Logo
FULL SITE MENU

First Post! Exhaust Risers Different Temps

M

mowinkid37

New member
Engine: 1998 Merc 5.7L 2BBL, Alpha One Gen II

First Post and new to boat ownership. hope to learn alot more here!

Im having issues with my exhaust risers being different temperatures. while I was riding today, I checked the risers and the left one (looking at the stern from the boat) was hot while the right one was warm. The temperature was ~175 and I had no power loss issues. i pulled the boat out of the water and pulled off the raw water supply line to the riser and hooked my hose up to it. water freely flowed though to the exhaust on the gimbal housing. I put everything back together and ran it on the muffs (engine was still warm but the risers had cooled off). after a minute or two, the left side was hot again and the exhaust coming out of the housing was hot on the left. on the right side, the riser was cool and the exhaust was cool.

im curious if this is normal or this is an issue. i though the risers were supposed to be the same temp and it doesnt make sense why both sides wouldnt be warm. is there some kind of casting in the thermostat housing that will route raw water differently. I did get a new impeller this year.

Anyone have any thoughts or run into this before.
 
Also when I ran it on the muffs and grabbed the riser supply line, it was hot while the other side was cool. That’s why I was thinking thermostat housing.
 
Post a picture of your thermostat housing showing all hoses and where they go, a good image can help understand your cooling system design.

Also, there are typically two components on the exhaust.
Manifold
Elbow. Some call this a riser.

Make sure the Elbow is what you are referring to.

A riser based on many descriptions is a spacer between manifold and elbow thus a "riser"....

what we need to see is how water is distributed from thermostat housing. Two outputs or four outputs.
 
Last edited:
So the picture I attached is the thermostat manifold and I pointed to the hot side. When I said I hooked the hose up to the riser, I meant the line I pointed too and I should have said exhaust manifold. This is the only supply line to it. I checked the temperature (with my hand) at the top of the riser where it 90s to the elbow. I’ll attach a picture where I point to that too. It’s weird to me that it would have no flow issues when I isolate it individually but as part of the whole, it is the hot side.
a67b53b7-5008-4e27-bc46-6fbb69706fcb
 
im having difficulty attaching. when i go to the insert image tab there is no ok button for the local upload. when i click the ok button from the URL upload, it doesnt do anything.
ill keep trying
 
Based on images you have single cooling which routes to bottom of manifold and runs up and through elbow.

When I get time I will look at water flow and parts diagram and get back.
I am actually working now and am not on my pc
 
"""Also when I ran it on the muffs and grabbed the riser supply line, it was hot while the other side was cool. That’s why I was thinking thermostat housing. ""

Based on the pictures which hoses are you referring to?


Two hoses closer to Carb are Outputs to manifolds.

Two more forward hoses,
Largest on left is engine re circulation and goes to Engine water pump.

Next largest on right side is Incoming water supply. This goes thru power steering cooler and then to transom.
 
Ok now we are getting somewhere.

Remove hoses at thermostat housing (two smaller ones only). Look at holes and make sure they are not clogged or rusted and closed in by scale.



The way the system works is,

COLD water comes in thru main water hose from transom, thru power steering cooler to Tstat housing.

The cold water flows directly out the two small hoses to manifolds as exhaust must be cooled at all times.
Once thermostat opens at temp, warm engine water mixes with the cold and it is now a "warm" mixture which flows to Manifolds.

If you have had the thermostat housing off, There is a Plastic piece that I believe is a diverter and is shaped a certain way.
That has to be in correctly if not the water may not be equally shared to the two sides.

As I said I need to look at my manuals later
 
i havent had it off. but ill prob take it off to check if its become dislodged or something. it is 23 yrs old after all. btw always fresh water, never salt
 
So you have no idea if this issue existed before?

Also one other thing, most raw water cooled engines typically one side elbow does run warmer that the other. Nature of the beast.
As much as 15 ish degrees difference
 
So I took off the line from the thermostat to the water pump and found a big spring in there. Is that normal? Also a bunch of flakes and rusty water came out. I blew out the housing with a water hose. Do you know what the thermostat housing should be torqued to?
 
If you are not sure about a spring in a hot water hose with a 90* bend or what to tighten those bolts to,
I am not sure we can help you any more.

You may want to seek some professional assistance.

Also, if you are determined to work on your own boat
Get the apprpriate service manual.
It will have bolt tightening specifications.
 
Hey I’m no expert and you can’t learn without asking. To me, a spring in a hose is bad but I’ve never pulled apart a boat before. It may be normal. And luckily the google machine had my back for the torque, it’s 30ft/lbs.

thanks again for for the help!
 
Back
Top