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Question for you - If it's a sub-standard rad...why would revving the engine (e.g. pop into 2nd) immediately drop the temp from 'critical overheat' to 'everything good' in no time flat, and why would the problem not exhibit at stop, only highway speeds that involve hills? Those don't point to a crap rad, or thermo.
if the radiator doesn't flow as much as it should (i.e., not the same though-put at a given pressure as the oe radiator) then it won't cool engine adequately. raising rpm raises pressure from the water pump pumping more so the radiator now lets some more pass through it.
i'm suspect of both the radiator and themostat. the bottom hose should not be cool to the touch, to me this indicates in-adequate coolant flow. since the water pump is oe Nissan (and you had the same issue before changing the pump change) my bet's still on tthe radiator (with the thermostat right behind). I guess it could also be some sort of physical obstruction in there but that seems less likely (but who knows what the emergency mechanic did).

I think air in system being the issue is unlikely. with all going on, most air should have bled off by now or at least be at top of radiator leaving it visibly low which would have been notied by now.
 

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Discussion Starter #22
MurandoSL2003 - Problem with doing it when it's overheating (which I assume is what you mean?) is the temp gauge doesn't go past normal when the car is still, only when going up hills at highway speeds (and MAYBE when going around tight curves at high speed, circumstantial evidence points to that).
 

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The problem may be elsewhere, as noted above by others.

I just think it's a good idea to start with making sure there isn't an air pocket, using a well respected tool like the Lisle Funnel, before troubleshooting the other cooling system components...
 

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Discussion Starter #24
Update. Got home after a drive where it overheated a bit. Turns out sweeping left turns at high speed really kick off the overheating, I drove around most of the afternoon, but there is one sweeping turn on the highway near my house where I usually notice it start...tested today, yup, that turn makes it start (and hills in general).

Got home, left engine running with temp gauge at about 50% (40% is normal).

Immediate readings - Top hose entering rad @ 175F, bottom leaving @ 135F.
Over the next 5 minutes the top hose slowly heated up to 200F, bottom one kept staying in between 135F and 140F.

So that strikes me as something is progressively getting hotter and hotter. The rad is successfully cooling it but whatever is overheating keeps overheating...probably until it overwhelms the rad's ability to cool...once the engine is revved above 3000RPM (at standstill, or when driving), or allowed to coast down a hill, then temp returns to normal for a while.

Have a Lisle Funnel in hand. Will use it in the next few days.
 

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I'm fishing for ideas like you are.

Have you checked the oil to see if there's any coolant mixing in there? I think this unlikely, but you've eliminated the obvious.
 

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It sounds like something is still restricting the flow of the coolant. Have you used an OBDii scanner to see the temperature of the coolant in the engine block? I don't know what Nissan thermostat "open" temperature is but in general it's about 190-195 degrees F (considering the optimal working temperature is about 190-200F. So at 175F., the thermostat may not be open, and there is little flow. If the coolant is not moving, the outlet temperature can be low, but at 200F, the thermostat should be fully open. When the coolant is moving through the radiator at normal rate, the temperature differential should be only 10-20 degrees I believe. Your 60-65 degrees may indicate there is not enough flow.
 

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Discussion Starter #27
No coolant detected on the last oil change. No oil in the coolant from self-visual inspection.

I have not used a scanner yet. Interesting that the drop only supposed to be 10-20deg....that would indeed indicate very slow flow
 

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Here is the possible fix you can try that seems to have worked with zebs issue....2006 Murano Overheating, new radiator same problem

If that does not work, I would change the radiator and stat again for $100 including using prestone longlife concentrate coolant and distilled water-walmart. Last effort before giving up. Or at least pull out the stat and check its functioning in a pot of boiling water.

Radiator with lifetime warranty should be good like this....New Radiator For Nissan Murano 2003 2004 2005 2006 2007 3.5 V6 Lifetime Warranty | eBay

And this stat.....New Thermostat Housing for Nissan Maxima Altima Murano Quest Infiniti I35 02-04 | eBay
 
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