Runs hot 190-240 but heater blows cold air

Asked by Jan 16, 2007 at 05:47 PM about the 1995 Pontiac Firebird Base

Question type: Maintenance & Repair

the temp guage goes to 185 then raises to 240 then back to 180. meanwhile the heater is blowing cold air. Then I park it for five minutes, the temp goes down to 180 and stays there and the heater starts blowing warm and hot air.  Someone says the head gasket is blown.  But I have no oil in the water or vica versa. What causes the eingine tempuature gage to fluxuate like this? I have a 3.4 L V6 and runs great for five minutes and the temperature gage goes nuts. Any feed back?
Willy the C.

23 Answers

The temperature sending unit could be bad. This screws into the top of the engine is inexpensive to replace. That could be why the temp gauge is going up and down so much. For the issue of the heater blowing cold air go to this link: This tells you how to check to see what the problem is.

15 people found this helpful.

well does water come into the passenger side floor when you turn sharp?

7 people found this helpful.

my Sunfire did something similar,, turns out that I had the antifreeze to strong

5 people found this helpful.

Check your coolant, if your car is blowing out cold air when hot it could turn into a serious problem if your low, look under your car for a leak. If you have any antifreeze inside the car it is most likely the heater core

1 people found this helpful.

oh and if your low add coolant!


you could possibly have air in your cooling system,have you flushed your system lately or change any part of the system? also you could have a bad thermostat.

3 people found this helpful.

i have a 3.8 v6 and my temp gauge constantly fluctuates from 185 to nearly 235 then back down again. when i turn on the heater it alternates blowing cold air and hot air with the temp gauge (when the temp is high heater blows cold, when temp is low heater blows hot) i recently changed the water pump and the antifreeze... then i drove 710 miles to phoenix and it made it perfectly fine. but when i got there it started doing the temp fluctuation... so i replaced the thermostat and changed coolant again and it still does it... HELP?

44 people found this helpful.

also... did you bleed out the air with the small screws when filling with coolant??? there could be air pockets in the top hose.. could be a heater core leak or airpockets in the heater core lines. how does the water pump sound ??? no unusual noises periodically?

9 people found this helpful.

buy a new thermistat.

1 people found this helpful.

I have a '96 3.8L Firebird and it started doing the same thing today. I was driving and happened to look down and see the temp gauge at a ridiculously high level...turned the heater on and it would blow hot and then cold and back again. Same exact ordeal. I just recently had the radiator, thermostat, and hoses replaced...and had the water pump replaced about a year ago. Any suggestions?

18 people found this helpful.

a lot of times is low on coolet if the heater gets warm one mintue then get cool it is low but if your gauge is going up and down it could be a bad ground most likely it is low

1 people found this helpful.

it sounds like a bad thermostat especially if it breaks stepping on the gas pedal will force the water pump to put pressure on it and force it open causing warm coolant to moveand drop the temp. have u checked engine temp opposed to the radiator? it really sounds like a thermostat issue

2 people found this helpful.

This is what you need to do - IF the thermostat is NOT new do it, it is always the first place to start. Make sure the cooling system is full and air bled off properly. Take it for a short highway trip 10 - 20 miles and see if it starts to do it again. If it seems fine yopu MAY have fixed your problem. Keep an eye on coolant usage and temp fluctuations aftward. If it comes back OR if the thermostat did not fix it you need head gaskets. It is not yet bad enough to effect runability but enough to 'airate' the cooling system with carbon dioxide ( Combustion gases ) - A thermostat is really an aquastat and needs to be in water/coolant to work properly and if set in a air /carbon dioxide setting it will open and close at will. This also relates obviously to the loss of heater function because of the lack of proper thermostat operation and the heater core getting airated. Do not drive this car too much like this. Good luck

7 people found this helpful.

sounds like an air pocket check the coolant level in the radiator (DO NOT OPEN WHEN UNDER PRESSURE----OPENING UNDER PRESSURE=GEYSER OF ANTIFREEZE) check pressure by squeezing top radiator hose usually pressure will fully relieve after being shut off for 20 mins. if ur not sure just let the engine cool completely.

