Money Down The Drain - From reading your magazine, I believe I am one of the few owners of an LS1-powered Firebird that just drives it and does not modify or race the car. It is a '99 model that now has 202,000 miles with no problems! It runs great and acts as if it has only 20K on the clock. I always service it myself and have used nothing but GM-Delco oil filters and Mobil 1 oil.
During my time in the military, I was a maintenance supervisor, and I keep excellent records on every drop of gasoline and oil that has gone into my Bird. I have noticed a slight decline in fuel economy over the last 10,000 miles of about 1-1.5 mpg. Everyone tells me the engine is getting old and using more fuel, but I do not believe it. The oil consumption between changes is exactly the same as when new, which is zero!
Everything else about the car is fine. But with the price of fuel I am looking for every mile per gallon I can get. What do you think? Henry Austin of Medford, OR
Ray Bohacz Responds: The slight loss in fuel economy your Firebird is experiencing is most likely caused by a degraded oxygen sensor(s). The component has not failed and is still functioning, but it has skewed slightly from age and high mileage. Think of it as a tire that is still good but near the tread-wear indicators.
As an oxygen sensor degrades, its switching or response time is slowed, thus the air/fuel ratio is driven slightly richer. The sensor is designed to maintain an air/fuel ratio of 14.7:1. A degraded sensor may create a mixture strength of 14.55:1 instead, causing the 1-2 mpg loss the car is experiencing.
With more than 200,000 miles on your car, you got your money's worth from the sensor(s). My suggestion is to install a new AC-Delco oxygen sensor on each bank of the engine. The post-catalytic-converter sensor does not need to be replaced since it is used to determine the catalyst's efficiency and has no impact on the mixture strength. With the new oxygen sensors installed, I am quite confident your fuel economy will return to normal or even be slightly better than before.
The last step before road testing would be to disconnect the battery ground cable for 30 seconds to allow the adaptive memory in the PCM to reset and learn the new oxygen sensors.