# HELP PLEASE! 91 sentra se-r overheats



## whirl (Apr 9, 2009)

I have a 91 sentra se-r in las vegas (not hot yet, about 70 degrees F).

A couple of days ago I noticed, when the car is idling (or stuck in traffic) the engine temperature gauge goes up all the way hot. So either I have to start driving to get air flow, or turn off the car. Sometimes the water is boiling out the overflow.

I noticed, that my engine fans don't turn on, so I start playing w/ the wires (moving them around to see if they get good connections). Now, the engine fans DO turn on, but only when the temperature gauge is almost at the top near the H. The fans will continue to spin until the gauge is 1/2 way in normal temperature. Problem is fans will only turn on again once the gauge is all the way near H.

So what do you think can be the problem? What options are there? Could it be the temperature sensor? Or could it be the thermostat stuck open that doesn't allow water to cool in the radiator enough? Does the thermostat in the SR engine affect the temperature sensor on when to kick in? I don't think thermostat stuck closed because it takes a while to warm up and cools pretty easily once I'm driving. 
Mechanic thinks it's loose wiring leading up to the relay box, but I'm doubting that. Or could it be bad wiring from the sensor? Although it does consistently kick in once the gauge in the dash hits near the H.

Please help, as I am leaving town, and want to fix it so my family members who need the car can drive it.
Thanks so much.
J


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## DannyNissan (Jun 3, 2008)

Upon re-reading it..

the first thing you want to do is check your fuid level,
make sure it's full, with no air pockets.

You want to remove the cap when cold, start the car up
add water, let the car warm up and the level should drop
as the thermostat opens up.. Then add more water.

I say water, but you would want a proper mixture of anti-freeze
and coolant. Usually 50/50 is recommended.

If you find that filling up with water doesn't solve the problem,
then maybe you have other problems...

Maybe the radiator cap is old where its' got a lower point at which
it will release pressure, thus causign you to go low on coolant.

Or the fan switch thermostat is bad, i'm not sure when it should kick on
but if it's kicking on at H and off midway sounds like it might be working right.

SO water level first, re-diagnose... then change the cap,.. re-diagnose..
then fan switch.. would be what'd i'd do.


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## whirl (Apr 9, 2009)

Thanks for the quick response danny. I dont think it's a water/ air pocket issue because the car runs fine when moving. Only when the car is idling does the temperature start to overheat. This is usually solved by the fans kicking in. However, it used to kick in when temperature is towards the middle of the temperature gauge keeping it that way. Now it only kicks in when the gauge shows the car is overheating.

So the problem is why are the fans kicking in only after the car is on the brink of overheating?


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## sonicgundam (Aug 15, 2007)

fans should kick in on their own when the temp sensor reads a bit of an increase in coolant temp. my gauge wont even move and my fans come on. i can hear them. especially during idle. they go on for about 15 seconds, then off, then on again another 30 seconds later.

i would say this could be a sticky thermostat, that is opening only when the coolant is way too hot, or its a fan issue. the temp sensor is probably alright, seeing as you're getting gauge readings.


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## whirl (Apr 9, 2009)

hey sonic, thanks for your response: yea my fans kick in only when the gauge is all the way near the top.
If it was a stuck thermostat, wouldn't the car overheat even when the car is moving? I'm guessing that the thermostat isnt stuck closed because, when the car is moving the airflow cools the radiator and goes in the engine. 
Whereas if the themostat was stuck it wouldnt get any cool radiator water. Also IF (and that seems to be the problem) the fans do kick in, the radiator water cools, and goes in the engine. That wouldn't happen if the thermostat was stuck.

By reading on other posts, the 91 ser has 2 coolant temperature gauges. One for the dashboard, and one for the ECU which controls the fans. So maybe the dashboard one works, but the ECU one doesn't?

Of course I'm open to suggestions. should i change the thermostat?


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## DannyNissan (Jun 3, 2008)

You said water is boiling out the over flow.. so it's possible you do have
an air pocket.

You always want to check the cheap, free and easy things first.
(unless you like wasting money replacing non broken parts
and doing non necessary work)

Check your radiator to make sure it's full before you 
start thinking something is broken. 

Fill it with water, drive it, and see if your problem is solved.

If it's solved and If it was low on water then either it's a random occurance, or maybe you do need a new cap after all. 

If it' not low on water, then yes, you need the fanswitch-thermostat.


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## whirl (Apr 9, 2009)

Yes Danny, I totally agree w/ you, check the free, easy, things first. Yes, the radiator isn't leaking and is full of water, so that seems to be fine.
Even if it was an air pocket, I don't think it will solve the problem of why the fans don't turn on. However, Im taking your advice and bought a radiator cap (low cost insurance). Tomorrow I"ll try to replace the temperature sensor $18 and see what happens.


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## DannyNissan (Jun 3, 2008)

Well if you checked the water then that shoudl do it.. but if you hadn't checked
the water i would have said:

OR it coudl be the gauge sensor is sitting on an air pocket and reading hotter than
what it really is.. because it's low on fluid... and thus you'd be wasting your
money switching out good parts.. because it's low.

but it's probably the $18 part you're buying


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## sonicgundam (Aug 15, 2007)

i don't know if i'd bother with the temp sensor or not because the gauge cluster on b13's is run by the ecu. that being said, the fanswitch is also probably run by the ecu. because you're getting a gauge reading from the temp sensor, its probably fine.

as for water boiling in the overflow, i'd expect that. b12 overflows, and i'm assuming b13 ones as well, are not fully sealed. old cars didn't have expansion tanks like new cars. with the coolant expanded, it just dripped onto the pavement. our systems are sort of like this in the fact that we do have an expansion tank, but it also, if too full, will drip out of an exit hole in the tank. therefore they aren't sealed. the pressurised coolant that the rad cap allows to pass into the expansion tank hits atmosphere and starts to boil.

as for the air pocket around the temp sensor, its still possible, but a temp sensor should read cold if in an air pocket. they don't read well, if at all, unless submerged in coolant.


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## whirl (Apr 9, 2009)

Danny, just playing devil's advocate here: if the air pocket showed hotter than it actually was.... wouldn't that trigger the fan's to turn on quicker? :fluffy: haha im just teasin'


Sonic, I just checked the manual. There are 2 temp sensors. One goes straight to the dash and nothing else. The 2nd runs to the ECU, which controls the fans. So what you see on the dash, may not match what the ECU sees. I'm not sure about all b13s, but the SR engine does have 2 seperate temp sensors. if the water is boiling out the overflow, and the gauge doesnt show overheating, i wouldn't trust the gauge 

so tmw, im going to replace the temp gauge that goes to the ECU, since the one that goes to the dash matches my overheating experience. hope that fixes it.
thanks guys


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## sonicgundam (Aug 15, 2007)

ok good stuff . didn't know about the second sensor, now i do. that could quite possibly be it, whether its covered in crap, or on the way out.


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## whirl (Apr 9, 2009)

Ok, I replaced the radiator cap AND jiggled the temperature sensor wire yesterday, and guess what? The fans turn ON. What a relief, but wait a second. Now, the fans are ALWAYS on. Even if the car has been turned off overnight, first thing I do is lift the hood, start the car, and the fans are on, even when engine is cold.
Today I changed the temperature coolant sensor, and fans are STILL always on. So now instead of overheating, the radiator is going to be cold all the time.
So I'm not sure if jiggling the wire did it, or the radiator cap? But what would cause the fans to always be on now?


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