Odds of ringland failure

[FaderUS]

Hi all, I just acquired a 2005 legacy gt 5eat, 160k miles, catless uppipe, Cobb CAI, stock downpipe and unknown brand catback. Stock tune. I have put 500+ miles on it in the last couple of weeks. Initially it had a CEL for the front o2 sensor which I replaced and the light went away. I also had to pull the TMIC and drill out a mounting bolt that was broken on the diverter valve. The wastegate actuator mount was also loose.

 

Per the PO the turbo, banjo bolts, radiator, and head gaskets and timing belt have been done within the last 20k.

 

All was well and the car has been running well following these patches but last night I suddenly got a P0304 misfire on #4. It is a solid CEL, no flickering and can be cleared with a reader but has come back 2 additional times in about 30 miles of driving. Each time the error popped on after the engine was warm, and went from being driven at 55-60 then came to a stoplight. The idle was a little low and rough at the stop light. The CEL popped on as I lightly touched the throttle after the light changed. It does not occur each time I duplicate this process.

 

I have no visible smoke from the oil filler.

 

I have not done any diagnostics other than the smoke test.

 

I had planned to do a compression and leakdown test, plugs, Blackstone oil analysis and oil change as part of the "new to me" assessment.

 

What should my next step be? Swap coil packs? Injectors? Is a Compression/Leakdown test a definitive way to rule out ringland failure?

 

Thanks for any input. -Chris

[Rhitter]

I did all that - swap plugs, injectors, coilpacks. It was a waste of time. Problem was a broken ringland. Compression / Leakdown is the definitive test to rule out a ringland failure.

 

Why did the PO swap out the turbo?

[Max Capacity]

If your a DIY kind of guy. do the free stuff first.

 

But I'm with Rhitter, if its been on the stock tune... I'd guess ring land.

 

Start saving your money...may be Santa will bring your some cash...

[FaderUS]

So if the compression and leakdown are good, it is very unlikely to have ringland issues?

[Rhitter]

So if the compression and leakdown are good, it is very unlikely to have ringland issues?

 

If they are good. Then it's not a ringland issue. But like Max said, if you want to spend a day swapping around coilpacks, injectors, plugs (see if the misfire follows) then that will also rule out a ringland. I think most shops charge around $300 for compression, leakdown, boroscope (at least bay area prices).

 

However, compression and leakdown can also show if it's a valve problem.

 

Here is what a broken ringland on 4 looks like from the test perspective

http://i.imgur.com/oSBIitk.jpg?1

[FaderUS]

That's fine, I can do all of that except the scope myself. I just wanted to verify that the leakdown could conclusively rule out a broken ringland. I had the test on the schedule anyhow, just making sure I wasn't missing anything else.

 

Other engines I've worked on typically allow a 10% variance between high and low cylinders...is this a reasonable variance for these motors too? -Chris

 

I guess I can put the shop labor $ to a scope if needed.

[xt2005bonbon]

If the misfire occurs only with a warm engine at idle, there could be a pretty good chance that you have a tight valve and ringlands are OK. Usually (emphasis on that word!), if there is a ringland issue that just started, it would show up at cold idle and may go away when fully warmed up (as the ring expands). Of course, if the ringland is really in bad shape, I would suspect to misfire in any conditions.

 

When you have a tight valve (i.e. little to no clearance), the misfire will usually only show up when the engine is warmed up. As the valve stem expands with heat, it reduces even more the little clearance left. As a consequence, compression gets low and misfire occurs.

 

In any case, check the usual suspects first as others have said (coils, plugs, injectors). And I would say check the valve clearance. Not hard at all, but a bit time consuming.

 

This happened to one of my cars on cylinder 2. I caught it early, and luckily did not burn a valve. It was an exhaust valve btw. Pretty common.

[Rhitter]

That's a good point xt2005, my misfire was worse on 35 degree mornings and got a lot better as the engine and temperature warmed up.

[FaderUS]

Thanks all. I have been babying the throttle and staying out of boost completely for the last couple of days to attempt to get my obd sensors to reset for a NYS inspection and so far I haven't seen another CEL.

 

I am in the 50-60 mile range and the sensors just went to ready, but of course it's the coldest day of the year so far and I now have a "Pending" P4020, probably due to the external temp. Hopefully it will clear and I can get the inspection later today.

 

XT, thanks again for the input. If the valve was too tight, would the CEL be so intermittent? The fact that it just showed up and isn't consistently reproducible makes me lean to a failing part (or something that has changed in some way) more than an adjustment issue. I am not familiar with the valve adjustment process on these cars, but is there anything that would have caused it to become tighter?

 

It is also possible that I am giving the PO too many honesty points and this thing has been a CEL prone/broken gremlin that just happened to run well while I test drove it...

[FaderUS]

To clarify, the temp is currently 9 degrees outside, and I'm not sure how cold it got last night. -Chris

[xt2005bonbon]

XT, thanks again for the input. If the valve was too tight, would the CEL be so intermittent? The fact that it just showed up and isn't consistently reproducible makes me lean to a failing part (or something that has changed in some way) more than an adjustment issue. I am not familiar with the valve adjustment process on these cars, but is there anything that would have caused it to become tighter?

 

I'd say it depends. If the issue has just begun to show up, then the CEL might be intermittent. In other words, one of your valves may be tight but borderline tight where compression might be ok sometimes and not other times. The best is to log a bunch of parameters when you drive (using this app for instance---specifically tailored to our car).

When I ran into this issue, one of my exhaust valves had zero clearance (the spec clearance is 0.35mm +/- .05mm btw). It had 0.04mm which is basically none!! CEL was not always showing up at warm idle but I was using the above mentioned app and I could see misfires occurring in cylinder 2 at idle, but not enough to trigger the CEL every time.

 

Exhaust valves may have a higher probability of getting tighter over time due to wear on the valve seat and/or the valve itself where it gets into contact with the valve seat.

[MTBwrench]

There's not much for me to add to the topic, as all of the regulars have already chimed in and said most of what needs to be said but I'll add two spots of information:

 

1) I had one engine with a cracked ringland and zero misfires that still ran fantastic, save for oil consumption. No smoke out the fill tube either.

 

2) I also had an engine literally melt a quarter of a piston out the exhaust, would not idle, but just barely threw a misfire code.

 

So I guess what I'm saying is seeing how these things are, a misfire frequency can have zero correlation to engine issues. I'd get a compression/leakdown test.

[NSFW]

I never had a misfire CEL with my cracked ringland either.