Is it common for oil leak to cause misfire in one of the cylinders? - JeepForum.com
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post #1 of 13 Old 02-15-2020, 07:20 PM Thread Starter
v1rok
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Is it common for oil leak to cause misfire in one of the cylinders?

I have been dealing with misfiring detected in cylinder 1 (with error code P0301 popping up).

I swapped coils, checked injectors, swapped spark plugs. Still the same cylinder 1 misfiring. The engine shakes more than normal and idle become uneven.

I read that it could be due to head gasket is failing and coolant leaking inside the cylinder. Though I did not notice coolant was getting low. Just in case, I bought K-Seal and added hoping it would help seal or minimize the leak. But so far it did not help.

But today I realized that I was getting very low on oil. I did not even thing about the oil. It never leaked oil before and I did not see oil on the garage floor under the car.

But when I checked oil dipstick today, I saw that it was below Low mark. Apparently I lost about a quart of oil without even noticing it.

Is it possible loss of oil is somehow connected to cylinder 1 misfiring? Can oil lean inside the cylinder and cause misfiring? Another gasket (oil gasket) maybe failing?


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post #2 of 13 Old 02-15-2020, 08:06 PM
LuthWJ77
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Which year WJ and engine? I think the misfire and low engine oil aren't related but the technical guys will chime in once they know which engine you have.
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post #3 of 13 Old 02-16-2020, 06:48 AM
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What is the condition of the spark plug in cylinder #1, if the misfire is due to oil contamination it should be apparent on the spark plug.

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post #4 of 13 Old 02-16-2020, 07:46 AM
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How's compression look on that cylinder compared to adjacent cylinders? Plug isn't going to tell you much compared to others in this case if it isn't being firing correctly over the driver circuit.

Plug an ignition tester into that coil and watch it fire compared to other cylinders, that is a high energy ignition and there may be an issue between the coil and the pcm either in the circuit or the pcm driver itself may be bad for that cylinder.
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post #5 of 13 Old 02-16-2020, 09:57 AM Thread Starter
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The engine is v8 4.7L HO. Car has 137k on the clock.

I pulled the plugs from #1 and #2 just after it started misfiring. See the picture below. #1 is at the top.

It is definitely "oily". Does it mean the plug is not firing?

I already tried swapping plugs between the cylinders. I also tried swapping coils. Same outcome.

So, it looks like it is not the plug or the coil that are bad. Something else making #1 misfire

Waterluvr:
"...plug an ignition tester into that coil and watch it fire compared to other cylinders, that is a high energy ignition and there may be an issue between the coil and the pcm either in the circuit or the pcm driver itself may be bad for that cylinder."

What ignition tester would you recommend for this high energy coil? And how can I test PCM driver for a specific cylinder (#1 in this case)?
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post #6 of 13 Old 02-16-2020, 12:01 PM
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I'm a marine tech by trade so we use multi cylinder ignition testers (I use the Stevens Instrument S48 spark checker) but there far more affordable tools for checking and seeing the ignition pulse when the coil fires (or fails to fire) like this single wire tester.



This will fire a blazing hot blue/white arc across the terminals when the coil is doing it's job properly, a weak ignition pulse will appear orange or not at all if the coil isn't firing.

If you have a weak ignition or none at all on that coil you can work it back from there starting with continuity and short checking the harness back to the pcm plug.

If that all checks out you know you have a bad driver in the pcm that controls that particular coil. I'd grab that ignition tester to start and check that suspect coil and try it on adjacent coils so you see what a working ignition pulse looks like on a healthy cylinder.

Not sure where you are located but I have loaner pcm's available at no charge for forum member's to use for troubleshooting issues like this your welcome to one if needed.

See where your at with the ignition tester and I'll help you from there, you may find that and a compression tester free of charge as a rental item from one of the chain parts stores I'd test that hole and a couple others just to compare psi numbers.

If that coil tests bad on the problem cylinder you can swap it and the tester to another hole and verify the coil is ok.

To test the driver circuit it helps to understand the basics of how it functions, you always have 2 wires for each coil one of them supplies + from the ASD relay and the other is the - driver wire from the pcm.

The pcm controls that coil by switching the ground, they are hot all the time via the asd relay. To test the driver circuit make sure you have power on the hot wire constantly and then you need to unplug the coil connector and test for continuity back to the pcm. From there you unplug the black connector on the pcm and check continuity on socket #7 that connects to the TN/RD ignition coil driver for cylinder #1 to the end of the coil connector.

