My brother in law has an '89 Conquest TSI that has an odd problem. Any time the boost gauge hits 0 on the boost gauge, it just pops, and wont rev. He replaced the engine with one from an '85 as his lifted the head and the '85 was a fresh build from the previous owner.
I'm guessing its a sensor or something as it started when the cars engine was swapped, and hasn't ran correctly since. Any help in guide me in the right direction would be appreciated.
Not enough fuel pressure? Wrong sized injectors? Plug gap too big? Other symptoms?
idles good and drives fine out of boost, just hits a wall anytime the boost gauge tries to go over 0.
Check the ignition timing on the new motor?
Good fuel mileage? Does it have good vacuum at 3000 rpm (with no boost) should be higher than Idle vacuum.
Bad map sensor/line?
Sounds like a map sensor issue. Does he have any kind of AFR monitoring?
I'm in WV & car is in Ohio. can someone show me where thd MAP sensor is? Also are the '85 Injectors the same as the '89? seems to me I read somewhere they are different.
No, the 85 had the same size primary and secondary while the 89 had a larger secondary. If he is running the 85 injectors it will be lean and could cause this problem if it's THAT lean. It's actually better to sit at 0 boost sputtering than go all the way to 12 while leaning out. 
IIRC the way those goofy things work is that at a certain point the computer turns on the secondary injector, and when it does this it drastically reduces the duty cycle on the primary injector and then the duty cycles of the primary and secondary go up together. So if it is trying to switch the 2nd injector on at 0vac / wot and cutting the PW on the primary and the secondary isnt doing anything or not enough, it WOULD get really lean which would cause the surging/popping.
Yeah if he is still running the '85 injectors that will definitely be your problem. Different secondary injector and tune on the '89
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