Need help with P0302 misfire (failed leak down test on cylinder 2)

S

Sinlessearth

New member
Hello fellow Jeepers. I am looking for a little help troubleshooting my P0302 misfire (there are no other codes). I have scoured the internet and still have some questions on the process. Here is the background. It’s 2016 JKU Sport with 115K miles. I have replaced the oil cooler housing, plugs, plenum gaskets around 100K, then housing again at 112K. This is the only engine work done so far. I always change the oil when the Jeep tells me to and use Mobil 1 full synthetic. It has never over heated or even went above mid-range. The misfire started just before the second oil cooler replacement. Also, the valve cover gasket now leaks on cylinder 4’s spark plug hole but it’s mild (discovered during 2nd cooler replacement). It is random and does go away sometimes. When cleared it will stay off for 1-3 days then pop up again until I reset it or goes away on its own. I never really noticed a rough idol until last night while sitting in a drive through however, the 1-hour drive today I didn’t notice any roughness like the night prior. Also, it still has power and will get up and go.

So far, I have moved the coil pack to cylinder 1, the spark plug (which looked fine and was gapped correctly) to cylinder 3, and the fuel injector to cylinder 6. The code still pops up, telling me these parts are not bad.

One thing to note is that when replacing the oil cooler housing the first time I noticed the wiring harness cable loom that runs to the left of the intake plenum over cylinders 1,3, & 5 (injectors and coil packs connections) had disintegrated and the wires are no longer really protected. I tried to put new loom on however, the heat destroyed that as well. I have inspected the harness connector that sits on top of the engine and didn’t find any pins recessed, bent, or corroded.

If I do a compression check and it’s good, then is it safe to assume it’s something with the wiring harness to the coil pack or injector?

Is there a good way to test the connection to the coil pack or injector for continuity or voltage drop without a special scanner?

If I do a compression check and it fails, then I need to do a leak down test or assume the head needs replaced?

If I do the leak down test, I doubt the leak would be the crankcase or coolant, but you never know. If the leak happens to be at the valves (intake or exhaust) could it be the rocker bearings or lifters? I don’t hear a real noticeable knock, but it could be faint.

What I am concerned about is doing a compression test and assuming the head is bad (not sure I can tackle that job in the amount of time I would have) and don’t want to replace the head if the lifters and rocker bearings are the real issue (which I could do). How do I know for sure it’s the head and not the rockers/lifters without replacing them and potentially wasting money and time?

Also, is the compression test done cold or warm? I have seen/read both. I believe I should pull the fuel pump fuse and burn the gas, but do I need to do anything for the spark or will leaving the coil packs unplugged be enough?

Any other recommendations/suggestions would be much appreciated.
 
A lot of times when the lifter goes, you'll visually be able to see some wear or scoring on the camshaft lobe for that cylinder.
 
Which cylinder is misfiring? Don’t assume the cylinder that’s getting oil leaked on it is misfiring. The code reader should display which cylinder is giving the code.

Skip the compression check. It won’t tell you much. Do the pressure differential test (leak down test). It’s best to do the differential test when the engine is hot. If there is a leaking cylinder, the differential test will help determine where the leak is. If the exhaust valve is leaking, air will be heard hissing in the exhaust manifold. If it’s the intake valve, the hissing will be heard in the intake. If it’s the rings, squirts of oil into the cylinder will stop or slow down the leak by temporarily sealing the rings.

When my 2012 started throwing a misfire code, it had a bad igniter module. I replaced all six modules and spark plugs at the same time. I figured if one is bad, it’s just a matter of time before the next one fails.

A bad rocker doesn’t always score the cam. If you hear any kind of ticking from the rockers, buy the rocker/cam tools and use them to inspect each rocker one at a time. The rockers have needle bearings that fail. If a bad rocker is found, replace the rocker and the lifter. It’s best to replace all the rockers and lifters at the same time. If one lifter is bad, chances are it’s just a matter of time before the next lifter fails.

The upper and lower intakes must be removed to get to the lifters. The lifters must be removed to check the bearings for play. It’s work, but not complicated. There are videos on YouTube showing how to remove the lifters without the special tools but there’s a risk the cam bearing surfaces will get damaged. As the cam bearings are machined directly into the heads, a damage bearing means the head must be replaced. The tools don’t cost much and make the job easier.

Chances are, if there’s no tick, the rockers are fine. If there is a tick, check the rockers right away to avoid costly damage. Worn rockers can score the cams. At this point, the engine is starting to make metal which will damage the cam bearings and the heads will need to be replaced. The metal will also get into the block and the whole engine will need to be overhauled or replaced.

This isn’t meant to scare you. If action is taken as soon as the engine starts ticking, major damage can be avoided. Labor at the shop will be expensive. A lot of money can be saved if you do the work yourself. Again, if there’s no tick, there’s no worry.
 
