2003 Nissan Pathfinder misfire help!

[edugan97]

Hello all.

I have a 2003 Nissan Pathfinder that has a VQ35DE V6 engine, Automatic with 248,252 miles. And it has a major issue. It’s misfiring on Cylinder 5. This all started in may, when all of a sudden it started acting up. Now I knew that I needed to change out some parts like spark plugs and such. So I changed out all 6 plugs and Valve cover and intake gaskets. And this engine status improved, but was still misfiring on 5. So I changed out the coil on 5. Not much of a change. So I changed out the Injector on 5, and it did make a difference... but still misfiring. So I take it to my normal mechanic. He can’t figure out the problem, but determined that it is getting spark, and the injector is injecting. He also did a leak down test, and finds that that is all good too. So now I’m stumped. Please help me figure this out.

[mdmellott]

Low compression can cause a misfire. At over 248K miles, it's possible #5 piston compression rings or valve seats are faulty. A basic compression test of that cylinder should confirm or rule out that possible cause. For comparison, test the others as well. For the test, the specifications for your cylinder compression pressure at 300 rpm are: 185 psi standard for each cylinder and a minimum of 142 psi and a maximum difference between cylinders of 14 psi. I know you mentioned that your mechanic did a leak down test, which is a compression test in reverse, using air pressure to pinpoint where leakage is occurring but it won't give you any indication of the compression specifications.

[edugan97]

Low compression can cause a misfire. At over 248K miles, it's possible #5 piston compression rings or valve seats are faulty. A basic compression test of that cylinder should confirm or rule out that possible cause. For comparison, test the others as well. For the test, the specifications for your cylinder compression pressure at 300 rpm are: 185 psi standard for each cylinder and a minimum of 142 psi and a maximum difference between cylinders of 14 psi. I know you mentioned that your mechanic did a leak down test, which is a compression test in reverse, using air pressure to pinpoint where leakage is occurring but it won't give you any indication of the compression specifications.

But if there was no air leaks, how would a compression test prove anything? Not that I doubt what you are saying, but wouldn’t it be obvious if it was low compression because the air from the leak down has a ton of blow by?

[mdmellott]

I suppose that is true if there was no major amount of pressure lose detected in the leak down test but there is always some air leakage, even on a brand new car, it's just a matter of how much pressure loss is seen and how fast that lose occurs that helps pinpoint a problem area. A leak down test is typically done when very low compression is identified during a compression test in order to help find the cause of a low compression issue. However, that test isn't a substitute for the compression test. It's a complimentary test. Your misfire trouble code on #5 could occur (for example) if all 6 cylinders had an acceptable compression test pressure but #5 was significantly lower (>14 psi lower) than the other 5 cylinder compression test pressures.

[mdmellott]

Carbon fouling of cylinder walls can increase compression on some cylinders, but not in a good way, and at the same time this carbon fouling in the compression rings can make other cylinders lose pressure. In older engines like ours, mine with >220K and yours at >240K, keeping that carbon build up from occurring and getting rid of what may be there is critical if we are to see >300K. On my old '91 Pathfinder and my current '02 I used a healthy overdose of Techron Complete Fuel System cleaner at least once a year to help remove carbon deposits. I'm just trying to see a bright side simple solution for you. Even if I'm wrong with this presumed diagnosis example, it couldn't hurt and it might help. Getting the cylinders and rings cleaned up with chemicals might help balance out an out of balance compression issue and eliminate the misfire. Wishful thinking.

[edugan97]

Carbon fouling of cylinder walls can increase compression on some cylinders, but not in a good way, and at the same time this carbon fouling in the compression rings can make other cylinders lose pressure. In older engines like ours, mine with >220K and yours at >240K, keeping that carbon build up from occurring and getting rid of what may be there is critical if we are to see >300K. On my old '91 Pathfinder and my current '02 I used a healthy overdose of Techron Complete Fuel System cleaner at least once a year to help remove carbon deposits. I'm just trying to see a bright side simple solution for you. Even if I'm wrong with this presumed diagnosis example, it couldn't hurt and it might help. Getting the cylinders and rings cleaned up with chemicals might help balance out an out of balance compression issue and eliminate the misfire. Wishful thinking.

This is very interesting. Just the thought of a compression test scares the hell out of me. That and the fact that I do not currently have access to a compression tester, I'll absolutly give this a try. I did notice that top of the piston head on 5 does look fairly black(as in pretty carboned up). And the exhaust smells like firecrackers, which from what I've read is carbon burning. So I'm hoping the fuel system cleaner at least gives me a sign that the situation is improving. Thank you so much for these suggestions. I'll post again with the results.

[edugan97]

Update:

I put in some Lucas fuel systems cleaner, and filled up the tank. I started it up and took it for a little drive around the yard. It’s still misfiring, however it seems to be acting a lot better then it was. Once again it starts up every time, and I’ve only driven it about 2 miles, but it seems to be doing something. I’m planning on taking it out for a short drive tomorrow during the day, primary to see how it reacts on the road instead of exclusively off road. One this I noticed especially is that the SES light only flashed at 1500 rpm almost exclusively. If I let off the throttle or had it rev up to about 2000 rpm while in gear it never flashed. Any kind of advice or other suggestions will be greatly appreciated, and I will give another update when I actually put some miles on it with the cleaner in the tank.