I don't think the X-pipe could do that but when installing it you may of mess some thing on the engine may be the O2 sensors are not working? I really dont know
I hope you get it working!
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1988 LX 5.0 Was 3060lbs (full Tank) 14.65@101.3 On a Gtech Drivetrain: 3.55, World Class T-5, Lakewood Bellhousing, Adjustable Cluch Cable, AL Driveshaft, Manual Steering Rack Suspension: MAC Control Arms,Maximum Motorsports Front Grip Package, H&R Sport Spings, Bilstein Shocks
Engine: Steal Mounts, K&N, Under Drive Pulles Exhaust: 1 5/8 Un-Equal Length Mac Headers, X-pipe W\cats, Edelbrock cat\back Cooling:3 Core Radiator