I have a overheating problem (not bad but enough to raise my concern) and everything has been checked out like hoses, themostat, head gasket (loss of coolant), air in the system and radiator.

What the service person tells me is the mechanical gauge on the dash may be reporting wrongly or the sensor feeding the gauge may be broken.

Now, I want to understand which sensor feeds this gauge. There are 2 sensors right next to the ATF fill tube, which one of them actually is the one giving the reading!!!

 

Not sure about the sensors, but, did you check your waterpump? You didn't say in your post. A water pump going out might make your overheat.

 

he better hope is isnt. the water pump is internal (behind timing chain) and a pain to get to. according to nissan they dont go out very often

 

Forgot to add that, I mean everthing was checked for a fat bill. Radiator, thermostat, heater core, water pump, hoses, coolant loss, head gasket, whole cooling system was pressure checked for leaks, etc.etc.etc. When I mean overheating, the stupid gauge goes up on uphill drives, comes down, randomly goes up, comes down etc. with a mind of its own, and all this with no hard acceleration. I am running out of options to root cause the problem.

 

Did you authorize them to spend a "fat bill" on diagnostics? If you didn't authorize them to spend that much and they didn't find the problem I would raise hell.

What happens if you just sit there @ idle?? After it warms up does it blow out heat @ idle or blow cold??? Also does it seem to fluctuate more when there is an extra load. (going up hills is an added load), or does it seem to fluctuate more over bumps. (going up hills also makes the car point at a different angle)

 

Like 5thgen. said....you got a big bill? Any dealer that can't confirm if the gauge or the transmitter is the problem or not should be reported to Nissan. Did he take both the thermostat and the coolant control valve out and actually test/check them? Are the temp. gauge movements rapid, senseless bounces or.... you seem to indicate that going up a hill (higher load) that the temp. indication rises, but how fast, how high etc.? From a cold start does it rise slowly like it did previously (normally) or what? The sensor is the front one (single wire) of the two and can be simply checked for temp. to resistance readings.

 

OK, this is how it goes. I did see the gauge go high (less than 3/4 mark) "only" during uphill drives under load. I replaced the thermostat and hoses myself while doing a flush. This didn't fix the problem, then I took it to my mechanic (not dealership) and he did everything as explained before. This is after 3 tries to root cause the problem, now he is out of options becoz he is convinced it has nothing to do with the cooling system as a whole and thinks it could be faulty gauge/sensor feeding the gauge. This is where we stand, anyway root causing the problem in the cooling system seems very complicated.

At idle, there is no fluctuation on the gauge or on bumps, it stays below the 1/2 mark. It fluctuates only under load and when the engine is working hard.

 

I'm not sure of the symptoms that an inoperative coolant control valve will give you (never seen it even discussed anywhere) but it has to do something abnormal. Now might be the time to change it. It's quicker and easier than the thermostat.

 

Sounds to me like you are fighting an actual overheating problem since this sensor actually raises when you put it on a load... You can verify this by cranking the heat on high when it starts to go up. (it will probably not go up as fast)

Fighting a non-dealer bill is going to be tough. Especially since you already paid it. I defiently wouldn't go back to this shop. I'm guessing that it's one of those places that "always hook you up and give you deals" most of the time.. I'm still sticking with a coolant problem. HeadGasket maybe (internally leaking so the "tech" at this "shop" didn't see a leak.

during the pressure test did they say it held the pressure indicated on the cap or did they just say that they didn't see a leak? Has your fluid level changed at all recently? Next time it overheats pull over and pop your hood to see if the fluids gurgling over into the overfill. (The level will probably rise.)
Check your oil and see if it looks watered down. If it's diluted the antifreeze/water should drip off the end and look runny.


BTW.. This "shop" should have been able to check the operation of the temp sensor with any scantool in less than 5 minutes. If they can't even do that, I say stay far away.
The reason I say they should have been able to check it is because (every car I have seen, so I'm assuming maximas too) have two temp sensors. The generic obd-II info shows the temp (calculated be the resistance of the sensor) of the sensor that controls everything except the gauge. If the sensor that controls the gauge goes up and down during this hill AND the sensor on the obd-II goes up and down accordingly then that would automatically rule out what they "think" is the problem...Like I said it should have taken them 5 minutes or less to do that.


Most oil change shops carry a coolany pressure tester. (The locally owned ones around here do anyways.)
If you stop by one of these shops on a slow day and ask the manager if you can test your system to see if it holds pressure, they will probably do it for you for $5 or $10 bucks.

 

the sensor sits in the coolant pipe

 

Did you test or replace the coolant control valve? It definitly can affect the cooling capacity/efficiency of the system.

 

Do you know where I could find the coolant control valve on the car?

This weekend I intend to check the resistance and voltage from the coolant temp sensor. The sensor before the coolant temp sensor is flimsy and shaking and on the connector there is some white gluey stuff. Something seems fishy on the first (one wire) sensor...

 

Absolutely make sure the connector/wire is good on/to the temp. sensor. That could explain the problem if it's not a good connection etc. Maybe someone bucksheed something (the white stuff??). If it's not the sensor connection etc., the coolant control valve is in the coolant pipe right behind the temp. sensors in the housing with the system bleeder bolt. It should start to open at 203 deg. F (just when the thermostat is fully open) but you won't be able to check full open (226 deg.). It allows coolant to flow through the engine block under high loads (high coolant temp.)