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Check engine light cylinder number six

3K views 34 replies 5 participants last post by  Mopar_ 
#1 ·
Twice now when I crank my Tj in the mornings before work to let it warm up my check engine light comes on. This is the second time in a little over a month now this has happened. After work both times I stopped in an Autozone and both times the scanner says misfire on cylinder number six. The first time I had the guy to clear my code. The second time after driving it for a couple of days the check engine light goes out. The guy at the parts store says it could be any of the following vacuum line, spark plug, or coil pack. Has anyone been thru this before? Which one is the most common? I pulled the first spark plug since it's the easiest to get to, and it didn't look to awfully bad.
 
#3 ·
Spark plugs are a good place to start but not necessarily your problem. Pull them all out and compare the #6 plug to the others. Does it have deposits on it, is it cleaner than the rest, is it white or black, look for things like that. You didn't mention what year or engine is in your TJ. Look for carbon traking on the plug wire/coil boot or on the porcelain part of the #6 plug.

A cold only misfire can be a number of things. A leaking injector can cause a cold misfire because the cylinder will be filled with fuel until it burns off. Same goes for coolant with a leaking head gasket or oil with leaking valve seals (both extreme cases). Sometimes coils will fail cold but work warm and vice versa. A low compression cylinder might raise once the engine warms. Could also be cam or crank sensor issues.
 
#5 ·
Your cooling system builds up pressure when the engine is hot. When you shut off your engine the pressure in the cooling system is higher than the pressure in the combustion chamber and in some cases when the head gasket is blown it can push coolant into the cylinder when the engine sits for an extended period of time. That coolant in the cylinder will cause a misfire on cold start up until it is pushed out of the cylinder and is burnt off the spark plug.

When the engine is running the pressure in the cylinder is much higher than the pressure in the cooling system so a leaking head gasket will let exhaust gases into the cooling system which can be seen as bubbles in the radiator or overflow tank and over heating when the problem gets worse.

Like I said before if this is the case you will likely be able to notice with a visual inspection. Coolant entering a cylinder will clean all of the carbon out and leave deposits on the spark plug. It will look different than the other cylinders.

If you suspect your head gasket leaks there are multiple ways to test for it. For your small coolant leak I would suggest to start by pressure testing the system for an extended period of time and looking for leaks.
 
#6 ·
Thanks for the explanation. I have had it pressured check by a mechanic. He says it held pressure for a long time, and he could not find a leak. He did not say how long, and I did not think to ask. A friend followed me after work the other day and said he did not see any steam out the tail pipe. I did not know the misfire problem and the coolant leak could be related.
 
#7 ·
The misfire and coolant loss could be related, but that does not mean that they are related. Your coolant loss and misfire could very well be two different problems. I am just explaining possibilities to you.

A slight head gasket leak can make your coolant level slowly go down and you will not notice it because the coolant is getting burnt off while the engine is running in very small amounts which will not cause a misfire or noticeable smoke out of the tail pipe. Only when the head gasket gets bad enough will it start leaking into the cylinder like I explained above, and usually by that point there are other symptoms such as the engine running above normal operating temperature, fully overheating, or pushing coolant into the overflow tank.

Just because a leaking head gasket can cause both of these issues does not mean that is the case with your vehicle so don't be to alarmed just yet.
 
#15 · (Edited)
That spark plug is in beautiful condition, something else is going on. Could be a bad coil rail. Does the #6 connector inside the rail look ok inside where it grips the spark plug, when compared to the other 5 connectors?

When you do decide to change your spark plugs and if you want platinum (which is a good choice), make sure to ONLY install dual-tipped platinum plugs like the Autolite APP985 or Champion 7034. Single-tipped (platinum on just one side of the gap) spark plugs like the Autolite AP985 or Champion 3034 will cause problems later due to the distributorless ignition system's waste spark design.
 
#17 ·
That spark plug is in beautiful condition, something else is going on. Could be a bad coil rail. Does the #6 connector inside the rail look ok inside where it grips the spark plug, when compared to the other 5 connectors?
Did not think to look. I could not get the plug for the coil unplugged. So I just left the coil laying there while I changed the plugs. I wished The factory would of left some slack so it can be easily unplugged. I also have a slow coolant leak and was afraid cylinder number six had a head gasket leak. I don't think it's leaking by looking at the plug. What do you think?
 
#18 ·
So can we rule out the head gasket letting coolant into cylinder number six. I'm not mechanically inclined, but from time to time the jeep shakes at 50-60 mph but that never gives me a check engine light. What gives me the check engine light is when I start the engine and let it warm up before work in the mornings. Which so far when it happens it's pretty cold for around here.
 
