CTMechE

Ok, so my wife's co-worker's Subaru (Not sure the year, but it's ~02 Outback 2.5i) has a misfire. CEL and rough running caused her to bring it to the local shop. They hosed her out of $80 to check the code, and told her it needed a "$500 tune up" for new plugs and wires. That also included a fee to reset the computer.

So, relaying it 3rd person, I said that's a bit ridiculous. Out of spite, I'd go to another shop, and just tell them to change the plugs first and see if that solves the issue. It has 70k miles anyways, so new plugs can't hurt.

My next step would have been to check coil packs, then wires, then check compression, and check the injector. This is a Cyl #4 misfire, not the whole engine.

So she had another shop change the plugs. They called today and said that didn't fix the issue. I'm not sure exactly what that means. But now they want to change the PCV valve. No quoted price yet.

But my main point is, while the valve may indeed be bad, I just don't see how changing it will have any effect on the Cylinder 4 misfire.

I'm fairly knowledgeable about engines, but I haven't done extensive repair work... which is why I'm asking the more experienced folks.

I don't want to dictate to her by proxy what to tell them, since it's her car. But I hate to see someone get screwed into replacing random parts rather than solving the main problem.

ScottyBallistic

I would check for a vacuum leak ..

CTMechE

Wouldn't a vacuum leak or faulty PCV valve trigger another fault code in the OBDII computer? Apparently the first shop says the only code is the misfire one.

ScottyBallistic

Nope ,

But can cause Misfire as it can be running lean .

I never run into a Bad PCV causing misfire so I could not answer that one for you . But You are on the right track with you other options above .

Just some other things I copied and pasted here for you ..

"Symptoms

Symptoms may include:
the engine may be harder to start
the engine may stumble / stumble, and/or hesitate
other symptoms may also be present
Causes

A code P0304 may mean that one or more of the following has happened:

Faulty spark plug or wire
Faulty coil (pack)
Faulty oxygen sensor(s)
Faulty fuel injector
Burned exhaust valve
Faulty catalytic converter(s)
Running out of fuel
Poor compression
Defective computer
Possible Solutions

If there are no symptoms, the simplest thing to do is to reset the code and see if it comes back.

If there are symptoms such as the engine is stumbling or hesitating, check all wiring and connectors that lead to the cylinders (i.e. spark plugs). Depending on how long the ignition components have been in the car, it may be a good idea to replace them as part of your regular maintenance schedule. I would suggest spark plugs, spark plug wires, distributor cap, and rotor (if applicable). Otherwise, check the coils (a.k.a. coil packs). In some cases, the catalytic converter has gone bad. If you smell rotten eggs in the exhaust, your cat converter needs to be replaced. I've also heard in other cases the problems were faulty fuel injectors."

CTMechE

Thanks for the info.

Apparently it's stumbling noticeably, so no doubt the code would come back if cleared.

And if it was a bad O2 sensor or cat, wouldn't other codes be showing in the computer scan?

That's why I'm more inclined to check the coil packs, wires, compression, and injector before the other things.

While the PCV valve may need to be replaced, I just don't see it solving the issue... just the shop trying to find more things to do.

ScottyBallistic

OBD codes are only for emissions related items .

There is not a code for all the separate sensors on the car . If they have a real scan tool they can read the o2 sensors on a graph and see if they are working properly . You can also tell if the Cat was working properly ..

dlaio

Mike, I have a ODBII scanner you can borrow if you want it.

I agree with Twizid, sounds more like an electrical problem.

A vaccum leak is pretty easy to find, on the jeep the damn thing whistled loud enough so I could hear it inside the driving compartment.

To check the PCV valve, just take it off the car and rattle it. If it rattles, probably ok, otherwise replace it. Heck they cost $1.65 each, I might just replace it.

To check the fuel injector, get a long handled screwdriver and put the blade on the offending injector. Put your ear on the handle and you will be able to hear it opening. Should sound like a tap-tap-tap.