99 Legacy OB Misfire All Cylinders

[sctcrash]

I used to really want a Subaru Outback. And I finally got what i wanted, man am I lucky.

 

I have had my 99 for about a year and a half now, and for at least the last 8-9 months I have had CEL problems. The current problem is I get a Misfire Code on each Cylinder. So here is the history.

 

I got a CEL some time last fall, took it in to the dealer. They said i had a bad ignition coil in cylinder 4 i believe, maybe 3 and 4 because I remember a p303 and p304 i think. They also replaced the plugs and wires. About a week later "BING" CEL is back on. The ingition coil they put in was bad. OK, all set.

 

Two weeks later "BING" CEL is on. They got a misfire on all cylinders code, cleared the code and checked it out, found nothing, sent me home. Week later "BING". It has gone on like this for a while now.

 

They did a Top Engine clean and that made the CEL go away for 2 weeks. Between each CEL the length of time i wait before bringing it in gets longer and longer.

 

Last time in they did a full engine clean, filled it with some fluid and let it sit for two days while i was out of town.

 

That lasted about 2 days.

 

Car has about 100K on it, bought it at 72, had spedo replaced about 3K ago and hate to do the math. I think it was around 91K at the time.

 

It seems to always come on when I am driving in 4th at about 3200 RPM. May happen in 5th, havent figured out RPM yet as it doesnt happen as often. Usually when I have been driving atleast 10 minutes or so. Before full engine clean i could drive it for 5 minutes at 3200 and watch it flash once every 30 seconds or so. Now its not as consistent.

 

A guy at Autozone just checked the codes again for me, he mentioned an EGR valve but didnt know much about Subarus.

 

HELP PLEASE, i want to love my Subaru as much as i know i would if this stupid CEL would go off.

[Cougar]

If you don't hear any misfire you may try replacing the knock sensor. These can sometimes be too sensitive and cause a code error that is not really there. Changing the gas you use may help also.

[ferret]

Because you have a misfire on ALL cylinders, let's get a little 'technical' here about what a 'misfire' is.

 

First for combustion, we need the basics, air/fuel , compression , and ignition. (I'm aware we all know these). Then with the help of the Camshaft pickup, we ignite spark at the proper moment. NOW the Crankshaft sensor MUST see the next pulse within a certain time, (and it is RPM dependent, ie faster RPM, ECU expects that pulse sooner).

Bearing this in mind, AND the fact you have a misfire on ALL cylinders, you can have a fuel / delivery problem (common for the first few starts after replacing a fuel filter, but then goes away). Still can be the filter, or a 'cranky' fuel pump. If you can, test the Fuel Pressure.

Can also be the Coil assy they replaced. I know there are 2 seperate coils inside, but it may have been the wrong p/n.

Can be the ECU, very expensive, and a long shot, but also a candidate.

Also the Crank or Cam sensor, May not be putting out a 'clean' signal.

OR something mechanical, Timing belt, Tensioner (allowing the crank / cams to move a few degrees at certain RPM / Loads).

Or a connector on any of the component not making good contact.

Are there any other sounds heard at the time, I know of a damaged Flex plate on a Dodge I serviced causing a misfire, but there were also other 'noises' associated with that vehicle.

Misfire on ALL cylinders is a tough code to resolve. You just have to eliminate suspects one at a time. Sorry I don't have a HERE's THE FIX answer.

[jhelm_waterw]

Ferret,

I liked your reply. I am having my own cylinder 4 misfire issue also.

My question is this:

how does the computer (ECU) detect a successfull detonation?

(What sensor or mechanism tells the computer that a cylinder fired off?)

It's not like there are 4 individual cylinder head sensors on the system.

(and just because a spark plug itself did fire, does not mean there was a viable cylinder combustion)

Thanks in advance for any insight.

[ferret]

Mis-fire accoring to the Theory of operations is detected after xx.ms AFTER the plug is fired. The CRANK sensor MUST see a pulse within this window. The window length varies according to the RPM. (Faster RPM...Smaller window to detect the crank pulse).

 

So ANY thing that causes a cylinder not to 'detonate' will cause a miss, no matter how slight, the PCM / ECU / Logic unit will detect it and post a misfire.

Most common I have seen across many vehicles in order are:

Plugs, Wires, Fuel filter, Fuel Pump, Fuel Regulators, Coils, Crank / Cam sensors.

