Leakdown test revealed my engine problem

Oh, I forgot two things.

I burn about 1 quart of oil per thousand miles. I always chalked this up to the rings. But now if the problem is leaking exhaust valves, what is the oil doing?

Two, I'd like to figure out the second weak link in the system. That is, by plugging the exhaust system, both the tail pipe and the leaking header, pressurize the system and see where air leaks. Is that a viable plan? Perhaps the rings are leaky too, but overshadowed by the massive leaking in the valves.

Thanks, Doug

Sounds like worn rings to me. When you yank the head, you'll see whether or not the valves need work as well. 104k seems awfully soon for either to wear out, unless the engine was severely abused or run low on oil.

One possibility you may not have considered: when the engine overheated, the head gasket may have failed between cylinders #3 and #4. In other words, compression may be leaking from one cylinder to the other. Head gaskets are funny--you may not necessarily get leakage into the oil or coolant.

So you're saying worn rings in the two low compression cylinders, as well as the exhaust valves.

The last two years of ownership before me did not have the greatest maintenance. The oil was run for 7-10k miles between changes, mixed brands and weights of oil, etc. An idiot driver.

I double checked the oil consumption numbers. 10-30 gave me roughly a quart per 1000. I switched to straight 30 on the last change and have used 3/4 qt in 1500 miles. So that's a qt per 2000.

But if the compression numbers return to the 160-170 level, why need I worry about rings allowing some oil burn anyway? That's good compression in my book, no?

I guess to test for this I'd have to get cyl 4 on the power stroke, then plug the intake and exhaust routes and feel for air coming from the cyl 3 spark plug hole. Likewise down the line testing for a gap between the other potentials.

I imagine some are wondering why I don't just pull the head and see, then do the rings while I'm at it. The reason is time. I need to know what I'm facing, budget out the time, then tackle it with little leeway to deviate. So knowing what I'm facing is the most important. Specifically, do I need to do the rings. That'll triple the time I suppose.

Thanks Lanny for the comments.

Doug

That shouldn't be enough to trash your engine in only 104k. Letting it get more than 2 quarts low might be. Your overheating was probably the worst thing to happen.

I have no idea how my '94 was treated for its first 3 years/54k miles. It was a lease return, obviously a commuter car with lots of highway miles, and probably received perfunctory maintenance. It was still 100% original, except tires and battery. But it uses no oil now at 122k, and sees the rev limiter frequently.

Those are pretty significant dry/wet differences. I would certainly suspect rings, except for your statement that there was no air coming from anywhere except the exhaust. Should have been coming from the PCV port. One possibility is that you put in enough oil to raise the compression; should be no more than a tablespoon, or less. Looks like you could have a head gasket leak between 2 and 3. As for the valves, my guess is that there's probably carbon build-up on the faces and seats preventing them from sealing properly. They probably can be cleaned up by hand lapping (you could clean the valves first by chucking them in a drill press and using emory cloth). At that mileage I wouldn't expect there'd be any reason to replace the seats. I would stick to lapping them with a single angle. The chances that the springs were damaged by overheating are exptremely slim, I mean we're probably talking about well less than 300ºF. I wouldn't worry about replacing them unless you have evidence that one's bad. Check the free height and the spring rate (a machine shop can do this for you).

John McClary ('94 Miata) jsgmcclary at cox dot net

John, thanks for the reply.

You bring up two interesting points.

1, did I conduct the wet compression test correctly? I'm planning to repeat it now, with more carefully measured amounts of oil precisely put into the cylinders, and less of it. I put in one capful of oil, which may have been enough to raise compression by volume reduction. I'll go for a measured 3/4 tablespoon, say. Or perhaps a teaspoon even.

2, lack of air from the PCV. But if the exhaust leak is great enough to mask the ring leak, wouldn't that explain it? Hence my thought to plug the exhaust system somehow and see what I get out of the PCV.

I've been hoping for this from day one. How can I pinpoint this with air, since one or the other valves are open in the opposite cylinder? I guess with cyl 2 on the power stroke, pressurize. With cyl 3 on the intake stroke and the sparkplug in, feel for air from the intake. Viable?

I was thinking more along the line that lack of cooling allowed the head to overheat closer to exhaust temps. But the head is also cooled by oil of course. So the temps it experienced were probably not as serious as I was thinking.

Doug

Rich, I was planning a last ditch unstick-the-rings attempt while I'm gone for a few days. You recommend MMO over ATF? Why? I'm learning this as I go.

Thanks for the post, Doug

Forgot to mention... yes, stuck rings seem to be a chance due to the overheating. Perhaps some oil caked due to the heat. Worth a shot. I'd be a seriously happy camper.

Doug

That might explain it. Might be a pretty good trick to plug the exhaust. As for your cracked header, there seem to always be a few stock exhausts for sale on miata.net, or there're always the junkyards.

Possibly. You might even try this with the #3 plug out and feel there. Smaller hole than the intake duct/throttle body, higher velocity might be easier to feel. Don't know if it would be a big enough leak to feel or not. If you want to dedicate a day to this, you could remove the camshafts (loosen all the journal bolts, after removing the timing belt etc.) to close all the valves and then do your leakdown test with the adjacent plugs removed and feel in the plug holes.

Unless you're planning forced induction or something, I'd probably skip the cooling reroute. I thought about it, but look at the access and think about if you ever have to replace the thermostat or anything back there (at least if the 1.6 is similar to my 1.8). You've got a new radiator right? Make sure the cooling system is clean (flushed out), consider replacing the water pump when you've got the engine apart and make sure your fans and ther mostat work properly. Plenty of these running around just fine without the reroute (just my opinion). Replace the timing belt of course, consider a little clean-up work on the ports (great link about this:

and while the head's off isa good time to do anything you've been wanting to do on the right side ofthe engine, like installing that remote oil filter kit... John McClary ('94 Miata) jsgmcclary at cox dot net

MMO has more effective solvents than ATF I use ATF for diesel engines and MMO for gasoline Either will sometimes free up stuck rings by cleaning out the crud/carbon from the ring grooves, etc. I use MMO on marine engines that have aspirated water back into the combustion chambers or on engines that have not been run for many years, etc. - frees up the stuck rings and valve stems. With either, remember to soak first then run engine AT NO LOAD: normal idle to high idle.

Hope this helps.