Morning,

I appreciate this perhaps isn't the right place but I'm hoping the collective wisdom on here might be able to help me with an annoying problem!

I have a 2004 Forester STI (2.5) which has developed a slight hesitation on part throtle - when cruising at a fixed speed for example. It's just had a cambelt service, always runs on super, etc. The car is mostly used by my wife for the school run so this might be contributing to the problem?

My dealer says it's likely to be the airflow meter but I wanted to check some of the cheaper things first. It feels like the car is being starved of fuel for a second and this produces the hesitation - hard to describe really.

I was thinking spark plugs origionally but now I'm not so sure...

any ideas?

 

has the hesitation only started since the cam belt service ?? too check the air flow sensor un plug it while engine idling the idle should rise or fall slightly if not the sensors dead also check coil pack un plug leads check for cracks and tracking marks and finally check turbo pipes for security and leaks good luck m8

 

It happened after the cambelt service but not straight away. I'll try those things and see what happens - need to google them first, I'm not good enough under the bonnet to know what they are!

 

Could also be the Knock sensor.. mine developed a fault (07 FSTI) and seems common on STI Foresters.. costs little for Subaru to check and change(moving the sensor to a different position)

 

That's interesting, don't think I have heard of the knock sensor fault (I have MY05 FSTI). How did you find this out? Looking at some logs or did the Subaru dealer detect and sort it?

This would certainly cause hesistancy if the knock sensor gave a load of false positives at a certain RPM.

I had a service recently and had the spark plugs changed, it made a real difference in pickup. If you haven't had them changed in the last 15K miles (think the official interval is 30K?) it might be something to try.

 

Okay, it's been fixed

The fault code was P0172 (running rich on bank 1) The MAF was dirty but the garage managed to clean it out.

The cause was my aftermarket (Green) air filter not fitting quite right in the airbox and allowing dirty air to seep through over time. It also had a slightly loose air feed hose which they tightened.

1 hours labour + a new air filter and it's a different car, super smooth and much more responsive. It was interesting to see the gap created when the Green filter sat in the airbox as compared to the standard STI fitment - I'm going to think twice about using aftermarket stuff in the future.