97 tracer P0401 p0402

97 mercury tracer, 100,000 miles.
Check engine light is on, ewe have replaced gas cap, egr valve and the pressure sensor valve.
P0401 = EGR flow insuffiecnt
P0402 = EGR flow excessive

Please advise

97 Mercury Tracer, 120,000 miles. Check engine light on. P0401 and P0402 codes (excessive and insufficent egr flow). We have replaced the gas cap, pressure sensor valve, egr valve, several hoses, the front oxygen sensor and welded a crack in the tube between the manifold and the catalytic converter. Codes still present. Finally took the car to an emissions specialist to diagnosis ($85.00). They tell us that the the two passages that run under the egr valve to the intake manifold are completely clogged with carbon and advised an 'air induction cleaning' for $300.00. Apparently, this involves removing and cleaning the throttle body, remove egr valve and clean passages, 'air induction cleaning' with a 'special' chemical and replacing the egr gasket and throttle body gasket. We would like to do this ourselves to save the $250.00 labor charge. Any advice on doing this? What chemicals or cleaning method to use? Thanks~