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 wrote: ↑Fri Nov 08, 2019 9:06 pmLow 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.
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.mdmellott wrote: ↑Sat Nov 09, 2019 12:10 amCarbon 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.