Very Specific Misfire Issue

[kensei]

Hey all, thanks for sticking with me as I am working through the issues...

My '05 NA 5MT is having a very specific misfire issue. Above 3000 RPM at a steady speed while on Cruise Control, such as a freeway, when the cruise lets off ever so slightly, I will get a misfire code for cylinders 3 and 4. This only seems to occur above 3K, I can be going 55 or 60 on cruise, for example, and the problem does not occur. Also, if I am just driving using the gas pedal over 3000 RPM (or under), i cannot replicate the problem no matter how gingerly I try to let off the gas. Additionally, when the cruise control comes back on the power, such as to maintain a steady speed on flat ground, or to climb a hill, the MIL light goes out and there is no misfire issue...

I replaced the spark plugs and plug wires, which were both in dire need of replacement, but it did not make any difference in the issue.

Could this be some type of vacuum issue related to the cruise control?

It has 166k miles and had head gaskets at 100k. When I changed the plugs, they were well worn, but had no signs of any oil leakage or oil burning, and, obviously, all new plug/wire connections are good.. I had seen a few previous topics about cleaning the coil pack connector, so I will try that. Next on the list will be coil pack replacement, especially since 3 and 4 are on the same half of the pack, but was holding off on the $150 for some more input...

Thanks for any ideas, other than just driving under 3K RPM!

[ZUBAJA]

I strongly recommend further diagnosis before "throwing money at it". Start with a compression check and a vacuum check. This brings up another thing I have wondered about. As far as I can tell, Subarus have strictly mechanical valve trains. On my 2006 Forester, when I had the heads rebuilt, I did a valve adjustment. I haven't found where there is a recommended valve adjustment interval. I have found where many owners never check or have them adjusted. A tight valve (usually exhaust) might cause some of your symptoms. Another possibility could involve the EGR system. EGR cleaning and service isn't all that difficult (or expensive - maybe a gasket). Dennis

[kensei]

Thanks for the feedback, Dennis. I don't have a compression tester, so I will have to rent one from Autozone or something along those lines, or maybe Harbor Freight, don't need an excuse to go there...

I did complete an EGR cleaning tonight based on your suggestion, but wasn't able to go for a drive. It was dirty, though the valve was fully closed. Based on stuff I was searching for is the problem is that very dirty ones cant fully close the valve. Nonetheless, a good cleaning is going to prevent any issues further down the road.

I did do some more searching/reading on NASIOC after I made my original post, and there was a ton of postings there about misfire issues, though most were with turbo engines. However, from what I could gather, when a coil starts to go bad, it does go through a kind of regression in which it's ability to not have a misfire gradually becomes less and less, with the misfires occurring at lower and lower RPM. Since this sounded similar to my issue, at least being RPM related, I did measure the resistance across the coils as several posts indicated, and found mine to be out of spec.

Normally, I would wait to have the Subaru part shipped, but in an effort to get the issue solved, I checked the local parts houses (Advance, O'Reilley and Autozone) and much to my surprise, Autozone actually had a Duralast (their house brand) in stock for $100. I did a quick check in the store to make sure it looked similar, and headed home. Imagine my surprise when I examined it more closely at home and discovered that it was an actual Subaru OEM part, complete with the same Subaru and Diamond names and number cast into the housing as the old one I was taking off. This new one went on just before the EGR cleaning, but like I said, the test drive will have to wait until the morning.

I know, fix one thing at a time so you know what solved the problem...based on the resistance readings and the cleanliness of the EGR valve, both needed to be done, so if the issue goes away I'm not questioning it... I think a compression test will still be in my future, though. Since I just recently got the car, good baseline numbers would be nice for any future issues that arise.

[kensei]

Just to keep anyone updated who is interested...

This morning's test drive revealed that changing the coil pack and cleaning the EGR valve did not solve the misfire problem. Still getting codes for 3 & 4 cylinders when letting off throttle above 3K RPM.

I was able to get a stretch of highway with little traffic and sampled a few different speeds to get some info. Under the same conditions as the original post only with the cruise set at 2750 RPM, the misfire issue does not occur. Bumped it up to about 2875 and would get an occasional misfire code, but not as consistent or long lasting as the original post. Anything above 3000 replicated the original post. Today, though, I was able replicate the issue above 3K while just using the gas pedal. Though I don't consider that a victory, at least I would think it eliminates a cruise control related issue.

