I recently have bought a 99 deville, had it about a week before car over heated and had the delearship replace gaskets. It has been a about two months and a low coolant light comes on, i filled it back up and now around two weeks later it has came back on. Ever since I got the car back from being fixed it runs rough when I start the car and goes away after fifteen minutes or so of driving. I am afraid my gaskets are going bad again and need to get an opinion of someone who knows more.
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It is probably a head gasket issue if you have replaced the thermo and the pump and it still overheats. You probably can drive it 15 minutes or so and then the gauge starts soaring towards overheat shortly there after. Under completely normal driving conditions. Northstar engine's actually don't overheat not from bad head gaskets (they aren't the defect issue) but the inferior bolts holding the heads to the block. The bolts have been found to be pulling out of the Northstar's soft aluminum block(this is the actual problem but it's blamed on the gaskets). Causing spaces between the block, gaskets and the head that allows hot exhaust air to seep into the coolant flow chambers. This why your have coolant bubbling out of the top of the overflow valve under the hood after only short drives. Under normal operation the engine is literally boiling it's own water supply. The head bolt issue is a design flaw by GM and one that leads to a great number of Northstar engines being crippled at well under 100K miles of driving. Only remedy is to have both heads pulled off the engine, the head bolt replaced with new gasket and you're all set. Bad new very labor intensive whcih = a several thousand dollar repair. I had it happen to my 1998 Concourse Deville. I feel your pain.
A head gasket leak is usually indicated (among other things) by continous overheating of the motor, and subsequent loss of coolant.
It can also be indicated by the presence of engine combustion gas bubbles in the coolant resevoir.
Ensure the coolant system has no external leaks, such as faulty hose/s, radiator etc.
Ensure motor tune or other motor mechanical condition is not the cause of overheating.
I assume the motor is actually running hot.
Check cooling fans are operating ok (as necessary when car warms up):- if not check 1) fuse, 2) relay operation, 3) heat sensor switch.
Check/replace the thermostat,
Check the coolant pressure cap is serviceable & of correct rating, Ensure cooling system is correctly filled, bleed off any airlocks as necessary when re-filling, Ensure radiator hose on suction side of waterpump is not collapsing at engine revs above idle (due softening), If the motor has been severely overheated, the head gasket could be blown - look for combustion gas bubbles in the coolant resevoir.
If you are not loosing coolant on the ground. You may have a problem of a bad intake manifold gasket or worst a bad head gasket. The two things to look at are coolant in your oil, manifold gasket. Coolant out the exhaust, head gasket.