Engine light while slowing down

Just got a 94 Grand Am 3.1 V6 automatic. Car runs great, and just had a used engine put in with approx 120 000 km (75,000 miles). The engine light comes
on when I am on the highway and I drop in speed by about 10 km/h. If I travel around in local traffic even upto 80 km/h it does not happen. Only at or above 100, and then coast down by 10 km , the light comes on. If I turn the car off, it is reset and the light goes off. It has the OBD1 diagnostic connector, and the garage said something about a map sensor or possibly O2 sensor, so they replaced both of those. The problem persisted, then I believe they said they replaced the ECM. The problem went away for 1 day then returned.
Any thoughts ?
Regards, KDawg
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Did they say what trouble codes they found?
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Robert Hancock Saskatoon, SK, Canada
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I didn't get the codes from them, but they said something about the code indicating that the exhaust was running too rich. Not sure if it has anything to do with the problem but it did pass an emission test, or at least they gave me a paper which says it did.
Regards, KDawg

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It could very well be an EGR Valve clogged with carbon, maybe?

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