Just another misfire/rough idle thread...

[StkmltS]

This morning on the highway I saw a few misfires several times. This was the first time that I can remember seeing any misses while driving. They happened in 3rd gear at around 2000 RPM. That's too low of an RPM to be putting much load on the motor, but I was going down a hill in stop-and-go traffic and sometimes shifting isn't fun. I'm going to be pissed if my motor breaks only a few days before I tear it apart.

[StkmltS]

Update from my rebuild: My misfire was caused by leaky valves. The valves weren't cracked or burnt, they were just dirty/pitted enough to not seal properly. The valve seats were also rather dirty. The "crack" that I thought I found in my piston must have just been dirty that looked like a crack.

[xt2005bonbon]

I now wonder if seafoam would have helped in your situation. I never used that product though..

[StkmltS]

I tried seafoam and it didn't help. I sucked about have a bottle through a vacuum hose and the other half went in the gas tank.

After my rebuild is finished I'll take a closer look at my valves and post any relevant info here.

[StkmltS]

So...

 

Two days ago I finished rebuilding my engine, and I'm extremely happy to say that it's alive and running... just like before. Talk about a doh! moment. Posting again in this thread is like admitting defeat. So lets just pretend that my rebuild fixed the problem and now I have a very similar, very new, problem. I'm not sure how I'm going to explain it to my wife. Wish me luck. Back on topic.

 

Driving the car yesterday I noticed something I hadn't before. Or it really is a different problem. Whatever. While the car is moving the motor VERY rarely misfires. No news there. Upon stopping the vehicle (0 mph) it starts missing. I don't understand why, but it's predictable and repeatable. I have a few logs to look at then I'll post a graph showing what's going on.

 

Does the ECU make any changes based solely on vehicle speed?

 

I'm not sure exactly how to move forward, other than to start looking at everything that I haven't already checked.

Edited September 20, 2016 by StkmltS

[xt2005bonbon]

So you say that it is missing at idle. But in which conditions? cold and warm OR cold only OR warm only?

 

As for troubleshooting. I'd just do the usual suspect again (coilpack, spark plug--I know she is new but still, injector). Go from there.

[StkmltS]

Cold and warm.

 

The injector isn't the one that came with the car. I bought a used OEM injector about a year or so ago. At this point I don't know what to think other than a wiring problem.

 

Sent from inner space.

[birkhoff]

No rebuild and install is without hiccups. Scary stuff in the oil, rough running, leaks of all types are possible and at least some are probable. The key is to arm yourself with all the new information you have so you can focus your efforts!

 

Cyl #2 was the problem before, right? There was never any evidence of a systematic problem with any other cylinders. Now cylinder #2 again. This cannot be a coincidence even though your mind may wander into that territory. You have to concentrate on things that didn't change during the rebuild. For example, you have all new pistons (isn't that lucky) so you can eliminate that thought. It is inconceivable that you built the exact same problem back into the motor due to sloppy assembly, or incorrect valve clearance or any other things like that. Maybe new problems, but not the exact same one!

 

I think you are bang-on to concentrate your efforts on electrical, but only things related to cylinder 2. Rereading your introductory post, there was a lot of effort on injectors, coils, vacuum system bore scope and whatnot and you really don't want to follow that path again. Electrical seemed to take a back seat, in retrospect.

 

Did you thoroughly check the coil connector for #2. Did you try cleaning and gently crimping the contacts?

 

If you check the wiring diagram, all four coils share the same ground point, and the same power wire (except at some point they separate in the loom and head to the coils). If you have any reason to suspect the power wire or gnd in #2, you could jumper over from an adjacent coil. And/or set up a direct ground for that coil.

 

Each coil has its own trigger wire back to the ECU. Could you bridge the trigger for #2 just in case there is an intermittent break somewhere in the loom?

 

Except for checking the connector, none of these are terribly simple steps but they are at least consistent with what you know, after the rebuild.

[StkmltS]

^ that's a good description of what's next on my plate. It won't be easy or fun, but it'll certainly be interesting.

 

I drove to work early enough this morning that I missed all traffic congestion and I didn't even see one red light (I rolled through the stop signs.) I didn't see any misfires until I stopped in the parking lot at work. Very strange that it seems to only happen at 0mph.

