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Oil in Antifreeze

How serious or expensive is cost of repair for problem causing oil in antifreeze for 1996 Bonneville with PO300 code?

Could be a very big problem.

This engine falls into the 10+ years GM was having problems with all their V6 and V8 engines and leaky intake manifolds. You need to get this taken care of. There was a recall on this from GM. If GM won’t cover the cost…it’s NOT super expensive…But get it fixed before it WILL cost you a LOT…or in this case because of the age…you’d probably just junk it.

Take care of this SOON. If not it could result in a catastrophic engine failure.

I already had UIM & LIM replaced, which I was told by mechanic would prevent this from happening.

UIM Upper Intake Manifold, LIM, Lower Intake Manifold. In your other post, you stated the gaskets were replaced, not the manifolds themselves. If you had the manifolds replaced, then I say you have a workmanship problem with your mechanic.

Maybe the 2 issues are unrelated. The 300 code could be due to a problem with the crank sensor, etc. and maybe the oil in the antifreeze is actually transmission fluid or Dexcool sludge that is being mistaken for oil.

Just something for consideration anyway.

I have seen anti-freeze in the oil, oil on the ground, anti-freeze on the ground so many more times than oil in the anti-freeze(from manifold issues). Look long and hard if this is or is not ATF in the anti-freeze.