Pending Cylinder 4 Misfire

Background: January 2018 I had a cylinder 4 misfire. Took it to my mechanic who replaced the coil and problem appeared to be resolved. A couple of weeks ago the engine was running rough and a pending cylinder 4 misfire code was logged. Took it back to the mechanic and he was unable to reproduce the problem. I've been driving it short distances since then and no issue. Today was headed on a longer distance drive and it started acting up again (slight hesitation, rough idle). Going to take it to my mechanic on Monday.

 

I'm wondering what possible things could be causing a misfire. Since it's intermittent it makes diagnosis difficult so it may be just a matter of throwing some parts at the problem. What parts could be suspect? Injector? Spark plug?

 

I don't have any logging tools. Some recent things I've done:

 

• Replaced the number 2 and 4 coils (1 and 3 were done a couple of years ago, all are now OEM)
  
• Replaced the battery with a new AGM battery...it's literally less than two months old)
  
• Compression test performed within the past year. Readings were OK for age. Nothing indicates any major issues with the engine.
  

It consumes about 1 quart of oil every 1K miles if I drive it harder than normal. About 1 quart every 1.5K with normal driving. Always had an intermittent, slight idle but nothing of concern. But I think whatever it is might be getting worse.

 

 

Mechanic says the intermittent nature indicates something less serious (i.e. major engine issue would not be intermittent). Thinking of just having him replace the injector but what else should be on the list? I'm sure he has his ideas but I want to be thorough.

 

 

 

Love the car and willing to spend some money on it. Primary concern is to solve the problem as I don't like having a vehicle which is marginal.

 

 

Thoughts?

How many miles are on it? How do the spark plugs look?

How many miles are on it? How do the spark plugs look?

Approximately 155K miles. No idea how the spark plugs look however they were replaced in May of last year (as part of the 150K mile service). Given how little I drive they probably have less than 5K miles on them.

ASSUMING you have a good coil on #4, I would move on to plugs and injectors:

Swap the #4 plug with #2 & see if misfire follows.

Do the same with the injectors.

Doesn't matter which you do first, whichever you find easier.

ASSUMING you have a good coil on #4, I would move on to plugs and injectors:

Swap the #4 plug with #2 & see if misfire follows.

Do the same with the injectors.

Doesn't matter which you do first, whichever you find easier.

Given the coil was just replaced on cylinder 4 and that was the problematic cylinder back in January I think it's reasonable to assume the coil is OK. Since the plugs were just replaced last year (and less than 5K miles ago) I think it's reasonable to assume it is OK too. While new parts can fail I think right now I would focus on parts which have not been recently replaced. Sounds like the injector may be the next item to replace.

 

What are everyone's thoughts on fuel injector cleaner? I've never been a fan of these things but, for $35.00, it may be worth a try. Any other possible causes for a single cylinder misfire other than plug, coil, and injector?

Valves being out of adjustment, ringland failure, bad valve.

I've never seen injector cleaner be a bad thing, but never had it fix a misfire either

Valves being out of adjustment, ringland failure, bad valve.

I've never seen injector cleaner be a bad thing, but never had it fix a misfire either

Would these result in an intermittent issue? The thought is if one of these were the issue the problem would be consistent and not intermittent.

They can, yes. Especially with rough idle, I'm curious What the valve lash is

They can, yes. Especially with rough idle, I'm curious What the valve lash is

Are these something a compression test may reveal? I ask because last year, as part of my 150K mile service, I had a compression test performed and everything came back good. The number four cylinder was inline with the other three. I've added 4,600 miles since the compression test was performed.

Leak down test would be a better indicator. Was that done?

Leak down test would be a better indicator. Was that done?

 

It was not. I had the compression test performed because it was in for the 150K mile service and I was wondering about the oil consumption.

Thought I would provide an update. I dropped the car off on Monday morning and my mechanic was unable to reproduce the problem. Given the intermittent nature of the problem I decided, proactively, to have him replace the injector for the number 4 cylinder.

 

I picked the car up this morning and drove it to grab lunch. Wouldn't you know it as I was coming home it started running rough and the check engine light illuminated. As I pulled in the CEL went out even though it was still running rough. As he was driving me home it came back on again.

 

The positive is the car is now exhibiting the problem so he can work on diagnosing it. The negative is the car continues to exhibit a the problem despite the spark plugs, coil, and injector having been replaced.

 

Due to the intermittent nature of the problem these three items were consider top on the list of failure items. It seems the easy stuff has been addressed. He now has the car and I'll update everyone with what he finds.

Another update: It turns out the new injector itself was faulty and my mechanic replaced it with another new one. I haven't had an opportunity to put many miles on the car but this new injector has lasted longer than the previous new injector. I'll update once I have many more miles on the car.

If the problem persists, time to check the valve clearance and do a leakdown test as well.

If the problem persists, time to check the valve clearance and do a leakdown test as well.

 

This is what worried me when the issue occurred after replacing the injector. We have replaced all the likely (plug, coil, and injector) items. Mechanic felt it was something electrical due to the vary intermittent nature of the issue.

 

If it persists I have a decision to make as any future repair is likely to be at a few thousand (i.e. $4K or more) for repair. I've toyed with the idea of buying a new, electric vehicle but so far have held off because I don't want to get rid of my Outback. But if it's going to cost me $4k or more that might be all the justification I need to move forward with the electric vehicle. Sadly Subaru doesn't offer anything new which is comparable to the fourth generation Outbacks. I'd jump on a turbo charged Crosstrek.

