kevin_p

Hi everyone,

I finally managed to get out in my UK MY99 Impreza Turbo last night having changed the battery as the old one had given up, it hadn't been driven for about three weeks.

I noticed when idling at traffic lights etc the revs start to fluctuate by say +- a couple of hundred revs. If I am stationary for a short period (approx less than a minute) it's fine but any longer and the symptons show again.

Any ideas on what the problem is please?

Thanks

DJ_Jon

iTrader: (7)

It's probably the MAF sensor, don't drive the car hard!

Unplug the MAF while its still running, if it stalls then the MAF is ok, if it doesn't then its fooked. (its early, so that could be wrong, but doing a search will reveal more answers)

apac

iTrader: (2)

happened to me on my 99 classic. As above. get the the MAF sensor changed. I had mine done under warranty, as the dealership would rather change tthat quickly than have to fork out fora new engine

Splitpin

Kevin: Do not carry out the procedure Jon has outlined above. As he says himself, this "test" is seriously flawed and doesn't conclusively prove one way or the other whether the sensor is functioning proprely or not. It will frequently result in dangerously degraded sensors "passing", which is a major problem.

What might be worth trying is getting the engine fully warmed, switching it off, unplugging the airflow sensor and then starting back up. You should see the check engine light on. If the idle stability is noticeably improved, then you almost certainly need a new airflow meter insert. If the idle is still up and down with the sensor disconnected, the explanation is almost certainly elsewhere.

Plug the sensor back in before driving the car, but as has already been said, stay off boost completely and replace it ASAP. The part number is 22794AA010, main dealer will charge you just over £100. Reset your ECU after fitting the new one too.

DJ_Jon

iTrader: (7)

Nice explanation Splitpin

When my MAF died I carried this this out without a problem, but I would definitly agree that it isn't going to give you a definitive result on whether the MAF is defective.

Hope you get it sorted.

kevin_p

Thanks for the replys DJ Jon and Splitpin, appreciate it. Splitpin, I'll follow your procedure and will let you know how I get on. Can you describe where the airflow sensor is please (I want to make sure I'm unplugging the right component!), I assume it's somewhere around the airbox/air intake?

Splitpin

No sweat Jon/Kev. Kevin, yes, you're right on the money. Look at the tube bolted to the airbox top and you'll see the sensor sticking out the back, held in with 5 security torx screws through a metal plate. Remove the grey connector and see what happens.

topshot

iTrader: (4)

I had the exact same problem when i got my MY00 a few weeks ago. Changed MAF and cleaned ICV, reset ECU and since then its be ok. fuel economy is better too!

kevin_p

Just a quick update to say I performed the test and the results pointed to a faulty MAF sensor as you all predicted. I removed and cleaned the existing unit and it's now running perfectly but I will order a new one to be on the safe side. Thanks again to Splitpin and everyone else for your help, you've saved me a lot of hassle and money, I'm sure Subaru would've charged a fair amount to diagnose the problem!

Splitpin

Good to hear you've got a handle on it but do not assume that the sensor is now working correctly under all load conditions, even though the idle stability has improved.

The common failure mode on these Jecs hot film sensors cannot be reversed by "cleaning", and a sensor that you cannot guarantee to be 100% reliable is a potential cause of serious engine damage.

Unless you have the ability to datalog your car (and thus know how the sensor is performing as the car is being driven), it is much safer for your engine to assume that it's duff and replace ASAP. And stay off boost til you have.

kevin_p

Thanks Splitpin, I'm picking up the new one at the weekend and won't use the car until then. Is there anyway of testing a MAF sensor other than with datalogging, I'm concerned that my new one may fail eventually but if it doesn't give any warning signs I may damage the engine. I've read that some people change them annually, I assume this is why.

joz8968

iTrader: (13)

You'd need to hook up it's connector to voltmeter/multimeter. You'd need someone in the passenger seat to monitor its voltage outputs whilst opening the throttle through all of its travel.

At idle, it should read around 1V and at WOT/max revs, around 5V (and proportionally in between those 2 voltages, for the rest of the rev range).

Splitpin

There is, although that's even less accessible as it involves testing the sensor performance on a flowbench.

However, there's no need to monitor the sensor directly if you want a bit of insurance. The way most people go is to fit an AFR gauge or (better) a knock monitor. Neither gives a direct indication of the airflow sensor's health, but will in practice be able to detect the symptoms of a failing one - as well as a range of other problems.

Provided you're using the standard airbox arrangement and a good quality panel filter, there shouldn't be any need to replace them anywhere near that regularly - the revised version of the sensor currently sold by Subaru appears more reliable than the older ones.

Splitpin

Those numbers aren't really correct Joz.

These sensors output over 1 volt with the engine off!

Idle once fully warmed is around 1.2-1.3 volts, and you'd never actually get one to output 5 volts, especially on a standardish car. 4.3-4.4 volts or so is probably the highest you'd see under these circumstances. The output isn't "proportional" either.

In any case, to be honest there's not a huge amount of value in the "passenger with a multimeter" approach. You really need to log the output alongside engine speed, boost pressure and throttle position in order to get any sense of whether the sensor is performing accurately, and in addition, many common or garden digital multimeters won't have a fast enough count rate to pick up the transient signal dropouts that are the hallmark of a failing 99/00 sensor.

joz8968

iTrader: (13)

Fair points - but doing that test could at least eliminate if it's actually shagged i.e. stuck at a low voltage when at high rpm (i.e. the IMPORTANT one! ). Although I expect it's unlikely for MAF to get that bad before noticing really bad running first...??

Yeah sorry, you're right, 1.1-1.2V at warm idle is what I see on my PFC. And also it's normally 4.3-4.5V at max throttle/rpm... but today it did get up to 4.9V!