#19 ·
There were a couple of plugs including number six that were not gaped .035, number six was .040. Don't know if it makes that big of a differance or not. Also if I get the code again, and I believe I will because the plugs looked good, could the problem be my fuel injector? Or is the coil rail usually the culprit?
 
#20 ·
.040 is not going to cause any problems.

The cause could be a number of things. It can become difficult to track down a misfire code that sets intermittently only when cold/cranking when you have the car in a shop full of diagnostic equipment. Even more so when trying to do so over the internet ;-)

Is the light coming on while cranking or is it coming on while it is warming up?

Possibilities could be a coil/boot, injector, cam/crank sensor, some sort of valve sealing/compression issue, etc.

On the #6 plug you pulled out inspect the porcelain part of the plug for any black line that run might be running down it. You are looking for carbon tracking (google images to see it), it will look sort of like a streak of pencil lead/burn mark.

Also another thing you can do is listen for even compression while cranking the engine over. Next time it is a cold morning like the mornings the code sets when you get into your jeep put the gas pedal to the floor before you turn the key on. This puts it into 'clear flood mode' where the ECM disables the injectors. Now turn the key on and crank the engine over (with the pedal still on the floor). It will not start but you can crank it for a little while and listen to the engine compression pattern. It should crank/sound very consistent. If there is an inconsistent cranking pattern that would indicate compression loss.

That said I think low compression is unlikely to be a problem in your case, but it is a quick easy test to listen for. If you had an oscilloscope you can do this test with it hooked up to battery + and - to get a visual representation. It is called a relative compression test.

Really I'm just speculating here and giving you possibilities of what might cause a cold misfire but I have no way of actually knowing with out you starting to run some tests and get more information. If I had to outright guess I would lean towards a cam or crank sensor because they are pattern failures for misfires on these, but because it is reappearing on #6 cylinder that leaves me a bit skeptical. Tomorrow once I am into work I will look on some technician networks and see if I can find anything.
 
#22 ·
So I did some digging and the only thingsI came up with that was a pattern failure, aside from the basics that we have already mentioned, was the possibility of an injector wire rubbing through the harness and shorting out. Check the wire harness from the injectors to the ECM and make sure there are no sections that have been damaged. It is most common to happen at the valve cover studs at the back side of the head.
 
#28 ·
Thanks. I did not have the time to check for a bare wire yet on the fuel injector like you told me to check. I did look right there at the plug that goes on the fuel injector, and it had electrical tape on it like the other five does. Maybe tomorrow I'll have time. I'm hoping the boot that connects the spark plug to the coil rail will fix it. When I pulled the spring off the rail some dust flew out between the spring, and the rail.
 
#30 ·
In case anyone Googles this question I am posting my solution to fixing this issue. Since I never got a misfire on cylinder number five only on six, I replaced the rubber boot and spring on cylinder six. This is the rubber boot that goes from the coil rail and covers the end of the spark plug. The old just pulls right off and the new one pushes right on the rail. It only gave a code on mornings in the twenties. I have cranked it up for a few of these mornings now with no check engine light. Last week was the best test at five degrees and now I think it's fixed. I'm glad I did not listen to the people at the parts store and bought a hundred dollar coil rail. The boot only cost $2.50 at the parts store.
 
#31 ·
Thanks for reporting back your findings.

I'm curious did you happen to look at the inside of the old boot? If so did you see something that looked like a line someone drew with a pencil (carbon tracking) inside of the boot?

If this one boot caused your problem then it is a safe bet that all of the other boots are in the same condition as the old one and might start causing problems also. It may be a good idea to replace them all, for preventative maintenance's sake.
 
#32 ·
I did not think to look. When I removed the spring some dust puffed out. I wished I would of kept it until after I posted. I threw it away a few days ago. I'm going to wait about replacing the rest until they give me a check engine light. I'm finding out why people say Jeep means just empty every pocket.
 
#34 ·
I just went thru this. Check engine light & misfire in cylinder #6. All my plugs were ready to be changed... especially #6. AND I could feel this misfire when going slow on the side streets.

That plug looks new.... but sometimes they can be defective. For the cost and time it takes I'd just replace that plug. Then the coil pack. But if it is misfiring you should be able to feel it.
 
#35 ·
I done fixed it. It was not the plug. I drove it for a little while after I changed the plugs, and it was not my plugs the light came back on. I could never feel the misfire. It only did it on cold mornings while warming up when I was getting ready for work. It was only after I changed the boot that this problem went away. I gave it time between changes before I tried something else. I also gave it time before I posted back just to confirm. I figure if the check engine ligh never came on in five degree weather it was fixed.
 
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