I was curious what the compression was on my 92 xj 4.0 5 speed with 174,000 miles,so I did a compression test today. the 1st 3 cylinders were 140-145 psi. cylinders 4 and 5 jumped all the way up to 160,and cylinder 6 was back down to 145. is this ok,or should I be worried about the 2 cylinders that were higher than the others? also,the plugs had a tan/white residue on them. Is this normal? I didn't replace the plugs,they were new about 6,000 miles ago. I just cleaned them off,checked the gap and re-installed them. thanks,Ben