At idle 2005 Elantra temp guage goes to cold using heater blower

Addendum: I did purchase a bleeder system/funnel and made sure to get all the air out of the coolant system!

It sounds like your thermostat is faulty. It's like it's stuck in the open position and not letting the coolant reach 180 degrees. You also may have an air bubble in your cooling system. When you replaced everything did you buy an air bleed kit to remove all air within the system? If not, I would try that first.

Oh, okay, you submitted your addendum as I was writing my response. When your temp is up into the normal range, squeeze your radiator hose and see if it's got pressure. If it's soft, your thermostat is stuck open.

I have recently felt the upper hose while the engine is hot,and surprised to see how "Rock Hard" it feels.I thought it was so hard that it could burst! I pointed this out to the mec and his response was that it was to be expected.

The only other thing I can think of is when you turn your heat to the ON position, the valve that lets the hot fluid into the heater core is not fully opening. Look for the two hoses that go into your fire wall. Is there a valve right there that you can see? If so, have someone turn on the heat and watch it to see if it moves. This would explain why you lose heat in the car, but not why your engine temp gauge would fall also. Unless you also have a bad engine temp sensor.

Mine can take up to twenty minutes for the temp gauge to even rise to the edge of the 'C' at idle when the temp is under 10 degrees. It warms up better once its on the road, but without the warmup it can take ten miles at 55 to get any warm air from the blower at all. This is not a 70's Ford or Chevy. Hyundai's have aluminum engines that naturally run cooler and being much more efficient don't waste as much energy in heat loss an the older cast iron types. In subzero temps, I often block the radiator with a bit of cardboard. I recommend a good winter synthetic oil, a sump heater and just what you're doing, letting it warm up a bit. Just don't expect comfort or defrosted windshields unless you install an electric cabin heater/defroster.

Again I want to reiterate that if I leave the fan OFF(blower for the interior heat) the guage comes up to a normal expected temperature "EVEN AT IDLE". When I get into the car and the guage has risen to an expected range, I can then turn the fan ON and the heat blows out nice and hot! It's ONLY when I have the interior fan set to the ON position that the temperature guage does not move. In that case the guage has not moved and there is no heat anywhere in the car and no heat built up in the coolant !Please remember that leaving the fan ON or OFF is done the night before so I can use the remote starter in the morning!

According to Quickdraw, it takes forever to warm up unless you are flying down the interstate. I believe what you are saying is that since you have installed the auto start, the heat is almost no existent. If what Quickdraw said is true, that these engines cool much quicker than the old cast iron ones, if you are turning on the fan at the same time you start the engine, then yes, it will never warm up. Try not turning on the auto start. Get up like you used to do, start the car, and let it warm up like it did prior to you getting the auto start installed. If you find that things are back to normal, then all the money you just spent on an auto start is essentially worthless. It's no fault of yours...just a car that is not meant to have an auto start due to the fact that it runs cooler than most cars........

Then the coolant has frozen. The fan position may be a red herring. Get a tester and check your antifreeze strength.

You might also want to check for bubbles at the radiator cap too. Sometimes aluminum heads get spongy or a head gasket can leak high pressure exhaust gasses into the coolant. It is occasionally an intermittent problem, based on temperature driven contraction and expansion of the head verses the top deck. This pressure leak creates a big high pressure bubble in the water jacket or return hose where the temp gauge pick up is and prevents the water pump from doing its job or the gauge from registering. There will be odd discolorations on the rad cap and little bubbles popping up, with smoke. Sometimes the overflow cap pops loose all the time and you wonder why. I had this happen on a 3 cylinder Chevy sprint and cold retorquing the head cured it. Look for simple first. Are you sure that the coolant is actually full? CHECK! You have to tip it in slow once its hot, so the jacket fills when the thermostat is open. You might have some debris in the hose defective new hose or collapsed rubber from the old hoses that randomly plug things up. That's why you flush. Are the hoses properly moulded or did you kink one trying to go cheap? You might have two things happening at once, like a damaged or wrongly modified temperature pick up, Stay calm and think logically. Did any of this temperature madness happen before the start gizmo got strapped on? What did they tinker with? What wires did they accidentally cut and scotch tape back together? Splices? Most of those guys aren't so smart. Does the heater box changeover baffle switch to hot based on a computer input telling it that every thing is hot now? Crack that radiator cap and see if coolant is flowing, stick your finger in the coolant when its saying that everything is cold after its been idling. Assume nothing. Maybe your new water pump is bad.

I and another 2005 elantra owner I know had a faulty thermostat issue that sounds exactly like this.