How hot should exhaust manifolds be?

[cloud_master]

Just got done putting on all new manifolds, risers, and shutters on my twin ‘90 4.3lx motors. Fresh water motors, original manifolds. They were over due.

Ran the motors in the hose today to check for leaks. Even at idle, after the engine came up to operating temp, I was putting my hand on the manifolds and risers. The risers were noticeably cooler than the manifolds. I could keep my hands on the manifolds, but after about 3 seconds or so, I had to take my hand off.

I don’t have an IR gun, but how hot should these run on the hose when engine is at temp? Both engines were exactly the same.

 

[achris]

Configured as 'warm' manifolds or 'cold' manifolds? Sea water cooled or close cooled?

Post engine serial numbers...

 

[cloud_master]

0C758101
0C758094

open cooling. Not sure on the warm vs cold manifolds.

Also, I never really paid attention before I guess, but when running them, most all the water was exiting at the bottom of the transom , I only saw a small trickle of water coming out of props. Both engines were the same. I was scratching my head wondering how I could feel exhaust coming out of prop hub at idle, but no water with it. How is that possible?

 

[tpenfield]

If you have the cooling system design that has hoses leading directly from the thermostat housing to the risers/elbows, then they get cold water coming to them in addition to the warm/hot water from the exhaust manifolds. So, the risers will be cooler to the touch.

I don't think I've ever been able to keep my hand on the exhaust manifold, but have on the riser.

 

[Lou C]

The warm manifolds set up is designed to run warmer that the standard cool manifold set up. The idea behind the warm manifold design is to keep them warm enough to prevent condensation in the exhaust which can happen at low engine temps at idle speed. So you need to know which system you have. Warm manifolds likely would run at 160 or so, close to engine temp because the water that flows out of the engine first then to the manifolds. The elbows are fed by cool water from the stat housing so they will run cooler.
In the cool manifold system cool water flows directly from the stat housing directly to the manifolds and then to the elbows. So these will run cooler, like at 100* at idle and maybe as warm as 135/140 just after coming off plane but after idling for a few min they will usually drop down to approx 100 or so. Pretty much all OMC/Volvo systems were set up this way but Merc had both cold and warm systems.