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Oil Pressure is Dropping and Sludge Present in High-Mileage Pickup
 
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Question:
Oil Pressure is Dropping and Sludge Present in High-Mileage Pickup
I've used Mobil 1 full synthetic with 10,000-mile change intervals since break-in was over in my 1999 GMC Sierra, 5.3 liter gas pickup. Now at 100,000+ miles, oil pressure drops off from the initial 55-60 psi to about 40 psi after about 4000 miles on the oil. I've reduced the change interval to 5000 miles for the last two changes. I'm also noticing sludge in the oil pan. Any idea what is going on?
-- Wayne Zarnecki, Loudon, TN
Answer:
Question: Are you experiencing any other problems such as lower fuel economy, loss of power, loss of coolant etc.?  It is difficult to diagnose the issue with a limited amount of information, but if we had to guess, we would say you may have a coolant leak or a bad fuel injector. The sludge in the oil pan could be indicative of an internal coolant leak. Coolant and oil do not mix and the resulting combination is usually sludge. On the other hand the loss of viscosity may be indicative of excessive fuel dilution. So if you have poor fuel economy or have added coolant more than normal lately, we would recommend that you have these issues checked.
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