Mini Cooper S Backfiring
June 6, 2011 9:32 AM   Subscribe

My mini ('09 S, automatic) is backfiring after I turn it off. Yes, I know about the "burble" you can hear when you shift gears in a manual, this obviously isn't my problem. Details inside.

I use premium fuel, get the oil changed when I am supposed to (only once since owning it - just following instructions) and generally take care of the car. Is there a reason for the puttering after I turn it off? It typically only happens in the morning after about a 10 mile drive. Not a loud backfire, but definitely a noise. I am terrified that this change the oil once a year madness is the problem. I'm at 32K and don't need an oil change for another 7K. Please help.
posted by courtyard to Travel & Transportation (12 answers total) 1 user marked this as a favorite
The oil change interval for Mini's are at 10,000 miles per Mini. They are using a synthetic oil that takes quite a while to break down. I suspect the oil is not the source of your issues.

Backfires, at their core, are caused by fuel being in your exhaust and igniting. It can be caused by a rich air-to-fuel ratio, exhaust system issues, timing issues, wiring, fuel pump/lines/injector issues, or a catalytic converter issue.

That all being said: Mini Coopers should be covered under a 4-year/50,000-mile warranty. Take it in. It's not going to get better on its own.
posted by Mister Fabulous at 10:51 AM on June 6, 2011 [1 favorite]

This behavior is known as dieseling and it usually points to a fairly serious problem with the air/fuel ratio in the engine. What is happening is that after turning off the ignition their is enough fuel left in the engine's cylinders to keep stuff burning for a few more engine revolutions. Now that virtually every car is fuel injected (and this includes minis) this problem has mostly gone away. This problem is probably not connected with the engine oil in any way. It is probably a malfunctioning sensor (or multiple malfunctions) that is sending bad data to the computer but not reading as bad to the computer. You don't say if you have a check engine light coming on or not but if you do this is certainly the case. If the car is still under warranty take it in, and they will fix it. Are you also changing the air filter? This is very easy to do and a really, really dirty air filter can cause some problems like this. Good luck.
posted by bartonlong at 11:28 AM on June 6, 2011 [1 favorite]

Response by poster: Thanks for the answers. No lights coming on yet. The dealer is about 100 miles away, and yes, I had all my filters changed about 6 months ago.
posted by courtyard at 11:45 AM on June 6, 2011

nthing bringing it into the dealer. I did a quick search on NAM and didn't immediately see any similar posts, but might be worth posting on there as well before you make the drive to the dealership.

Good luck!

Also, not to nitpick, but the oil-change interval isn't 10k miles. It's generally 'whenever the car says it needs one', which usually falls between 15-20k miles, or 1 year.
posted by Tu13es at 12:40 PM on June 6, 2011

Nota bene: I haven't worked on a Mini (of the current crop) but I've worked on a lot of cars and what follows is some non-specific advice to help you find the problem.

Is it a single loud bang or pop? Then this is gas igniting in the exhaust and is is due to an over-abundance of fuel in the engine and not being cleanly ignited before being forced out of the cylinder. Possible issues: Fuel injection computer putting too much gas in, o2 sensor going bad and resulting in too much gas being injected, vacuum valve on fuel rain failing in pulling too much gas into cylinders, gas in your oil reservoir (smell the dipstick to check for this, but not very common). I should note that all these things are quite uncommon with a car this new, all these issues should be covered by any respectable dealer. A quick matchbook test for this issue is: has your gas mileage gone down lately? If so, one of these is your issue.

Is it more like a single sneeze or wheeze when you kill the ignition (physically turn the key off)? Then: distributor is off timing and injecting spark into dead cylinders (ie- one on the exhaust stroke), crank timing is off causing spark into dead cylinder, coil is not firing correctly. These are again, all issues more common to engines much older then yours and should be covered under warranty. Quick test: does the car have notably less power than you recall? Then this is your issue.

A third option is the engine is overheating and igniting left over gas, if this is the care, you'll hear "pinging" on the cylinders while running hard (accelerating uphill or in a hurry to get up to speed) but usually this is caused by bad gas. Buy known good gas and see if the issues goes away. Your engine knock sensor should prevent this from happening, check it if issue persists. Again, power will be notably less if this is an issue.

Really outside stuff in decreasing order of likelihood: oil in exhaust, stuck injector, bent valve, bad valve seal, bad rings, bad head, bad block. I'd be really shocked if any of these are an issue.

My money's on the o2 sensor, personally, but I haven't seen or heard your car so this is me sitting on a couch and typing out loud. Take that for what you will.

Any good mechanic should have already checked all these issues, but it should give you a place to start your conversation with your dealer on how they are going to make this right. Modern cars don't do this and the onus is on the dealer to make it good.
posted by 1f2frfbf at 2:29 PM on June 6, 2011

the "burble" you can hear when you shift gears in a manual

That's your problem. Yours is an automatic. The shifting is different and it's not allowing those gasses to build up and fire off in the exhaust the way they would in a standard. I'm guessing that your car isn't burbling as much as a standard and is firing off in the exhaust after shutdown. (Only one or two pops, sounds right to me.) You should call your dealer just to verify this, but I read that the S model gets some extra sprays of fuel to cool the valves and that results in the little backfires and burbles.

So, considering that your car isn't miss-firing, lagging, stuttering or shakey during idle, I'd venture to say that there's NOTHING wrong with your car. Seriously. It's just awesome.

And yea, a quick phone call to your dealer should clear this right up! Cheers!
posted by snsranch at 5:32 PM on June 6, 2011

No fuel injected car should do this. As soon as you turn the key off, the fuel stops and there is nothing to ignite. So, it is getting fuel when it shouldn't be.

(It's got nothing to do with burbling during gear shifts, because, you know, the car is "on" during that time. Neither has it got anything to do with the oxygen sensor, or any other sensor for that matter, because those sensors are equally "off".

Likely, an injector is leaking. If it is still under warranty, make them fix it. If not, run a couple bottles of Techron through it.
posted by gjc at 6:49 PM on June 6, 2011

Response by poster: After I shut down the engine and sit in the car for about 30 seconds there are several pops. Not a loud noise at all, just a slight pop. If I didn't sit in the car I would never hear the noise. Thanks for all the car talk, I obviously don't know what I'm doing. I'll likely take it in as warranty is up at 36K and I don't want to miss out on that.
posted by courtyard at 4:56 AM on June 7, 2011

FYI, warranty is 4 years / 50,000 miles. The 3/36k is for maintenance, which is things like oil changes.
posted by Tu13es at 5:45 AM on June 7, 2011

Oh! That noise? That's the fuel system pressurizing, from what I gather. Or at least that's the consensus on NAM from what I remember. You can occasionally hear it in the right rear quarter just after you turn off the car. It's not loud like a gunshot (i.e. a backfire), it just sounds like a light thump.
posted by Kyol at 3:00 PM on June 7, 2011

Response by poster: Kyol - so, you think it's harmless? I can only describe it as I did before, like a puttering. Is that what you are talking about? I'll poke around NAM again.

Tu13es - thanks for all your clarification.
posted by courtyard at 8:40 AM on June 8, 2011

Yeah, if it were more metal-on-metal I'd say it was the exhaust system pinging as it cools down, but if it has a real plastic grunt to it I'd go with "harmless fuel system pressurization". What it's pressurizing, I couldn't say.
posted by Kyol at 10:46 PM on June 9, 2011

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