matpat19

I have a check engine light on my 96 cherokee. When I read the code, the reader says “P0300” for random engine misfire. But the problem is, the check engine light seems to like to come and go as it pleases. One day it will be on and stay on for a couple days, but then it turns off and stays off for 2 days max. We have replaced the cap and rotor, spark plugs, spark wires, as well as tried around different fuel types. Any ideas?
Also forgot to mention in the first place the misfire happens in all cylinders, but most commonly #2. Thank you for replies.

BruceB

P0300 (M) Multiple Cylinder Misfire Misfire detected in multiple cylinders.
P0301 (M) CYLINDER #1 MISFIRE Misfire detected in cylinder #1.
P0302 (M) CYLINDER #2 MISFIRE Misfire detected in cylinder #2.
P0303 (M) CYLINDER #3 MISFIRE Misfire detected in cylinder #3.
P0304 (M) CYLINDER #4 MISFIRE Misfire detected in cylinder #4.
P0305 (M) CYLINDER #5 MISFIRE Misfire detected in cylinder #5.
P0306 (M) CYLINDER #6 MISFIRE Misfire detected in cylinder #6.

Well..my thoughts... it could be a number of things that cause a "random" misfire in multiple cylinders.
Sensors going bad like the crank position sensor, camshaft position sensor, temp sensor to the ECU...ECU going bad, bad fuel injector, short in wiring to fuel injector, bad coil, bad spark plug, bad spark plug wire, bad ECU main ground etc.. etc... etc.....
This doesn't mean to roll out the parts cannon and start replacing anything.
You do want to stick your noggin under the hood and do some eye pokin looking for tell tale problems, loose or frayed wires, real dirty electrical connections......
Remove and refresh your grounds, many write ups on that here.
Remove your spark plugs and take a good look at them....
Make sure your battery cables are corrosion free and your battery is good...,
And after all that, just drive it until you get a different code identifying a more direct issue.
Hopefully some of the more skilled Jeep guys will add in anything I missed or overlooked or just plain out don't know.
Hope this helps.

tjwalker

With the P0300 code stored, you're looking for one common root cause, rather than a problem with an individual cylinder or the parts that involve them. But because you also have a P0302 (cylinder 2), you need to look at everything related to that cylinder a little closer.

While there certainly are plenty of possibilities, for the random misfire code I always start with checking fuel pressure. Must be tested with a fuel pressure gauge which can be rented from most parts stores if you don't have access to one, call around for availability. 96 was a year of change in the fuel systems for the Jeep Cherokee. Early 96 builds with the fuel pressure regulator under the hood had a fuel pressure spec of 31 psi. Later 96 builds with no FPR under the hood (all the guts in the fuel tank) had a fuel pressure of 49 psi, plus or minus 5 psi.

If fuel pressure checks out in spec, I would try swapping the fuel injector from cylinder 2 to a different cylinder to see if the misfire follows to that cylinder.

Also never a bad idea to test compression, a good snapshot in time of the internal condition of the engine. The compression spec for the 4.0 engine is 120-150 with no more than a 30 psi variation between cylinders.

This is a process of elimination. Let testing guide the way for you. Good luck and keep us updated!