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Misfire Driving me mad....
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Old 11 May 2009, 04:02 PM   #1 (permalink)
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Question Misfire Driving me mad....

Ok, so I have researched and looked and tested various things but I cannot get rid of a misfire under decent acceleration.

Impreza WRX STI 1994 Version 1 Import - No Mods.

Under reasonably hard acceleration I get an intermittent stutter, mid to high rev range, no problem at all at idle/low revs/speed. Not at a particular point in the range. No problem if speed built up slower.

Boost is steady and constant just under 1 bar even when misfire occurs so I don't think its a boost issue. Stutter clears after a second or two and continues to accelerate. It may or may not occur again up the range.

Things I have done:

Checked fault codes, all clear
Replaced Spark plugs
Replaced fuel filter
Replaced air filter
Checked all coil packs for cracks and attempted to clean the sprung contact face - none found, all clean
blown out the MAF housing interior with an air compressor
Reset ECU
Tried different fuels, v-power down to bog standard unleaded, no difference

It feels like a spark is failing and not igniting a cylinder. I think it could be one of the following. Gents, please give me the benefit of your vast knowledge to help regain my sanity though I fear its too late for my hair....

1) Timing out? Garage I bought it from not a Subaru garage and replaced timing belt and tensioners for me before I bought it? Highly possible they did this but would there be worse symptoms of this???

2) Coil pack internals breaking down? But no visible damage? How do I test their effectiveness under load???

3) Failing to fuel/air enough? (Can't believe this is the case)

4) Anything else?


Please help!!!
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Old 11 May 2009, 04:12 PM   #2 (permalink)
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Re: Misfire Driving me mad....

it could well be a coil pack breakdown....
the maf can really only be checked by plugging it in at a garage/tuner n their software should flash it up(won't show on fault code reading)..also it could be the fuel pump with an intermitant fault...
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Old 11 May 2009, 04:36 PM   #3 (permalink)
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Re: Misfire Driving me mad....

Yeah, I guess I figured that the fuel pump would be more subtle in its effects. It feels like one cylinder "forgets" to fire but it could well be the fuel, but then I would expect that to start to show up through out the rev range. Would a run on a rolling road track the problem down? Or any other ideas about diagnosing the problem?
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Old 11 May 2009, 04:44 PM   #4 (permalink)
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Re: Misfire Driving me mad....

sticking it on the rollers will surely find it and is probably the only hassle free way of finding the problem...if the pump is failing it will have a "fireing on 3" effect,as the pressure is not up to its usual level,hence a "stammer" type effect...
my maf went n had no lights or anything indicating the fact,only found out when,whilst being mapped,wouldn't punch over the 275bhp mark!!...i'm sure you know how to test for a weak spark on a coil pack,so won't go into details on that one..
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Old 11 May 2009, 04:55 PM   #5 (permalink)
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Re: Misfire Driving me mad....

I know how to check for a weak spark on a plug, would it be the same for the coil pack? I guess so, I just remove the rubber boot to get access to the contact.

I guess I might as well check this first... however the prospect of discovering just how many horses have bolted from under the bonnet from my aging scoob slightly puts me off putting it on the rollers....however it might surprise me and this may be the lesser of two evils and the only way for finding out for sure.... I *really* want a decat and then a remap but I am following advice I have read about getting sorted before I do this...
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Old 11 May 2009, 05:00 PM   #6 (permalink)
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Re: Misfire Driving me mad....

same principal bud

it sure is the best bet bud,get it running right n safe and the got the mapping n modding line,if your unsure about decatting and also with MOT's being a bit tighter than they were,maybe look into a sport cat(100cpi unit),saves the hassle of swapping when that time of year approaches
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Old 11 May 2009, 05:24 PM   #7 (permalink)
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Re: Misfire Driving me mad....

Ahem....that part might not be a problem

Presumably though I would still get a bit of a boost from a sports cat without the remap being necessary and less MOT hassle. I am guessing about 15 ponies as opposed to about 20-25 with a full decat... then upwards with the remap etc...the slippery slope.....

