The Nissan Path

I bought my 2001 3.5l a few months ago. After a little while I noticed the cabin heat was very weak to non-existent--seemed to fluctuate a bit. Later I was moving out to a new location and so was driving over high elevations with the car loaded and the a/c on. Under those conditions I noticed the car started to run warm. It never got above the high mark on the meter, but would fluctuate between that point and normal. Luckily this was right at the end of the drive, so I was able not to stop, let it cool off, and limp into my destination without it running that hot for an extended period.

Over the next couple of months, I used the car for normal around the town driving and few longer trips. Most of the time it ran at normal temperature, but every now and again it would exhibit this same behavior of running warmer than normal but not really overheating. I tried making sure the system was bled multiple times and replaced the radiator, neither of which did anything.

Last week I finally got around to replacing the thermostat. Lo and behold, when I took the housing off, there was no thermostat in there. Someone had broken the thing out of the housing and then reinstalled the latter, no doubt to cover up whatever the real problem is. I installed the new thermostat, bled the system, including running it at idle for 20 minutes or so until it got up to normal operating temperature. Then, I went for a drive. As soon as the engine was spinning at more than 2k rpm consistently, the temperature quickly pegged all the way to the top of the scale and stayed there. So the car is now undriveable.

So then I took it into a mechanic. He ran through the bleeding process again and got the same result (no improvement). He tested the coolant for exhaust fumes and found none. He also checked to make sure the water pump was circulating coolant, which it is. The car will run indefinitely at idle and stay at normal temperature, but as soon as you actually drive it, it overheats. The mechanic is now stumped and I have the car back.

I'm about to replace the thermostat again and also the fan clutch with OEM parts for good measure, but I have a feeling these aren't the real problem, and I don't know where to go from here. Does this ring any bells for anyone?

Any advice greatly appreciated.

Thanks,

Aaron

This engine has a trick to remove all the air from the cooling system... You cannot do it while the truck is sitting level because the coolant filling cap is not the highest point on the system.

In order to bleed it properly, the best method I always use is to put the truck on ramps (only the front wheels). This will bring the front of truck at an angle and make your filler cap the highest point.

open the cap, start the truck, let the bubbles come out and fill as needed... when no more bubbles and the level does not go down anymore, screw the cap and see if this fixes it...

boys over at thenewx have a nice walkthrough.

http://www.thenewx.org/forum/showthread ... ir+bubbles

Thanks for the tips. I just tried the first method. There were a few small bubbles for the first 5 or 10 minutes. After the truck was up to normal temperature and the bubbles disappeared I replaced the cap. During the bleeding I had the cabin heat running, but since there was no pressure in the system to push the coolant into the heater core there was no heat. After replacing the cap, still no heat. Revving to 2500 rpm, the heat came on stronger and stronger, and the engine maintained normal operating temperature. Slowing the engine back to idle resulted in the cabin heat getting cooler again.

I let the truck down off the stands and drove around the neighborhood a bit in low gear to keep the revs up around 3k (since this was the range where the engine usually started to overheat). All good at lower speeds. Then I got on the freeway, up to about 70, and basically as soon as I got up to speed the temperature pegged to the top again. Cabin heat had disappeared at this point. The lower radiator hose is cool (since the coolant is just coming out the radiator at that point, I guess that makes sense).

I'm fairly confident that the system has been bled properly. But is it possible that air is getting introduced into the system at some point? Could the water control valve be stuck partially open?

two things;
1) there is a rubber cap in the middle of the firewall, just behind the intake, has a small clamp on it. You need to remove that to bleed the air out.

2) thats not your prob. You likely have a leak at the back of the engine on the tube the connects between the heads. Check for a coolant stain on the passenger side (usually) of the trans. Pressure test it and look from below. You need to tilt the engine and replace the gaskets.

Thermostat mounted, with up up? And out out? That little rivet is a vent. The stat should just make the motor run too cold and no cabin heat in the winter. If it is missing, it would not make the motor get too hot. The other person left it out because they couldn't fix it either. Water pump/belt. If you are getting toward time for a timing belt, I'd go in for the pump, new stat and belt and do anything you need to vent it. You are getting too little cooling. The fluctuations are when ambient temp and or load are changing the need. A pocket of air will stop water travel, same as in house heating. Maybe flush fittings with hose? That is done on houses.