Need help finding coolant leak on 1999 Pontiac Grand Prix


Asked by Jan 02, 2016 at 11:17 AM about the 1999 Pontiac Grand Prix 4 Dr GTP Supercharged Sedan

Question type: Maintenance & Repair

I've got a 1999 Pontiac Grand Prix 3.8 Supercharger which has had some odd coolant issue for a while. About two months ago I noticed the car seemed hot when running and especially when idling, but the thermostat didn't show anything to be worried about. One day I noticed a small puddle of coolant under the car and after doing research online figured it must be the thermostat, so I replaced it and the gasket, which seemed to help. Now I can see the temp going up to 180F, then dropping down. The problem is that the last couple of days I've seen a lot of coolant under the car. My husband put the old temp gasket back on (not sure why, he seems convinced it was in better condition than the brand new one) and now when we stop and park in the driveway coolant practically gushes out. The thing is, it doesn't seem to be coming from that gasket but above it. I think the fluid is coming from what I would call the top of the engine, near the transmission fluid dipstick, but I can't be 100% sure. Before we get to digging in there can anyone give me some ideas on what to look for? The coolant itself looks fine, no discoloration, though it is a little over full (miscalculation on my part). After the car sits for about 60-90 seconds the dripping stops. Thanks for any help you can give.

6 Answers


Check your radiator where the main radiator core connects into the side pieces on each sides of the radiator when they build them at the factory those to Ends or mounted on each side of the core and I have been known to leak from the seam between the two parts usually a small quart of radiator stop leak will fix it for about 30 days but the only way to really stop it is replace the entire radiator because no one repair radiators anymore just get it Hot up the hood and take a flashlight and look down both sides of the radiator inside and out you should see where the steam is coming from

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Sounds like the intake manifold gaskets. The 3.8s are notorious for this. If you decide to fix it replace the plastic upper intake manifold. Don't reuse the old one. They tend to crack. Make sure you change the oil twice after the repair to remove any antifreeze from the engine. You can also pull the oil dipstick and check under the oil filler cap for white milky "goop". If it's there, it's definitely the intake gaskets. Be advised that it's NOT going to be the head gaskets. The 3.8s rarely, if ever, blow a head gasket! HTH. - Jim

1 people found this helpful.

Thanks Chris -- we tried that trick but could hardly see any steam, just what may have been a little up top near where the leak seemed to come from. Thanks as well JS -- we're not seeing foamy oil but we think it's the gaskets. Got some parts on order and will hopefully get it in a couple days. Only thing to update is that the low coolant light comes on when there's obviously plenty of coolant, so the coolant is probably not circulating very well. The light doesn't stay on, though, it comes and goes. Then when we park about 1/2 cup of coolant drips out for a minute.

1 people found this helpful.

You're welcome. Glad to help. Sometimes those intake gaskets will leak externally only. But as a precaution, if that's what it turns out to be, I'd still have the oil changed twice. Better to be safe than sorry. Those coolant level sensors can be a pain in the neck! I'd suggest removing it and cleaning it. You'll find it just below the radiator filler neck with an electrical plug on it. The light may stay on a day or two after but that's normal. The Factory Shop Manual for my '00 Grand Prix talks about that. HTH. -Jim

1 people found this helpful.

Thanks! The mileage tells us it's time to do an oil change anyway so that's not a problem. We'll check on the sensor too. I'll update after we do the deed LOL


So the part came in Thursday and we replaced the upper manifold gasket set, but there's still some leaking going on. It's improved and the leak that I saw up near the steering fluid dipstick is gone, but below it there is some pooling which appears to be coming from a lower area, so I think the lower gaskets needed to be replaced, too. We just can't do that now -- a cold front is coming in and it's supposed to drop to 3F, and we don't have a garage. (We should have replaced them when we were elbows deep in the car this afternoon but I'm nursing a cold and the cold front came in so we had to stop.) The good news is that the oil looks good, there's no frothing or "turkey gravy" look to it. Thanks again for your help!

1 people found this helpful.

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