fatboy_coach

iTrader: (1)

Hi,

Picked up an 08 Sti back in August and since then it's had an intermittant hesitation/stumble between 2000-2800rpm, esp in 6th where it doesn't seem keen to rev at motorway speeds. It's especially noticable going up an incline, even on cruise control.

I've had subaru for you look at it twice but it's always behaved when it's there (like a dog going to the vets!), however this weeks cold snap has brought this stumbling on in spades. In I mode it's terrible (almost like my old metro when it was damp), S is better but not great and S# makes it barely noticable.

Once on boost it pulls like a train and there's no CEL.

I'm hoping for a bit of direction as to where to start looking, MAF is my guess but having done some searching on here there's something about the CTS?

Any advice is greatly received

TIA

Ian

fatboy_coach

iTrader: (1)

Forgot to add, car is totally standard

Coleyo2c

Front lambda sensor. When on boost your ecu ignores this sensor because it's on closed loop. hence your problems go away on boost.

abre

So an obvious question is what fuel flavour (Brand/Octane) are you running and when you took it in, did they do an ECU dump and reset?
I had hesitancy on mine but that was under load around 4K revs after investigation the dealer asked me to change fuel brand (Tesco Momentum to Shell Nitro) it got worse not better so after a month went back to the cheaper stuff which mine runs better on!
They did a further full ECU dump (nothing of any value to report) and a reset and it has kind of cleared up. Now and then I'll feel a slight missed beat but its so infrequent I've stopped worrying.
However all of mine was originally linked to a suspected ringland failure diagnosed by the dealer (which it wasn't).
Is yours using oil or has your fuel consumption gone up as these can all be symptoms of a big problem when associated with hesitancy.

fatboy_coach

iTrader: (1)

Abre-I did the same, runs better on 99 than on Shell?!?!? Didn't do an ECU dump but they did reset the ECU. It's not played up for months but this last week it's come back with avengence.

I was concerned about ringland and it does use oil. The fuel economy has dropped but I put that down to me having fun and all of the routes to work being full of roadworks, hence no steady runs.

abre

If you get the logs checked I believe that if there are an increased number of knock detections logged that too can be an indication of potential ringland problems. If you do suspect ringland then a compression test will give an indication also... the problem normally shows up as reduced compression in cylinder 4.

Hope it's not ringland for you, but worth getting it double checked. Rob84 on here might be able to offer some advice from a knowledge/experience perspective.

fatboy_coach

iTrader: (1)

Just been reading through this:

https://www.scoobynet.com/general-te...e-wrx-ppp.html

And it confirms what you were saying, I'm going to play with romraider/freessm tomorrow and see what logging I can do

MrNoisy

iTrader: (28)

I would imagine if you're running it on non Momentum or Shell fuel then the likelihood of knock will be increased - the car won't like poor quality fuel; the standard map is probably for '97 fuel but I wouldn't put anything other than the two above in my car.

Few questions:
- how many miles?
- how long since the last plug change?
- what's your plug gap?
- have you checked all your coil packs are secure and the wiring looms to them snugly plugged in?
- is it modified?

fatboy_coach

iTrader: (1)

Always run on Tesco 99 or V power, infact it runs better on 99 than V power IMHO.

56K, serviced in August (60K service so included plugs, dunno about the gap but trust S4U did it correctly. To rule that out I'm getting new plugs fitted next weekend to see if that helps.)

Coils and leads all good from what I can see!

No mods, std UK STi.

Apostle

08 Sti's and the famous 'stumble'... Some do & some don't.

Check out the Cobb tuning USA website looking at the 08-14 Sti Fuel Pressure Regulator Kit. That'll fix it.

Some mappers try to map around the issue - with some success.

Don't waste your money on anything else.

A

I run an 08 Sti owned from new - had forged 2.5 - now on a 2.35 stroker.

Andaru

Hate to be the bearer of bad news, but mine was like this sometimes and it turned out to be block warped at number 1 and warped heads. Luckily it could be machined out, but it still came to £2800 with a lot of discount from my Bitz Motorsport and Alyn@ASP!

I did plugs, coils and MAF before the rebuild too.

fatboy_coach

iTrader: (1)

Apostle, did you fit the kit when you rebuilt?

Andaru, how did the garage diagnose warped block? Compression test?

MrNoisy

iTrader: (28)

Sounds like you need to talk to a mapper, specialist or both mate.
I'd say a mapper as they can normally stick it on the dyno to try and reproduce the fault whilst plugged in to see what causes it.
If it's not something simple like plugs it could be 101 different things, and we can all guess away but some fixes are cheap and some expensive, and none of them might be what's causing your fault.

Andaru

There was combustion gas in the coolant and when it was taken apart the gasket was fine but you could see that it was leaking at no.1, when the head and block arrived at the machine shop they confirmed it.

Apostle

The infamous 08Sti stumble - read all about it on the USA sites - lots of info.
Only hesitant on part throttle NOT on full throttle. A

fatboy_coach

iTrader: (1)

Update - 2 hours and a lot of tinkering at Subaru 4 U this morning showed that compression was fine on all cylinders but the plugs on cylinder 2 and especially 4 are showing signs of running lean. The plugs are only 4500 miles old, so it was a little surprising to see the discolouration.

New plugs fitted and now looking at getting the injectors taken out and tested next. Don't want to remap until I've ruled that out as it will only mask the problem.

MrNoisy

iTrader: (28)

If it's running lean I'd be inclined to suspect the fuel pump not the injectors mate. Len should be able to test the fuel pressure from the pump easily enough by temporarily inserting a gauge inline between the fuel filter and the engine.

fatboy_coach

iTrader: (1)

Update 2 - new injectors fitted to cylinders 2+4 (ran out of time to fit them all) and for a week it ran like a beast but now it's back to it's old ways. Not as bad but still noticeable.

It appears that whatever the problem is, disconnecting the battery fixes it for about 250 miles then the ECU 'relearns' the problem and it starts driving like there's a misfire.

Might try the fuel pump but am also going to get it remapped to see if it's a bad map.

fatboy_coach

iTrader: (1)

To close the thread, the final answers are all here but in short it was 2 damaged pistons....

https://www.scoobynet.com/engine-man...uld-i-log.html