Throttle problems under light load - driving me crazy...

[Guest]

Hi guys,

I've got a 2002 Legacy 250S, which has a subtle problem with the throttle under very light load -open road use mainly. Basically under very light load when the foot's just touching the gas, between 2000 & 2500 RPM I feel the throttle lightly flapping, or closing prematurely - so a very light surging on & off, almost like the electronic throttle is flapping open & closed.

I don't notice it around town, but definately on long trips this light surging ends up driving me crackers -definately doesn't feel right.

My mechanic checked the ECU codes & found a faulty fuel/air sensor, so that was replaced & it did help it run alot better -but still didn't sort this out.

Any ideas what this could be? I read somewhere about a throttle position sensor, but the ECU doesn't show any codes & my mechanic thinks I'm going mental. I hate cars that don't run right, you know what I mean

cheers

A

[gazzy2000]

Ask if he reset computer after changing the 02

Plus check that tyre sizes, wear and pressure are all even and the same etc

[Guest]

Yep, mechanic reckons he did reset it after changing the sensor. Tyres are all the same size, brand, condition & pressure -albeit a bit old & knackered.

Definately feels wrong at that certain point on the throttle, I can feel it mechanically doing something strange, always when just touching the pedal.

[Shale]

Mine did similar crap recently. It's got a broken coil, old spark plugs, and had a misbehaving AFM. It's an old turbo though, so a little different.

MAY be a spark issue, or AFM issue.

[gazzy2000]

not sure if your model has a IACV or if so, what kind, maybe that needs a clean?

How many KMs has yr car done?

Fuel filter been done?

Upper engine cleaner?

Plugs and leads been done?

You cud try cleaning the AFM with CRC air flow meter cleaner ($20 from ripco)

[Guest]

Spark plugs will be getting replaced next service, but apparently are currently in good nick. Mechanic has checked coils, plugs & leads, reckons they are OK so far. Seems strange, the fault seems so distinct. Like when I throttle off slightly from that cruising point, it feels like it's closing a bit prematurely. THen when I slightly increase, it pops open -feels very electronic/mechanical if you know what I mean.

The car's done 102,000km, recently had a leaking head gasket replaced - luckily no overheating had occurred. Heads were fully serviced & cleaned. Then had to replace fuel/air sensor -expensive. Pain in the arse, should have just gone out & bought another turbo for all the trouble this thing's been worth -but that's another story!

Not sure if it's got the IACV, will go away & do some research (aka google) on whether the EJ25 has it or not.

[gazzy2000]

dare I bring this up again, but when had a 97 STI with bad surging i used this:

http://www.imprezasport.net/forums/index.php?showtopic=785&st=0

Its one of those things to DIY and save $$$, plus you can eleminate it as an issue.

[Guest]

Looks interesting... I'll have to give it a go. In that thread they also talked about vibration, I'm sure this 2.5 has never felt quite right, more vibration than my last 2.0 TT, smooth as silk it was...

[funkytown]

TPS will cause this just like you mention.. always gets overlooked. The sensor gives variable voltage and the ecu reads this as trottle on trottle off when its constant.

[Guest]

Yeah someone else mentioned the TPS, is it easy to replace? How can you test it?

[funkytown]

 Ashwina said:

Yeah someone else mentioned the TPS, is it easy to replace? How can you test it?

put a multi meter over it.. its just a little black sensor that sits on the side of the trottle body, its "easy" to change but somewhat a PITA to calibrate once its on (with a stock ecu).

[Guest]

Sorry to be a dumbarse but ...once I've got the multimeter hooked up what should I be trying to measure? Changing resistance whilst pushing on accelerator? Or voltage?

[log1call]

It sounds like TPI adjustment to me as well. Some have a swithch that tells the ecu that your have your foot right off so the idle can stabilise, if that's out of adjustment they have a switch on and off effect as you open and close the switch and the ignition timing is being effected by revs and airflow etc. When they are new, if the resistances are correct then the switch part will be in adjustment as well. As they wear the switch and the resistance no longer coincide. I'd adjust the TPI so the throttle problem was cured and disregard the standard settings.

Have a look at this... http://www.ultimatesubaru.org/forum/showpost.php?p=861589&postcount=4

[Guest]

Thanks Log1call that's awesome, makes a heap of sense. I might dust off the old multimeter & see what I can find (or break)...

[Guest boostin]

There are 2 oxygen (or air/fuel ratio) sensors on those engines. Did the mechanic replace the front or rear one?

[Guest]

What fuel are you using... I am not sure but I think 98 should be used? If it is this knock problem, could a higher grade fuel solve any issues? Doubtful, but I have to ask

Also I agree with the TPS idea, as it plays a huge part in the ECU's calculations. Does anyone think a dicky MAP sensor could have any part in this? In the sense that if it is registering a vacuum (falsely) and TPS is light, does it assume an over-run fuel cut of sorts.

[Guest]

Not sure which one it was, was an expensive part $600+