The vehicle in question is a 1989 Chevrolet S10 pickup, 2WD, 4.3L V6, automatic overdrive transmission (700R4 I think?), mileage estimated somewhere above 150K (odometer doesn't work).
A few months ago a problem began to develop where the engine would lose power and just sputter at different times, similar to fuel stavation. Usually the episode lasts no more than 2-3 seconds and then it resumes running normally.
It seems to be more frequent when the vehicle's torque converter is locked; in fact, on a number of occasions the sputter condition started immediately at TC lock, roughly 30-35 mph in third gear.
The vehicle has stalled completely a couple of times when the sputter condition occured at lower speeds; it has started up again immediately each time.
During this time, the ECM has reported no codes, so I've gone through the usual gamut of suspects: tune up (including new cap, rotor, and wires), ignition module, EGR valve (which was in fact not working, but replacing it did not make any difference in the problems), fuel pump. I even replaced the battery cables at the suggestion of a mechanic who noted they were moderately corroded and said that they might be affecting ECM voltage (it didn't help at all).
I've tested the ignition switch, and made sure that all electrical connections were good and tight. I've noticed no unusual smoke from the exhaust The exhaust isn't plugged as far as I can tell. Yet the problem persisted.
Yesterday, on the interstate, the engine began sputtering the way it had been, but this time it didn't sort itself out as it had in the past. Instead, it continued to lose power until even at wide-open throttle, the best the vehicle could manage was 5-10 miles per hour. I was fotunately able to limp the vehicle to an exit, but when I came to a stop and put it in neutral, the engine at idle was on the verge of stalling. It would not rev past about 2,000 rpm (and even at that speed it was really, really struggling). Disconnecting the EGR valve smoothed out the idle somewhat but did not help the poor-rev problem.
I had the vehicle towed home, and I was able to check the ECM for codes again; this time I got a 33. I've gotten conflicting information about code 33, but most often the answer seems to be MAP sensor - is this correct? Can a faulty or failed MAP sensor really cause all this grief? Is it possible to test a MAP sensor, or is it just some item you replace and just hope that was the problem? I really, really don't want to spend any more money than is absolutely necessary on this truck, and I'm tired of throwing parts at it without results.
Any assistance would be greatly appreciated.