Mass Air Flow Sensor?

[JStamberg]

My check engine light came on recently (96 Legacy LS 2.2 4EAT), and I decided to pick up my own scan tool, rather than paying someone else to do it. (By the way, if you are looking for one, I got a China-made knock off of the ElmScan 5 for about $85 w/ shipping and handling from a guy on ebay... it hooks into my laptops usb port... works great... free software available online... http://cgi.ebay.com/ebaymotors/ws/eBayISAPI.dll?ViewItem&ih=014&sspagename=STRK%3AMEWN%3AIT&viewitem=&item=330163025508&rd=1)

 

Anyhow, I have had problems with low gas mileage (18-20 mpg in town) and with overheating (see previous thread: http://www.ultimatesubaru.org/forum/showthread.php?t=76566).... so I've been curious to see if I could learn anything from the data the sensors are putting out.

 

In the Haynes manual is says that the MAF sensor should be reading between 4 and 7 grams per second when the engine is idling, and that it should increase steadily with throttle increase. My car is only showing about 2.7 g/s at idle.... Is that unacceptable?

 

(I have tried using MAF Sensor cleaner on the sensor... the sensor looked fine beforehand, however.)

 

Any thoughts on what is normal?

 

It seems to me that if the MAF sensor is detecting less than the actual amount of air coming into the engine, then it will not put enough fuel into the engine... resulting in a lean mixture... detonation... knock sensor retarding engine timing... hurting performance, fuel economy, etc.

 

Let me know if you want some of the other sensor data... I logged about 15 minutes of driving on city streets and highway.

 

Thanks in advance!

[subaru360]

2.7 g/s is low. You might have a hose off on the intake ducting between the mass airflow sensor and the throttle body or a vacum leak. If all that looks good and the wiring looks good it's probably a bad sensor.

[JStamberg]

Ok, I'll take a closer look and see what I can find....

 

I just checked out the price for a new MAF sensor (1stsubaruparts.com)... $272!! I never guessed it would be such an expensive part.

[subaru360]

I have a used one if you end up needing it.

[jon38iowa]

Ok, I'll take a closer look and see what I can find....

 

I just checked out the price for a new MAF sensor (1stsubaruparts.com)... $272!! I never guessed it would be such an expensive part.

 

Is that price just the sensor? They gave me quote similar to that for my MAF. However, I was able to purchase just the (reed) part of the sensor and it cost

$55.00 from Subaru. Additionally I had to buy a tool to remove it.

My car might be different than yours, but I would look into it.

[JStamberg]

Excellent... good to know. It seemed a bit steep.

[JStamberg]

I have a used one if you end up needing it.

 

That rocks... thanks for the offer.

 

I looked over the intake, I don't see anything obvious. Is there any sort of effective way to check for leaks or anything beyond just looking at it?

[subaru360]

That rocks... thanks for the offer.

 

I looked over the intake, I don't see anything obvious. Is there any sort of effective way to check for leaks or anything beyond just looking at it?

 

With the engine running, spray some carb cleaner around all the hoses and see if the idle goes up.

[JStamberg]

With the engine running, spray some carb cleaner around all the hoses and see if the idle goes up.

 

I don't have any carb cleaner handy.... could I use the MAF sensor cleaner I recently purchased?

[JStamberg]

I went ahead and tried using the MAF sensor cleaner (pretty much the same thing as carb cleaner.... but gentler, right? still flamable...), but didn't get any surges/increases in RPM.

 

Would replacing the sensor/reed be the next step? Any other way to test it and make sure that's the problem?

[idosubaru]

i don't know how combustible MAF sensor cleaner is. i'd go ahead and get a can of carb cleaner at your earliest convenience. the MAF sensor stuff is probably similar to brake cleaner in some sense and probably has flammable liquid precautions on it though.

 

i'd recommend spraying around all your intake hoses, vaccuum hoses and hopefully you'll notice a jump in the revs or fluctuations from spraying. the gases from the spray get into the intake and change the air to fuel ratio artificially so that's sort of how it works.

 

if it ends up being a MAF sensor, i'd feel perfectly comfortable installing a used one in my wife's vehicle. they fail so rarely that it doesn't make any sense to buy new.

 

nice hit on the code scanner you bought, you're finding it helpful and easy to use? that's great, thanks for sharing!

[Manarius]

Uh 18-20 around town with overheating isn't bad mpg, especially if you're running the A/C all the time and your car is AWD.

[JStamberg]

i don't know how combustible MAF sensor cleaner is. i'd go ahead and get a can of carb cleaner at your earliest convenience. the MAF sensor stuff is probably similar to brake cleaner in some sense and probably has flammable liquid precautions on it though.

 

i'd recommend spraying around all your intake hoses, vaccuum hoses and hopefully you'll notice a jump in the revs or fluctuations from spraying. the gases from the spray get into the intake and change the air to fuel ratio artificially so that's sort of how it works.

 

if it ends up being a MAF sensor, i'd feel perfectly comfortable installing a used one in my wife's vehicle. they fail so rarely that it doesn't make any sense to buy new.

 

nice hit on the code scanner you bought, you're finding it helpful and easy to use? that's great, thanks for sharing!

 

Yup, it works well... There is commercial software out there, but I found a nice combination of freeware/GNU software. Specifically: "scantool.net", "wOBD" ("obd2crazy"), and "OBD II Logger." Between these 3, I pretty much have most of what the commercial stuff offers. They're all super easy to use.

