4.3 one exhaust elbow hotter than other?

[lr3guyjoe]

I have a new to me 1996 4.3 L MerCruiser. The boat was given to me by an older gentleman who’s a friend of a friend that said he was told the exhaust manifolds were bad and that it had some electrical gremlins.

I got the electrical sorted out and I got the boat up and running. The manifolds appear to be a little crusty around the manifold flange is between the riser and exhaust manifold but other than that appear to be okay. I rang my buddy up and asked if he could ask the man in more detail what was wrong and he said one side was running hotter than the other and it blew a hose off the manifold.

Today I got the motor up and running to operating temperature, I let it idle for about 15 minutes then I took it up to about 1500 RPM and took temperature readings all over the manifolds.

-The manifold temperatures in the center of the manifold opposite the mounting face were about 100° and left and right or within 2-3° of each other.

-The risers were around 95° and again left and right 2-3° of each other.

-The elbows showed a larger temperature difference one side Was about 105° and the other side was about 125°

- The lower exhaust tubes after the elbows were about the same temp from left to right around 105°.


Any ideas why one elbow might be so much higher then the other? I’m also not really seen where any hoses could blow off from other than the rubber connectors on the elbows.

 

[Bondo]

The manifolds appear to be a little crusty around the manifold flange is between the riser and exhaust manifold but other than that appear to be okay.

Ayuh,..... That's where they fail, at the gasket flanges, which rust away,......
Only way to inspect the thickness of those flanges is to pull the risers, clean both surfaces to clean bare metal, 'n then measure their widths,.......

The manifolds/ risers never, ever run the exact same temps, so that's not any indication, of anything,.....

 

[Scott Danforth]

usually which ever side is higher is the hotter one

if you walk to the starboard side of the boat, the port elbow will be hotter. if you walk to the port side of the boat, the starboard side will be hotter.

 

[wingless]

On my boat the raw water is provided by a single inlet / single pump.

That raw water path goes through all the engine heat exchangers, then to the engine antifreeze heat exchanger.

The engine antifreeze heat exchanger splits the raw water path, into two paths. That split means one side makes an additional pass through that heat exchanger, with that raw water side exiting at a higher temperature than the other side.

That higher raw water temperature fed to the exhaust is also a factor in the temperature difference for the two sides on the exhaust.

 

[harringtondav]

At idle on the muffs, my '96 4.3 tends to run a bit cooler on the starboard side, as well as a little more water coming from the starboard exhaust relief port. My guess is my raw water comes from the drive on the port side into the thermo hsg, and passes directly across into the starboard manifold. The port water has to take a 180 deg turn back to the manifold. So maybe more flow resistance (?).

Once underway each riser feels the same. The drive water pump provides more than enough flow at cruising rpms.
BTW, my engine manual states to not run that engine over 1500 rpm while on muffs. Your seawater pump should deliver over 3 GPM at 1000 rpm. More rpm = more flow. If you were on muffs, you may have been starving the seawater pump, depending on your garden hose flow.

 

[achris]

....BTW, my engine manual states to not run that engine over 1500 rpm while on muffs. ....

And if you have a vortec engine, you can't run at idle on muffs either. The valve overlap is enough to allow the cylinders to pull water back form the ends of the elbows with the exhaust valve open and into the cylinder. You need to run at around 1200-1400 rpm when on muffs. (It's in the owner's handbook. You know, the one everyone reads before operating their engine.)

Chris.........

 

[tank1949]

Most Alpha pickup pumps do not provide adequate water above a certain RPM. Bravos require steel reinforced water hoses from pick up pumps (usually crank of pully driven) to water source because pumps really suck water much more than Alphas, but I still do not believe I would maintain high RPMs on muffs or any device supplying home water via a garden hose to my motors. While on muffs and under 1500 RPMS, slowly disconnect exhaust manifold hoses and examine flow. Reconnect ASAP or you will burn up rubber. Also, an inferred thermometer could more easily determine hot spots. Good luck!