Ok so I recently had my 1998 jeep grand cherokee 4.0 rebuilt. Now it is running but once you drive it for a lil while the check engine light comes on and gives an upstream o2 sensor code. But when hooked to a scan tool both o2 sensors are going crazy. We checked the timing at the distibutor and it shows to be about a tooth out but if you put it were it is suspoded to be it wont run. The scan tool says it is firing at 14-17 degrees before top dead center at idle. Can anyone tell me what it is suspoded to fire at.