Q: My son purchased new a 2009 Chevy Silverado with a 5.3 liter engine. He’s always used synthetic oil and changed it when the oil life minder indicates. Recently while on a road trip the oil pressure dropped to about 10 psi and the low oil pressure light and stop engine warning came on. He checked the oil level, which was OK. He restarted and the oil pressure was fine. He drove about another 10-15 miles and the oil pressure dropped again by 15 psi and the service engine light came on. He reduced speed and drove to the nearest Chevy dealer. They diagnosed a faulty oil pressure sensor and replaced it at a cost of $463. Two months and 1,800 miles later the service engine light came on. The oil pressure gauge read a normal 40 psi. The Chevy dealer where he purchased the truck found the oil pressure read by a scan tool was lower than that measured with a mechanical gauge at the filter. They determined the screen next to the sensor was clogged with sludge. They replaced it and flushed the engine for $483. Did the first dealer screw up by not checking and replacing the screen? Is sludge unavoidable with just a three-mile commute to work even with synthetic oil? The truck had 49,000 miles when these problems occurred. Would more frequent change intervals help? Or should he plan on doing a flush every 25,000 miles?

A: Sludge build-up inside an engine occurs when moisture in the air inside the engine condenses during a cold start and contaminates the oil with water. Combined with fuel contamination and oil oxidation over time during normal use, this can form a “glop-like” sludge that can block or restrict oil passages, screens and filters, causing a loss of oil pressure. Under normal driving with the engine fully warmed up, the moisture evaporates and is drawn out through the PCV system and burned. Short trips after a cold start may not allow the engine to reach fully operating temperature long enough to eliminate all this contamination.

So, while the oil life monitor system may not call for oil changes for 10,000 miles or more, in this case more frequent changes would remove much if not all of the contamination before it has a chance to jell into “glop.”

Should the first dealer have caught this? Based on your son describing the repeat scenario of dropping oil pressure, the pressure sending unit is suspect but it might have been wise for them to replace the valve lifter oil filter screen, mounted just below the sensor, at the same time.

More frequent oil changes should prevent additional sludge build-up. According to TSB 04-06-01-029G, GM does not endorse any type of engine flushing.