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WJCO said:
A lot of times when the lifter goes, you'll visually be able to see some wear or scoring on the camshaft lobe for that cylinder.
Click to expand...
Thanks for the help. Sounds like I need to go ahead and replace that valve cover gasket and take a look at those lifters.
 
GP NOIR said:
Which cylinder is misfiring? Don’t assume the cylinder that’s getting oil leaked on it is misfiring. The code reader should display which cylinder is giving the code.

Skip the compression check. It won’t tell you much. Do the pressure differential test (leak down test). It’s best to do the differential test when the engine is hot. If there is a leaking cylinder, the differential test will help determine where the leak is. If the exhaust valve is leaking, air will be heard hissing in the exhaust manifold. If it’s the intake valve, the hissing will be heard in the intake. If it’s the rings, squirts of oil into the cylinder will stop or slow down the leak by temporarily sealing the rings.

When my 2012 started throwing a misfire code, it had a bad igniter module. I replaced all six modules and spark plugs at the same time. I figured if one is bad, it’s just a matter of time before the next one fails.

A bad rocker doesn’t always score the cam. If you hear any kind of ticking from the rockers, buy the rocker/cam tools and use them to inspect each rocker one at a time. The rockers have needle bearings that fail. If a bad rocker is found, replace the rocker and the lifter. It’s best to replace all the rockers and lifters at the same time. If one lifter is bad, chances are it’s just a matter of time before the next lifter fails.

The upper and lower intakes must be removed to get to the lifters. The lifters must be removed to check the bearings for play. It’s work, but not complicated. There are videos on YouTube showing how to remove the lifters without the special tools but there’s a risk the cam bearing surfaces will get damaged. As the cam bearings are machined directly into the heads, a damage bearing means the head must be replaced. The tools don’t cost much and make the job easier.

Chances are, if there’s no tick, the rockers are fine. If there is a tick, check the rockers right away to avoid costly damage. Worn rockers can score the cams. At this point, the engine is starting to make metal which will damage the cam bearings and the heads will need to be replaced. The metal will also get into the block and the whole engine will need to be overhauled or replaced.

This isn’t meant to scare you. If action is taken as soon as the engine starts ticking, major damage can be avoided. Labor at the shop will be expensive. A lot of money can be saved if you do the work yourself. Again, if there’s no tick, there’s no worry.
Click to expand...
Thanks for the info.

The code is P0302 which is cylinder 2. This cylinder is known for being a bad head at least in 2014 and older but is present in some newer models as well. The cylinder that has the valve cover leak is cylinder 4.

Could a bad lifter cause the intake or exhaust valves to leak?

I know it isn’t the igniter module (coil pack) or plug because cylinder 2’s has been moved to other cylinders and the code did not follow. I believe the compression test or even the leak down test will validate the cylinder vs an electrical issue going to the coil pack.

I don’t hear an obvious tick however, starting yesterday it is starting to idle rough and is more noticeable.
 
A bad rocker should not result in a leaky valve. A bad rocker would result in less valve lift, not more. A sticky lifter could cause a valve leak by holding the valve in the open position.

The problem might be the connector from the wiring loom to the igniter. If you can run a code reader while the engine is running, the live data should tell you if that spark plug is getting voltage. Check continuity of each prong of the connector to its corresponding lead using a multimeter.

Don’t waste your time with a compression check. It can give you a false reading. Do the differential test using a setup like this-

FCE1B0EB-0D97-4F5F-9315-172A98E1DC6B.jpeg
 
Well, I did the leak down test, and I am not excited about the results. I was able to push 70 PSI into the cylinders but no more due to leak rate and air compressor limitations however, here are the results. I checked cylinder 2 3 different times just to be sure. The air on cylinder 2 is coming out the exhaust so there is something with the exhaust valve.

Cyl – PSI out/in – loss %
1 – 69/70 – 2%
3 – 69/70 – 2%
5 – 70/70 - <2%

2 – 54/70 – 23%
4 – 66/70 – 6%
6 – 68/70 – 3%

When I started it up after the test the engine was loud for about a second, but I assume this is because the air pushed oil out of the cylinders. Anyway, I drove for about 1.5 hours before it detected another misfire. I know loss is high on cylinder 2 but if it has a consistent 23% loss wouldn’t that trigger a consistent misfire? GP NOIR mentioned a stuck lifter could cause a valve to leak, could this be the reason.

I’m new to digging this deep into an engine but what options are there to get this fixed… I know the classic Chrysler answer…. replace the head. But what else is there? I can still check the lifter but would hate to go through the process of replacing the valve cover gasket, just to do it again if the head does need replaced but I will if needed.

Would it be possible a can of seafoam could help if its due to carbon buildup? It's not likely but maybe worth a try?

What would happen if I let this go for a while until I could get a head? This is my daily driver, so a lot of down time isn't good.

Any recommendation will help. Thanks
 
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