Edited September 21, 2016 by StkmltS

[heiche]

Can't believe the amount of work you've done already! Trying to think of what hasn't been covered is tough! Are there any other odd symptoms your car has that might somehow tie in with this?

 

I wonder if there could be any damage to your crank sprocket and/or crank position sensor?

 

Also I remember some discussions about your ER-HC error, and talk about the BIU. Wonder if there's any possibilities there, or if it's a dead-end?

 

Did you try the slightly-twist-the-coil-pack-terminals trick, so they have more of an interference contact with the connector?

[StkmltS]

Heiche, you're about as in-tune with our ECUs as anyone on here... can you explain (1) how misfires/roughness is measured/detected and (2) what it means when the roughness count increases/resets?

 

I haven't tried swapping in a new/used MAF sensor (I have cleaned it though), but that wouldn't explain why the miss only happens in #2. Other than that I don't know what to do other that test the wiring in 1,000 places.

 

The ER-HC/BUI issue was a poor ground connection at the main grounding bracket on the rear side of the DS head. I had inadvertently swapped one of the bolts with a longer one from somewhere else and it was bottoming out before tightening down the bracket. I'm very confident that was the sole cause for the ER-HC problem.

 

My crank sprocket looks good, and I haven't tested the sensor.

I'll try the terminal-twist trick next time I'm in there.

[xt2005bonbon]

Since your long term fuel trims look great (from your other post), I would not worry about touching your MAF, vacuum hoses and stuff like that.

And yeah, I would 'play' with electrical stuff and see what happens.

How does your fuel dampener on the driver side fuel rail look like?

[birkhoff]

 

I haven't tried swapping in a new/used MAF sensor (I have cleaned it though), but that wouldn't explain why the miss only happens in #2. Other than that I don't know what to do other that test the wiring in 1,000 places.

 

--- snip ---

 

My crank sprocket looks good, and I haven't tested the sensor.

I'll try the terminal-twist trick next time I'm in there.

 

Now we're talking. If it isn't specific to #2, it should be passed over for now. MAF is a good example. Checking again for vacuum leaks another.

 

A sloppy crank gear should give intermittent problems with timing in all cylinders, and I don't see how a bad crank sensor could isolate to a misfire on one cylinder. Two cylinders maybe. More likely all four.

 

Wiring section of FSM has a testing procedure for the ignition leads. It also shows pin-outs at the major connections for the power, ground and trigger leads. That's the next step after cleaning and tightening up the connector.

[heiche]

Could the crank gear be responsible for throwing a cylinder-specific misfire? maybe. Although I would think if it's something like a bent tooth, it would cause misfires in TWO cylinders (1 and 2 -or- 3 and 4).

 

My understanding of how misfire detection works (not Subaru-specific) is that it is mainly between the ECU and the crank position sensor/crank trigger gear, and to a lesser degree the cam sensor so it can tell which phase the crank is in (is it cylinder 1 or 2? the crank position is the same, so the ECU relies on the rough cam position to know which one it is). The ECU monitors the crank position signal to determine what the crank position is, in degrees, at any given time. The ECU basically waits until it fires a plug, then for a very short time it monitors the number of degrees of crank movement. If that number is less than a specific threshold then it considers a misfire to have just happened, so it increments the misfire/roughness counter for that cylinder.

 

So let's say your crank gear has a slightly bent tooth that corresponds to when cyl 2 is positioned a few degrees before top dead center, that could fool the ECU into thinking that more or less degrees passed when it is monitoring cyl 2. However, that would also affect misfire detection for cyl 1, so I'm not sure how viable the idea is that you have a damaged crank sprocket.

[xt2005bonbon]

So while we are on that topic, what is the unit of the misfire/roughness counter? I never got a straight answer to this question (see bonus question below).

 

Bonus question: what does the 'roughness count' parameter provided by the ecu really report? I understand it is a count, but a count of a number of misfire per what?, per second or per ---- fill the blank please.

[heiche]

So while we are on that topic, what is the unit of the misfire/roughness counter? I never got a straight answer to this question (see bonus question below).

