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Cooling Problem

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Okay so I’ve got a 2008 RM-Z 250 that has cooling problems and I can’t seem to figure it out. Beforehand i have replaced the cylinder head, valves, piston, rings and timing chain. As well as top end gaskets to go with, I have just replaced the water pump seals to keep the coolant out of the oil. I’ve got a temp gauge radiator cap. Every time I start it up just to see if I maybe fixed the problem, coolant just comes running out of the overflow. I don’t know where to go next any suggestions?

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bad radiator cap, or over filled radiators

Before I messed with it the radiator itself smoked. But it stopped after I put a different cap on it

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How long do you run the engine with the bike at a standstill before it expels coolant?

It doesn't take much time to warm up a 4-stroke MX'er to start pressurizing and expanding the coolant,

especially if you've overfilled the radiator or, have a bad cap.

 

With a cold engine, remove the cap and start up the engine,

If you get excessive bubbling in the coolant (some is normal due to flow)

either you have a bad head gasket, warped mating surfaces or,

a crack somewhere in the cooling jacket which allow compression/combustion gases into the coolant.

 

A cooling system pressure tester is also a handy troubleshooting tool

Edited by mlatour
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How long do you run the engine with the bike at a standstill before it expels coolant?
It doesn't take much time to warm up a 4-stroke MX'er to start pressurizing and expanding the coolant,
especially if you've overfilled the radiator or, have a bad cap.
 
With a cold engine, remove the cap and start up the engine,
If you get excessive bubbling in the coolant (some is normal due to flow)
either you have a bad head gasket, warped mating surfaces or,
a crack somewhere in the cooling jacket which allow compression/combustion gases into the coolant.
 
A cooling system pressure tester is also a handy troubleshooting tool

It takes about 20-30 seconds for it to start running out of the overflow. I have just replaced gaskets. Not familiar with the warped mating surfaces part (explain if you would)

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How long do you run the engine with the bike at a standstill before it expels coolant?
It doesn't take much time to warm up a 4-stroke MX'er to start pressurizing and expanding the coolant,
especially if you've overfilled the radiator or, have a bad cap.
 
With a cold engine, remove the cap and start up the engine,
If you get excessive bubbling in the coolant (some is normal due to flow)
either you have a bad head gasket, warped mating surfaces or,
a crack somewhere in the cooling jacket which allow compression/combustion gases into the coolant.
 
A cooling system pressure tester is also a handy troubleshooting tool

I replaced the cylinder head so I wouldn’t think it would be cracked anywhere

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Had the engine ever severely overheated in the past?

 

'mating surfaces' meaning the machined flat portion on which the head gasket sits (cyl. and cyl. head)

Your service manual will have the acceptable tolerance for mating surface trueness,

Also the instructions to properly tighten the fasteners steps, pattern and torque values which I hope you followed.

 

Warped surfaces can happen during a severe overheat, 

despite a new gasket the sealing is compromised when either the cylinder head and/or cylinder surfaces aren't perfectly flat.

Just to give you an idea how small the tolerances can be, on my CRF250X, any more than 0.002" is beyond the acceptable limit.

 

 

You mention putting a different cap but, have you tried a brand new radiator cap?

Any visible bubbles as per my simple test suggested earlier?

Edited by mlatour

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Had the engine ever severely overheated in the past?
 
'mating surfaces' meaning the machined flat portion on which the head gasket sits (cyl. and cyl. head)
Your service manual will have the acceptable tolerance for mating surface trueness,
Also the instructions to properly tighten the fasteners : steps, pattern and torque values which I hope you followed.
 
Warped surfaces can happen during a severe overheat, 
despite a new gasket the sealing is compromised when either the cylinder head and/or cylinder surfaces aren't perfectly flat.
Just to give you an idea how small the tolerances can be, on my CRF250X, any more than 0.002" is beyond the acceptable limit.
 
 
You mention putting a different cap but, have you tried a brand new radiator cap?
Any visible bubbles as per my simple test suggested earlier?

I don’t know if it’s been severely overheated in the past, and I did follow all directions and torque specs. I haven’t gotten to the bike yet but i will let you know what the results are when I do your test.
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Had the engine ever severely overheated in the past?
 
'mating surfaces' meaning the machined flat portion on which the head gasket sits (cyl. and cyl. head)
Your service manual will have the acceptable tolerance for mating surface trueness,
Also the instructions to properly tighten the fasteners : steps, pattern and torque values which I hope you followed.
 
Warped surfaces can happen during a severe overheat, 
despite a new gasket the sealing is compromised when either the cylinder head and/or cylinder surfaces aren't perfectly flat.
Just to give you an idea how small the tolerances can be, on my CRF250X, any more than 0.002" is beyond the acceptable limit.
 
 
You mention putting a different cap but, have you tried a brand new radiator cap?
Any visible bubbles as per my simple test suggested earlier?

Alright I did your test you mentioned. One or two bubbles rose right when i started it but that is all. I filled it up just above where you can see the radiator down inside and the level didn’t rise enough to make it to the overflow, just below. The coolant just sloshed around in a circle inside. Maybe a clog somewhere in the hoses or radiator? Let me know. Thanks

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Actually all sounds normal, time to try a new radiator cap.  (stock pressure one, not a 1.6 etc.)

Yeah I figured most of the cooling would come from air while I’m riding.. but it’s really sketching me out how quick the header gets too hot to even tap with my fingers.

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Actually all sounds normal, time to try a new radiator cap.  (stock pressure one, not a 1.6 etc.)

Oh and the radiator cap that I swapped out for the stock one was a 1.8.. maybe that had something to do with it I’m not sure?

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No unless the 1.8 was defective, 

it would actually hold more pressure inside before excess was purged out the overflow tube.

 

Is the sealing surface inside the radiator neck (where the cap's seals sits) damaged?

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No unless the 1.8 was defective, 
it would actually hold more pressure inside before excess was purged out the overflow tube.
 
Is the sealing surface inside the radiator neck (where the cap's seals sits) damaged?

Id have to go out and look but I’m about 95 percent sure it isn’t.

My header gets too hot to touch in around 45 seconds - 2 min. Is this normal for a four stroke? I’m relatively new to how quick the four strokes get hot and how hot is TOO hot. Converting from a two smoker

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Yes as you've described they warm up quite quickly.

100 degrees fahrenheit within a couple minutes is 100% normal... just making sure

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Don't know the exact temps but lets just say too hot to touch within a minute.

Too lean carburetor settings also contribute to a hot running engine.

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Don't know the exact temps but lets just say too hot to touch within a minute.
Too lean carburetor settings also contribute to a hot running engine.

Now that you mention it I may have a lean condition. The bike always dies when I try and give it gas without the choke on.. sound like a lean condition?

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Dies as the engine is warming up or, even after the engine is up to operating temps?

 

If it once ran fine on the same jetting specs, then it's not the settings but rather

blockage or, a wear item that is creating a lean condition.

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Dies as the engine is warming up or, even after the engine is up to operating temps?
 
If it once ran fine on the same jetting specs, then it's not the settings but rather
blockage or, a wear item that is creating a lean condition.

While it’s warming up, i can blip the throttle only while choke is on and it won’t die but once choke is off it dies immediately no matter how much throttle.

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