Question about 1999 GMC Suburban

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Oil is in my engine coolant reservoir as well as my radiator

99 gmc yukon 5.7 v8 and I have oil in my engine coolant reservoir as well as my radiator. not sure what causes this but i need some help in figuring this out

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Most likely the intake manifold has a broken gasket or iteslf is broken.

take it to a shop and have them diagnose it by checking for HC's in your coolant. If there are non present, then it could be inside of the timing cover or the intake. If there are HC's present, you will need a new Head, Engine or just Head Gaskets, but if it's over 150,000 miles, you may want to just buy an engine if that's the case. You can't repair a high mileage engine with new top end components because it will "**** out" or ruin the bottom end shortly after installation.

Good luck! Hope that helps.

P.S. - I have mroe GM's come in with that same symptom than any other vehicle. After your repairs are complete, get rid of the DEXCOOL and go with a good, green antifreeze. It works much better than DEXCOOL.

Kevin, ASE Certified

Posted on Sep 12, 2009

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Are you sure it's engine oil and not ATF? Our 99's transmission''s cooling pipes run through the radiator. Had the tranny power flushed and the fluid was blown into the radiator as the pipes ruptured. New radiator another tranny flush, one month later the tranny failed altogether.

Posted on Oct 06, 2009

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Get this to a mechanic you can trust right now. Oil and coolant should not be in the reservoir. It may have gotten there becasue of a gsket that has failed allowing the two to combine. You will need the mechanic to find the source and then do a complete coolant system flush, and complete oil change after repairing the reason for the failure of your components from keeping the two separate.

Posted on Sep 14, 2009

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Your description may actually be your answer, so to speak. You mention 'generally as engine is cooling or if coolant is added too quickly'. It sounds to me like the reservoir is over-filled.
When the engine is cooling, coolant tends to flow back to the reservoir (plastic 'jug' where you add coolant). A bad radiator cap or one that was replaced with too low a PSI rating can easily allow too much to fill the reservoir and and occasionally overflow it. Most reservoirs use a rubber tube to drain excess coolant so it may appear to 'leak' near the front left wheel or left side of engine.
Replace your radiator cap. (did they pressure test the cap too?)
Make sure you check your owner's manual for the proper PSI. My sources show 15-16psi, but that sounds a little high to me. Also, make sure you have the correct (50/50) anti-freeze/water mixture.
After you replace the cap, make sure the reservoir has the right amount of coolant and isn't over-filled. (To 'fill cold' line) Drive the engine until hot, then park the truck and see if the problem is fixed. Good Luck!

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