99 Grand Prix Overheating What Now

[snowman4evr]

Hi, I have a 99 Grand Prix with about 70,000 miles that is overheating, blows cold air. But when I bleed it at the thermostat it will do fine for about a week or so, and then its the same thing. I understand that this could be either the intake gasket, or a head gasket. My question is how do I check to see which one it is? Thanks ahead of time. will

[Ripn12s]

Have u replaced the tstat???

[knucklebuster2]

If you are getting a lot of brown sediment, the Dexcool is breaking down. It can clog your system, and cause overheating. It also probably is clogging your heater core, therefore no heat. GM add cooling tabs from the factory which help seal any leaks, also, but this isn't the problem. I would do a system flush, blow out the heater core, and use green antifreeze. There are many people who believe Dexcool also affects head and intake gaskets. I changed over and never had a problem again.

[snowman4evr]

Thx Ripn, and Knucklebuster for your replys. Ripn I have replaced the thermostat about 4 to 6 mths ago to no avail to the problem. That was my first thought though also.
Knucklebuster I understant what you are saying about the dexcool. But would the system being stopped up, cause it to get air in the system? Like I said once I bleed it from the bleed valve on the thermostat it will be fine for a week or so, maybe longer. And then its the same old thing. No heat, overheating.

thx will

[Ripn12s]

Snowman,

Its really strange that the system will work for a week or so and then fail.

Might want to try and have a pressure test done on the radiator. The cap might be failing to seal or keeping the seal.

[Ripn12s]

I think if u do a compression check on motor you should be able to determine if the head gaskets have failed.

Might try www.clubgp.com for more answers. They have tons on people over there that have probably run into every problem possible on these motors.

[jay hullett]

is car loosing any coolant, if it is suspect intake gaskets-look at the driver side corners for signs of coolant also take oil filler cap off and look at bottom of cap to see if it has a brown jell or take finger and run around inside edge of valve cover where oil cap goes.intake gaskets are a common problem i average putting on 2-3 sets a week also on 3.1 3.4 engines make sure u replace oil pump drive oring common place for oil leak -will be oil leak on driver side

[knucklebuster2]

Originally Posted by snowman4evr

Thx Ripn, and Knucklebuster for your replys. Ripn I have replaced the thermostat about 4 to 6 mths ago to no avail to the problem. That was my first thought though also.
Knucklebuster I understant what you are saying about the dexcool. But would the system being stopped up, cause it to get air in the system? Like I said once I bleed it from the bleed valve on the thermostat it will be fine for a week or so, maybe longer. And then its the same old thing. No heat, overheating.

thx will

No, the system being clogged will not cause air to get into the system. the only thing that will cause air is a leak. It may be so small, you are not noticing it. Did you check the passenger carpet/floor mat? The heater core may be leaking slightly. (Mine went bad too.) If you have the 3.8L, another common problem is a plastic elbow that runs from the intake to the belt tensioner. Yes the coolant goes through the belt tensioner on the 3.8L! I have never seen a more stupid design. Anyway, this elbow is a common failure problem and usually leaks slowly at first. Until it gets bad enough to short out the alternator-which is right next to the elbow. Take a look. Is the base of the alternator rust colored? This problem may seem like the water pump went bad. The elbow leaks down onto the water pump. Take a look, it is there. Motormite (Help!) makes the elbow and it costs about $3.00. Even the package says it is a common problem on the 3.8L. Good luck!

[snowman4evr]

Guys I really appriciate all your help. First off It is a 3.1L (3100) I have pulled out all the plugs and they are all normal, I was thinking if it was a bad head gasket, that one of the plugs should be white, but they were all normal. The car runs fine, not even a miss. I believe though I am going to take a little bit of everyones advice though, and change the coolant over to the regular green stuff after I flush the system, and I intend to go ahead and change the intake gaskets, while I allready have the coilpack off. If this dont help it, I will be at a total loss. Again I really apraciate everyones advice. I will keep you posted if this takes care of it or not. thx will

[rpreston2]

Idea, Since the radiator cap lets out extra fluid and pressure, when the engine cools it sucks the fluid back in through a straw (rubber hose in overflow) If the straw has a hole or lose seal with over flow hose or radiator, then it will suck in air. So I would suggest inspecting and or replacing the radiator cap, and overflow hoses to keep the air out.

My hose was laying loose in the overflow tank one day an have been having issues since. I need to bleed my system.

[95ragtop]

Hope you haven't changed the coolant over to the green stuff yet. The problem is in the gasket design not the coolant type. I have a '95, the 1st year of this engine design, and it has the green coolant. I think the cooland was changed on the "M" code engine late 95. Suprise..Lower Intake Gasket went bad. Looking at the design of the original gasket, not suprised. The ends of the original have no support during thermal expansion of the engine. The new design does. Hopefully you don't have an intake problem. I would however change the Dexcool coolant at 50K rather than the 100K suggested. Good luck

[MoshimoGTP]

What is the normal compression for a 97 GTP with 97K miles???... I wanto to check mine!!

Thanks