2005 Pontiac Grand Prix POOR MILEAGE

2005 PONTIAC GRAND PRIX
50,000 MILES • 6 CYL • FWD • AUTOMATIC
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RAYJACK8950
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  • 1 POST
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?
Sep 16, 2009 at 6:01 PM
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MMPRINCE4000
  • ✓ASE Certified Mechanic
  • 8,548 POSTS
  • ASE Certified Mechanic
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.
Sep 19, 2009 at 7:31 AM
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