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1996 Mercruiser 3.0 overheating at idle

Kryptonics

Kryptonics

New member
1996 Mercruiser 3.0 slowly overheating when idling, the temperature increases to over 200F and up, if I rev up the engine to 2000-2500RPM the temperature goes down close to 140F pretty fast.
What I already did:

  • Replaced the impeller
  • Checked water supply
Here is the video of water supply hose detached from the thermostat and running at 800RPM. Please let me know is the incoming water flow looks good enough


Next things I'm going to do is to check the thermostat and exhaust. If the issue is clogged exhaust, any suggestions on how to clean it without removing from the engine?


P.S. I'm new here so will appreciate any help
 
What you describe as your issue points to the water pump impeller in outdrive.

Describe who did what and what was replaced and when.

Low flow at idle and higher flow as rpm increases is 99% of the time the impeller.

After it was replaced did someone start engine out of water with no water to drive while on trailer? If so you smoked the impeller.

It is not a clog/blockage at this point.
 
Did you had a chance to check the video of incoming water flow I have at 800RPM? Is it looks okay or weak? I replaced the impeller during Spring maintenance, didn't run it without the water
 
You probably right, I removed the thermostat yesterday, nothing changed, checked water channels for corrosion, they look pretty clear for a boat that used on fresh water. Now I am reading complains about Sierra impellers, people have too many problems with them, so I decided to recheck the water supply and I got about 1 gallon in 15 seconds at 1000-1200RPM, but then I decided to drop the RPM as low as possible (probably around 600) and water flow stopped. I think my Sierra impeller might be a problem so I ordered new OEM impeller kit together with the housing, will need to get the boat out of the water (this is the hardest part, that's why I hoped that the problem is somewhere in the engine) and change it up later this week. Will let you know if it fixes my overheating problem in case someone curious too.
 
Kryptonics said:
View attachment 20700

My overheating on idle problem was caused by this little guy.
Click to expand...

Good catch,

Jeff,

On the Gen 1 there is a short plastic "guide" tube I call it, that sticks into the impeller housing that the copper tube slide into and into the impeller housing.

On the Gen 2 (in his picture) this is what is used. Just a different design for gen II

This was most likely allowing exhaust gasses to enter the raw water stream, Speaking of which.
Make sure that rubber DAM is in place if not or it is battered, REPLACE IT!.
I will post a pic of the assembly in another post
 
Last edited:
Considering the GEN II kit comes with everything needed, why only change the impeller?
W-12043-Water-pump-service-kit.JPG
 
That is why it is always best to buy Mercruiser for most parts.

It may cost a bit more but it will have everything you need in the kit and you know it will fit properly.


Once you become familiar with what is what and what is needed then you may be able to go off brand for a specific item.

But in the end. OEM!
 
Kryptonics said:
Did you had a chance to check the video of incoming water flow I have at 800RPM? Is it looks okay or weak? I replaced the impeller during Spring maintenance, didn't run it without the water
Click to expand...
I watched your vid. You're supposedvto hold the hose upright & see how much water comes out. It should shoot up about 6 inches or so. Just for future reference.
 
kghost said:
That is why it is always best to buy Mercruiser for most parts.

It may cost a bit more but it will have everything you need in the kit and you know it will fit properly.


Once you become familiar with what is what and what is needed then you may be able to go off brand for a specific item.

But in the end. OEM!
Click to expand...
When checking water flow thru the t-stat hose is it best practice to hold the hose upright or down so that the water runs into the bilge & guess at it?

BAHAHAHAHAHAHA
 
kghost said:
WiteA$$,
Actually you are wrong as usual.

The correct method is not the one you describe. Wrong again!

But unfortunately I aint gonna tell ya.


Maybe see what OMC reccomends...
Click to expand...
Hi Ace! On an OMC Cobra why is it difficult to seperate the lower half from the upper drive half or vice versa even though the bolts have been removed, Ace?

BAHAHAHAHAHAHA
 
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