Mariner 150hp 2.5L Overheating – Need Advice

[procraft150]

Hi all,

I’m looking for some advice on a persistent overheating issue with my Mariner 150hp 2.5L outboard (serial 0B384105). Here’s what’s been happening and what I’ve done so far:

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**Symptoms:**
- Motor idles fine and “pees” strong from the telltale until it reaches normal temp.
- Once warm, the telltale stream gets **very hot** and sometimes even smokes/steams.
- Engine triggers the overheating alarm after a few minutes at temp.
- I noticed **one cylinder head runs much hotter** than the other.

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### **What I’ve Done So Far:**
- **Replaced thermostat** and **water pump impeller** – no improvement.
- **Checked for blockages** in cooling passages and telltale line – all clear.
- **Ran compression test** – all cylinders within spec.
- Suspected an internal leak or gasket failure.

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**Latest Work:**
I recently **removed the divider (water jacket) plate** and gasket to inspect for issues.
**Below are pictures of the divider plate, gasket, and mating surfaces:*

Based on these symptoms and photos, could a leaking divider plate gasket cause the severe overheating and hot telltale I’m experiencing?
- Is there anything else I should check before reassembling with a new gasket?
- Any tips for ensuring a proper seal or cleaning these surfaces?
- Could this also explain why one side of the engine runs hotter than the other?

 

[Dukedog]

first its a 2.4.. first 2.5 was letter "d" in '91... did you jus change tha impeller?.. needs ta be a complete water pump kit.. you need a water pressure gauge ta be able to diagnose easier.. it will pretty much tell all.

no photos showin' on my computer?.. look for any small cracks on both pieces of tha divider.. check for being flat.. both divider pieces and block surface.. do a poppet kit jus ta be sure..

 

[procraft150]

first its a 2.4.. first 2.5 was letter "d" in '91... did you jus change tha impeller?.. needs ta be a complete water pump kit.. you need a water pressure gauge ta be able to diagnose easier.. it will pretty much tell all.

no photos showin' on my computer?.. look for any small cracks on both pieces of tha divider.. check for being flat.. both divider pieces and block surface.. do a poppet kit jus ta be sure..

thanks for reply
I've already replaced the impeller and poppet valve kit, but i'm still getting the exact same symptoms. right now, i'm installing a new water pressure gauge to help diagnose the problem.

for those with experience, how do you interpret the readings from the water pressure gauge? what should i be looking for at idle and at higher rpms? any advice on what pressure numbers indicate a healthy cooling system versus a possible blockage or restriction?

 

[Dukedog]

'bout 4 (3 @ very min.) ta 7/8 at idle.. 14 to 'bout 21 max at speed.. preferably 14/18.. gauge will show actual pressure (pump condition), any type blockage and how tha poppet is working.. theses are my opinion only and worked for many years.. you may have one of those "ya gotta be here" ta do diagnostics on it.... jmo.

pics look good.. no water intrusion visible on any parts at all....

 

[procraft150]

'bout 4 (3 @ very min.) ta 7/8 at idle.. 14 to 'bout 21 max at speed.. preferably 14/18.. gauge will show actual pressure (pump condition), any type blockage and how tha poppet is working.. theses are my opinion only and worked for many years.. you may have one of those "ya gotta be here" ta do diagnostics on it.... jmo.

pics look good.. no water intrusion visible on any parts at all....

Thanks for the info and your opinion, really helpful. I’m going to swap out the pressure gauge next, and I’m currently waiting on the divider plate gasket to arrive. Once I get everything installed and tested, I’ll report back with the results. Appreciate your input!