z28pete
Posts: 5699
Joined: 8/7/2005
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The following tip is from a RockAuto customer. To find and share more tech tips, visit the RockAuto Part Advice Wiki.
I have a 1987 Olds Cutlass Supreme with a 5.0 liter engine and an OBD-I control computer, something shared by many GM cars of that vintage. The car started to slow down at idle speed and either stall, or the computer went "open loop" and the idle speed returned to normal, in which case the computer went "closed loop" and the process started all over again. The cause of this was a real "Rube Goldberg" setup, as follows: The lowest oil return hole in the valve rocker compartment is 1/4" higher than the lowest point on the head-to-valve-cover seal, so there is standing oil against this seal. The car is sufficiently old that this seal started to leak. When the car is shut off, oil leaks out of this seal and runs down the outside of the engine. Some of it gets onto the outside of the oxygen sensor. When the car is started, because of this oil the computer thinks the mixture is too lean, and enriches it until the engine is putting out lots of oily carbon, which coats the inside of the oxygen sensor.
Meanwhile, the oxygen sensor gets hot and evaporates the oil on the outside, so the outside returns to normal. The carbon on the inside makes the computer think the mixture is too rich, so it proceeds to make it leaner and leaner, which is what was causing the idle speed to drop and the engine to stall if the computer didn''''''''t go "open loop". Using a propane torch to burn off the carbon layer on the oxygen sensor, plus re-sealing the valve cover on the side with the sensor, made the problem completely disappear.
Ron in Virginia
< Message edited by z28pete -- 9/12/2008 7:31:34 PM >
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Pete
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Printable Version
Something for 5.0 owners to consider - 
