Mass Airflow Sensor Problem

Download a video on how to replace the timing belt, it needs it by now anyway.

Remove both oxigen sensors and test them with a torch, they should give you a few mV at high temp (download a video on this).

Take your vehicle to a shop and have the catalytic convertor removed and checked for obstructions.

I don't know about this car, but you look to see if there's a procedure for resetting the ECU and letting it "re-learn" your driving ways.

P.S. before you start all this, first do a vacuum leak test, (by ear, visualy and reaching out for fryed/cracked hose connectors).

Knock sensors are known to fail on the Subarus. They are merely a piezoelectric transducer, I believe, and can randomly fail. At the age of your car, it may not be unlikely that both could have gone bad.

I had a code for knock sensor trigger the CEL on our 2002 Forester. My mechanic said that you would probably notice rough running (it wasn't) if there really was a knock condition, because the ECU can usually deal with it. He replaced the sensor and it was all good.

As for the knock sensors affecting the MAF, I have no idea, but it is possible that the ECU trying to accommodate the supposed knock condition could affect the airflow somehow?

Probably just knock sensor went bad....

http://www.ultimatesubaru.org/forum/topic/116520-knock-sensor-101-and-replacement/

Since you seem to work on the car your self and you have a air flow restriction: do you miss any tools lately?

Just asking. Would not be the first one to leave tools in the wrong spot.

Ideas?

Yes how about the normal visit to a garage to have them look at it?

You post well, really good infos and very clear, most unusual on CR4!!!

Firstly, I would have also replaced the MAF as a first (and also been wrong it would seem) as I have had to replace one on every car I have had over the last 18 years, usually within 2 - 3 years of purchase (new)....

Needed, but still not really reliable....they were very expensive then too!! Now there are "repair kits" for most models.....

I tend to agree with the "knock sensors" being the next best to replace now, except maybe an easy test for a blocked Cat, simply at home (after telling the neighbors), remove it and run the motor for a short time, with a cheap bit of pipe installed so as to NOT burn the car underside, if the engine then runs fine/much better, you know what the fault is!!

Best of all is obviously a proper Cat "box" with the oxygen sensor built in, but no monolith....its then far quieter....but probably impractical to do!! Though I have heard that many build one in between the exhaust tests in the USA!! Naughty, naughty!!

Best of luck.....

Thanks to all for your suggestions. This car has been somewhat of a lemon since I bought it for my daughter, but it is somewhat of a rare car and looks cool, so I resist divesting myself of it.

Think I'll look at the cats next (sorry Del, no pun intended). I've learned in certain circles, that you can remove the cat, chisel out the material inside, then reinstall it. I've never done that, of course! Then I'll move on to the knock sensors. They're still pretty cheap to replace. We'll see what codes get thrown up after that.

Thanks also to AG for the complements. Learned from the gurus!!

You have raised a point here that we should all notice, that is if someone posts a question, yours is a good example, where all or most of the relevant info is supplied from the start, its almost worth giving you a GA (GQ?) for, as it makes helping SO much easier...

No snarky comments as well!!

I have a question, how did you read the codes, was it at a Subaru workshop, or one of those units available for small money nowadays?

If the latter, you might get a better idea from the Subaru unit.....I have seen that happen on certain VW models....far more detail and depth....

Just a thought!!

If it turns out to be a Cat, its always a good idea to occasionally do a high speed "burn" of the cat. At least 15 minutes fast drive on the highway...

If that is not legal where you are, then find a long steep hill, drive up it in a gear one too high, (heavy load maybe) but foot on the floor, that gets the engine and the cat warmed up!! Watch the water temp.

I used to do that years before cats were a requirement, just to clean out the built up "Coke" that occurred then.....fuel quality has dramatically improved since those days...

Hi Andy,

This car is old enough, that it is before OBD2. There is no port to plug a code reader into. The way to read codes is to open the access door to the fuse block under the dash, and there are unattached, spaded wires not connected to anything, along with an accompanying unattached connector. You have to plug one of these wires into a certain connector hole. You then turn the ignition key to the "on" position (but don't start the car), and count the number of blinks the odometer makes. Kind of like morse code. The number of blinks correspond to a certain failing component. To clear the codes, you plug the second wire into another connector hole and repeat. It made 3 series of blinks, corresponding to the MAF sensor, Left Knock sensor, and Right Knock Sensor. There are 3 sets of wire/connector scenarios on this car that I have dealt with, one for the ECU, one for transmission, and one for airbag system.

