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First place to check is what you have done . Check for the correct firing order of the plug leads as it is possible to cross 2 wires. Next make sure that the leads are not touching each other as this will cause cross over and fire the wrong plug. Next check the sensor position as there is an air gap to the rotor that has to be correct. Check that the sensor is tight and that there is no dirt in the air gap. check that the wires are properly connected . Lastly run the fault codes to determine if something else is not causing the problems.
i had a similar problem with a 1998 grand cherokee 4.0 had a no start condition changed the crank sensor the car started idled fine no codes till you drive it then it was giving p0430 p0460 misfire cylinder 4 and 6 and ran rough no speed didnt make sense ran good before sensor went bad removed new crank sensor on the bottom it has a round cardboard glued to it so it does not go in all the way and get caught by the flywheel and break its for the air gap . paper says it will shear off when you start the car problem was it wouldnt let the sensor go in far enough for the proper air gap so when i re-installed the sensor the second time i held it in with left hand tightened the bolt with right hand started the car and it idled fine again but when i drove it it was fine and no codes
Backfiring can occur in engines that are running lean because the air-fuel mixture has insufficient fuel (too much air in the ratio) OR whenever the timing is too advanced. On modern vehicles unless the timing chain or timing belt has slipped (not likely) you simply have an air leak that is permitting too much air to enter the motor, causing hard starts and poor driveability, especially at idle. Check for a hole or loose fitting connections in the air duct from the air filter to the throttle body. Make additional checks for holes in the rubber hoses between the PCV valve and the motor (PCV valve hoses run to the intake of the motor). Even small holes or gaps can cause significant issues. if you can't find the leaks with visual inspection and feeling or removal of the parts and replacement, try spraying carburetor around the intake and hoses. If motor revs it means there is a "leak" there letting air into the motor, (bypassing the mass air flow sensor that regulates air volume).
When installing a crank shaft sensor, you need to use a "gauge feeler" to set the gap for the balancer. It should be spaced about .24 - .26 hundredths of and inch. If you do not have this tool, nor can not obtain or purchase one, you can use a match bood (open, flat part, folded over).
DO NOT try to start car until this is gapped or you will damage the sensor and will have to purchase a new one. I learned this the hard way on my Buick Centry LImited.
Also, if you have properly gapped the crank shaft sensor - you may need to replace the cam shaft sensor, which should located just above the crank shaft sensor.
Please let me know if you need further assistance.
Guru, Premium Expert @ Fixya.com
I still think this is a timing problem, most likely with the crankshaft sensor. However, it could also be in the throttle positioning sensor, since you had raised the idle to get it to work momentarily. Also, the mass air flow sensor cannot be ruled out entirely. I would start with the crank sensor, then the throttle positioning sensor. Try these and let me know.