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Greg A
Greg A, Master Troubleshooter
Category: Pontiac
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Experience:  Working with Electronics & Cars for 25+ Years
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2003 Pontiac Grand Prix GT: 3.8L..the cooling system..bleeder..factory

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On a 2003 Pontiac Grand Prix GT (3.8L non-SC) where are the cooling system bleeder screws located? I have the factory service manual that indicates there screws (plural) but I have only found the one on top of the thermostat housing. Also, which of the heater core hoses is the outlet? I see two, one on top of another. Thanks in advance.

Hello & Welcome,


If this is like most of the GM engines, there should be a bleeder screw at the thermostat housing (as you have found) and another one above the water pump, in the 'plumbing' that takes the coolant across the engine.


Here is a diagram.




As for the heater inlet and outlet hoses, I have checked both the procedures for replacing them, and they do not say if the top or bottom connection is the in or out. If you have the factory manual, you probably have the same diagram as below. But the input usually comes from the side that leads to the thermostat connection, and the outlet goes toward the water pump.




I will do a little checking in another source and see if it says which is input and output.


I hope this helps. Please let me know if you have more questions.


Take Care, Greg A.

Customer: replied 8 years ago.

Hi Greg A,


Thanks for your help. I'll look at it in a few hours and let you know. At the end of the day, the problem is that the car cabin never gets warm. The engine will reach normal operating temperature (not at all overheating) but the vents only produce lukewarm air at best. Actually the air alternates between lukewarm and cold. The thermostat was replaced last week (thought it may be stuck open) and yesterday the heater core was backflushed (checked out o.k.). I'm certain (based on empty hoses) that there is air in the system that needs bled. Will this air cause the problem as described or am I on the wrong track?

I think you are on the right track. It does sound from those symptoms that there is air in the system. I was going to suggest that you flush the core in both directions, but I see you have already done that. Hopefully it is not something else that is getting air into the system, like a leaking head gasket or cracked head.


For now, I think you should try to go through the drain & fill procedure (fill really), and try to get as much air out of the system as possible. That is where those bleed screws come in. When the engine is cool, fill the system at the radiator, with the screws open, until coolant comes out of those screws. Then - while coolant is coming out, close the screws quickly. If coolant comes out of one first, close that one, then do the other one.


Also, make sure that the overflow bottle is at least half full all the time. Then, over a couple of heating and cooling cycles, coolant should be pushed into the overflow bottle, along with any air trapped in the system, and only coolant is pulled back in. This is kind of a 'progressive bleed', as I call it. It may take a few cycles of the engine heating & cooling off to get this done, and the overflow bottle has to have coolant in it at ALL times.


You should also check to make sure the area of the radiator cap is clean, and if the cap has not been replace in the last couple of years, that should probably be done too. The objective here is to get the system FULL with no air, and keep it that way.


I will also check my system for any other notices that might involve this problem and get back to you if I find anything.


Please let me know how it works out, or if you have more questions.


Take Care, Greg A.

Customer: replied 8 years ago.

I'll be headed back to work on it shortly so I'll let you know how it pans out, and if I can find that second bleeder.


I've already purchased the new radiator cap, but failed to put it on yesterday (doesn't do much good if it's still in the box).


I'll keep you posted. One last thing (for now, at least) -- do you recommend putting the car on ramps when filling? It seems that for whatever reason (slope of the driveway, etc.) the car is situated such that the radiator fill is not the high point on the cooling system so I'm thinking that ramping the car would help get a bit more air out.

Hi again,


The ramps wouldn't be a bad idea. I have done that kind of thing in the past.


You could also 'pre-fill' the heater core as an extra step, but that might be too much work right now, as it is probably full enough at this point. What I mean is to take the hoses off and put coolant in the top core line until it comes out of the bottom. Then put the botXXXXX XXXXXne on and keep putting coolant in until it won't take any more. Then connect the top line. You could even try to pre-fill that top line too, but it gets messy and it only gets the lines full.


If the system is working properly, coolant should be flowing through there anyway, and the air will come out over a few heating & cooling cycles. What I briefly mentioned in my last message, about a leaking head gasket, can be tested for. If there are exhaust gases detected in the coolant, you will be looking at another repair. Let's hope that is not the cause, and filling the system completely will take care of the problem.


Let me know how it goes.


Take Care, Greg A.

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Customer: replied 8 years ago.
Thank you very much for your help. All is now well.

In case this is helpful for anyone, here is a recap of what was wrong and what was done:

ISSUE: Intermittent heat on a 2003 Pontiac Grand Prix GT (3.8L normally aspirated). Temperature reading on instrument cluster fluctuates, and heat comes and goes randomly (ranges from lukewarm to cold).

SOLUTION (scroll down for a recap if you don't have time to read): Partially drained cooling system. Replaced thermostat, and coolant temperature sensor (since the thermostat housing was removed and coolant was drained I figured I might as well save some time down the road). I then placed the car on ramps (a floor jack may be equally effective but I'm not sure). Backflushed the heater core by disconnecting the two 5/8" hoses on the passenger side of the engine compartment near the water pump. The hose farthest left (when standing in front of the car facing the engine) is the outlet hose (it's also the highest hose in the bracket that's attached to the firewall). The hose to the right is the inlet hose. I connected a 5/8" garden hose adapter (one end is a barb, the other is a female fitting for a garden hose) to the outlet hose, then connected a garden hose. I used a 5/8" fitting (barbed on both ends) connected to about 5 feet of 5/8" clear hose and attached this to the inlet hose. The free end of the clear hose was placed into a five gallon bucket. The garden hose was then turned on and warm water was sent to the heater core at the outlet, through the heater core, then out of the heater core at the inlet (and into the bucket). I left it running for about 8 minutes, emptying the bucket when it was getting full. I filled the engine with coolant using the technique described about. Note two things: (1) I could only locate one bleeder screw (the one on the thermostat housing), and (2) the upper radiator hose seemed to be installed in such a way that part of it was the high point of the cooling system. To compensate for this, I simply pressed down on the middle of the hose while filling the system with the bleeder screw open. I got quite a bit of air out this way. Also, it may be a good idea (and this is how I did it) to pour coolant into the engine where the thermostat goes (saves time having to wait for it to open when you turn the car off, then wait for cool down, then top off). After all of this was done, I started the car and let it warm up to operating temperature. The heat worked flawlessly, the needle didn't fluctuate, and there were no leaks detected.

RECAP: Backflush heater core; replace thermostat, temperature sensor, and radiator cap; be sure to bleed as much air as possible out of the system; warm car up and enjoy heat!

Hi again.


Great job on getting it working, and that was an excellent summary of what was done.


Enjoy the warmth!


Take Care, Greg A.