 

I'm not really sure. But based on observations, it seems like it is a misfire counter - so if cylinder 2 misfires 10 times in the last few seconds, the misfire count will be 10 for cyl 2. Then at some point the ECU simply resets the counter back to zero, and I'm not sure if that's based on elapsed time since the first misfire, or what. Doesn't seem to retain the misfire count for very long though.

 

Andrew, if you think the crank sprocket is a possible offender, could you get your borescope in there to take a peek, maybe if you remove a top TB cover? You could slowly rotate the crank with a wrench (plugs out would be smoother, ugh).

[xt2005bonbon]

I'm not really sure. But based on observations, it seems like it is a misfire counter - so if cylinder 2 misfires 10 times in the last few seconds, the misfire count will be 10 for cyl 2. Then at some point the ECU simply resets the counter back to zero, and I'm not sure if that's based on elapsed time since the first misfire, or what. Doesn't seem to retain the misfire count for very long though.

 

here is an interesting thing though. But first let me ask this: say you are idle with an rpm of like 750. How often do you think a given cylinder will get spark per second?

[StkmltS]

Thanks for sharing your knowledge.

 

Based on that info:

I don't think it's my crank sprocket/sensor because I can feel and hear the motor miss. I'm absolutely not just "hearing things" as my wife suggested many times. With the hood open you can even see the slight back-forth shudder at the same time as hearing it miss. Earlier today another car guy here at work looked at it and he also saw/heard it missing. The miss is real and being reported accurately to the ECU.

 

Why does it happen when the car stops, but not when the car is moving? It makes no sense and I bet whatever the cause is has something to do with speed. I need to find something connected to the engine that changes with vehicle speed (vs engine RPM).

Edited September 21, 2016 by StkmltS

[xt2005bonbon]

Why does it happen when the car stops, but not when the car is moving? It makes no sense and I bet whatever the cause is has something to do with speed.

 

with my 06 and her tight valve, I experienced and observed exactly the same thing: the car could move super slow (while idling) and not register any misses. But once I was at a complete stop, THEN it started counting. Still super strange to me. I don't know if this influences the misses or not, but I view the OCV angles live when I drive. And I have noticed that even if you are moving super slow or getting to a stop, these values will get to zero ONLY when you are at a complete stop. So it may be connected. Who knows.

[birkhoff]

Ok, I'll take a stab at it. 750 RPM means 375 ignition events per minute on each cylinder, or approximately 6 ignitions per second. I think. That's at idle. Imagine at 5000 rpm! Clearly Herr doktor professor Otto was a madman to think this could ever work.

[StkmltS]

here is an interesting thing though. But first let me ask this: say you are idle with an rpm of like 750. How often do you think a given cylinder will get spark per second?

 

It takes four strokes (two full revolutions of the crank) for each piston to fire (spark). Starting at TDC in cylinder X: Down1/180° = intake, Up1/360° = compression, Down2/540° = combustion, Up2/720° = exhaust.

 

So, each full rotation of the crank produces 2 sparks.

Cylinders 1 and 3 fire on the first 360° of rotation, and cylinders 2 and 4 fire on the second 360° of rotation.

 

TLDR

At 750 RPM each cylinder sees intake/compression+fuel/combustion+spark/exhaust roughly (pun intended) 6.25 times per second.

(750rpm divided by 2 rotations per spark, divided by 60 seconds per minute)

Edited September 21, 2016 by StkmltS

[xt2005bonbon]

ok. Then if your numbers are correct, it really looks like the misfire/roughness count reported by the ECU is really just that. Cause I recall when I was swapping injectors, one of them was not sitting right, and the roughness count was going crazy. IIRC, I was seeing approximately 6 counts per seconds. Once it reached 99, it reset and kept going.

[StkmltS]

So you're saying the roughness count is misses per... what?

Currently mine seems to be resetting to zero after anywhere between 2 and 30 misses.

I'm still thinking about it.

Edited September 21, 2016 by StkmltS

[heiche]

Have you tried any "live" tests when it is misfiring? Wiggle the #2 coil plug to see if it stops missing. Wiggle the #2 injector plug to see if it stops missing. Turn on the AC to raise the idle a little. Move the shifter from park, drive, reverse, etc. Just looking for ANY more variables

[xt2005bonbon]

^+1.