Various/intermittant Cylinder Missfires Jeep Cherokee 4.0.

I know I've mentioned this before, but I'm still having issues, even after doing some work. Ok, this has officially stumped me. Here's the story...

About 3 years ago, I noticed one or more of my cylinders were missing at idle. This happened right after running some seafoam through the vacuum line on the brake booster. I took it to several shops and they all had a different diagnosis.

1 . My spark plugs were no good...

-My problem with this diagnosis is that the plugs were only 3 months old (were replaced after it started missing) and were Bosch Platinums. He changed the plugs out for Autolites, and the engine ran better, but still not quite right.

2. Timing appeared to be off...

-Had the timing set professionally and put in brand new distributor (I'm not sure what they call it on the 2001s, but I only know it's where the distributor would go) and cap. No change.

3. Crank Position Sensor was bad...

-This seemed the most plausible cause, however one day my Jeep failed to start. Took it to the shop and they replaced the CPS and it fired right up. However, it's still missing... After firing it up, I had them check the compression of the cylinders... seemed one cylinder was pretty low when first starting, then pressure climbed to 95lbs after a few seconds. Which leads me to believe oil is getting in between something and sealing up the cylinder. Possibly the piston rings?

Another thought I had was maybe the valves need reseated or replaced. Also was thinking about replacing the head gasket or even getting a brand new head with valves already in place.

This is something that's been going on for almost 4 years. And I'm afraid to drop $400 for a head assembly to find out it's not the problem. Also, replacing pistons and rings is something that I have not done myself yet. But I'm not afraid to try and tackle it if I need to.

The only other option I've thought about is just getting a new 4.0 longblock and putting the exterior components I have onto it and go from there. But $2000 is money I don't currently have. Trying to fix the problem rather than work around it.

I would appreciate any ideas or advice!

 

Almost al 4.0 have a slight miss at idle mine does it, my roomates does, it his girlfriends does it. Although the 2000 and 2001 4.0's had problems with the heads cracking so keep an eye out for that.

 

I think I've heard somewhere before about the heads cracking. Hmm... If the head cracked, that may explain the missing...

...and...

I've also noticed a drop in HP. My mom has the same Jeep but her's is a 99 and her Jeep has more giddyup than mine. Way more. And her's is all stock. I upgraded the air intake and put a flowmaster 40 series muffler on mine. I noticed an increase in HP but my mom's is still better. Very curious.

Now that you mention it, my mom's Jeep has a very slight miss every once in awhile too. A friend of mine also had the same Jeep and the same problem. Thought maybe there was epidemic of missfires in Jeeps lol!

 

I'd try cleaning the throttle body. Make sure to only use cleaners made specifically for throttle bodies. Make sure the PCV valve, hoses, and the connecting ports are clean.
I've found that doing the simple stuff first often solves the problem!

 

This may sound like a dumb idea, but I'd often considered getting a 302 and a tranny from a 4WD truck and sticking that in place of the 4.0... Would this be plausible?

 

Have you replaced the coil pack?

 

Jeeps don't have a pcv valve they have what a ccv. On mine I've tried everything you can thing of to get it to go away even changed the injectors but still has a slight miss at idle.......I've just learned to live with it.

 

Thats an affirmative. I replaced plugs, coil pack, fuel pump, cleaned the injectors... No difference at all.

Same here. I've pretty much given up. At first it seemed electrical, but now I'm starting to wonder if it might be mechanical.

But I'm still wondering about switching out the engine to a 302 and get a 4WD tranny behind it... Wonder if there's a way to do that and keep the 4WD.

 

CCV ( closed crankcase valve ) is like a PVC valve, but with no moving parts. It's just an orifice in the fitting. It can clog up and make the engine run rough.

 

Yes but it requires a new transfer case also. You can probably use the YJ conversion mounts because the 302 is as common a swap into jeeps as the 350 anymore.
The biggest problem is you have to use an as new or newer 5.0 and you have to go through a whole IM certifications process in some states, in other states you might not even be able to drive it on the road after the conversion is done, unfortunitely I dont know OH law so you would have to check with your DMV.

The 4.0 is one of the best engines AMC made i was really sad to see Heep discontinue it and between the horrible looks of the new wranglers I wouldnt consider buying a new Jeep product..

 

Yeah I agree the new Jeeps look terrible. I know the 4.0s are good engines but I'm looking for a bit more pep from my Jeep. Since I know the 302s pretty well, I figure I should go with an engine I know something about.

EDIT: And as far as the transfer case, I was figuring on finding a donor tranny and transfer case from a 70s F100 4x4. My only concern, is the gear ratios out of the transfer case... would they be the same as the Jeep ones? Or doesn't it matter? Last thing I want is for the front wheels to spin slower/faster than the rear end.

What is IM certification? And why would it matter what engine I put into my Jeep so long as it passes emissions and drives? I would love to bypass the computer and go straight to a carburetor setup and replace all the gauges with older manual ones.

 

How would that work that'll cause th transfer case to bind up and break something. The 4.0 has almost as much up as a 302 and probably the same torque. You should look into building a jeep stroker motor for more power that's what I'm doing to mine when I get enough money to finish it.

 

I meant that as that's what I don't want to happen.

 

Unfortunitely its a federal law that you cant put a carbureted engine in an EFI vehicle even if it passes the sniffer test..

Here you go with the exact verbage

For light-duty vehicles, installation of a light-duty eng~ne into a different light-duty vehicle by any person would be considered tampering unless the resulting vehicle is identical (with regard to all emission related parts, engine design parameters, and engine calibrations) to a certified configuration of the same or newer model year as the vehicle chassis, or if there is a reasonable basis for knowing that emissions are not adversely affected as described in Memo 1A. The appropriate source for technical information regarding the certified configuration of a vehicle of a particular model year is the vehicle manufacturer.​

http://www.epa.gov/Compliance/resources/policies/civil/caa/mobile/engswitch.pdf

This is just the federal documents , you need to check with your state agencies to see if the state has added anything on to this.

 

Well, I guess that tosses that out the window... I'd rather not put a fuel injected 302 in my Jeep anyway. I'd be worried about hood clearance. Now what if I waited until 2016 and got Classic car plates for my Jeep. Would it matter then?

Or I could just put my money and engine into a Maverick.

Was trying to think of an alternative for an engine. But maybe stroking the 4.0 and doing a rebuild might not be a bad idea now that MaverickDan brought that to my attention. Hmm...