what cause backfire on mx5

[wun911]

At winton I hear large pop and crackly fron the exhaust especially when pushing it > 6000 rpm.

Is this common for MX5?

Hole in exhaust? Dirty fuel?

On the way home car is fine < 4000 rpm all the way home no problems...

what should I ge looking at specifically

[Hellmun]

When does it occur? When you shift up, when you heel and toe down. Anytime you hit a certain RPM? Be a bit more specific.

When I put my extractors and intake in my car it used to backfire a lot on 0 throttle going downhill on the over-run with the standard ECU. Now with the non-standard ECU it does it occasionally on heel n toe downshifting.

[mx52nv]

Is this common for MX5?

It is for SPs and cars that are running a little rich.

Hole in exhaust?

Possible - check when you get a chance.

Dirty fuel?

Possible - although I doubt it in this instance.

what should I ge looking at specifically

Most common reason in your case is due to a temporary imbalance in the air to fuel ratio (running rich= more fuel/less air) during gear shifts, causing the unburnt fuel (fumes) to ignite in the exhaust system.

Colloquially this is called backfire but I think from memory, it originated from the days of when carbureted engines were running lean and the fuel ignited within the intake system (hence the term).

I'm sure there are better explanations out there than my cursory and probably technically simplistic version

And yeah my MX5 does it as well (track or otherwise) but I know I am running rich

[SuperMazdaKart]

anti lag turbo systems do it too, but somehow i don't think that'd be the reason this time

[bruce]

Hole in exhaust is the first thing to check.

[Steampunk]

As Hellmun stated, be specific as to when this happens.

In addition to what the others stated, IF it happens at WOT and NOT shifting gears, it may be fouled/old plugs and/or leads that isn't keeping up with the combustion demands.

Although an overly rich mixture is the most common diagnosis for "backfires"/ popping, an overly lean condition will also do it.
Don't ask me the reasoning behind it.

When I had the overrun fuel-cut active in my MegaSquirt (ie. it cuts fuel delivery at closed throttle to save fuel), I used to get popping often. When I turned it off, it hardly happens.

[wun911]

It happens on the main straight at 6000 rpm (im in 3rd gear im thinking about changing to 4th but I havent done so yet) just happens all of a sudden my foot is planted to the flor. Loss of power exhaust pops loudly crackles and it stalls, I turn the key it starts up I ease it off the track.

I scratch my head, 20 min later get back onto the track, happens again after after about 8-9 laps, same thing on the straight, friggin frustrated now, lots of track time but car not lasting the distance happens a few times thoughout the day etc.

I was thinking heat issue?

Can it be timing? Nube question but is timing related to "running rich"?

I take an easy drive home in 6th gear around 3000 rpm drove around 300 km to get home no problems all the way.

I will check the plugs, order in new leads, replace the fuel filter and look for holes in the exhaust FFG method.

Thanks for the sugestions...

[stevesports]

wun. My car does it aswell. It is possibly a leak inthe exhaust somewhere. It is more prominent when the car goes into closed loop. This is i believe because the car is running a little rich, or incomplete combustion of the fuel.

[Hellmun]

That's quite odd, on WOT your running off your maps rather than adapting according to the EGO sensor to my understanding. So I wouldn't think it could be exhaust or leads. If the car is actually stalling I'd think it was something more like the immobiliser. If it was a plug/lead it would be a misfire and you'd just lose power but it'd keep going. To actually stall your losing power on all cylinders.

I'd first want to work out if I was getting too much or too little fuel based on the spark plugs after it happened. Either all cylinders stopped firing or they just stopped getting fuel.

[sliq]

it could be a rooted catalyzer as well.

[manga_blue]

Wun, just as a general rule for any car:
- backfire in the exhaust = rich mixture dumping into it,
- backfire in the intake side = either lean mixture or high back pressure in exhaust causing flame front to enter inlet side

Many MX5s backfire in the exhaust when you go quickly from full throttle to no throttle, such as during a gear change, resulting in a very short period of rich charge going into the exhaust and burning there.

[Rotary]

Wun, I’d be looking for a fuel or ignition system problem with this one, it’s unlikely to be on the exhaust side as failure there would give a permanent unusual or faulty condition,
Would be good to get it checked because it may get worse and start playing up even under less extreme conditions,

Does it start immediately after stalling, or does it require some cranking before it fires?
When you say 20mins later it’s ok, do you pull over and stop the engine or let it idle for twenty then drive off?
If idle does it, idle smooth? Rough? Stalls?
Did you try some hard revving or to drive it around straight after, or always after stopping?
How it behaves in these situations may help to narrow it down a little
Matt.

[Red Dragon]

Hi Wun it was good to see you at the new members night.
I had a similar problem last week the car sounded like a WRX or a V8 on 6 cylinders
Number 1 plug lead was the problem, changed all leads & new plugs while at it.
Result- good as gold
Cheers

[corners]

Wun, just as a general rule for any car:
- backfire in the exhaust = rich mixture dumping into it,
- backfire in the intake side = either lean mixture or high back pressure in exhaust causing flame front to enter inlet side

Many MX5s backfire in the exhaust when you go quickly from full throttle to no throttle, such as during a gear change, resulting in a very short period of rich charge going into the exhaust and burning there.

Mine does this all the time!!!

[sirbob]

My NB8A used to do this after i put on a 2.25 exhaust and high flow cat. Personally i liked the snap, crackle pop on decelleration into corners...

However, my car never stalled and it only occured when i was using compression braking.

Your problem sounds more like a fueling issue and is most likely caused by a blocked/contaminated catalytic converter IMO.