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No heat and temperature gauge going up and down

I have a 2002 Nissan Sentra. In Nov. 2010 had inspection done and replaced upper radiator hose at that time. Right after that, I had no heat and the temperature gauge would go up to hot but then come right back down. Brought it back to mechanic, changed thermostat and both things ok only part way home. Started riding with coworker only driving to her house 7 miles away and still temperature gauge up and down, stopped driving completely in Feb. Just brought it to dealer who replaced thermostat and worked fine all the way home and then started all over again the next day. Seems to happen faster at higher speed but will eventually do it in any driving conditions. Today service engine soon light came on. Obviously not thermostat problem. Could it be water pump? Dealer did pressure test and that came back fine so no head gasket problem although don't have a lot of faith in that since problem still exists and they thought thermostat would fix it. Don't want to keep spending money on possible problems, only actual ones. Any help would be greatly appreciated.
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Comments

  • edited May 2011
    Have you personally checked the coolant level? Not just the overflow bottle but the radiator also. You have air in the cooling system. I am not familiar with your Nissan engine but I have seen your exact symptoms in other "known to be difficult to burp" cooling systems. It needs to be filled properly and "burped" if it has an air pocket in the system. On some cars this involves removing a plug in the block until coolant flows out. Again I am not sure if Nissans have a big issue w/this.

    How does the coolant look? Another cause for air in the cooling system can be a head gasket issue, combustion gasses getting into the cooling system. Shops can test the coolant for this but you said that a pressure check was done and ok. It is still possible that a head gasket leak into the coolant may not be found by a cooling system pressure check but if done carefully it will usually tell. You don't mention if the engine is running any different, is it? The check engine light makes me suspicious but we need the code. A lot of chain autoparts stores will pull the code for you for free (AutoZone, Advance Auto etc.) Get the code read and report back.
  • I agree. This sounds like an air pocket in the cooling system. You'd think, however, that a Nissan dealer would be able to figure this out and correctly bleed the air from the system.

    Have you checked the coolant level in the radiator? If not, you need to do so right away. When the engine is COLD, the radiator should be completely full.
  • Something to try:
    Park on a steep incline, pointed up.
    Let the engine cool.
    Fill the radiator.
  • All good suggestions, but also try replacing the thermostat. Twice in one night for me with the same suggestion, but if it barks, it's a dog.
  • kizwiki - thermostat has been replaced twice at this point. I could believe one bad thermostat but 2 with different mechanics installing them. Possible but not likely.

    circuitsmith - how long would i have to let engine cool? Just trying to think of where i could go near me with a incline that wouldn't have to sit for hours unattended.

    Checked the radiator this morning, not full, added antifreeze, service engine light went off. Also regarding head gasket, I have checked oil and tailpipe (on advice from others) and oil is clean (no foam or milky color) and no stickiness on tailpipe to indicate a leak anywhere.
  • edited May 2011
    One thing you could try is to disconnect the top heater core hose, and fill the heater core. Then, using the highest point you have on the cooling system, loosen the hose clamp, and start it (the motor). Keep your eye on the loosened clamp, and the area around it for antifreeze to start bleeding through. Once the air has escaped, tighten the clamp.

    I had a 3-series BMW that had the water jacket through the intake, and the hose leaving the intake had to be loosened to get the air all out. It was a PITA, but it worked. had a Renault 5 that was similar, but it had a tiny stopcock buried back there I had to loosen. Took me hours to find it.

    Good luck!
    Chase
  • "how long would i have to let engine cool?"

    Cool enough for safety.

    You could make some makeshift ramps out of lumber etc. since you won't be getting under the car.
  • Coolant level...coolant level...coolant level. Head Gasket...Head Gasket.
  • I suggest you have the coolant tested to see if exhaust gases are getting into it.
  • And the likelihood of the head gasket failing on the same day the radiator hose was replaced?

    The OP hasn't posted back, which tells me he got the air purged out of his cooling system and moved on.
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