schuss

Since late spring, May or so.

Great tip, thanks! Going to give that test a try on Saturday and see what happens. You're right, at 205k miles I probably would be wise to replace it. I did replace the fan blades 20k miles or so ago because there was a small crack in one.

That's a good call - do you know a way to test them?

Interesting, thanks, I will check that! I sent an oil sample to Blackstone (http://www.blackstone-labs.com) and it came back with traces of coolant in it. Not enough that I could see it but enough that they clearly detected it. So I apparently have a leaky head gasket. At least that's something I know how to fix.

Interesting - thanks! So is the factory one not designed to fail in the open state? Is there a different brand that you found? That makes sense for me, too, as I'm not in a super cold climate and it's always in an underground garage anyway, so even if it did fail open that wouldn't be a huge risk for me.

Didn't even know about that other one - thanks! I'll try to find it. Does it make sense that it would be blocking water flow? If running the heater cools the engine down pretty well, that seems like water is circulating - at least between the heater core and the engine block. It doesn't overheat at idle, only when the engine is working.

I have a 2001 QX4 (VQ 3.5 4WD) with 206k miles on it. It keeps overheating and I can't figure out why. It will run for 20 minutes or so OK at about 45% on the temp gauge, and then the temp will start to climb. It climbs faster if I'm going up a hill, or going 70 on the freeway. I can turn the heat on full and that controls the temp, so I can usually get up a moderate hill with the heat on full (and the windows down). Sometimes it's not enough and I have to stop partway up the hill and let the heater cool things down. Turning the AC on will make it heat up even faster, switching to heat (and turning off the AC pump by going to ECON mode) makes the temp drop immediately. So the coolant is definitely moving through the system and through the heater core, otherwise the heater trick wouldn't work. First, I did the obvious - made sure it's full of water. I bought one of those radiator bleeder funnels so I can be sure I'm filling it and removing all the air. I open the bleeder valve on the back of the motor (fluid comes out of it when the tank is full). I also make sure the overflow tank is above the Min line but below the Max. I live in Seattle and it has been warm this summer, but nothing extreme. I'm running 50/50 that I bought pre-mixed (so I can't screw it up) and added a bottle of Lucas Oil Super Coolant. Next, I replaced the radiator, again making sure the coolant was full afterward. Since that wasn't the issue, I then replaced the water pump and thermostat. Neither of them looked bad; the water pump spun smoothly, I didn't actually test the thermostat but it wasn't obviously broken. But that didn't fix it, either. The engine isn't losing coolant - it stays full once it's full. I see no signs of coolant in the oil, or oil in the coolant - the coolant is perfectly clean. There's no signs of burning coolant - the exhaust isn't cloudy at all, it runs clean. The engine doesn't seem to have lost power and from what I can tell there isn't something dragging on it, making it work hard. I have 18" wheels but they aren't overly heavy and they've been on there for 100k miles without issue up until the last couple months. I replaced the wheel bearings 20k miles or so ago. I'm running in 2WD mostly so the transfer case and 4WD system shouldn't be causing the engine to work extra - although there's no noticeable difference in power or temp issues when in 4WD. What else about these engines fails that causes them to overheat? What else can I check? Has anyone else had this problem with a VQ35?