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Multiple Cylinder Misfire P0300 mystery

pqrSTUVWxyz

NAXJA Forum User
Location
Beaverton, OR
I've read through and searched several of the past threads on this problem and cant seem to solve it as of yet.

I have a 1996 Cherokee
184k
4.0L
Automatic

What it seems to be doing is misfiring mostly in higher gears when cruising on the freeway, or at lower RPMs in general. You can hear it misfiring at idle once it warms up between 750 and 1000. When I'm on the freeway if I drop it out of OD and into 3rd and drive near 55mph and the rpms are around 2200 its barely noticeable.

I started with the easier stuff and have progressed from there.

Ran a bottle of Injector cleaner with a full tank of new gas
New Spark Plugs
New Plug wires
New Dist Cap
New Rotor
Replaced TPS
Replaced CPS

It has helped a little overall, but not fixed the problem. It still misfires at the above mentioned times and still throws a check engine light.

From this point what would you all suggest it may be?

I do not have an Ohm Meter to test with.

I was thinking that the way it is acting that it could be a plugged injector? What would you recommend to test for this? I didn't want to throw new injectors in it if I dont have to.
 
Throw a new distributor at it first, then a new cat because the old one has probably turned to stone from the un-burned gas that got dumped in it.
 
Try looking for a vacum leak in the evap system. Under crusing conditions the evap system will purge the vapors collected by the charcoal canister back into the intake to be burnt. If its not working properly it could be stuck open or have a leak in a hose. An easy way to check is try spraying some kind of non flammable liquid on the plastic hoses and around things like the intake gasket. If the idle speed changes you have a leak in that area.

Also you said you ran injector cleaner thru it. I kinda doubt its just one bad injector as that would only throw a code for that cylinder not a mulitple cylinder misfire code. You looking for something that effects all the cylinders not just one. Did you clean the throttle body at all? I'd try scrubbing that with an old tooth brush and some carb/choke cleaner as well as spray inside the IAC motor passage both above and below the throttle plate.


I really doubt that the converter is bad. You would notice a rotten egg smell or it would run like total crap all the time.

A possibility is that the timing chain is stretched out with that many miles on it.
 
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So you are advocating troubleshooting instead of just throwing expensive parts at a problem? :laugh:

You caught me! :cheers:
 
i just had the same thing on my 96, one of the new plug wires I put in had a crack so the plug was sparking to the block, then i got the 420 code for the recall, if you haven't done the cat recall yet I'd look into it
 
The way I approach a single cylinder misfire is to concentrate on all things related only to that cylinder:

1. Spark plug
2. Wire
3. Distributor cap
4. Fuel injector and associated connector and wiring
5. Internal condition of the misfiring cylinder (run a compression test on all cylinders)

You've done 1-3, but not 4 and 5.

For #4, you could try testing the #3 fuel injector electrically (time to buy that ohmmeter, you DO need one) or swapping the fuel injector on cylinder 3 with a different cylinder and see if the misfire code follows.

For #5, it is time to get that compression gauge out. Internal condition is often overlooked.
 
A quick check for free-

after dark - motor idling, look at distributor, and coil, plugs, wires for arcing, should NOT be any.
 
A quick check for free-

after dark - motor idling, look at distributor, and coil, plugs, wires for arcing, should NOT be any.

Refinement: using a spray bottle on "mist" do as orange suggested but add in "misting" the plugs/wires/cap/coil with water.
 
Warning, the previous post may lead to curling your hair. :gag:
 
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