Error Code 42 on a 1991 sierra

[Mr.Griz]

Every now and then when i start my truck, the service engine soon light comes on, and as soon as i shut it off and re start it goes away. When it is on it seems to idle a bit rough. The error code is 42. I have replace just about everything under the hood, even tried a different ecm. But no luck. This may soud stupid but i notice it happens more when it is damp or has rained. Anyway any suggestions on how to fix?

Thanks

[steve in alaska]

you might try this link..I'm sure you know code 42 is EST (electronic spark timing)..maybe it'll help..
http://www.fieros.de/en/v6help/code42.html

Best of luck,
Steve