1999-2004 Grand Cherokee WK - 4.7L Engine Help Needed


Expedition Leader
My son's 2002 WK has 200,000 miles on, with a freshly rebuilt motor. We bought it a few months ago, and didn't rebuild the motor ourselves, so we aren't sure how complete a rebuild it was, I suspect not very, because in the past few months we've had to replace all the major sensors, including crank sensor, TPS, etc....

Currently, it's not running, or barley running. It has a significant misfire on Cylinder #7, indicating P0307. It sounds terrible at the tailpipe, though it idles okay, with only a minor hickup audible from the engine bay. However, there is a lot of white smoke coming out the tailpipe once it warms up, and its got no power at all. It is loosing coolant, but not huge amounts, there is a small coolant leak between the hose and the radiator that could be the source of lost fluid.

We pulled the spark plugs for #5 and #7, and #7 (the misfire cylinder) has a lot of oil on the spark plug head, and is fouled and gunky.

There aren't any bubbles or gasses coming up through the radiator, but there is a smell of oil near the rear of the engine (near Cylinder #7), and some white smoke drifts up from near the exhaust manifold in the same general area

So I'm confused, it doesn't seem like a blown head gasket (no bubbles, not enough coolant loss to cause a misfire), and it doesn't quite seem like a fallen rocker arm (no tapping noises), and it doesn't seem like cracked manifold (no exhaust noise near the engine).

Any thoughts? I am really not sure what do next? I'm going to remove the valve covers and see if there is any visible issue with the rocker arms, but I suspect they are okay (recently rebuilt motor...). I'm wide open to ideas or suggestions. I am not a mechanic, and have no experience to draw on other than a Montero with a bad cylinder that I was unable to fix and eventually sent to the crusher.
Here are a couple of things you can check before pulling the valve cover.

1. Change the spark plugs. I had a similar issue several years ago with my wife's '99. I switched plugs around and tried to get the problem to migrate but the misfire stayed in the same cylinder. New plugs eventually solved the issue.

2. Similarly, you might have a bad coil pack that is causing the misfire. Switch the packs to see if you see any change.

3. You might also have a clogged injector. Pulling the fuel rail is a little more difficult but doable. You can buy all new orings on Amazon for about $10 but I have had good luck reusing them. Drop a little penetrating oil around the injector before pulling. Also, use compressed air to blow out the accumulated dirt in the injector hole before you pull them.

I have also heard of people having trouble with the injector pigtails and causing a misfire, but I have never experienced this issue.

Good luck
Compression test it. If there's a significant mechanical problem with #7 (like a broken valvetrain part), that'll show up. If compression is good on #7, the issue is either fuel, spark or something else along those lines.


Expedition Leader
Thanks for your response. We tried all the swapping of coil packs and spark plugs and wires, with no success. So we pulled the valve cover and sure enough the rocker arm for Cylinder #7 was missing. It was actually broken into two different pieces distributed throughout the upper valve guide area. Cam looks OK but the valve lifter rod or whatever it's called has zero compression in its spring plunger assembly, and will need to be replaced. About $20 in parts. Will have it back together soon


Expedition Leader
Well, rocker arm and new valve lifter/lash adjuster installed. Put everything back together, and now it won't run at all! So weird. Sounds like its not getting gas, or air, or spark. I have no idea really. But it's not starting. So frustrating. Went from running (poorly) to not running at all after fixing it. I have no idea how to proceed from here.
Let's start with the repair.

Did you do it yourself? How did you get the new rocker arm installed? Did you remove the cam?

Did you replace all lifter/lash adjusters or just the one? Did you soak them in oil overnight before installing?


Expedition Leader
1. Yes
2. Slipped it under the cam and popped it into place. No need to remove the cam. We did rotate the engine through one full cam revolution to ensure the cam lobe engages the rocker arm properly. We did this by hand by turning the bolt holding the timing chain pulley in place.
3. Only replaced the one lifter and rocker arm. The other ones are due for replacement, but this was not a planned (budgeted) repair.
4. We did not soak them overnight, they I did dunk them in fresh motor oil prior to install.

I don't think the repair caused the problem, or at least not directly. Pretty sure the new parts work, the new problems (no spark, or air, or fuel - not sure which), was not a previously existing condition.
You said this is a problem with cylinder #7 so we are talking about the driver side of engine. Correct? Just clarifying. No offense intended.

Let's determine fuel first.

Did you remove the fuel rail when you removed valve cover? It isn't necessary to remove it but I am curious about fuel pressure. A spontaneous fuel pump failure is not unheard of. Can you hear it prime the system when you turn on the key? Is there pressure at the rail? The relief valve is on the passenger side. You can press the schrader valve to see if fuel squirts. This isn't a foolproof way to check pressure but is indicative.


Expedition Leader
Yes, originally the issue was with the broken rocker arm and stuck lifter on #7.
We did not remove the fuel rail. I don't smell any gas when trying to start it, not sure about hearing anything. I may not know what to listen for. I will look for that Schrader valve. I am not familiar with where it might be off hand.


Expedition Leader
We had one guy suggest doing a compression test on all 8 cylinders. Kind of a PIA to do that, not sure if it would find my current problem.
As soon as you turn the key to the run position you will hear a whir or a whine from the rear of the vehicle. It is an electric motor sound that lasts for a second or so. It is distinctive. If you do not hear it, it means the pump is bad or not getting power. Check the fuses and relays first.

My finger is pointing to the schrader valve. I put a brass cap on mine but yours will be black or gray plastic. When you pull the cap you can press the pin just like a tire valve. Fuel should spray out. It shouldn’t dribble.

We had one guy suggest doing a compression test on all 8 cylinders. Kind of a PIA to do that, not sure if it would find my current problem.
Compression check at this point would give you more info about condition of engine. If the reason for the original dropped rocker was a dropped valve seat you might identify a continued problem. You said that you rotated the cam and confirmed the rocker was working properly. If I were doing a compression check I'd probably only check the drivers side rather than all 8.

My experience with dropped rockers always meant a dropped valve seat and or a bent valve. Sometimes the valve had hit the piston and damaged that as well. Lots of other people claim no additional problem other than the bad rocker arm. If it started before, though, it should start again. If the seat has dropped or the valve is bent you will have the same problem you had originally.


Expedition Leader
Quick update, determined the following:
1. After reassembly, car alarm was triggered somehow and immobilizer prevented it from starting/running
2. Days later, engine started up no problem, but ran poorly, no change in symptoms
3. Removed valve covers again, #7 rocker arm dislodged again
4. Noted that valve stem not rising up, and appears out of alignment with other valve stem/seat assemblies.
5. Bent Valve!

Will require complete dissassembly and a head job. Boo.