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1999 GMC Sierra 2500 SLE 4x4


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I have a 1999 GMC Sierra and the other day I got a low coolant light on. So I filled it,then a check engine light came on. Today LOW . Coolant light comes back on and check engine light shuts off? Any ideas? Thanks.

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Ok, I checked the oil and its not milky looks pretty clean still. I searched around the water pump and don't see any signs of leaks. Low coolant light and check engine light have both now shut off. I'm baffled.

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These years in particular are notorious for intake gasket leaks where they start out slooowly losing coolant. As I've mentioned in a few posts elsewhere here, I still own a good ol' 98 Yukon and will bet yours has the exact same 5.7 engine as mine. You don't mention how many miles you have on it but if it's got anything over about 120,000 miles and the intake gaskets are the originals, you've got what will appear to be a very minor problem now that will turn into a really big one if you don't address it and soon.

 

The problem is the original intake gaskets on these years of 5.7's get eat up by the original Dexcool antifreeze that was installed at the factory and what starts out as a seemingly minor issue of coolant loss will in time turn into a major catastrophe! My Yukon started out needing coolant added about once a month, as time went on it got to the point of needed coolant once a week and was taking anywhere from a quart to about a half gallon, never once was there any indication of coolant in the oil because the gasket(s) were leaking around the intake ports and it was going out the exhaust instead. I knew I had a problem but chose to ignore it and take a chance. Well, one rather cold -15 F morning last winter after starting and letting it idle for about 5 minutes the gasket in question completely failed and I ended up with a very nice milkshake in the crankcase! Not good! Luckily I realized in time what had happened and shut it down before any damage to the engine took place. I spent the following Thursday night and most of the following Saturday changing the intake gaskets which had completely crumbled and came out in bits and pieces, then giving the engine a very good flush, but it lives to tell about it today.

 

My advice is address it now! When you go to buy the intake gaskets at the parts store they will likely offer you the budget gaskets and the top shelf ones, don't skimp on them and get the good ones which will run you around $70 for the set. The set I bought was made by Victor but I'm sure Felpro and a few others offer comparable quality intake gasket sets for it. It's a fairly fun job (not) getting all the wires, hoses and so on out of the way to get the intake off and out of the way, so plan on at least a good Saturday's worth of work to get the job done, but at least in your case you haven't had the full on blow out yet so do it now before it's too late, you'll be glad you did! Yeah, it's a hassle but it beats not only having to do the intake gaskets but also having to goof around dealing with a crankcase full of oil and antifreeze as well. :mad:

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A code for crumbling intake gaskets and your crankcase is about to be filled with a milkshake? :fart:

If the check engine light is coming on, yes there is a code to be read & diagnosed. May not be a code specifically for intake gasket but still will give a better idea of what the problem is instead of guessing.

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If it's losing coolant it's one of a few things, none of which I've never needed a computer or a code to tell me and was figuring out problems like this when point ignitions were the norm and that's about as sophisticated as things got. Computers and fancy electronics aside, the internal combustion engine is still designed and operates in much the same way it did 80 years ago, and to this day are still plagued by many of the same problems that can develop over time. It's either a head gasket leak, intake gasket leak (unless the intake is dry which it is not on this truck), water pump leak, radiator leak, hose leak or heater core leak. If there are no external leaks and it's not leaving any evidence of it in the driveway, then its internal and is either intake or head gaskets and or a cracked / warped head which is another known issue with some of these engines. That or the carpet on the front passenger side is completely soaked and you haven't yet noticed the smell of antifreeze and fogged up windows inside the truck, that would be a leaking heater core. I never got any codes or check engine lights prior to my intake gaskets letting loose and the problem of loosing coolant went on for quite some time (like a year) before the gasket(s) completely failed that one fateful morning.

 

Million dollar questions: How many miles are on the truck? To the best of your knowledge are the intake gaskets the originals? Even if the Dexcool was changed out for conventional (green) coolant at some point in the past (mine was a long time ago), if the intake gaskets are the originals (mine were) the damage has already been done and time is not on your side.

 

One other friendly piece of advice: DON'T start adding 'Stop Leak,' there is NO 'mechanic in a can' that will fix it.

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