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Misfire when hot under load
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Old 24 Mar 2013, 04:12 PM   #1 (permalink)
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Misfire when hot under load

If this is posted somewhere else then i apologies, but from what I've read, nothing seem to be my problem, or doesn't sound like it anyway.

Ive got a Classic WRX import 1995. The car is standard, apart from a HKS back box, no dump valves or decats, or remaps etc...

It runs perfect when just pottering around and under medium acceleration.

But give it full acceleration esp when you have been on a long run and engine is hot, get the revs to 4000rpm on full boost and the car judders, shakes, backs off etc.. But theres still power, its not like boost or fuel cut with the total power cut feeling. But then on the odd occasion power does drop off but then trys to come straight back in

Boost gauge reads 0.9 bar and doesnt budge.

If i leave the car overnight with battery disconnected or reset ecu via the plugs under the dash then go for a run, then the car is perfect hot or cold. But then it slowly starts coming back. Seems worse the hotter the engine is.

Now ive changed all 4 coil packs for new ones as the tips were all cracked. (Aftermarket ones)
Changed knock sensor for a new one due to old one looking corroded
Changed MAF sensor for second hand one (green label type) tried the one of my mates car also.
Even tryed my Mates Z4 ECU from his car.
Cleaned boost solenoid
Looked at spark plugs, BUT not checked the gap. Spark plugs are new, but if i remember right the part number related to a 2004 STI....

The car has done it since i got it last year, but not really driven it properly as been doing bits to it, but ive got it one the road now so need it sorting so i can enjoy it.

Am i looking at possible fuel issue, poss fuel pump, split hose somewhere, or even a Earth issue??? Is there a lambda sensor in the manifold on these, that could be causing the issue???

Any help would be mucho appreciated!!

Andy

Last edited by Andywrx1980; 24 Mar 2013 at 05:01 PM.
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Old 24 Mar 2013, 07:41 PM   #2 (permalink)
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Re: Misfire when hot under load

check the coil packs, esp under the rubber cover, they crack when old and cause said misfires.
hth
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Old 24 Mar 2013, 08:05 PM   #3 (permalink)
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Re: Misfire when hot under load

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Originally Posted by bugeye_bob View Post
check the coil packs, esp under the rubber cover, they crack when old and cause said misfires.
hth
Thanks for the reply bugeye_bob, but all four coil packs are brand new as i stated in the list of parts ive changed above. The old ones were indeed in very poor condition under the rubber covers. But problem is still there

Im going to get the right spark plugs tomorrow and gap them to 0.6 as recommended, like i said, when i checked them they looked new but part number related to the likes of a 2004 STI etc....

Just would like to know other possible causes incase the spark plugs dont cure it. I dont mind changing sensors etc as everything on the car looks original or genuine and are prob 18 years old!
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Old 24 Mar 2013, 08:34 PM   #4 (permalink)
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Re: Misfire when hot under load

check the plug gap to start with
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Old 24 Mar 2013, 09:14 PM   #5 (permalink)
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Re: Misfire when hot under load

Quote:
Originally Posted by Tidgy View Post
check the plug gap to start with
Thanks Tidgy! Been reading up on the spark plugs that are in it, and default setting is 0.8!! So if the plugs have just been put in, then hopefully this could be my issue!

So whats best for mine, ive read so many different settings from 0.6 0.65 0.7. I was going to go with 0.6

Im guessing if the gap is to big this is what "blows" the spark out under hard acceleration, creating juddering/hesitation ??
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Old 25 Mar 2013, 12:07 PM   #6 (permalink)
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Re: Misfire when hot under load

pretty much sums it up, gap depend son spec, sure someone like bob can advise best gap for that spec, i just get it done for me lol
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Old 25 Mar 2013, 05:14 PM   #7 (permalink)
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Re: Misfire when hot under load

I'd run .65 - .7
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Old 13 Apr 2013, 07:43 AM   #8 (permalink)
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Re: Misfire when hot under load

Hi guys, just thought i would update this thread.

Problem all fixed. Turned out to be a combination of rust in fuel tank, degrading rubber fuel hoses and under performing fuel pump. Got to expect it really as all the parts were orginal.
Uprated the fuel pump to a walbro, clean out the tank as best as i could, replace a couple of rubber fuel pipes and the fuel filter for good measure!
And she goes like a rocket ship, completely transformed the way the car drives.

Im now a happy bunny, so off for a blast today to Oulton park while the suns out!!

Last edited by Andywrx1980; 13 Apr 2013 at 07:46 AM.
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