Solving intermittent misfiring issue - help?

[Flat4]

Hey guys,

Just entered in the Mt Vic hillclimb which is on the 16th March and my Legacy has developed an intermittent misfiring problem. It isn\'t bringing up any codes but my mongrel car has a V5 STi engine wired into it and the diagnostics aren\'t exactly "working".

It started with the car not idling a couple of weeks ago driving thru town. This was a problem driving in bumper to bumper traffic. Has happened again a couple of times since. However this issue seemingly fixed itself.

I then noticed a misfire on the motorway sitting at just under 100kph in 5th. Didnt think much of it.

It escalated quickly as next time I drove in to town, after a 5 min coast down a hill from my house, it started misfiring badly in 1st, 2nd and 3rd (not under much load and even off boost) and rapidly losing power to the point where I had to pull over because I couldn\'t keep up with 20kph traffic. This has happened again several times to varying degrees. It will sometimes run perfectly and sometimes misfire like a mofo.

To sum it up:

- 2 weeks ago it was cutting out and not idling

- Now it doesn\'t seem to cut out but sometimes has a rough idle

- Intermittently misfiring, sometimes minor and sometimes like a motherf**ker, progressively losing power until none at all

I tried unplugging the AFM when it was running like a dog yesterday and it cut out straight away. Plugged back in, restarted car and it was happy as larry!

Before I go and buy heaps of new parts can anyone suggest what could be causing this? AFM? Fuel pump? Spark plugs? Coil pack?

Thanks in advance

[rex-leggy]

did you put new plugs in when you put the motor in?

[Flat4]

 rex-leggy said:

did you put new plugs in when you put the motor in?

Nah just checked visually and cleaned. Will buy some new ones this weekend, but I don\'t have a lot of time to sort it so if it sounds like it could be the AFM or fuel pump or something else then I\'ll replace them with new ones this weekend as well

[Marky]

 Flat4 said:

Before I go and buy heaps of new parts can anyone suggest what could be causing this? AFM? Fuel pump? Spark plugs? Coil pack?

Yep, that that that and that haha

Would expect spark related stuff to be more apparent with boost added, but then it is a subaru and they do some weird shit

[Flat4]

 Marky']

[quote name='Flat4 said:

Before I go and buy heaps of new parts can anyone suggest what could be causing this? AFM? Fuel pump? Spark plugs? Coil pack?

/quote]

Yep, that that that and that haha

Would expect spark related stuff to be more apparent with boost added, but then it is a subaru and they do some weird s***

True that haha. Seems to be running all good on boost, when it stops misfiring and revs out past 3.5k

[soopersubaru]

Good earth to the engine? Also check alternator. No AC current as this can cause ECU brain malfunction too.

"free" checks.

[boon]

I would borrow an AFM and a coil pack from someone to diagnose.

[Rosssub]

 boon said:

I would borrow an AFM and a coil pack from someone to diagnose.

+1, most likely causes.

[Flat4]

All the earthing points seem to be all good. Will definitely look into borrowing AFM and coil pack this wkend.

Air filter is also on the list. Discovered airbox also full of fluff, obviously the result of the grass autocross the other weekend. Probably not helping the situation