3 people found this helpful.

i have the same problem can you help me changed water pump and thermostat and anti-freeze

5 people found this helpful.

My only suggestion is and I do not have that decade of a firebird but my mother did and according to her she mentioned to me that she had a vacuum leak due to a check valve permitting vacuum diaghrams to not hold open or close depending if you are speeding up or slowing down and that would add or take away vacuum from the diverter valve to the heater core. Try that on for size and have a looksy there. All that I can think of not seeing the car at hand. One more thing be sure that the vacuum resivor is not cracked and actually holds a vacuum, (that is the supply to opperate all vacumm assisted diverter doors in the cab as well as under hood sensors.)

1 people found this helpful.

My 98 Firebird V6 did the same thing.T hoses to heater core were clogged. Also if I add alot to raditiator I have to bleed it or it runs hot.I still have issues wit it but it runs cooler more & heater works great


i have the exact same problem on my firebird v8 and i changed evrything a new radiator the coolant temp sensor the coolant level sensor new coolant fan motor only thing i got left to check are the hoses and the temperature switch i bled the top hose and all and my waterpumps still good unless the bearings goin bad but i recommend checking your top radiator hose un screw it enough to leak out the air til wat squirts out if that doesnt work make sure ur radiator is filled properly and dont put water in your radiator breather hole thats just for when it gets hot the fluids have somewhere to go and not fly out like a geyser but if that doesnt work put a coolant temp switch on which is under the thermostat ull have to go under the car to get to it and if that dont work put in a temp switch aka temperature sending unit and if that dont work put bigger stuff like a radiator ...hoses...water pump and so on

2 people found this helpful.

my v8 has the same problem its just the way the block was built the high horse powered motors need more heat in them to run better just not a whole lot should run any where between half and three quarters

4 people found this helpful.

Mine is doing the same....filled both radiator and overflow....Took on highway and the needle went way high...Turned the heat on high and got cool air....Parked the car till it cooled down, opened the radiator cap and was half full????? The overflow looked liked it had a bit more than went i filled to line....Where did the antifreeze in my radiator go???? I think maybe water pump leaking or thermosate sticking??? If the therosate is sticking would the radiator golow within a 10-20 min. ride??????

5 people found this helpful.

I finally figured out I had to bleed it every time I added anything to radiator or coolant reservoir,Even if I was just topping it off.There is a screw or bolt in the thermostat housing pipe ,while car is running & both caps off turn the screw/bolt a little & it will bleed the air out.It helped me & hope it works for you

3 people found this helpful.

I have a 1994 Firebird it running hot just change water pump and the thermostat still runing hot so too thermostat out still running hot and yes passenger side floor is wet idk what to do and im changing bottom radiator hose tomorrow but I doubt it


I had similar issues with my '99 Firebird. If the floor is wet on the passenger side (coolant leak, not rain water or dog pee) then your heater core is shot. Replacing it with a new one is your only option if you want your heater to work. If you live in a warm climate and never see snow, then you can clamp off the heater hoses or run the inlet hose directly to the outlet, bypassing the heater core altogether. Also, the heater core gets hot coolant from the engine as soon as the coolant warms up. Unless its airlocked. the thermostat only regulates when coolant is diverted through the radiator, because its becoming too hot and needs cooled down. So if you have over heating issues or a heater wont blow warm air, and you topped off the coolant, and replaced the thermostat, and bled the system properly, and your cooling fans are kicking on, its probably a head gasket. But replace or adjust the bypass valve by the heater core first. Thats why your heater wont work. Tip: jack up the front end of the car at least a foot when topping off the coolant, makes bleeding the air out much faster. I remove the top heater hose for a sec and bleed it there as well. not while the engine is running of course.

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