If all is still well the last test is checking for continuity on the driver wire to ground, with both the pcm and coil connector unplugged see if you get a path to ground checking from either end this would cause the coil to fire randomly without the pcm being able to switch it on and off as needed.
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post #7 of 13 Old 02-16-2020, 09:39 PM
KRAZY8
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I see that you're using NGK spark plugs. If I were you I would use the original Champion spark plugs that is made for your Jeep. That might be part of your problem.
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post #8 of 13 Old 02-17-2020, 01:16 PM Thread Starter
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Krazy8, I might try to put in different spark plugs, just to make sure. Though I strongly suspect this is not why #1 is misfiring. I replaced plugs about two years ago and the engine purred nicely with NGKs. Until now that is... But I might give it a try -- spark plugs are very ineexpensive

Waterluvr,
Thanks for the details on the tests to do with the ignition coil and PCM driver. I had a spark plug tester, but I did not have coil-on-plug ignition tester. I just ordered one. They are also inexpensive and it is good to have one handy in the garage. Will do the tests once it arrives.

I am still puzzled by excessive loss of oi; though. The truck never leaked oil before. So, I never even thought of checking it initially when misfiring started happening. But when I finally checked, I was surprised to see that more than 1qtr of oil was gone. And I did not see any oil spots on the garage floor. So, the oil must have been leaking from the engine and burning in the exhaust?

Could this be somehow related to misfiring in #1? Can the oil somehow leak inside the cylinder?
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post #9 of 13 Old 02-17-2020, 02:53 PM
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Quote:
Originally Posted by v1rok View Post
Krazy8, I might try to put in different spark plugs, just to make sure. Though I strongly suspect this is not why #1 is misfiring. I replaced plugs about two years ago and the engine purred nicely with NGKs. Until now that is... But I might give it a try -- spark plugs are very ineexpensive

Waterluvr,
Thanks for the details on the tests to do with the ignition coil and PCM driver. I had a spark plug tester, but I did not have coil-on-plug ignition tester. I just ordered one. They are also inexpensive and it is good to have one handy in the garage. Will do the tests once it arrives.

I am still puzzled by excessive loss of oi; though. The truck never leaked oil before. So, I never even thought of checking it initially when misfiring started happening. But when I finally checked, I was surprised to see that more than 1qtr of oil was gone. And I did not see any oil spots on the garage floor. So, the oil must have been leaking from the engine and burning in the exhaust?

Could this be somehow related to misfiring in #1? Can the oil somehow leak inside the cylinder?
As I advised earlier grab a free loaner compression tester from a chain store if you don't have one and get psi numbers on that suspect cylinder versus the others. You could could have an issue with the piston/rings, cylinder egged out or scored badly etc. with enough blow by from the bottom end to be the problem.
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post #10 of 13 Old 02-17-2020, 10:11 PM Thread Starter
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Good suggestion. While removing/swapping spark plugs, I might as well do compression test.

I have a compression gauge already. I did compression test on another engine (jaguar) about 3 years ago. For that test, I remember, I needed to pull the fuel pump relay in order to stop fuel delivery to injectors so that the engine can crank without firing.

For this engine (4.7L v8), should I do the same? Kill the fuel pump before the compression testing? Where is the fuel pump relay located?

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post #11 of 13 Old 02-17-2020, 10:12 PM
KRAZY8
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If you have been using those plugs for sometime then its probably not your problem, but it may be worth a try since spark plugs are inexpensive anyway. Good luck!
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post #12 of 13 Old 02-18-2020, 06:55 PM
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Quote:
Originally Posted by v1rok View Post
Good suggestion. While removing/swapping spark plugs, I might as well do compression test.

I have a compression gauge already. I did compression test on another engine (jaguar) about 3 years ago. For that test, I remember, I needed to pull the fuel pump relay in order to stop fuel delivery to injectors so that the engine can crank without firing.

For this engine (4.7L v8), should I do the same? Kill the fuel pump before the compression testing? Where is the fuel pump relay located?
You can pull the fuse for the fuel pump or just unplug the connector at the injector(s).
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post #13 of 13 Old 02-19-2020, 07:31 AM Thread Starter
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A little bit go Googling helped me find where the fuel pump relay should be. Top right corner in the diagram below
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