Ummm maybe check your ground? Have the ECU checked out as well. The 4.0L had issues with piston slap untill 91'. The best thing would be to get a new 91+ jeep. I'm asuming that your talking about a cherokee. The 4.0l was not put into yj's untill 91. Befour that they used a horrable vershion of the 258 4.2L I6.
Better to be "dead and cool" than alive and uncool.
1999, Sport, 4-door, 4L-I6, AW4 auto, np231, 8.25 rear, dana 30 h-p.
BFG T/A KO P235 tires, KC 130w slimline lights, skid-row t-case skid more to come!