Can't remember if you've mentioned it before... When does it misfire? mostly at idle? if yes, warm or cold, or both?

 

Are you mechanically inclined?

Can't remember if you've mentioned it before... When does it misfire? mostly at idle? if yes, warm or cold, or both?

 

Are you mechanically inclined?

 

Quick background:

• All spark plugs were replaced less than 5K miles ago as part of the 150K miles service interval. The were not replaced due to any issue with the car.
  
• In late January of this year, in cold weather, the vehicle developed a cylinder 4 misfire. Mechanic replaced the coil and the misfire, along with the CEL, disappeared.
  
• About a month ago (Saturday morning) I experienced a rough idle and a slight hesitation while driving down the road. It was colder out that morning. No CEL but a pending code for a cylinder 4 misfire was set. Problem exhibited itself on Saturday and Sunday (Sunday is when the pending code was set). Took it to my mechanic who couldn't reproduce the problem.
  
• Last Saturday, three weeks from the above symptom, the car had a slight roughness. Temperature was colder (32 degrees with snow). Took it to my mechanic on Monday and explained to him the symptoms and the weather conditions. The next day (Tuesday) was going to be cold and snowy so I thought it would be ideal to replicate the conditions for which I experienced the issue. My mechanic was unable to reproduce the issue. Being proactive I decided to replace the injector just to rule it out.
  

 

The problem seems to pop up when it's colder out but I cannot say for certain. Especially since my mechanic couldn't replicate the issue when it was cold out.

 

I am mechanically knowledgeable but not inclined. Unfortunately I do not have the time to work on my vehicles. I've reached a point in my life where I can afford to pay someone to do the work or, if too expensive, buy an alternative. I think I've reached a point where if this car is going to cost $4K plus to repair I'll probably look into replacing it. I love the car, it's a lot of fun (I think I have more fun with it than my X5-M). But reality is it might more sense to replace it.

I see.

 

Could very well be valve clearance issue, a burnt valve, or cracked ringland. So yeah, do a leakdown test (warm) and post the results.

 

If the issue was a tight valve and you'd be willing to fix the problem yourself, then it may cost you your time and maybe 300-400$ worth of parts. But since you said you'd rather pay someone, then it may not be cheap .

Also, I should add that typically, a cracked ringland would throw more misfires when it is really cold out since the metal has 'shrunk'. But then, as the engine gets warm, it may get better, where the metal has now expanded and compression is a bit better.

 

On the other hand, if you had a tight exhaust valve it'd be the opposite: the warmer the engine, the worse it'll perform particularly at idle. That is because the valve stem itself may expand in length and since the clearances are tight, it can no longer provide a good seal. This results in low compression = misfires.

 

Now, this is my understanding. I am not a pro mechanic . And these theories are not full proof either. Cause I did have a tight exhaust valve once that exhibited the symptoms of a cracked ringland on my 06. I also had a tight exhaust valve on my 05. But that one exhibited the symptoms I described above. Go figure. Sooooo yeah. time for warm leakdown test and check clearances.

I see.

 

Could very well be valve clearance issue, a burnt valve, or cracked ringland. So yeah, do a leakdown test (warm) and post the results.

 

If the issue was a tight valve and you'd be willing to fix the problem yourself, then it may cost you your time and maybe 300-400$ worth of parts. But since you said you'd rather pay someone, then it may not be cheap .

 

Those have been suggested as possibilities and, if the problem recurs, will be something to consider. That's where the dilemma lies: How much am I willing to put into diagnosing the problem? My mechanic states that such problems would be in the thousands of dollars to repair. Do I want to spend thousands of dollars to repair (given the engine is consuming 1 quart of oil per 1K miles)? Or abandon ship and buy something else?

 

Since I've been toying with the idea of buying some form of electric vehicle I may take this as a sign to move on.

 

I'd love to work on it myself but there are several factors working against that. To do such major engine work would require more resources, including time, which I do not have. I'm not against spending $4K plus to fix this car...I really love it and it's been a very practical car. But there are some other things I need to consider.

 

For the moment everything is positive and I don't have to make that decision. However if it acts up again it will be decision time.

OK. Good luck. Hoping that it won't return.

OK. Good luck. Hoping that it won't return.

 

Thank you and I appreciate your feedback.

If the problem persists, time to check the valve clearance and do a leakdown test as well.

 

I agree - if compression test checks out and a leakdown test don't show anything special then it's probably the valve clearance and that shall be taken care of as soon as possible to avoid burnt valves.

 

And when doing that - check and correct it for all cylinders.

I agree - if compression test checks out and a leakdown test don't show anything special then it's probably the valve clearance and that shall be taken care of as soon as possible to avoid burnt valves.

 

And when doing that - check and correct it for all cylinders.

Does the engine need to be removed from the car in order to check the valve clearance?

 

For the moment it appears to be running fine. However it has been an intermittent issue so I'll need more drive time. I'll be taking it on a 200 mile drive next weekend and if there are no issues I will be very encouraged. If it gives me problems I may just decide now is the time to pick up an all electric vehicle (something I've been considering but haven't pulled the trigger on as I love my Outback and don't want to part with it).

Does the engine need to be removed from the car in order to check the valve clearance?

 

Not at all.