Please Read, Help Needed! QX4 Overheating Issues

[gordon.su]

Hi everyone,

Hope all is well. Thank you all for constantly being so helpful.

I have a 2001 QX4 at 250,000. She runs great however still overheats.

Overheat occurs:

- driving over 1 hour in temperatures above 18 degrees celsius
- even on a cooler day <18 degrees celsius driving over a hour and being stuck in traffic (bumper to bumper)
- when overheating occurs thermostat runs very close to the top of the gauge where 'H' is
-**only way to cool down the car is to either drive faster allowing wind to hit the rad OR turn on the heat on full blast**


I have exhausted all options and believe changing my radiator may be the only solution:

1) Changed OEM radiator cap (last summer)
2) Coolant Flush (last summer)
3) Changed OEM thermostat and gasket (last summer)
4) Clutch fan (last month)

Please share any tips of experiences you may have to this related issue.

Thank you!

Gordon

[Leo1998]

Did you check the other thermostat?
overheating-1-thermostat-2-thermostat-t580465.html

[gordon.su]

Did you check the other thermostat?
overheating-1-thermostat-2-thermostat-t580465.html

Hi Leo thank you for your reply!

I have read about the 2nd thermostat/water control valve. Do you think it is a probable cause? It seems like a hard place to get into.

Ive searched throughout this forum and noticed some people having success with clutch fans (which did not work for me), and replacing the whole radiator, but not so much about the 2nd thermostat.

Could my head gasket be another possible cause as well?

I feel like just bringing it to my mechanic and having him replace both the 2nd thermostat and radiator and crossing my fingers for the best...

[Leo1998]

If it's never been replaced with that many miles I would change it and the radiator.

It is in a tough space and hopefully someone with experience will chime in

[donald]

From the sounds of it, you covered all the basics first. Looking at the schematic in the FSM, it sounds plausible that it'd be the water control valve (ie. thermostat #2).

Looking at the diagram from FSM (attached below, the bottom image), the cylinder block is only cooled when both the thermostat and the water control valve is "open". So, since you changed the thermostat already, we can assume that's functional. We'll also assume no bubbles in the system (after a flush), nothing is leaking as you've changed gaskets and radiator cap, and the water pump is properly producing the proper amount of pressure to push fluid around.

So in the image below, lower diagram, there's a black path, white path, and a hash/striped path.
I'd believe the hash path is what you want to confirm is working properly - based on your description of driving, the engine is thoroughly warmed up and at operation temperature, it should be running in the hash path.

Your cylinder head may be cooling properly (your functional thermostat is working), but it could be that your water control valve is malfunctioning and/or not opening to allow fresh coolant to flow through and back to the radiator. So, if the WCV is not allowing flow, HOT coolant is just pooled up in the cylinder block collecting heat with now where to send it. And wherever the temperature sensor(s) is, it's picking up that the engine is not properly cooling and sending the signal to drive up the temperature gauge on the cluster.
Based on CourtesyParts.com, it's a rather inexpensive part $9 (http://www.courtesyparts.com/21230-valv ... 98290.html), but it sounds like a lot of work to get to. FSM notes that the fuel pressure needs to be released, upper intake manifold removed, and injector "tube" needs to be removed... just to get to this $9 part

also... just double check that your temperature sensor is functioning properly too. Make sure there isn't any dust buildup on the connectors that might send a faulty signal.

[gordon.su]

If it's never been replaced with that many miles I would change it and the radiator.

It is in a tough space and hopefully someone with experience will chime in

thanks Leo, ordered and replaced the radiator... ordered the watervalve from dealer but they nevered got them in on time...

however after noticed that around the head gasket appears to be some leaks =(.

is it worth it to go through the tedius labor to replace the head gasket or just swap in a new engine, vq35de or an exterra engine?

[gordon.su]

From the sounds of it, you covered all the basics first. Looking at the schematic in the FSM, it sounds plausible that it'd be the water control valve (ie. thermostat #2).

Looking at the diagram from FSM (attached below, the bottom image), the cylinder block is only cooled when both the thermostat and the water control valve is "open". So, since you changed the thermostat already, we can assume that's functional. We'll also assume no bubbles in the system (after a flush), nothing is leaking as you've changed gaskets and radiator cap, and the water pump is properly producing the proper amount of pressure to push fluid around.

So in the image below, lower diagram, there's a black path, white path, and a hash/striped path.
I'd believe the hash path is what you want to confirm is working properly - based on your description of driving, the engine is thoroughly warmed up and at operation temperature, it should be running in the hash path.

Your cylinder head may be cooling properly (your functional thermostat is working), but it could be that your water control valve is malfunctioning and/or not opening to allow fresh coolant to flow through and back to the radiator. So, if the WCV is not allowing flow, HOT coolant is just pooled up in the cylinder block collecting heat with now where to send it. And wherever the temperature sensor(s) is, it's picking up that the engine is not properly cooling and sending the signal to drive up the temperature gauge on the cluster.
Based on CourtesyParts.com, it's a rather inexpensive part $9 (http://www.courtesyparts.com/21230-valv ... 98290.html), but it sounds like a lot of work to get to. FSM notes that the fuel pressure needs to be released, upper intake manifold removed, and injector "tube" needs to be removed... just to get to this $9 part

also... just double check that your temperature sensor is functioning properly too. Make sure there isn't any dust buildup on the connectors that might send a faulty signal.

hmmm the wcv never came in on time as i was replacing the rad, my mech doesn't suspect the wcv to be the issue, we think it may be the head gasket now... i did pretty much everything an am very disappointed, not sure what to do anymore, i haven't been loosing any coolant anywhere lately, except for those times it overheats and coolant comes out of the coolant reservoir, however i just replaced the reservoir cap which was previously cracked (not sure if it can still leak out with it seal).

i also used OEM coolant for the new rad.

[donald]

did you see any oil or oily film in the coolant when you drained the fluid? Any browning in the coolant?
Anything noticeable in the oil when/if you drained it recently?

I'm curious and interested to learn how the mechanic (or anyone else) connects overheating with a leak in the head? And if it were a leak, wouldn't a significant leak cause such drastic temperature swings?

[Leo1998]

I would be surprised if it was the head gasket because i would suspect you would have white smoke coming out the tail pipe. Here's a case of what could happen when the thermostat fails at the Water control valve.
Read this thread...
vq-w-possible-blown-headgaskets-how-to- ... 87375.html

vq-w-possible-blown-headgaskets-how-to- ... 75-30.html

[gordon.su]

Hi everyone,

Thank you for all pitching your great ideas.

I waited a couple of days till it got hot enough to really see what has changed or what has not.

Today in Toronto, Canada the weather was pretty hot 28 degrees celcius, with the humidex it was about 32.

I drove my QX4 in rush hour traffic for over 1 1/2 hour. Normally that would be the the tipping point when it usually overheats.

There was no overheating yet... I will keep my fingers crossed that this overheating issue is done and dealt with!

What I recently did again was my:

a) new rad (seems to be the main issue)
b) OEM rad fluids

I believe my previous diagnostics only helped alleviate some but not the main issue which was the rad:
c) clutch fan
d) 1st thermostat
e) rad cap

what I haven't done and do not hope to do is
f) water control vale
g) water pump
h) head gasket
i) new engine

I'll keep you all updated!

thanks,

Gordon

[donald]

A little over a week later... Still good I presume?

[gordon.su]

She's still running like a champ!

Thanks for asking Donald

Cheers