94 Sonoma Check engine soon

I keep getting the check engine soon warning on my 94 Sonoma, 4x4, 4.3, 5sp. It only comes on after about 30 to 40 minutes of driving. The temp is good,
oil pressure is good, oil level seems fine but it's hard to tell exactly because it splashed up the stick real bad. When I shut the truck off it resets immediately and I can go another 30 to 40 minutes before it comes on again. Anybody heard of this before. Brian
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egr valve stuck or the vacumm port feeding it is plugged with carbon
snipped-for-privacy@ns.sympatico.ca wrote:

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Really should get it scanned and see what the code actually is, the only sure way to diagnose.
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Thank you very much for your replies, this is an awesome newsgroup and I need more help because this isn't going very well. I tried to read the codes according to the Haynes manual and it said I have to short pins "A" and "B" on the ALDL terminal in order to get the "Service Engine Soon " light to flash to the proper code sequence starting with code 12; which is the code to say the diagnostic is working properly. My 1994 4.3L is a TBI manual tranmission so I think I am reading the manual correctly in that I have an OBD 1 model. When I do the diagnostic the code twelve should flash three times on the SES but it comes on steady as if I haven't shorted the pins; which leads me to believe that the PCM is not functioning properly and should be replaced. I checked pins "A" and "B" and I do have the 5V reference voltage. The manual doesn't tell me what to do if code 12 does not appear. Any suggestions?
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Are you turning the ignition on first and then jumping pins A and B?
Or;
Are you jumping pins A and B and then turning the ignition on?
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snipped-for-privacy@ns.sympatico.ca wrote:

94 is an oddball year, 93 was the last ODB-I and 95 was the first real ODB-II, 94 was sort of a transition year and is somewhere in between, some call it OBD 1.5. You will need something to read the codes there, the short A&B won't work.
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I've tried every combination possible to read the codes, including a reader but no luck. I did put a vacuum pump on and the EGR valve is shot. That should resolve my SES light coming on but I still can't read any codes. I guess the next step would be to change the PCM. I hate to do that since all the other functions appear to be working but it's useless if you can't read the codes. Anybody have any last resort theories before I change it?
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Start at square one...
Does your truck have a 12 pin ALDL?
If yes, are there terminals occupying cavities "A" and "B"?
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Yes 12 Pin ALDL with "A" , "B" and "M"
"A" has the 5 v reference signal, "B" is ground and "M" is the data line.
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