Check Engine

Never mentioned or asked, how many miles on this car? Has it been determined whether it’s a bad ring or a burned valve or simply a tight valve causing the low compression? Doing a valve job might be a feasible repair…

The car has 168,500 miles on it.

The mechanic that did the compression test said that he added oil to the combustion chamber with the lowest compression reading and that by doing this it caused the compression to come up. He said it was because of the ring being worn.

Is it possible that it’s not the ring?

Its the rings. Normally rings would not be bad with only 168k on them. But you’ve been driving around with a misfire on that cylinder for 8 years, that means that there has been a lot of gas washdown on that cylinder, causing excess wear on the rings for that cylinder.

I don’t mean to be harsh on you, but if you had taken care of this 8 years ago, you wouldn’t have the problems you are having today. Learn from this and do not repeat this mistake in the future.

I think the bad ring has been causing the misfire, not the other way around.

The car was tuned up at 131k/8 years ago and the CEL was on before and after the tune up.

I did try to take care of this 8 years ago but nobody was able to find the problem. One place said the spark plug wire was loose and that he went ahead and pushed it into place and turned off the CEL. Came back on shortly afterwards. Another place (Mazda dealer) tried to sell me a set of plug wires but I knew that wasn’t the problem since It was tuned up recently.
So after I moved I decided to try again with a different mechanic. Luckily he had enough sense to do a compression test after seeing the misfire codes and that nothing else was wrong with my plugs, wires, cap and rotor. So I don’t think the misfire has been caused by anything other than the weak cylinder. Plus, if the misfire wasn’t caused by the low compression wouldn’t the codes have reported an ignition component as the cause?

And what’s really baffling is that it doesn’t run rough. At all. I just read in another post that a misfiring engine is obvious. Certainly not true for my car if it really is misfiring.

I’m starting to wonder if there is something wrong my OBD system.

With 85 PSI #3 isn’t misfiring so much as just weak output.
You might not feel it, but the computer easily senses it.
The head needs to come off to do a meaningful fix, that’s the bottom line.

You’re right circuitsmith…and so is everyone else that says to do all or none… But I’m hard headed and inquisitive so I think I’m going to pull the valve cover anyways and check the valve stems, clearances, and replace the seals. I’ll obviously still have the compression problems though.

At a minimum I’ll learn a few things and have fun as I venture into new territory. And at a max I’ll extend the life of the cat and o2 sensor/s. And hopefully l don’t screw it up and make it worse.

Wish me luck!

And thanks for the input everyone.

Hopefully I’ll be able to return the favor someday.

Your misfire could have been due to a faulty injector 8 years ago. It isn’t always the spark. If you have an adjustable valve lash and it was too tight, that could also have caused the problem.

That’s a good point keith. I’m going to check those things out.

I just pulled out the invoice from the tune up that was done in 2003 and at the bottom of the page, under the “Notes” it says: “PO421 CAT Converter code set in computer. Also worried about injector pulse.”

Too bad I didn’t notice this at the time. Actually I didn’t have the tune up done, it was the previous owner/family member so I didn’t look at the invoice since I didn’t take the car in. Wish I would of.

So yeah keith it looks likely that an injector was the cause. Good call.