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Thread: Car running hot

  1. #1
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    Default Car running hot

    I have a 96 civic

    It was a pretty hot day yesterday. I ran the car for about half hour, went into a store and came out as I was idling I noticed the temp gauge going up to about 3/4, not in the red zone. Typically it never goes that high, as I started to drive it came back down. I parked again and sat there, as it hit about 3/4 again the fan kicked on to bring down the temp.

    This morning I added a little coolant and the overfill tank has plenty of coolant, no noticeable leaks under the car.

    Question can it be the thermostat, or if the thermostat is bad will just overheat?

    Any other ideas?

  2. #2
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    Default Re: Car running hot

    It *could* be the thermostat, but that's not always the cause. They usually fail in the open position, but its possible its sticking somewhat or not opening completely. Take out the old thermostat and put it in a pot of water with a thermometer. It should open completely about 192 - 202 (depending on the vehicle, don't know the specs for this car). If not replace it -- they are cheap.

    If that isn't the problem it could be a number of things:

    1. Lean fuel mixture. Will usually set off an engine code but not always.

    2. Failed temp sensor.

    3. Head gasket. Take a look at the coolant. If it has any milky white substance that's a good indicator but not fool-proof. You can get test strips which indicate the presense of exhaust gas output into the coolant.

    4. If its an electric fan, is the fan working? Let it get up to operating temp and as the guage goes up you should here it turn on. I suspect this as the #2 cause if the thermostat isn't the problem.

  3. #3
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    Default Re: Car running hot

    Quote Originally Posted by The Chariot View Post
    It *could* be the thermostat, but that's not always the cause. They usually fail in the open position, but its possible its sticking somewhat or not opening completely. Take out the old thermostat and put it in a pot of water with a thermometer. It should open completely about 192 - 202 (depending on the vehicle, don't know the specs for this car). If not replace it -- they are cheap.

    If that isn't the problem it could be a number of things:

    1. Lean fuel mixture. Will usually set off an engine code but not always.

    2. Failed temp sensor. Know how to test temp sensor, I know there is one going to the thermostat housing, it has a plug lead on it.

    3. Head gasket. Take a look at the coolant. If it has any milky white substance that's a good indicator but not fool-proof. You can get test strips which indicate the presense of exhaust gas output into the coolant. I looked in the rad no milky white and also, no white smoke out of exhaust another sign of head gasket.

    4. If its an electric fan, is the fan working? Let it get up to operating temp and as the guage goes up you should here it turn on. I suspect this as the #2 cause if the thermostat isn't the problem.
    The fan did kick on when I sat in the car idled, but came on about 3/4 way up instead of say 1/2 way, seemed late.

    thanks

  4. #4
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    Default Re: Car running hot

    In many cars this is normal, if the car sits in traffic for a while especially on a hot day, or if you park for a short time on a hot day then restart, the temp will be higher than normal.

    Here's the scoop: When the engine is cold, the thermostat closes, so coolant only circulates through the engine, not through the radiator. When the engine warms up, the thermostat opens, and coolant now starts hitting the radiator, which cools the coolant. The thermostat is designed to maintain a set temperature by opening or closing slightly so that the temp. gauge stays more-or-less in the same spot, under normal circumstances.

    Under certain circumstances, the temperature can rise above normal. For example, when idling for an extended period and not moving (parked with engine running, or stuck in traffic for 10-15 minutes or more), the air around the radiator will get hot. When this happens, the gauge will creep upward until a preset threshold is reached, then the radiator fan will kick on, pulling cooler air over the radiator and causing the gauge to drop down a few clicks, then the fan will shut off. You'll see this cycle repeat over and over on a hot day when idling. This doesn't happen when driving because the air naturally flows when the car is moving, and the fan doesn't have to kick on at all. In many cars the radiator fan is also the A/C fan, so you can test the fan's operation by turning the AC on and see if the fan runs then.

    Also, if the engine is warm and you park the car on a hot day, the heat from the engine will transfer into the coolant which is no longer being circulated. For a period of time, until the engine cools, this coolant will be hotter than the normal temperature the thermostat maintains when the car is running. This will cause a hotter-than-normal reading when you start the car up after a short period of time, but it will drop back down as the coolant circulates and (if needed) the fan runs, or airflow from driving will bring the temperature back to normal.

