Antifreeze in oil MPG way down?

Hello, I have a 1999 Grand Prix GT with a 3.8 about 106000 miles. About 6 months ago I did the upper and lower manifold gaskets and replaced the plenum with a new Dorman plenum due to antifreeze disappearing. After finishing everything was perfect. No more antifreeze loss and power back to normal... until a couple weeks ago and began having the same problem. So I replaced the upper plenum and there was alot of antifreeze filled up in the intake. Everything great again but just the other day I noticed slight power loss, MPG's way down, about 5 MPG and antifreeze in the oil. (milky oil under cap) What could it be now? Could I have another defective plenum? Head gasket? Lower manifold gaskets again?Except for the slight power loss and MPG down the engine runs GREAT! No smoke out exhaust. But with antifreeze getting into the oil I know something is wrong! Any suggestions anyone can give is GREATLY appreciated! Thanks!:banghead::banghead::banghead:

Are you actually seeing antifreeze in the oil or just white material on the oil cap? Moisture alone will cause the white residue if you only drive short distances. This is also more prevalent in the winter due to moisture formation in the engine as it cools down after begin run. As for the lower MPG there can be a lot of reasons for that. First of all cold weather can cause a drop in the MPG due to extended idling and cold weather.

Mine developed a HUGE leak at the wall where the water pump screws into. When I replaced the pump I tried to clean that wall with a small knife and it went trough as soon as I touched it.

One more thing could be a cracked head and coolant leaks into the chambers when pressure builds up.

I'm sorry about your problem. I really hated when mine was leaking, I can feel your pain :(

Best regards,

Oscar.

EDIT.- One last thing, if you didn't/couldn't clean all of the coolant in the engine, that could also cause the milky oil under the oil cap. If you extract the dipstick and the oil is milky too that's a big problem.

Thanks GTP Dad. I guess it could be the moisture from the cold temps. I drive it about 15 miles each way to work and thats about it. But unfortunately it never that easy for me. Since the MPG's are down so much I am afraid its a bigger (only to get worse) problem. I am not really seeing antifreeze on the dipstick but I figure its only a matter of time before I do. Is there a leak test I could easily perform on it to pinpoint the exact location of the leak?

there a leak test I could easily perform on it to pinpoint the exact location of the leak?

You could pressurize the system with a coolant pressure tester, and watch for a leak. I'm not positive, but I'm pretty sure Advanced Auto rents them out.