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post #1 of 11 Old 10-12-2012 Thread Starter
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Universal M25 Overheating Issue

Hi Everyone...

I know this is a common issue and problem but have been through everything on the internet and this forum and am still stumped.

I have a Universal M25 my problem is the engine operates perfect at under 1800 RPM... Above 1800 RPM the water temp quickly rises to 190-200+ and keeps going unless I back down. I would really love to get this up to normal operating speed but can't figure it out.


The engine status so far:
New Coolant/Water running 60% water with cooling additive
170 Degree Thermostat
Checked all cooling lines for blockages
Replaced Raw water pump (was Oberdorfer N202M-03, replaced with N202M-07)
Replaced heat exchanger with updated 3"
Checked fresh water pump (may replace anyway but looks ok)
Pump belt tension is good


Looking for any tips someone might have to troubleshoot this and identify the problem.

Right now my next plan is to run through some coolant flush/radiator cleaner and refill and try again. But beyond that I haven't a clue what to approach next. The raw water flow out the exhaust seems fine, there was a noticable increase when I upgraded the raw water pump so I'm reasonably certain that system is functioning properly.

Thanks all!
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post #2 of 11 Old 10-12-2012
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Universal M25 Overheating Issue

You might want to check and see how plugged your mixing elbow is...
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post #3 of 11 Old 10-12-2012
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Re: Universal M25 Overheating Issue

It does sound like it's functioning fine.

Before dumping too much time and energy into fixing an overheating problem, I'd confirm that there is an overheating problem. You can buy an infra-red point and shoot thermometer for under $50 bucks now. Buy or borrow one and aim it at the block and head to get a temperature independent of your instrument gauge.

Ray
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post #4 of 11 Old 10-13-2012
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Re: Universal M25 Overheating Issue

How long ago did you put in your 3" HX? I did the same thing on our M25. But after time, salt drops out into one of the ports. Consider atking the HX off and checking everything. You'll have to burp the engine when you replace the coolant.

Engine Overheating 101 - How to Burp Your Engine (Reply #6) Engine overheating problem (with Instructions "How To Burp!")

How old is the thermostat?

That's all I can think of right now.

Good luck.

This info may be helpful to/interesting for you, too.

Diesel Engine - C34

Stu Jackson, C34, 1986, M25 engine, Rocna 10 (22#)

Last edited by Stu Jackson; 10-13-2012 at 11:56 PM.
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post #5 of 11 Old 10-14-2012
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Re: Universal M25 Overheating Issue

Excessive exhaust back pressure will cause engine overheating at higher speeds. Check the exhaust back pressure and make sure there is no blockages in the muffler, trap or outlet.

Wasps setting up home in the outlet is a possibility.
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post #6 of 11 Old 10-15-2012 Thread Starter
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Re: Universal M25 Overheating Issue

Thanks guys! I checked the temp sender and guage this weekend, basically by throwing the sender in a pot of hot water at varying temps and it seemed to resond well (the guage read generally exactly the same as the termometer in the water). Also checked the resistance on that, so unfortunately I think the easy fix is out.

Next up I guess will be to take a look at the exhaust system probably take the whole thing apart and make sure it is clean and clear.

For Stu - The heat ex was replaced about 3-4 years ago, guess I will have a look at that as well. The thermostat is brand new, was the first thing I started with a couple weeks ago.
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post #7 of 11 Old 10-15-2012
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Re: Universal M25 Overheating Issue

Quote:
Next up I guess will be to take a look at the exhaust system probably take the whole thing apart and make sure it is clean and clear.
the good news about exhaust restrictions is that they typically occur at or close to the mixing elbow.

Ray
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Re: Universal M25 Overheating Issue

Might be on to something at the mixing elbow... It looks really clogged up with carbon and other stuff... THere are even a few chunks of something stuck inside. Any ideas about how I can clean this out? Or should I just replace it? Where do I get a replacement?
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post #9 of 11 Old 10-16-2012
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Re: Universal M25 Overheating Issue

I'm a cheap ba$tard, so you might be asking the wrong guy. I've had mine apart before and just scraped out the scale and chunks with a flat blade screw driver. My high school shop teacher would have had a fit.

Ray
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post #10 of 11 Old 10-18-2012 Thread Starter
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Re: Universal M25 Overheating Issue

Ok, well gave it a good washing in muriatic acid.. After removing the carbon and rust the inside doesn't look super great and probably needs to be replaced. Of course I can't seperate just the mixing elbow from the rest of the exhaust riser even with the largest pipe wrenches I could find so I need to replace the whole thing... For now i'm going to slap the whole thing back on and see if it makes a difference.

Cleaned out the heat ex too while I had everything basically apart anyway. It wasn't looking too bad but can't hurt either. Will let you guys know the next time I get her out how it goes.
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