The Flex plate was rare, but others with mopar experience put me onto that one.

These have been my experiences, but every car and problem can / will be different.

[jhelm_waterw]

Mis-fire accoring to the Theory of operations is detected after xx.ms AFTER the plug is fired. The CRANK sensor MUST see a pulse within this window. The window length varies according to the RPM. (Faster RPM...Smaller window to detect the crank pulse).

 

Ferret, thanks for reply.

So... it is the CRANK SENSOR that detects a 'Kick' (so to speak) on the crankshaft ? Is the crank sensor a device which (much like the knock sensor) detects a vibration or a 'sound' on the crankshaft? So the crank sensor is NOT a positional sensor? I think I was originally confusing the crank sensor with a rotational position sensor (like the flywheel sensor on my old audi).

Ok. Now I can see how it can (by listening to the crankshaft) detect if a detonation took place. Thanks

[1 Lucky Texan]

Wow, Ferret's post is really good!

You don't mention the car's mileage, could it be due for a timing belt and could that be the problem? Also, is the failure more likely on a damp day or during rain? Does the vehicle have any rusty/corroded ground connections under the hood? Would it ever throw the codes stationary but revved to 3200 rpm?

 

just thinking out loud

 

Carl

[Setright]

Fuel mixture leaning out over 3000rpm is a symptom of a drop in fuel pressure. The regulator lives on the end of the fuel rail on the passenger side. Replace this.

I would also suggest replacing the knock sensor if it has the grey connector - old type. (New is white)

[BigMattyD]

The crank sensor is a positional sensor.

 

 

I believe that the computer can detect a misfire because it knows the engine rpm and the time of spark, so it knows when to expect the crank sensor to report its position.

 

If there is a cylinder misfire, the crank will slow down slightly, so there will be a delay in the time it takes for the sensor to report its position.

 

 

Matt

[Cougar]

It is the knock sensor that detects misfire I believe. Some of the sensors were too sensitive and gave a false error code. Replacing the sensor was the cure for the problem.

[sctcrash]

Thanks for the great information everyone. Thanks Ferret for all of the detail you provided. I am not a mechanic unfortunately so my ability to do most of this stuff is verry limited.

 

Since the car has almost 100k now I wonder if I should just start with getting the Timing Belt replaced.

 

I have to take the car in anyway, i think my throw out bearing in the clutch, or some bearing has started to go. I am getting a lot of whine from what seems to be my tranny. I had that in another car and it was my bearing.

 

Is there anyway I can check the Fuel Pressure, or have someone check it for me without taking it to the dealer?

Also how difficult/easy would it be for me to replace the Fuel regulator and Knock sensor, and how much do those parts cost. I know a mechanic who may do it for me if I cant. But he doesnt work on subarus so shoudl I just take it to the dealer and suck it up.

[1 Lucky Texan]

Thanks for the great information everyone. Thanks Ferret for all of the detail you provided. I am not a mechanic unfortunately so my ability to do most of this stuff is verry limited.

 

Since the car has almost 100k now I wonder if I should just start with getting the Timing Belt replaced.

 

I have to take the car in anyway, i think my throw out bearing in the clutch, or some bearing has started to go. I am getting a lot of whine from what seems to be my tranny. I had that in another car and it was my bearing.

 

Is there anyway I can check the Fuel Pressure, or have someone check it for me without taking it to the dealer?

Also how difficult/easy would it be for me to replace the Fuel regulator and Knock sensor, and how much do those parts cost. I know a mechanic who may do it for me if I cant. But he doesnt work on subarus so shoudl I just take it to the dealer and suck it up.

 

If you do have the TB replaced, there are good reasons to have several items in that area addresssed at the same time due to the labor cost. A good Soob mechanic can suggest what else needs replacing or try a search here.

 

Carl

[Setright]

First: I must agree with Cougar. The knock sensor is constantly listening the engine as the cylinders fire. An ECU signal to send a pulse to a spark plug that is not followed by a "bang" is what triggers the misfire code.

 

sct: Replacing the knock sensor is easy. You need a good socket wrench with an extension to reach past the intake manifold. Search the forum for pictures and more good advice on this.

 

Fuel filter and regulator require you to relieve the pressure before removing the hoses. Again, search and ye shall find loads of info on this. I would suggest the disconnected fuel pump (or relay) because it will empty the system and gives the least spill.