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88 mj backfires at WOT

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O2 sensor could cause this too if its bad. My 2.5 would act like that sometimes while accelerating. I tried several things with the fuel system hoping to get it to go away, finally put a new O2 sensor in it just because, and as soon as I took it out on the street, it smoothed out and has been running great ever since. But like the other posters said, check the EGR valve for proper operation and closing ( it would cause a vacuum leak if it didnt close all the way), Vacuum lines, fuel pressure, fuel filter, check that the intake manifold bolts are tight... My 258 I had in My Grand Wagoneer would do this, tightening them regularly would keep things working properly. But I don't think the 4.0's had much of a problem with this. Good luck!

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