2003 Honda Civic Si Coupe Temp Gauge problem

Tiny
CURTISMARTELL
  • MEMBER
  • 2003 HONDA CIVIC
Hey. I own a 2003 Honda Civic Si Coupe with approx 130,000 kms on it. It works fine when the car is driving, but if I come to a stop, and but the car in park and let it run idle for 5 minutes or so, the temperature gauge inside the car starts to creep down towards the "C" mark. As soon as I put it back in drive and get going, the temperature will slowly rise back to about the normal midway point. I checked the antifreeze and it is fine, and not leaking but I don't know what could be causing this problem. Please help. As well, the heater seems to work fine, hot air coming out. Any help is appreciated, thanks
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Sunday, December 17th, 2006 AT 5:05 PM

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Tiny
INTEGRAGUY29
  • MEMBER
This sounds an awful lot like a partially broken thermostat and due to it being a reltively new car it is most likely

The thermostat reroutes coolant to the radiator when the engine reaches operating temperature (heats up). And closes as it begins cooling down to keep it there, it is constantly cycling.

They are designed to fail open, IE if they wear out they stick open and always send coolant to the radiator (the alternative being catastrophic overheating). They are very simple devices, a piston filled with wax that melts as it heats up pushing it open.

Typically this means car never wont ever reach operating temp unless your really putting your foot into it, however it could cause this to if its only partially failed.

Luckily thermostats are cheap and easy to replace, thats the first thing to change. Thermostat + gasket roughly 10 dollars, labor probably around 50 bucks, I do them on integras in about 10 minutes.

Other possibilities include a clogged radiator or a leaking head gasket. You dont want a leaking head gakest.
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Sunday, December 17th, 2006 AT 5:14 PM
Tiny
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Thanks for the response, I hope it's not a leaking head gasket. Believe it or not, I replaced the head gasket roughly about 6 months ago, maybe less. I got it done at the Honda dealer where I bought the car. They replaced the head gasket, and planed the head. They also replaced the thermostat housing gasket. I had taken it to my local mecahnic a few days ago and he put it on the computer and told me it was the temp sensor. So I replaced the temp sensor, but it is still causing the same problem. Do you think it could be as simple as replacing the thermostat?
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Sunday, December 17th, 2006 AT 5:18 PM
Tiny
INTEGRAGUY29
  • MEMBER
Cooling problems can be confusing to diagnose, but I can almost guarantee this is failed thermostat and they're so cheap its the first thing to change. Nothing else im aware of causes engine overcooling.

Scenario works like this, at idle the thermostat should be closed most the times since little heat is generated. But its stuck partly open so the radiator cools the engine below operating temp.

When you start cruising much more heat is generated and the temperature rises. The thermostat most likely is still able to open fully so the car does not overheat, but again it is worn and does not fully close, so next time you stop the engine is overcooled again.

Take solace, its in fact very much extremely unlikely that its the headgasket, they dont cause overcooling, just overheating, im currently doing a headgasket job and it popped into my head.
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Sunday, December 17th, 2006 AT 5:32 PM
Tiny
CURTISMARTELL
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Thanks again for the quick response. I will get that checked tomorrow, do you have any other ideas you might be able to help me with in case that is not the problem? Could it be something electric, or maybe an air pocket? Any ideas would help, but I will certainly try the thermostat first as it sounds like you have a good idea what you are talking about, thanks.
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Sunday, December 17th, 2006 AT 5:35 PM
Tiny
INTEGRAGUY29
  • MEMBER
Those things cause overheating, only thing im aware of that causes an engine to not reach operating temp is a failed thermostat.
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Sunday, December 17th, 2006 AT 5:37 PM
Tiny
CURTISMARTELL
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But it does reach operating temperature while I am driving, it stays in the normal mid point range, and everything is fine, but as soon as I for example park in someones driveway and wait for them to come out, that is when the temperature starts to go down. Then when I leave and start driving again, it will go back to normal. Really think it could be the thermostat?
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Sunday, December 17th, 2006 AT 5:39 PM
Tiny
INTEGRAGUY29
  • MEMBER
Yes. Really really!
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Sunday, December 17th, 2006 AT 5:49 PM
Tiny
CURTISMARTELL
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Ok, thanks for all the responses. The only thing I was wonering, and correct me if I am wrong, but if the thermostat was stuck open, wouldn't that mean the car would not heat up to the normal driving temperature? I know it takes alot longer to heat up, but it does get to the normal operating temp. Also, while the car would be cooling off, should the fan come on or not come on?
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Sunday, December 17th, 2006 AT 6:09 PM
Tiny
CURTISMARTELL
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While the car is idling, and the temperature starts to go down, the fan comes on, but temp still continues to drop. Is that normal?
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Sunday, December 17th, 2006 AT 8:55 PM
Tiny
BRUCE HUNT
  • EXPERT
Stop procrastinating. The explanation of a thermostat not fully closing is a sound one and very logic. Do the work yourself it is easy.
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Monday, December 18th, 2006 AT 8:09 AM
Tiny
CURTISMARTELL
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Hey, thanks for the responses everyone. Sorry about being so obnoxious, I just worry alot. I have had a lot of different cars over the years, and a lot of probelms. I always think the worst. The thermostat and gasket will be in friday, so I will get it put in and will repost and update the situation. Thanks.
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Monday, December 18th, 2006 AT 2:58 PM
Tiny
CURTISMARTELL
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Well, got back yesterday from replacing the thermostat at the garage, and no improvement. Car takes extremely long to heat up, and when I am driving, it is fine, but when I put it in park and let it run idle, temp starts to go way down and it blows out colder air. Thermostat I got was from the honda dealer. Any other ideas anyone?
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Friday, December 22nd, 2006 AT 3:53 PM

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