I guess that I should also clarify that I can't actually physically tell when the misfires are occurring. They do not seem to have any effect on the performance or drive-ability of the vehicle and I cannot detect any change in engine sound or "feel." I only know that it is a misfire problem because the check engine light will flash while it is occurring and then go out when the car is back on throttle. I have an UltraGauge that I am able to refer to that is reporting the P0303 and P0304 codes. Thus far, it has only reported them as Pending codes; they have not switched over to active or stored codes.

So I will try to get those compression readings and get those posted, but am still open to suggestion in the meantime.

[kamesama980]

I wouldn't get to excited over the coil pack until you thoroughly look it over for any sign of prior use. Having worked at a parts store employee, I pissed off a few customers by actually checking the box they returned that the parts were ours and unused and the contents were not: customers trying to return their broken parts in our box for a refund (apparently a number of other employees did not have this practice). Since there was no change in your symptoms, though, I wouldn't worry about it too much.

FYI you don't have any separate cruise control systems or components. The electronic throttle body is controlled by the ECM whether the command is from the gas pedal or the cruise algorithm. That brings me to another possibility: cleaning the throttle blade. Can't think up an explanation why it'd only happen on cruise.

[kensei]

Yes, I initially thought, too, that maybe I got someone's old coil pack. However I feel good as there was a Duralast "D" embedded (not a sticker) in the resin on the bottom of the pack, so I hope that is a good indicator...

Thanks for the info on the cruise control, but since my Baja is a Normally Aspirated EJ251, I still have a separate throttle cable and cruise control cable that manually operate the throttle butterfly valve. I think only the Turbo engines have the "drive by wire system."

As a strange twist of fate, though I am still having the misfire issue as detailed above, it has now mysteriously migrated to the #1 cylinder only; I have not got a code for #'s 3 & 4 in two days of driving, but have received only a P0301 instead. Also of note, the new #1 code does not require the use of cruise control, it can be replicated though still requires the RPM to be at or near 3K.

This "migration" is making me think that I am having more of an electrical (sensor or ground) issue than an actual misfire, especially since I can still not feel any hesitation or difference in sound when the alleged misfire occurs.

Apologies to those who have suggested getting compression numbers, still haven't had an opportunity to rent/purchase one yet.

[kamesama980]

Yes, I initially thought, too, that maybe I got someone's old coil pack. However I feel good as there was a Duralast "D" embedded (not a sticker) in the resin on the bottom of the pack, so I hope that is a good indicator...

Thanks for the info on the cruise control, but since my Baja is a Normally Aspirated EJ251, I still have a separate throttle cable and cruise control cable that manually operate the throttle butterfly valve. I think only the Turbo engines have the "drive by wire system."

As a strange twist of fate, though I am still having the misfire issue as detailed above, it has now mysteriously migrated to the #1 cylinder only; I have not got a code for #'s 3 & 4 in two days of driving, but have received only a P0301 instead. Also of note, the new #1 code does not require the use of cruise control, it can be replicated though still requires the RPM to be at or near 3K.

This "migration" is making me think that I am having more of an electrical (sensor or ground) issue than an actual misfire, especially since I can still not feel any hesitation or difference in sound when the alleged misfire occurs.

Apologies to those who have suggested getting compression numbers, still haven't had an opportunity to rent/purchase one yet.

Only the early NAs have the mechanical throttles. they went to the throttle-by-wire in 04 or 05 for the NAs as well. Yes, all turbo Bajas are E-throttle.

Another thing to consider, having a 251, is the valves are easily adjustable and do require occasional adjustment. something else could be a failing harmonic damper (aka crank pulley) the missfire code is set by the ECM seeing a certain pattern in the crank position sensor signal at a certain point in it's rotation. if the damper's going bad and you're getting a funny harmonic at just the right time it's conceivable it could show up there. same thing goes for the torque converter, if you had one.

[ZUBAJA]

So what is the suggested valve adjustment interval? I have yet to see it in the "maintenance sections of my owners manuals. Dennis