bseries2093

i have an 88 integra with a poormans type r. i have had the swap done for a while now but i have this problem.
if i get on it hard and take the rpm to 7500 or so then get off the gas sometime the check light will come on. the light come on for the map sensor. code 3 .. ive done all the voltage checks on the sensor and it check out fine. i even replaced the sensor and still the same problem. im thinking the ecu is the problem but someone suggested maybe its mechanical. he said the exhaust may be to small ...
the exhaust i know is too small its the dc headers with 2in collector to custom 2.25in exhaust. i cant find any vacuum leaks .
everything is obd0 right now. i dont know where else to look. Has anyone ever had this problem before.? would timing cause problem like this.