I tell you, though, I've been "shadetree" wrenching on cars for 45 years, and never have I encountered so many varied electrical systems and so many areas that can fail as this one. I remember a line in one of the Star Trek movies that went "The more they over-check the plumbing, the easier it is to stop up the drain!"

Again, thanks for your input!!

OMG!!

Thanks for the full explanation....

Keep us informed and best of luck....

There is a test for restricted exhaust systems that involves just a vacuum gauge. Connect the gauge to manifold vacuum, and drive the car in the upper RPM ranges. If the exhaust becomes restrictive above say 3,000, the vacuum will move into the pressure side of the gauge.

Another easy test for restrictive exhaust is to just create an exhaust leak. Remove the O2 sensor, loosen the exhaust manifold from the head, anything that causes an exhaust leak will serve as a test. Back when we had air pumps, just removing 1 or 2 of the air injector tube fittings would change the performance of a restricted exhaust vehicle. Good luck.

Just curious, how old is the battery? And do you know how clean the battery terminals are? And is the alternator putting out a minimum 14 volts? I've seen a lot of problems with automotive computers giving multiple codes because of low voltage. Anything less than 14 volts at the battery posts will cause many expensive heartaches

Once the car engine (alternator!!) has stopped, the battery voltage drops quite rapidly on even a new battery to around 13 volts or even slightly less. It depends on age & quality......

Also, the load. Modern car audio and GPS systems take a specific time to finally shut down to minimum, usually in the milliamp range....

You should test it out yourself with a known accurate voltmeter......connect it with the engine running, let it stabilize, that will be the maximum voltage. Then turn off the engine and simply watch the voltage reading, often it will drop to 13 volts within 30 minutes....

Also, a "good" car alternator will usually generate somewhere between 13.8 to 14.4 volts when the battery is fully charged, 14.1 being a good middle value to achieve (could be a tick higher!).....this helps a battery to recharge after a start sequence quite rapidly, but does not over charge while on long runs.....

Sadly, some cars, some French ones for example, put out a higher voltage and "use up" batteries quite fast, often within 2-3 years, but I haven't tested any recent models......

The theoretical voltage of a good condition car battery fully charged and unloaded/unconnected is 13.2 volts if I remember correctly......so expecting 14 odd volts (other than just after charging!) from a car battery is overly hopeful I am afraid.....

I am of course posting for a nominal 12 volt battery and modern car alternator charging system.....

Andy, thank you for pointing out that I wasn't specific about minimum of 14 volts. I meant to include, with the engine running. I know this is CR4 and nobody is a mind reader. Your right that a battery at rest will show around 13 volts. In the 40 years being involved in auto electrical, I have noticed the post 1994 are more sensitive to running voltage. 2002 and newer are even more sensitive about voltage. With the Japanese car it's even more critical. Nissan has service bulletins regarding critical charging voltage. With the newer cars there multiple connections between the alternator/ battery hot wire that corrode over time, thus losing voltage. With Nissan its so critical they specify no more than .03 volts between the alternator and battery.

I specified checking voltage on the battery posts, which also meant with engine running, you can have 14.5 volts at the battery post terminals and only 13 volts on the post due to corrosion. Computerized cars are extremely sensitive to running voltage.

Absolutely correct.

My post was just to correct the impression you (accidentally) left for those here with no knowledge of this subject, they may have "found" problems that did not really exist.....

Have a great day.

You too my friend!

You can try higher octane gas. I have heard many stories of older vehicles needing higher octane gas as they age. There can be carbon build up that affects the compression ratio and the older computer systems may not be able to account for it properly hence the knock which causes the ECU to pull timing causing loss of power. Also knock can occur when mixtures are to lean so if there is more air entering the intake than the MAF thinks it can knock. Look for leaks after the MAF in the intake. Also MAF sensors are calibrated to the intake piping so they can be thrown off if you change there location or the amount of intake piping (if you have done any work on it).