    In cars with mechanical instead of electric fans, the gauge tends to remain more stable since there's always airflow over the radiator. Also, some cars with electric fans run them continuously, varying the speed as needed instead of cycling them on and off, with the same effect as a mechanical fan, a more stable temperature gauge.

    When you have a problem, is when the gauge reads higher than normal under normal driving conditions. If you're cruising down the highway, or even in town if you're not stopped a lot, or the weather isn't hot and the gauge still reads higher than normal, then you have some problem with the cooling system, such as low coolant, a faulty thermostat. Also, if the gauge ever goes into the red zone, turn the car off! If it goes into the red when idling but not when driving, it's probably the radiator fan isn't working.

    I used to have a Saturn SL2 (similar size/build quality to your Civic) and it did the same thing. When idling for more than 10-15 minutes or so, especially on hotter days, the gauge would creep up toward the 3/4 mark, and just before hitting 3/4, the fan would go on (computer controlled) and run until it was a bit higher than 1/2 way (it dropped fairly quickly with fan running), then it would stop, and repeat as needed. When driving, the temperature would drop back to the level the thermostat would maintain, which was at or slightly below the 1/2 mark. On my Audi A4 I drive now, the gauge stays in the middle no matter what, because this car varies the fan speed instead of just switching it on and off at certain thresholds.
    Last edited by kpatz; 06-03-2010 at 10:24 AM.
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  5. #5
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    Default Re: Car running hot

    Quote Originally Posted by kpatz View Post
    In many cars this is normal, if the car sits in traffic for a while especially on a hot day, or if you park for a short time on a hot day then restart, the temp will be higher than normal.

    Here's the scoop: When the engine is cold, the thermostat closes, so coolant only circulates through the engine, not through the radiator. When the engine warms up, the thermostat opens, and coolant now starts hitting the radiator, which cools the coolant. The thermostat is designed to maintain a set temperature by opening or closing slightly so that the temp. gauge stays more-or-less in the same spot, under normal circumstances.

    Under certain circumstances, the temperature can rise above normal. For example, when idling for an extended period and not moving (parked with engine running, or stuck in traffic for 10-15 minutes or more), the air around the radiator will get hot. When this happens, the gauge will creep upward until a preset threshold is reached, then the radiator fan will kick on, pulling cooler air over the radiator and causing the gauge to drop down a few clicks, then the fan will shut off. You'll see this cycle repeat over and over on a hot day when idling. This doesn't happen when driving because the air naturally flows when the car is moving, and the fan doesn't have to kick on at all. In many cars the radiator fan is also the A/C fan, so you can test the fan's operation by turning the AC on and see if the fan runs then.

    Also, if the engine is warm and you park the car on a hot day, the heat from the engine will transfer into the coolant which is no longer being circulated. For a period of time, until the engine cools, this coolant will be hotter than the normal temperature the thermostat maintains when the car is running. This will cause a hotter-than-normal reading when you start the car up after a short period of time, but it will drop back down as the coolant circulates and (if needed) the fan runs, or airflow from driving will bring the temperature back to normal.

    In cars with mechanical instead of electric fans, the gauge tends to remain more stable since there's always airflow over the radiator. Also, some cars with electric fans run them continuously, varying the speed as needed instead of cycling them on and off, with the same effect as a mechanical fan, a more stable temperature gauge.

    When you have a problem, is when the gauge reads higher than normal under normal driving conditions. If you're cruising down the highway, or even in town if you're not stopped a lot, or the weather isn't hot and the gauge still reads higher than normal, then you have some problem with the cooling system, such as low coolant, a faulty thermostat. Also, if the gauge ever goes into the red zone, turn the car off! If it goes into the red when idling but not when driving, it's probably the radiator fan isn't working.

