John Andrew Mazda was re: Backiring

[Nigel Hughes]

I may have to revise my opinion of franchise dealers.

Having spent 3 weeks on-and-off in garages and auto-electricians trying to
find the cause of the backfiring and replacing bits that didn't fix the
problem I finally took my car into JAF&M. They diagnosed the problem within
the day, groomed the car and didn't charge me anything. So thanks to Cameron
and the Green Team at JAF&M.

I'm told the problem is caused by insufficient back pressure in the exhaust
(I've got an after market header, done the spare-under-boot mod and replaced
the cat with a cobi resonator), and the backfiring is from a lean mixture
rather than unburnt fuel, so apparently won't do damage - any comments?

My options now are put up with it, reduce the exhaust pipe size or fit a
Link ECU - I like the last option... anyone got a spare one lying around
they want to sell for a couple of hundred bucks


Nigel.

JST4FN

[shaznandy lockyer]

Nigel! what size is your pipe and did they say why the fuel mix was lean?!!
Nigel Hughes <nigel@equinoxsoftware.co.nz> wrote: I may have to revise my opinion of franchise dealers. Having spent 3 weeks on-and-off in garages and auto-electricians trying to find the cause of the backfiring and replacing bits that didn't fix the problem I finally took my car into JAF&M. They diagnosed the problem within the day, groomed the car and didn't charge me anything. So thanks to Cameron and the Green Team at JAF&M. I'm told the problem is caused by insufficient back pressure in the exhaust (I've got an after market header, done the spare-under-boot mod and replaced the cat with a cobi resonator), and the backfiring is from a lean mixture rather than unburnt fuel, so apparently won't do damage - any comments? My options now are put up with it, reduce the exhaust pipe size or fit a Link ECU - I like the last option... anyone got a spare one lying around they want to sell for a couple of hundred bucks Nigel. JST4FN

[Nigel Hughes]

The pipe is just 2"...

My understanding was the fuel mixture is normal, except when doing a
"spirited" gear change in the 4000-5500 rev range where the lack of back
pressure resulted in a lean mixture and backfiring... I'm a bit mechanically
challenged so don't know if this was a reasonable explanation?

Nig.

[shaznandy lockyer]

Was the Oxygen sensor fitted to the header??I'm curious 'cause I'm about to install aftermarket headers and do the pipe(cat is gone already)and the O2 sensor screws into the stock exhaust manifold so if replaced the sensor needs to go into the new set you see...How did those dudes rectiy the problem? cheers Andy

[Nigel Hughes]

Yep, there is an O2 sensor fitted on the header (the sensor has been removed
and tested OK, and the readings getting to the ECU from the sensor are
reasonable). I'm not sure whether it was a standard fitting on the header
though - the rest of the header and pipe are ceramic coated but the bolt and
welding where the O2 sensor screws in aren't.

I think I may be looking at changing the muffler / resonator (again -
had it changed to try and reduce the noice)... but that will have to wait
until next month. You may not have a problem if you stick to the standard
muffler.

Pity, it performs noticeably better since the cat was taken out, but I don't
want to put up with the backfiring and $1500+ for a Link ECU is not on the
cards.

Out of interest, has anyone else done the spare-under-boot mod with a
straight through muffler/replaced the cobi resonator with a cat had problems
backfiring?

Nig.

[Gary Morrison]

Nigel, I'm unsure how you would get a lean backfire. A backfire as I
understand it needs excess fuel that ignites in the exhaust system. Maybe the
increased flow results in a lean condition that the O2 sensor sees, more fuel
is delivered and then when you lift off the standard ecu doesn't respond quite
quickly enough, and the increased fuel lasts for just long enough to cause the
backfire. Guessing...

[John&Sene Johansson]

Hi
I have the same setup as you, spare-under-boot mod with a straight through
muffler,replaced the cat with a coby resonator, and it does sometime
backfiring at gearchange 5-6000rpm. But I don4t think it does any harm
Except all teenagers think I have a turbo. If you are worried check the
sparkplugs if they have turned whitish/ grey when it could be problem that it
running to lean. Original ECU should manage small modifications as a more
freeflowing exhaust, and modified airinlets eg. coldair intake
JOHN

[jeff]

I love the backfiring noise!! Only mine's more like "thooop!!"