Shout out from Jeep Country Pennsylvania! My daughter has a hiccup!
 

My daughter saved up and bought her first vehicle - a beautiful 2001 Grand Cherokee. Unfortunately not long after having it it would start running horribly. No CE light but it did throw P 0301 and 0306 (cylinder 1 and 6 misfire). Usually once it warms up well it starts misfiring and continues until you shut it off, it will start and run well for a bit then have the same issue. If I floor it it actually cuts completely out for a bit. Some days it won't do it at all. To date I have replaced

Coil rail
ECM (used)
Crankshaft sensor
Cam Sensor

After reading about some similar issues on here, I believe the same coil powers cylinder 1 and 6 - so I thought maybe the input wire from the computer was damaged so I did an ohms check (actually ripped harness apart between coil rail and ECM but everything looked fine.

Any thought on where to go next?

THANKS for any thought you can share!

Welcome to CF!

Look for a bad connection on the pins at the PCM. The ignition coils ground thru drivers in the PCM. That's how they work; they are supplied with 12 volts and each coil is grounded thru a digital driver in the PCM, Actually ungrounding to fire. The coil magnetic field collapses to create the high voltage spark. You are correct in that there are only 3 coils and #1 and #6 fire together at the same time. It's called "wasted spark" and is designed to burn any fuel in the exhaust going to the converter.

Wasted spark is not a new concept. It was originally used on small 4-stroke utility engines like Tecumseh and Briggs because they mounted the ignition points on the crankshaft under the flywheel and fired every revolution for simplicity. That's why they "bark".

Coincidence?
 

Okay, I know this may be coincidental but it seems that when I don't turn on the climate control the jeep runs fine. Is there any possible way this could be connected to a 1&6 cylinder misfire?

I can't see any way they are connected, but you never know. I had a friend with a Saturn that lost a cylinder (misfire) after they installed a remote start unit. It seems they nicked an injector wire when drilling a hole for their unit. That was hard to find because it was in a bundle.

The climate control works thru the Body Control Module and I don't see how it could effect the ignition.

Thanks Dave. I'll keep digging!

If you're interested, you can download a Full Service Manual for your 2001 WJ from pacificcoastmanuals.com for $7.99+tax. It's 93mbs of compressed pdf files and the best source of information I've ever seen and is year specific so you don't have to wade thru useless stuff about other models. It has very good circuit diagrams (as good as any get!), diagnostic procedures, component locations, and access info. Even explanations of how the systems work. It's worth every penny. The one thing I've never seen anywhere is the internal circuitry of the PCM. Not here either.

Also check out wjjeeps.com for lots of interesting stuff about WJs with access links for WKs and WK2s.

Gotta throw my two cents in here:

Take both cables loose from the battery, and clean both the terminal and the post. The computer is finicky about constant voltage, and that little bit of crud (that you can't see from the top) can wreak havoc. It is free and easy to do, and could save you a bunch of wrath from Cruizer. (He hates when people buy parts unnecessarily. )

My 99 and my 94 both had goofy issues that were resolved by cleaning/replacing the terminals. The 99 acted like it had jumped timing and would stall at idle unless I had my foot in the gas. The 94... well I forget exactly, but I think it was shifting erratically or some such terrible symptom. It had a bad battery altogether, but the terminals were about rotted out, so both got changed and it was fixed.

Thanks Ranger! Sometimes it's the simple things that will get you. Very good point. In my case, it was corrosion inside the molded lead cable ends only visible at the ends of the wire insulation where it goes into them.

Okay guys, thanks for the input and ideas! Cleaned up the battery terminals. We will see what that does. Another idea I want to run by you : I have noticed a high pitched whistle once the vehicle warms up. Could be vacuum lines but have inspected them and research suggests that it could be a leak between the intake and the head. The location of the whistle seems to support that. Would it be possible that a leak there could cause a misfire? It does seem that the misfire doesn't happen unless the whistle is present. It was suggested that the manifold bolts be retorqued. Is this a reasonable step?

A good way to find an intake manifold leak is to use a propane torch. NOT LIT! Crack the valve open and play it around the gasket area. The propane enrichment will cause a rise in the idle and if you're good, you can isolate the leak to a specific spot. It's also dry and won't make a mess like carb cleaner will. Also, check the injector o-rings on 1&6 before you pull the manifold. Those can be checked with a couple drops of oil around the manifold o-ring to see of the whistle changes. Also, the oil will disappear.

That last manifold bolt in the back is a PITA to torque and is a probable source of a leak.

Some new info!!!
 

Question: 1) What position is cylinder 1 and 6 in? They fire together so I assume they are either the front or back two cylinders?

2) can a miss send the vehicle into limp mode?

3) If the high pitched noise I hear changes pitch when slightly step on the accelerator, does that help in diagnosis?

I went through the vacuum lines very well but did not find any leaks.

So apparently the incredibly rough running is because the vehicle is going into limp mode. It seems that once the vehicle warms up, as long as I keep the RPM's up it runs fine but if I allow the RPM's to fall that is when it intermittently goes into limp mode. I continue to have codes for cylinder 1 and 6 misfire. I have a new code - mass air leak (455 I believe).

Thanks for all your input.

The cylinders are numbered 1 thru 6, front to back. 1 & 6 fire together because when 1 is producing power, 6 is on top of the exhaust stroke. The first coil on the rail has a long wire inside the cover that goes to the #6 plug and that's the usually problem that causes a misfire. It shorts out internally. You've already changed the coil pack so I'm assuming that's not still the problem, but it could be the coil feed wire, connection, or the driver in the PCM. If those are the case, your original coil pack may still be okay.

"Limp Mode" refers to a function of the transmission that is turned on if there's something wrong with the electrical circuits in it. It locks the transmission in second gear ONLY so you can at least drive it home. There's no limp mode function for the engine. You obviously have a vacuum leak. Check the throttle body mounting bolts for looseness. Use a piece of hose up to your ear and more it around the engine when it's running to see if you can locate it. Check the first front and last rear manifold mounting bolts for looseness also. Make sure that whistle isn't behind the dash because the brake booster and climate control can do that.

Dude! You are so helpful! Thanks!!

That's what I'm here for, to pass on the stuff I know about. It gets very tedious at times, though! Some people need to be hit over the head with a 2x4 to get the point. NOT saying you do!

Haha. I probably qualify for the 2x4 once in a while.