Jeep Enthusiast Forums banner

Not firing on all cylinders

5K views 17 replies 7 participants last post by  Newtons3 
#1 ·
My 89 xj that I swapped an 87 motor into, they were both 4.0, is not firing cylinder # 2. When I pull either the plug wire or the injector plug, with the engine running, there is no change in the way that the motor is running.

Any ideas? It runs, but not smooth.

Thanks in advance.
 
#5 ·
If you know you have spark...make sure fuel is coming out of the injector. Could be something in the fuel rail or wiring harness going to the injector. Unplug the injector from the manifold, point it somewhere safe, and start the engine. If fuel sprays out of it......go back and check your spark.

If you are still 100% sure you have good spark, do a compression test on that cylinder. Could be a fubared cam lobe/spring/lifter or something.
 
#6 · (Edited)
Ok, here is where I am now. I checked the spark with a new plug in the spark plug wire, while another plug was still in the cylinder, and it sparked fine. I then swapped the injector plug from #1 to #2. At that point #1 was the cylinder not firing.

So I am guessing it is the connector. What is the best way to check it?
 
#9 ·
My guess is that the injectors get pulsed too fast to see accurately on a multimeter. You're going to need an oscilloscope. The factory service manual would tell you exactly what to do. At the very least you can do some continuity tests. I imagine the injector harness is directly connected to the computer. What you need to find out now is if it is the connector or the wiring, and if its the wiring.....you'll have to start search for broken/shorted/open wires.
 
#11 ·
Won't it pulse the injector once every other rpm? Even at 750rpm that is 6.25 pulses per second. Even the fanciest Fluke meter probably wont adjust that quick. You'd end up with an average.....the meter will yeild slightly varying voltages somewhere in the middle of the voltage range between 0V and the pulse voltage.

Still....this should be good enough. Anything is better than nothing. That means the injector is getting pulsed. If it is 0V...see if you have continuity to ground. That probably means the wire is shorted. If its not shorted, find the other end of the wire that plugs into the computer. See if you have continuity. If you don't...the wire is open somewhere.
 
#13 ·
Well it appears that things have righted themselves. I remembered that I had read on here that after replacing the Crank Position Sensor that you need to disconnect the battery for about 30 minutes to reset the computer.

That is what I did, and now it seems to be firing on all cylinders.

Thanks for all of the help. This forum is a definite wealth of information.

Thanks again.
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top