88 Chevy truck, service engine light

350 TBI, 700R4, about a jillion miles on it (seriously, at least 350-400k, maybe more). It normally runs like a truck with 20,000 miles, except for valve guide smoke at startup.

I drove it about 15 miles on the highway today, and then as I was slowing for a red light, it stumbled and the Service Engine Soon light came on. The engine continued to run. I didn't need to give it extra throttle, but I thought it might stall, and the idle was a bit rough.

I pulled into a lot and raised the hood with the engine running. It was still a bit rough, and the (recently replaced) belt tensioner was rattling from belt bounce due to the rough idle. I had to drive about 5 minutes to my destination, so I got back on the road. There was no change in the way the engine ran during this time. Temperature, oil pressure and voltage were all normal.

I shut off the engine and went into the store for about 30 minutes. When I came out, the engine started and ran normally, and the service engine light was off.  Fifteen highway miles home, and nothing abnormal.

I've had this truck since about 2008, and that's the first time I've seen this light; I didn't even know it was there. Is this OBD 1? Is there a way to follow up, by triggering a pattern from the warning light flashing to direct you to a specific system?

It's OBD1, you can get the code by putting a jumper wire or a paper clip between terminals A and B of the ALDL port under the drivers side of the dash.  I'd wager you will get a code 33 or 34, and it's probably a failing MAP sensor.

This site has the procedure for getting the codes Read GM OBD1 codes

In reply to 06HHR :

Thanks, that's what I needed to know. I'll reply again when I learn something.

Generally speaking with a tbi engine you’re looking at something in the distributor or the coolant temperature sensor.  

Yeah, I'd put money (like a dollar) on it being the coolant temperature sensor derping and cause it to run like butt. Happened to be 350 TBI-equipped wagon. It would be fine until the sensor decided it was -40°F and the car would dump in fuel to combat the issue.