Jerky power delivery
 

I wonder if you guys have any thoughts on this problem I'm having with my motor (MY99, JDM, STi V wagon, decatted 16 months ago).

Over the last few weeks the car hasn't been running at 100%. Its been a bit jerky between 3k-4k rpm (the feeling is like driving into a gusty wind or lightly applying the handbrake occasionally!), most notably in 4th and 5th gears. I did an ECU reset by disconnecting the battery over the weekend and car was noticably quicker and the syptoms were still there, but much less. However, they are a back to what they were before the reset. BTW, I can't hear anything strange coming from the engine and the car idles perfectly. The car runs on Optimax whenever poss but with no octane booster. Its done 66.6K of faultless miles. There is an amount of sooting around the exhaust too, but that has happened since I decatted it.

I have read about MAF/lambda sensors, coil pack, clogged filter etc but I'm not a mechanic wizard so don't have the experience to toy around with these parts. Some of what I read applies, but like any problem, there's not one that I feel is 100% my problem!. I don't have any monitoring equipment either, BTW. I'm hoping that Len at Subaru4You will look at soon just in-case its serious.

Can you give a few pointers on what maybe the cause?

Cheers

Matt

Hi Matt,

I am by no means an expert on such matters but, If this is a Japanese import car and it has NOT been remapped to run on UK spec fuel (95-98.?RON) you need to be using - at the absolute minimum - Optimax AND octane booster.
I think the Japanese imports "want" 100RON fuel.

I can't explain though, why you haven't had a problem before now.

I'm sure I have read this on here somewhere before.
If it is not a Japanese import - ignore everything I just said. ;)

Run a few tanks of Optimax + Octane booster through it (or at least Optimax) and see if it improves. If you have had poor quality fuel in the car recently it can take a few tankfulls of the "good stuff" to get back up to speed.

Hope you get it sorted

Mark.

Hi Mark,

Thanks for the reply. The car has always run on SUL, about 95% of this time on Optimax. I was told by several creditable sources at the time of decat that I wouldn't need a remap.

I suppose my worry is that in a worst case scenario, there's been gradual wear and tear on the engine due to a disparity in the RON expected and the RON I give the my JDM STi (yes its an import) leading to an expensive you-know-what :( . I would like to able to afford a remap, but I just can't seeing as my house DIY is taking all my cash ;)

Best case scenario is that its a dirty sensor, clogged filter, or a cheap part (<£100) replacement.

BTW, since the ECU reset the other day, the car goes like a rocket ;)

Cheers

Matt