Rkeenan

I'll start with the basics. 2000. Grand Cherokee. 4.0 4wd Laredo 190k miles.

Truck had a blown head gasket. Took the head off and sent to the machine shop to be checked. Sure enough the head was cracked. Ordered new head. Installed new head, thermostat, temp sensor, oil pressure sensor, spark plugs (NGK), all "PCV" grommets, hoses, elbows etc. lifters, pushrods, rockers, pivot ***** and saddles. During reassembly the intake manifold cracked so that was replaced as well.
After all was reassembled the truck fired right up and ran with no problems. Truck ran great for about 150 miles and now has a misfire on #6 when it gets up to operating temp.
-I swapped injectors around and mis stayed at #6
-Replaced coil rail. No change.
-Swapped plugs around. No change.
-Sprayed throttle body cleaner looking for vacuum leak. Nothing found.
-Leak down test on all cylinders when cold and when hot. Cylinder 6 went down a couple percent and is at 9% leak down when hot
-Pulled valve cover back off to check for stuck lifter. When #6 is at TDC compression both pushrods feel the same. (Neither has more resistance/movement than the other nor do they feel different from other cylinders when they are at TDC compression).
-Rechecked valve train and head torque.

I am a Honda tech so this isn't the first motor I've been into. However it's the first Jeep motor I've messed with and did it as a favor for a friends son. Murphy's law has taken hold and I'm at my wits end. I may have left out something but I'm looking for any advise from you guys to help get this truck back on the road.
Thanks in advance

dave1123

It's entirely possible one of the valves in cyl #6 isn't seated properly because of an incomplete grind. Buying a new complete head isn't always perfect and you shouldn't have to check it before you install it, but lately there have been problems caused by poor prep or bad parts.

I just watched a video where a guy checked the valves for seating on a new Dart head by standing it up and filling the intake ports with carb cleaner. All 4 valves leaked the solvent into the chamber! Checking them with Prussian Blue showed the seats weren't ground equally at their 3-angles to put the mating surface in the middle of the valve.

I bought a "Clearwater" head for my 2000 4.0 and never checked it either. It did it's job properly, but I wound up using an extraordinary amount of oil with smoke out the pipe within 30K miles. The compression was great, and except for the oil, I had a couple plugs carbon up. I went back in and changed the valve stem seals. It seems they must have used New-Old Stock seals and they were hard and not sealing. I installed Fel-Pro Viton seals and that cured the problem. When I say it was using a lot of oil, I'm saying it used a quart in 250 miles! After I fixed it, it took a week for the oil to burn out of the converters.

The point I'm trying to make is don't trust a new head to be prepared right. Check it yourself. You shouldn't HAVE to, but that's the way it is lately.

Rkeenan

I agree 100% you shouldn't have to check the head but at this point I wish I had. The machine ship I used has been in business for a long time and I have used them many times. I purchased thit head through them as the price was as good as any other place I could find and in doing so they didn't charge me to look at the original head.
What is stumping me is that it runs fine until it gets warm. It also ran fine before the work was done. Mechanical diag and with the parts I have replaced, I'm leaning toward something in the electrical side of the problem.
I picked up a whole noid set today (my Honda ones wouldn't work) so I'm going to check pulse at the actual connector when it gets warm.
Has anyone ever seen a bad PCM cause a single cylinder misfire?

dave1123

It's possible you've got a bad driver in the PCM. The ignition coils in the rail and the injectors are powered up constantly and they are grounded thru drivers in the PCM. That way the PCM doesn't have to handle heavy current loads.