that dead ended against the intake in the back so there isn't much water circulation there. Next pic is with the intake removed, I don't see anything in the 2 water passages that could be a concern, or cause overheating....and this was after approx 20 years in salt water....