But I am getting ahead of myself there, gotta sort out that bl00dy misfire first....!
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Old 11 May 2009, 05:32 PM   #8 (permalink)
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Re: Misfire Driving me mad....

the sportcat will get you good gains in the bhp and the torque departments
also you can do away with any other cats in the system too,if you take that route a straight through centre section n freeflow back box will cerntaily help things along nicely also if yor feeling flush a set of headers will give you even more...breath the engine and all will be good and it's safe to run with those forementiond items before remapping
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Old 11 May 2009, 10:39 PM   #9 (permalink)
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Re: Misfire Driving me mad....

My 03 STI did that, get a compression test done.
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Old 12 May 2009, 07:56 AM   #10 (permalink)
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Re: Misfire Driving me mad....

A compression test?! Er I hope that throws nothing up at all as it mostly would throw up one of three things...none of them good! What did yours throw up and what other symptoms did you have?

I do have a compression tester gauge, I suppose I could test it....
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Old 12 May 2009, 08:54 AM   #11 (permalink)
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Re: Misfire Driving me mad....

It's a long shot mate and I hope it doesn't show any faults but you seem to have done a lot so far.
I spent 4 months with a missfire slowly getting worse and we did everything to try to find the cause. In the end I had a compression test done by Subaru and they confirmed that I had no compression at all on one
cylinder
. Turned out to be the exhaust values had crumbled, all the unburnt fuel was running down in to the Cat. Cost about 3k to get it fixed
It may have been a coincidence but there was 2 other STI's in the garage at the time, which also had the same fault. SubaruAssist wouldn't help so I took the cost.
As I said, I hope it isn't but if you can rule it out then it's worth the effort.
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Old 12 May 2009, 05:04 PM   #12 (permalink)
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Re: Misfire Driving me mad....

Try testing your injectors as this can cause a misfire but not on idle as i have had this problem. I would also go with Matt-WRX-Import and do a compresion test. Hope you get it sorted. Also just to add my mates wrx wagon had a bad coil and the symptoms were that it was cutting off when it got to about 1.5 bar and back firing for fun.Hope this helps
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Old 14 May 2009, 05:36 PM   #13 (permalink)
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Re: Misfire Driving me mad....

Quote:
Originally Posted by Tom991981 View Post
Ok, so I have researched and looked and tested various things but I cannot get rid of a misfire under decent acceleration.

Impreza WRX STI 1994 Version 1 Import - No Mods.

Under reasonably hard acceleration I get an intermittent stutter, mid to high rev range, no problem at all at idle/low revs/speed. Not at a particular point in the range. No problem if speed built up slower.

Boost is steady and constant just under 1 bar even when misfire occurs so I don't think its a boost issue. Stutter clears after a second or two and continues to accelerate. It may or may not occur again up the range.

Things I have done:

Checked fault codes, all clear
Replaced Spark plugs
Replaced fuel filter
Replaced air filter
Checked all coil packs for cracks and attempted to clean the sprung contact face - none found, all clean
blown out the MAF housing interior with an air compressor
Reset ECU
Tried different fuels, v-power down to bog standard unleaded, no difference

It feels like a spark is failing and not igniting a cylinder. I think it could be one of the following. Gents, please give me the benefit of your vast knowledge to help regain my sanity though I fear its too late for my hair....

1) Timing out? Garage I bought it from not a Subaru garage and replaced timing belt and tensioners for me before I bought it? Highly possible they did this but would there be worse symptoms of this???

2) Coil pack internals breaking down? But no visible damage? How do I test their effectiveness under load???

3) Failing to fuel/air enough? (Can't believe this is the case)

4) Anything else?


Please help!!!

My 95 import is doing exactly the same. Driving me mad too, I've changed the plugs, coil packs, filters. No fault reading on ecu I dont know what else to do!
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Old 14 May 2009, 05:43 PM   #14 (permalink)
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Re: Misfire Driving me mad....

Claire.

have you tried anything else that's been mentioned above??
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Old 14 May 2009, 07:02 PM   #15 (permalink)
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Re: Misfire Driving me mad....

I've had her compression tested and that was fine. I'm having a complete exhaust system fitted next week cause mine is on it's last legs so maybe there is a small hole in it I cant see yet, But I was underneath her yesterday and didn't notice anything. As I said i've done the air filter, fuel filter, oil filter, 4 new plugs, 4 new coil packs, Knock Sensor, Reset the ECU but she is still doing it, I think she does it even more when warm. Just hesitates when you put your foot down but then jerks and she pulls fine then.
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