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Why is my truck overheating?


schuss
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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?

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Did you check/replace the second thermostat? It's located under the intake manifold in the rear of the engine (it's a pain to get to), If I remember correctly, Nissan calls it a water control valve and allows coolant to flow through the block.

 

Does it overheat at idle?

 

I'm wondering if you have a blockage or collapsed radiator hose.

Edited by mel.d
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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.

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The water control valve is just another thermostat. If it fails in the closed position, then no coolant will flow through the block/cylinder walls. However you will still get coolant flowing through the heads since the first thermostat is working properly. The heater core pulls coolant from the heads not the block.

In reviewing the cooling circuit in the repair manual, If in fact the water control valve is the root-cause, then your cooling system is probably only running at 70%.

 

Since the water control valve is a pain to get to, I made sure I replaced mine with one that is designed to fail in the open state.

 

 

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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.

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Factory thermostats do not fail open.

 

It's been over 4 years since I replaced the water control valve, so I don't remember the brand I bought. Since your original water control valve lasted this long, just buy OEM and you'll be good-to-go for another 14 years.

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  • 2 weeks later...

check your reservoir first if clog.

 



Last year my 02 Nissan Pathfinder 3.5 with 135+ starting overheating. In my investigation i have notice i am loosing coolant in my radiator but my reservoir is in the right amount level. After one week due to a very cold weather seems my overheating issue was settled. Until the month of February started overheating until my engine start to stall. Following day went to Auto zone store and order my water pump and 2 thermostat. After replacing all the parts and took for a spin about 30 minutes and so, i began to smell a anti-freeze coolant but no where to trace. For about 5 months every time before driving constantly checking my radiator to refill. My research never stop until i was curious about my reservoir that never emptied at all when ever i loose coolant in my radiator. In my curiosity i decided to pull the black hose from the radiator and decided to blow an air. :lmao: guess what ? my reservoir was clogged with a form of a gel. Don't ask me how and what or where? After taking apart and cleaned the ability to flow back and reserved the coolant goes out well..For all my fellow member in case you experience overheating, check your reservoir before anything else.

Edited by RedPath88
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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. :)

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

 

I just replaced the fan clutch and fan on my overheating BMW and it did the trick. The fan clutch was stuck fully engaged so that caused the fan to spin at the same speed as the engine. Loud and dangerous. The fan clutch is filled with hydraulic fluid and a valve (and some other black magic) that prevents it from overspinning to the point where the fan blades start shooting off through your fan shroud and hood.

 

As the engine warms up ---> too high of a temp. the internal valves react to the temp change (rising above a preset limit) and cause the clutch to engage, which spins up the fan to higher RPMs. This is what helps cool your engine by drawing more air through the radiator.

 

Here is a dangerous test:

 

Try spinning the fan blades by hand with the engine switched off. You should be able to spin it very easily. If the fan is locked up or gummy feeling, it's probably bad. Also, if it spins like its attached to a $200 bearing, it's also bad. There should be a slight bit of resistance, but not too much.

 

A 100% functional fan clutch should spin relatively freely at idle. You can fire up the engine, and after the initial engagement the fan clutch should disengage. Now you should be able to take a folded up piece of cardboard and lightly make contact with the tips/edges of the rotating fan blades to the point where after a few seconds you'll be able to stop the fan from spinning completely.

 

At this point, if I was you, I'd replace the fan clutch anyway, and measure the diameter of the fan itself to make sure you have the correct part. An undersized cooling fan will have too much gap around the tips of the blades and the edge of the fan shroud... meaning that you're not getting efficient airflow ---> cooling.

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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.

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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.

 

How long has it been overheating?

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