96 cherokee sport. Miss after HG change.

[72mino]

I have a 96 cherokee sport 4.0 4x4 auto. Oil + coolant were mixing, so I changed the head gasket. A few months ago the water pump went out and it overheated which I assume is what caused the HG to go.
I also replaced the plugs, wires, cap, rotor, and upstream O2 sensor (MIL code).

Now with everything done it drives fine, but at idle it has an eratic miss, which seems worse when its cold. Its showing codes p0303 (cly 3 misfire) and p0300 (random multiple cyl misfire).
Its also still showing the O2 sensor code, don't remember the #, but its high voltage, so I probably have a wire shorted out.

Any ideas on the the miss or where the O2 sensor wires would have shorted.


Dustin

[fredjacksonsan]

Multiple misfires is a symptom of the Crankshaft Position Sensor going out.

Do a search on the Cherokee forum and you'll find you are not alone! There are a couple how to's (one is stickied to the top) and it's a pretty easy repair.

[72mino]

Changed CPS, fairly easy with 2 people, one underneath with ratchet, extentions, swivel, and socket, and one on top keeping socket on bolts. I dropped 1 bolt in the bellhousing and had to remove all the bolts from the inspection plate I could (exhaust was in the way for one) to get it back, but we got the bolt, put the new CPS on and it didn't make a difference.

Any other ideas, also does anyone know where the front O2 sensor wires could get frayed.
Could the high voltage coming from the O2 sensor cause the miss at idle? What about a coil going bad?

Dustin

[gtmud]

WHen you had the HG done, did you have the head magna-flux( crack checked). When its cold, get the compression of # 3 and 4. see if they are about even.

[72mino]

Thank you for your input.
Yes I had it milled and checked for cracks. I have not check the compression, but I doubt that is a problem since it isn't all the time. I'll find time in the next couple days and post what I found. I might try another cap and rotor also, they're cheap and I could have gotten a bad one.

Dustin

[72mino]

GTmud - Is it important that the engine be cold? I checked compression, but my wife had been out eariler and it definately wasn't cooled back down. I tested all 6 cyliders and they were all 145 psi except cly 5 which was 148.

Dustin

[72mino]

Thank you for all the ideas so far, but...

The misfire is getting worse, now it sometimes sputters for a split second when taking off. Also, I am now getting codes for cly 1-4 misfires and random multiple cly misfires.

If the coil is going bad could it cause a misfire or would it just go from running to no spark??

What about the distributor itself?

Any way to test them?


Dustin

[72mino]

I thought I had the problem tracked down to a bad injector (no change when #1 injector unplugged), so I replaced all of the injectors with 19# mustang injectors. Now it runs even worse, and I am getting code p0201 - fuel injector circuit malfuntion cly1.
Now I'm thinking that a wire for injector 1 could be shorting with an O2 wire causing both the miss and the O2 code. I unplugged the O2 sensor, routed the plug to the top of the engine and started checking continuity...Bingo, maybe...One of the injector wires has continuity with 3 of the O2 plug wires, then I check it with a ground and they have continuity. Should 3 of the O2 wires be grounded? How do I find the short if there is one? Any other ideas?

Dustin

[72mino]

I finally found the problem. The #1 spark plug insulator was cracked.