misfire

i have a 1990 xj with 210000 miles 5 speed. it misfires in mid rpm range with the throttle about halfway . full throttle and if runs fine. putting around does fine but the average getting anywhere if backfires like a mother. i know its a misfire because i put a timing light on it and just to see if it missed out and like every other rotation of the crank it would fire. i already put a throttle position sensor and crank position sensor in it from what i read here but still missing out. im lost so please any info would be great. thanx

I see that you have put in a new TPS . Did you adjust it when you installed it ? Check and clean your all eletrical connections AND grounds under the hood . Also make sure that the Vac line going to the MAP is plugged in . Backfiring is a sign of a lean fuel mix , I would check to make sure that your intake manifold bolts are tight and that there are no vac leaks . Have you checked your fuel preasure ?

"every other rotation of the crank it would fire"

That sounds correct to me...

The four strokes of the cycle are intake, compression, power, and exhaust. Each corresponds to one full stroke of the piston, therefore the complete cycle requires two revolutions of the crankshaft to complete.

ive checked for vaccum leaks and have found nothing. i have sprayed starting fluid and very lighty on all vaccum lines and intake and such to see if it would pick up any from the fluid and nothing. but it has a misfire that i cant figure out. its not firing all the time. i think that the fuel pressure was at about 30 psi , give or take a couple

i checked the fuel pressure about a month ago. this has been a problem that i have had since i bought the jeep last febuary

I got the same problem but no backfire. It's not a friggin ground, my cam sensor, crank sensor, fuel pressure, fuel filter, my tps, or my cap, rotor, wires. The only thing ive left to try is different plugs. O, and i put a new computer in it. I also have a 94 5 speed 4.0L. O i also forgot the IAC, O2 sensor and all my vacuum lines are excellent. If you figure it out let me know. Yeah to add more the ignition coil was replaced and the whole distributer (asking alot of different mechanics gets alot of different guesses which in the ends costs a hell of alot of money!) All the above has been checked with various dealership tools, meters, and scanners and/or replaced. My skip presents itself at the same point, half throttle = skip, WOT = fine. I did pull my plugs and it seemed to be that my plugs may be too hot so i bought stockers hope to replace them tomorrow.

94XJ2door5speed said:

I got the same problem but no backfire. It's not a friggin ground, my cam sensor, crank sensor, fuel pressure, fuel filter, my tps, or my cap, rotor, wires. The only thing ive left to try is different plugs. O, and i put a new computer in it. I also have a 94 5 speed 4.0L. O i also forgot the IAC, O2 sensor and all my vacuum lines are excellent. If you figure it out let me know. Yeah to add more the ignition coil was replaced and the whole distributer (asking alot of different mechanics gets alot of different guesses which in the ends costs a hell of alot of money!) All the above has been checked with various dealership tools, meters, and scanners and/or replaced. My skip presents itself at the same point, half throttle = skip, WOT = fine. I did pull my plugs and it seemed to be that my plugs may be too hot so i bought stockers hope to replace them tomorrow.

Click to expand...

A 1990 has a Renix system . And ground problems are very common with 87 to 90 models . Head to firewall and down by the dipstick .

if its a fuel related problem then how does it not fire everytime its suppose to. it is NOT FIRING all the time in the mid range.

Backfire sympton can be caused typically by fuel starvation, timing problem, inconsistent spark and weak spark, carbon buildup, valve problems, ...probably a few other things...so, since the camshaft position sensor provides fuel injector timing I suggested that might be the problem, because if the spark is there without the fuel pulse there to be ignited, it will feel like it's not firing, and misfiring/backfiring....

If your confident the fuel system is working correctly, including the fuel injection timing and injectors, then I guess you need to focus on the spark timing.

i know its got a misfire in the spark timing. i put a timing light on one of the plug wires and had a buddy hold the button while i was in the jeep reving it up and the light was blinking fine and then got really messed up and when i floored it, it went back to normal

well i finally figured it out. the other day i was on here and on a thread someone explained with pics how to index your distributor. well i put the timing mark on the harmonic balencer as 0 degrees and pulled the cap off and it wasnt pointing the right direction. it was pointing to where number 5 spark plug wire on the cap was. the plug wires where on wrong also. i never pulled the distributor untill tonight so it wasnt me. they had number 1 plug wire where 5 should be. number 5 where 6 should be and so on. well you could see on the cap where it was just barly firing on the edge of the points on the cap and on the edge of the rotor. well when i gave it gas the computer tried to adjust the timing and the rotor bug wasnt in the right spot and wouldnt fire. so i pulled the distributor. moved it back 2 or 3 teeth, set it back down put the plug wires on right and it runs great now. im still not sure how it ran before but it did. but it finally runs great now.
thanks for your responses