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2005 Pontiac Grand Prix POOR MILEAGE

Tiny
RAYJACK8950
  • MEMBER
  • 2005 PONTIAC GRAND PRIX
  • 6 CYL
  • FWD
  • AUTOMATIC
  • 50,000 MILES
After changing and overfilling my crankcase by a quart of oil and taking a 450 mile trip with it overfilled. I started having poor mileage from 30 down to 22 hwy. II did drain the excess but it hasn't seem to help. Can you explain why and did I damage anything?
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Wednesday, September 16th, 2009 AT 6:01 PM

1 Reply

Tiny
MMPRINCE4000
  • MECHANIC
  • 8,549 POSTS
Overfilling the crankcase can cause oil to foam due to mechanical action and put excessive pressure on seals and can also get past PCV valve and into engine.

Remove plugs and look for oil fouling, replace if oil fouled.

If problem persists, start by checking compression on each cylinder.
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Saturday, September 19th, 2009 AT 7:31 AM

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