    I used to have a Saturn SL2 (similar size/build quality to your Civic) and it did the same thing. When idling for more than 10-15 minutes or so, especially on hotter days, the gauge would creep up toward the 3/4 mark, and just before hitting 3/4, the fan would go on (computer controlled) and run until it was a bit higher than 1/2 way (it dropped fairly quickly with fan running), then it would stop, and repeat as needed. When driving, the temperature would drop back to the level the thermostat would maintain, which was at or slightly below the 1/2 mark. On my Audi A4 I drive now, the gauge stays in the middle no matter what, because this car varies the fan speed instead of just switching it on and off at certain thresholds.
    Basically, that is what is doing say parked and idling the fan will kick on, and bring it down, just seemed later than normal.

    I am curious if the thermostat is bad, it will just run hot no matter what?

  6. #6
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    Default Re: Car running hot

    A bad thermostat will generally cause abnormal temperature readings (either too cold or too hot) whether the car is moving or not. If the gauge stays steady in the middle during normal driving conditions, the 'stat is likely ok. If it's sticky the gauge could go higher than normal as the car warms up and then drop down when the 'stat unsticks, or the car would take a really long time to warm up if it's stuck open.

    The point at which the fan kicks on could be controlled by another sensor that could be bad, if the gauge gets uncomfortably high before the fan kicks on. But it could also just be controlled by the ECU, like it was in my Saturn. When idling in hot weather, I could see the gauge move up in increments, and when it hit a specific increment the fan went on, and after it dropped down to a certain increment it would shut off, so the ECU controlled the fan as well as the gauge, using the same sensor. Some cars have more than one coolant temperature sensor; one may be used by the ECU to control fuel mixture while another controls the gauge and/or fan.
    If I'm passing you on the right, YOU are in the wrong lane!

    If speed kills, how come I'm still alive?

    Active Countermeasures: V1 3.858, Escort Redline, Beltronics STi-R+, LI Dual 7.1x CPU/8.7 Heads (front)
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  7. #7
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    Default Re: Car running hot

    In a car that age it is most likely the radiator is clogged from years of use. This is normal. Having it reconditioned by a radiator shop will make it cool like new. It may be cheaper to just buy a new radiator if it cant cut it in the heat this summer. I drive a 94 Nissan Pick up and have priced a New radiator at $100 bucks.
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  8. #8
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    Default Re: Car running hot

    Flushing the system and putting new coolant in might help too.

    But I think it's just the way the fan cycles. Many cars from that time frame did the same thing even when new. When idling, there's no airflow to remove the hot air around the radiator, so it gets hot until the fan kicks on or you drive and cause airflow. I think newer cars cycle the fan on/off more quickly so the gauge doesn't move as much. I've seen cars idle for a while and I can hear the fan cycle on, run for a few seconds (they tend to be loud), then stop, and then do the same thing a minute later.
    Last edited by kpatz; 06-03-2010 at 11:41 AM.
    If I'm passing you on the right, YOU are in the wrong lane!

    If speed kills, how come I'm still alive?

    Active Countermeasures: V1 3.858, Escort Redline, Beltronics STi-R+, LI Dual 7.1x CPU/8.7 Heads (front)
    Other/Backup Countermeasures: V1 3.813 (loaned to friend), Beltronics Pro RX65 M4 6.3
    Vehicle: 2002 Audi A4 1.8T Quattro
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  9. #9
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    Default Re: Car running hot

    Quote Originally Posted by Freebird View Post
    In a car that age it is most likely the radiator is clogged from years of use. This is normal. Having it reconditioned by a radiator shop will make it cool like new. It may be cheaper to just buy a new radiator if it cant cut it in the heat this summer. I drive a 94 Nissan Pick up and have priced a New radiator at $100 bucks.
    I priced a rad at advanced it was like $94 bucks.

  10. #10
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    Default Re: Car running hot

    Quote Originally Posted by focalcivic View Post
    Quote Originally Posted by Freebird View Post
    In a car that age it is most likely the radiator is clogged from years of use. This is normal. Having it reconditioned by a radiator shop will make it cool like new. It may be cheaper to just buy a new radiator if it cant cut it in the heat this summer. I drive a 94 Nissan Pick up and have priced a New radiator at $100 bucks.
    I priced a rad at advanced it was like $94 bucks.
    I would go with a new radiator and install a new thermostat while you have it apart.
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