87 GMC R1500 TBI 350 CEL comes on after pulling couple mile grade.

[Brock Griesbaum]

Hey new to the forum here, need some thoughts on an issue I'm having. 87 GMC R1500 TBI 350. After I pull about a 2 mile 10% grade just as I crest the top my check engine light comes on in the same place every time. I have no change in how it runs, no missing, no loss of power, no overheating. I've done a full tune up on the thing runs great. Timing dialed in. I do have the EGR plugged off but other than that everything works as it should. Would the EGR be making this happen or is there an issue being picked up because of change in altitude. Or that I'm keeping constant throttle for the length of the grade. It's not steep like I said 10 or 12% for a couple of miles. Note when I say constant throttle I mean like barely into it. Pulls like a champ. It's just odd that it comes on in the same spot on the same hill every time.. when I shut the truck off and restart it, SES goes out and stays out. Ran a diagnostic. Only code that I get is 12 ( diagnostic mode) and 24 (VSS inactive) but when it comes to code 24 wouldn't I get that regardless because the vehicle is not moving when I perform the diagnostic??? Any Thoughts. Thanks.

Brock

 

[gmbellew]

does your speedometer work?

 

[gmbellew]

but EGR does a self test and will throw a code. usually on highway at steady throttle it does the test. it would be code 32, though...

 

[fast 99]

Many codes SES will not illuminate unless problem is occurring at that time or hard fault like open circuit. Code will be stored usually for about 50 key cycles. If problem does not occur again in 50 cycles, it will be automatically cleared.

VSS codes can be set with an intermittent signal drop out, loose connection, wire break or rub, ect. First look at every connector and wire for pin push back, corrosion, wire rub/break.