 

Thanks for the info on the carb cleaner, I'll hop out and get some. Good thoughts to on not buying a new MAF sensor.

[porcupine73]

My check engine light came on recently

So what code(s) did you read?

[JStamberg]

So what code(s) did you read?

 

Well.... that's the embarassing part.... :-\ It was P0325... faulty knock sensor circuit. I ordered the part last week ($52+$15 S/H); it arrived today. Yesterday I was taking a quick look at the knock sensor to see what all was involved, and I noticed that the knock sensor was not plugged in completely. Obviously when I reset the code, it stays off now. So now I have a new knock sensor, but it appears I may have wasted my money...

 

It seems weird to me that it was unplugged. I had fooled with it several weeks before (after reading the knock sensor threads), just to check it out, but didn't see anything obviously wrong. I plugged it back in, but maybe it wasn't connected completely. After several weeks of driving it must have loosened up just enough to start setting off the code. I guess I win the bonehead award for not double checking the obvious thing before ordering the part.

 

From reading other threads, I guess the knock sensor can be starting to fail without setting off the light, so I'm contemplating changing it out anyways. The company I ordered it from will charge a 20% restocking fee... plus I have to foot the shipping bill (both ways). I figure by the time all that happens, I'm better off just throwing it in my car.

 

The car has 205,000 miles on it, so its probably about due. Of course, I'm always open to other ideas/advice.

[porcupine73]

Since you already ordered the new knock sensor, I would just go ahead and install it. The mating surface between the sensor and block needs to be clean and the bolt specifies a particular torque.

 

If the knock sensor became unplugged, the ECU was retarding timing to the maximum as a failsafe. Reduced power and fuel economy would be noticed.

[JStamberg]

That's the conclusion I came to after reading some of the other threads...

 

However, it still doesn't explain the low airflow rate.

 

Here's a bit more info on my timing from the sensors: stays at 20 degrees while idling. It topped out around 50 degrees while on the highway (just for a moment). In city traffic it stayed mostly between 34-46 degrees. These are all numbers from today, so the knock sensor unplugged shouldn't be effecting things at all. The Airflow rate was from today as well (after plugging the knock sensor back in yesterday).

 

Porcupine, I definitely noticed reduced fuel economy and power while the knock sensor was unplugged. Fuel economy got down to 15 MPG. Ouch! Nice to know the car was designed to protect itself from major damage.

[porcupine73]

Those ignition timing values you mentioned sounds just like what I see in my '96 Legacy 2.2L (using scangauge).

 

Are you having other issues leading you to be suspicious of the MAF, or are you just wondering since you're reading a value lower than what Haynes says the spec is?

[JStamberg]

Mostly just wondering. As I mentioned at the beginning of my post, the MPG has been a little low (not terrible, just lower than expected), and I have had overheating issues in the past.

 

Just finished installing the new knock sensor and taking it around for a spin. Nice and easy job....

[jon38iowa]

Since you already ordered the new knock sensor, I would just go ahead and install it. The mating surface between the sensor and block needs to be clean and the bolt specifies a particular torque.

 

If the knock sensor became unplugged, the ECU was retarding timing to the maximum as a failsafe. Reduced power and fuel economy would be noticed.

I Agree! And, I put my money where my mouth is. I replaced the knock sensor in my'99 two weeks ago.. The original was probably fine, but to me its getting old, moreover, I like to keep the car rode worthy(going out of state) at all times.

Seems to me like cheep insurance.

[JStamberg]

Yesterday I tried the carb cleaner as suggested.... I sprayed it all over the hoses and connections, but I didn't notice any changes.

 

Then I sprayed the heck out of the MAF sensor one last time with the MAF cleaner.

 

Lastly, I used Seafoam Deep Creep on the throttle body (just because I already was messing around with the intake hose).

 

I drove it hard for about 20 minutes... came home, checked the air flow again... now its between 2.3 grams/second and 2.5 grams per second. So quite a ways below the 4-7 g/s the book recommends.

 

My question is this: is there some way that I will hurt the car by not changing out the sensor? I.e. what are the consequences of the sensor reading such a low air flow rate? Or is it worth it just to go ahead and replace it with a used one (which I have an offer for), and be done with it?

 

One last curiousty question.... totally unrelated... what is the purpose of the curved plastic black piece located between the air filter and the front right headlight? Mine is not really connected, and I just want to rip it out, but I hate to do that not knowing what the heck it does.

 

Thanks!

[porcupine73]

Hi. If you're not noticing any issues and there are no vacuum leaks, I wouldn't worry about the reading being below what the Haynes manual says. I didn't see a spec for that value off hand in the factory service manual. Here's an article from EndWrench about Subaru MAF's: http://endwrench.com/images/pdfs/EWMAFAug05.pdf

 

Another possibility is if you can locate another MAF somehow, junkyard, etc., and compare to see what it reads, or maybe hook up your reader to another similar Subaru and see what it is reading.

 

Those plastic contraptions (someone has called them donkywongles in a recent thread about it) on the intake are to tune the intake to reduce intake noise and possibly induce Helmholtz resonance.

[Manarius]

Like I said earlier in this thread, I think your MAF is operating perfectly fine. You're making a problem out of nothing. Your MAF is NOT contributing to your bad gas mileage. You don't have driveability isssues, therefore your MAF is not broken.