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Manifolds and risers temp question

C

Captron51

New member
Folks, this is my first post in this forum, though I am fairly active on bayliner forum. I wanted to get input from outside that group for new perspectives. I am a very experienced boater and owned a lot of boats over the years.
I am a bit perplexed though regarding my current 1994 mercruiser 5.0 w alphaone gen 2. The issue is starboard exhaust components running much hotter than port. It is raw water cooled. I have replaced lower unit impeller, recirculating water pump, all new manifolds and risers, checked flappers, new tstat, confirmed good flow discharge very each side......and still the starboard side runs warmer than port. At idle and below 2000 rpms both sides remain cool enough for comfortable touch. But at planing speed of 3000 rpms, the starboard side raises to 185degrees (measured with ir gun) and I found top of manifold on forward end read 208 degrees. The engine temp gage and ir reading of block showed held steady at 160 degrees. No engine overheat.
I have seen a lot of conflicting information on the web about this, some saying it's a problem , others saying it is a normal result with merc raw water cooled system due to discharge route of water. I have noticed that the supply hose from tstat to manifold on starboard is warmer than very cool supply to port. At same time, the raw water in is next to port supply to manifold, while the engine block discharge from water pump is right next to the supply to starboard.
Bottom line question for fellow mercruiser owners is.....if the engine is running great and not overheating, how worrisome us the exhaust getting up to 200 degrees? Appreciate any feedback.
 
Here's what I'd do: Install a gate valve in the hose going to the COOLER side and cramp down on it until the hotter side runs cooler.

In other words, to hell with what's causing this; just do something to overcome it. Right?

Jeff
 
First question.
Do you have four water hoses running to exhaust, two to elbows/risers and two to bottom of manifolds? Or just two going to bottom of manifolds?

what you are experiencing is normal. Water follows the path of least resistance.
That bieng said due to design the water flows easier to one side under high flow conditions easier then the other side.
Other than carefully modifying the output thermostat housing casting to allow 100% equal flow to both sides there is nothing to do but enjoy the boat. As long as motor temp stays at normal operating temp (160ish) all is good.
 
First I would check that there are no obstructions in the hotter water path... Broken bit of old impeller in t'stat housing or in hose feeding manifold on hot side, partially collapsed hose or obstruction in hose. Due to differences in flow path, temps will never be the same unless you go to a lot of trouble to "balance" the flow paths. 203 does seem a bit hot. Check for obstructed flow starting @ the t'stat and going to the injection point into the exhaust flow in that elbow. If none, just live with it.
 
Thanks to you guys for responses. To answer questions..
I only have one supply hose to each manifold entering bottom center. I have checked tstat flow and appears to be equal and strong each side, but only did that at idle. Not at 3000 rpms. Starboard supply exits tstat warm, port exits tstat cold. After reading your comments, I am inclined to run the boat and keep eye on temp gage. Being a worrier, I hope to stop paranoia about exhausts catching fire tho. I suppose the temp gage will start to show me a problem developing if it starts to climb. I think I'm over thinking it?
 
You didn't say if this situation is new or an existing one, i.e. "the boat has always been this way for me".
Also... is it the correct T's stat, i.e. a MARINE one, is it installed correctly, and is it the correct t'stat housing.
Just for "fun", remove the t'stat ( buy another gasket first) and run it.
 
One thing to check is in the thermostat housing I believe there is a plastic diverter it has two large slots in it, I forget if it is above or below the thermostat itself but if that is not perfectly centered so the diversion occurs equally that may be a partial source of your issue if there is an issue at all.


Seeing as you only have one cold water to bottom of manifold, This design will generate more heat that the 4 hose design at the elbows.

The cold water passes thru the exhaust manifolds first before elbows. (typical exhaust is ~ 1500 degrees +/-) so the walls of the manifold get HOT and that water must take that heat with it as it passes by.

With the 4 hose design cold water goes to the elbows almost all the time directly so the port vs starboard heat issue is less evident.
 
Thanks for helpful ideas. I bought the boat a year ago and pretty much expected to do manifolds and risers based on my experience with other boats. At first, the port side was hot and starboard cool, so I only changed port manifold and both risers. Result was immediate port side cold but then starboard hot. I figured must be due to old port being clogged and with new one offering least be resistance, the water was now going easier thru port side. So I changed starboard side but that side remains 20-40 degrees warmer than port. Tstat housing is the factory marine unit and does not have check balls. I replaced 160 with 140 degree tstat and the plastic sleeve is centered. I have pondered what would happen if I teed off each supply hose to run water into each riser on front side where a plug is currently located, but my repair manual shows my current flow path as a legitimate application for 5.0 raw water cooled. Or like others have suggested.....just stop worrying about it as long as engine is not overheating?
 
Stop worrying about it.........

As long as temp gauge is at 150-160 ish and no excessive steam is present just run it..........20 or so degrees is no issue in my opinion!!

No need to change OEM design to make you feel better.................It works just fine and has been around for over twenty years.........
 
Please tell me is this temperatures are ok:

Right riser UP hottest: 77 F
Right manifold: hottest 149F
After exhaust bellow: 88F

Left riser UP hottest: 122F
Left manifold hottest point: 199F
After exhaust bellow: 160F

Thermostat housing temp: 163F

Mercruiser 4.7L MPI 2007 year DRY Joint

It this ok for this season ?

thank you
 
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