Logo
FULL SITE MENU

Exhaust manifold temperature

F

Fairliner62

New member
Repowered the boat with new marine 350 long blocks. New Mercruser center riser style exhaust mainifolds. The exhaust manifolds on the port (regular rotation) engine run around 100 deg on the manifolds and 110 deg on the riser. The starboard (reverse rotation) manifolds run around 110 deg on the manifolds, but the risers get up around 180 deg when running at speed.

This seems a bit hot, what could be my problem? Could there be air trapped in the top of the water jacket on the risers?

By the way they are raw water cooled.
 
Last edited:
That is too warm for risers..... and definitely too warm if in Salt water (salt crystallization occurs at 140* and above).
Properly working risers and sea water systems generally keep these cooler at the higher RPM.

If the engines are not over-heating, your sea water supply is likely OK...... but may be restricted somewhere.
The actual engine typically requires less sea water than what the system will typically supply.
(most sea water is by-passing the actual engine cooling demands and goes on out the exhaust)

I think that we need more info.

Open system (aka raw water cooled) or Closed Cooling system?
Single Sea Water discharge diverter (aka "T" fitting)...... or dual outlet H/E?
Diverters can cause an un-even discharge of sea water to Port/Stbd sides........ in fact, the dual outlet H/E's can even cause un-even flow.

Sometimes we can restrict the "cooler" side, diverting more sea water to the "warmer" side...... however, it is best to "open up" one side, rather than to "restrict" one side.

Could you have interior delamination inside of a hose? (rare, but it does occur)
Are the Manifold-to-Riser transfer ports clean and clear?

.
 
OK, engines are closed loop cooling and are running right at 160 deg., exhaust is raw water cooled (salt water). Each engine has its own raw water intake and strainer. As I posted, engines are new, manifolds are new. All hoses are new. I did reuse the raw water pumps from the old engines (sherwoods), both are a couple years old and did not change impellers. I was having no cooling problems with old engines or manifolds.

Both engines and related systems are carbon copies, only having problem with starboard engine (reverse rotation).

Thanks
 
Last edited:
The only thing that would be different with the RH rotation engine, would be the engine circ pump. This requires a circ pump that is bi-directional. However, this would likely not cause any difference to exhaust man/riser temperatures without some tell tale signs of engine temp being too high.

The composite material (rubber), on our Sea Water pump impellers, takes a "set" when left in the pump housing...... and particularly while not being used for any length of time.
These can actually last longer on a boat that is used more regularly, than on one that is laid up for the winter (impeller left in pump).
Maybe these should be replaced.
(I always remove my impellers for the lay-up duration)

I still maintain that you have a restriction, of some sort, somewhere in your raw water system.

Are the tube passages within the H/E clean and free from any debris?

Did you by chance loose any vanes from a previous impeller failure...... vanes that may not have been retrieved?

Don't discount interior hose delamination.

Even while new...... you may want to remove the risers and give the transfer ports a good inspection. With this being twins...... we can even swap risers from one engine to the other...... although a considerable amount of work.

What about portions of the exhaust system beyond the manifolds and risers?

A long shot: I recently read a thread whereby a person inadvertently received a bad manifold casting from a major manifold supplier. This caused damage to an exhaust valve. There was no mention of high heat during the failure. I don't know if the owner had even considered manifold temperature prior to the failure.
Point being..... try your best to not over-look anything, no matter how silly it may seem. Sometimes these are very small items that cause huge issues.

Remember........ the camel didn't realize that one small straw could break his back.

.
 
Thanks Rick,

I think my plan of attack will be to first replace the impeller in the raw water pump and see if that improves the temps - then on to pulling the risers if no improvement.

Again, everything in the raw water system is new with the exception of the pump. All fittings, hoses, exhaust hose were replaced so I doubt there is a problem with them.
 
Update on temps. New impeller for the pump is ordered. I went down to the boat today to troubleshoot a bit more. Manifolds are cool to the touch (both sides of engine). Risers are cool on sides and on rear, exhaust hose is cool (both sides). Risers are only hot on front and top (both sides). Does this temp profile lead to any conclusions? If the pump is not pumping enough, is the water path through the front of the riser the last to get water? Osco center riser mercrusier style manifolds.

Thanks,
Ron
 
OK, installed new impeller today. Checked for any kinked hoses, none. Engine and transmission HE's were dismantled and cleaned before going back into boat during new engine install. Sea strainer is clear, I can see good flow through it (glass top). Removed risers and found no obstructions. Removed cleanout plugs on manifolds themselves and found no obstructions.

Risers are still cool on sides, back, exhaust horn, exhaust hoses. Risers are still getting hot on front only and across the top!!!!

Gaskets used between manifolds and risers (no spacers) have full cutouts on side water jackets and small (3/16") holes in front and rear water jackets. I went as far as to enlarge the hole in the gasket for the front water jacket to approx twice the size to allow more water flow through the front water jackets of the risers. Still getting hot.

I completely stumped why only the front of the riser is getting hot. Again, this is exactly the same setup as the port engine has and the risers on that engine are cool as a cucumber on the front (of the riser).
 
Problem solved!!! I had the manifolds plumbed as a one supply hose configuration (hose in bottom of manifolds). Went to a two hose configuration (supply to bottom of manifolds and supply to riser) and risers are running cool.

Thanks to all that answered with suggestions.
 
Back
Top