Heater only works when RPMs above 3000

Tiny
AGENT6.2
  • MEMBER
  • 2001 NISSAN SENTRA
  • 4 CYL
  • FWD
  • MANUAL
  • 120 MILES
2001 Nissan Sentra - heater only works when RPMs are above 3000, like when I'm cruising down the highway at 60mph or more. Only cold air when driving through town or on country roads. Also, coolant not cycling properly between radiator and reserve - when driving home yesterday the engine ran very hot. After shutting the car off I noticed the coolant reservoir was full. I checked later that night and reservoir was back down and radiator was full. Car also leaks some coolant near cap (which doesn't seal as tightly as in other cars I've driven). Any ideas? Thanks!
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Wednesday, December 8th, 2010 AT 3:30 PM

10 Replies

Tiny
JACOBANDNICKOLAS
  • EXPERT
First, replace the rad cap so it seals. As far as the coolant, the overflow raises if the engine is hot and lowers when it cools, so that is normal. Have you ever replaced the thermostat? Have you ever tried flushing the system?
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Wednesday, December 8th, 2010 AT 3:33 PM
Tiny
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Thanks for the quick reply. The rad cap seals, I guess, it just doesn't seem as tight as I'd expect. Rad caps I'm used to have to be pushed down and twisted to seal, this one doesn't push down. Perhaps there is a problem? My first thought was the thermostat so I plan to replace that soon. I just bought the car and don't know if the system has been flushed. Thanks again!
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Wednesday, December 8th, 2010 AT 5:36 PM
Tiny
JACOBANDNICKOLAS
  • EXPERT
It never hurts to replace the thermostat. They are cheap. However, I don't feel that is the problem with not getting heat until 3K RPM's. Usually, the coolant level is low, the heater core is partially plugged. I have a question. With the engine at normal operating temp, running and the heater on high, do both heater core hoses get hot? Let me know. If the T-stat and flush doesn't help, my next concern (and I hate to say it) is a head gasket issue.

Let me know what you find.

Joe
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Thursday, December 9th, 2010 AT 12:16 AM
Tiny
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Thanks for the advice Joe. I greatly appreciate it. I'm getting the radiator flushed and thermostat changed today, and will check out the heater core hoses this weekend. I'll let you know. The heater actually worked for a little while this AM while traveling into work. I have a 10 mile commute on country roads and another mile on the highway; the heater usually only works when I'm on the highway. Today it started working after a mile or so into the drive then went cold 7 miles later. Weird. Thanks again!
- Dave
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Thursday, December 9th, 2010 AT 12:33 PM
Tiny
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Let me know what you find.
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Thursday, December 9th, 2010 AT 4:51 PM
Tiny
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After the radiator flush, cap replacement, and thermostat replacement yesterday my car was heating wonderfully. For a short while. On my way home the same problems came back: no heat and the engine was dangerously hot. I popped the hood and found the coolant reservoir overflowing. Major bummer. This morning I let the car heat up then did as you asked and checked the hoses going to the heater core. Only one was hot.
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Friday, December 10th, 2010 AT 6:47 PM
Tiny
JACOBANDNICKOLAS
  • EXPERT
It sounds like the heater core is plugging or there is an air blockage. What you can try is this. With the engine cold, loosten the return line from the heater core and start the engine. See if any air pockets come out. If that helps. I have to be honest. At this poing I'm questioning a head gasket.
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Monday, December 13th, 2010 AT 5:12 AM
Tiny
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My neighbor is a retired mechanic so I'll ask him to take a look at it. I'll let you know how it goes. Man, I hope its not a head gasket. Thanks so much for all of your help.

-Dave
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Monday, December 13th, 2010 AT 2:13 PM
Tiny
JACOBANDNICKOLAS
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I'll keep my fingers crossed. Let me know what you find.
Joe
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Monday, December 13th, 2010 AT 6:29 PM
Tiny
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Hey Joe. Sorry for the very slow follow up on this topic. The sentra is currently inoperable in my driveway. I took it to a garage a month or so ago and they thought it was the head gasket, but couldn't say for sure without further tests. They also told me that to repair it they would have to take out the converter which is so badly rusted that I should replace it anyway, but since there was so much rust underneath they thought I should consider getting another car. I continued to drive it slow to keep it from overheating but soon after my visit to that garage it lost much of its power and started sputtering. I'm assuming its the converter. My retired-mechanic neighbor isn't convinced the problem is with the head gasket so I'll probably pay a garage to run the necessary tests, and fix everything. I can get a few of the parts I need used and determined that the cost of the parts and labor will be high, but when added to what I paid for it will bring the total cost to the blue book value so I think it will be worth it. Anyway, just thought I'd let you know. Thanks again for your input. Cheers! -Dave
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Friday, February 4th, 2011 AT 3:36 PM

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