Hey there all... went to have the oil changed on my 95 16V sidekick... It has been sitting for about 4 months if that matters.  Anyway, got to the oil change place after running to O'Reilly to get oil and Lucas Additive and filter.  I pulled it into the bay and it was leaking coolant pretty bad.  I checked the water pump location and saw nothing. I could not tell where it was coming from.  The gauge did not show hot.  We took the cap off and it was low.  There was coolant everywhere.  SO we cleaned it off and started it until it started leaking again.  Still not showing hot... I checked the sending unit wire and connection and no change.  THe gauge just goes half way up.  We thought it may be coming from the cap.  The radiator is only a year old.  So the guy filled it to the brim with coolant replaced the cap with a new one and it stopped leaking.  It ran as we sat there and talked then started leaking like never before from the overflow. 

I am stumped.  I drove it 5 miles home and by the time we got home it was not leaking and the gauge is still at half way. However, the engine seemed hot just from the heat coming off of it.  I am wondering what would cause it to overheat or is it overheating?  Any ideas???

There must be a leak somewhere, it should be pressure tested. The pucking out of the over flow and not showing hot on the gauge could be caused by and air lock. If the coolant was low and you topped it up that could be your problem.  I like to run it with the rad cap off until I am certain the thermostat has opened. Then I top up one last time an pop the cap back on. Another thing could be your cap itself. If it will not hold pressure then it will cause it to boil over prematurely. Or it may just be overheating and the gauge or sensor is reading wrong!

thanks!  how hard is it to replace the thermostat?  I put a new sending sensor in... I also got a new cap... ran it in the driveway for 30 min and it did not boil.  could be okay? 

One other question is... there is a fan in front of the radiator, right?  When should that kick on?  It does not seem to be working properly.  is that an easy fix? 

    Follow the top radiator to the engine,where it bolts to the water neck.Drain some fluid out od the radiator to minimize mess.Remove the 2 bolts on the water neck.and the thermostat is right there,can't miss it.Make sure none of the old gasket gets into the engine when cleaning it off..\
  The electric fan should kick on when the air conditioning is on.

a pressure test is a real good idea.
it can help find small leaks that are tucked away in funny places(heater or bypass hoses) and confirm when you have repaired it.
perhaps your local parts place has one they will loan.

   You might want to have it flushed also.Could be some rust or debri causing a blockage.

thanks!  how hard is it to replace the thermostat?  I put a new sending sensor in... I also got a new cap... ran it in the driveway for 30 min and it did not boil.  could be okay? 

One other question is... there is a fan in front of the radiator, right?  When should that kick on?  It does not seem to be working properly.  is that an easy fix? 

As mentioned, the electric fan will come on when the AC is on.....and ONLY when the AC is on, not if the engine is running hot/boiling the water/etc.....it is strictly an AC only type fan.

When you replace the t-stat you use EITHER the rubber gasket the wraps around the edge of the thermostat OR the paper gasket between the water neck and the block, not both.  If you use the rubber gasket around the t-stat, I recommend putting some RTV between the water neck and block to make sure there are no leaks if the rubber gasket doesn't seal good.