2003 Nissan Altima 2.5L Thermostat Installation


I've heard it said that we learn better from mistakes than from anything else. Maybe not faster, but better. And it's true -- you'd be surprised what you'll learn after trying to put 200 horsepower's worth of nitrous through a 175-horsepower Nissan engine. For instance, you'll learn how quickly a car will overheat when you're running said nitrous on an engine with a busted thermostat. Then you'll learn a little something about Nissan's odd ideas about controlling coolant flow.

Replacing the Thermostat

  • Most thermostat changes start out about the same in terms of finding the thermostat. Trace the upper radiator hose, and it's usually right there in the housing bolted to the block. This one was no different. The upper radiator hose leads to the housing on the block, readily visible from the driver side of the car. After draining the system by opening the radiator petcock, I disconnected the large radiator hose, the medium-sized heater core hose and the throttle control actuator plug from the top of the housing. A couple of bolts later, and the housing was off the car. With the old thermostat pulled out and the gasket scraped off, I dropped the new thermostat in -- with a new O-ring sandwiched between the new gasket and the block. The book listed 16 to 20 foot-pounds of torque for the housing bolts, and the hoses went on just as they came off.

    After refilling and bleeding the system, and checking for leaks, I made the mistake of feeling pretty good about myself; a feeling that persisted until I took the car out for a test drive and watched the temperature needle slowly rise to the meltdown zone. Uh-oh. A thousand thoughts raced through my brain, most of which involved major failure relating to the irresistible use of nitrous oxide.

    I parked the car and broke out my repair book. I flipped through it, looking for answers as though it had a chapter on engine breakage resulting from stupidity. Well, it didn't have that, but on the page after the torque specs, it did have an exploded diagram of the procedure. And that diagram showed -- a second thermostat. After parking and popping the hood, I looked where the book directed, under the exhaust heat shield on the front of the engine block. Sure enough, there was a second thermostat housing hiding under the exhaust manifold.

    The second housing had two bolts holding it to the block. After draining the radiator -- again -- and burning my hand on the exhaust manifold, I got the second housing off, replaced the thermostat and gasket, and had it bolted back on. It's bolts took the same 16 to 20 foot-pounds. With the cooling system refilled and bled, the car finally ran cool. Which tends to happen when you replace one apparently perfectly good thermostat, and one broken one.

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