How to repair a 2009 mini cooper that started misfiring? What is the most common cause for misfiring?
Coil
Plugs, wires, coil, moisture. How do you know it is missing? What are the symptoms? Any check engine codes you can share with us?
I don’t know any more than what my son is telling me. It’s his wife’s car. I don’t think the engine light came on.
Ignition coils or carbon buildup inside engine requiring blast cleaning with crushed walnut shells are the 2 most common causes I see.
Thanks for the info. I’ll pass on the suggestions.
He says it’s a compression problem.
Hmm. Bad valve? Broke ring? Sounds expensive
Then have a leak-down test performed.
A leak-down test will indicate why the engine misfires.
Tester
The mini is now at the dealership where he bought the car.
Your Mini has coil over plug ignition system. A misfire can be cause from a fouled spark plug, a vacume leak, a failing coil or worst case scenario…low compression. Theres a few other causes but they get fairly serious and should not apply here. You should get an engine light if the coil is not firing bec the computer will see this…you will NOT get a misfire code if a spark plug has failed. If its a real misfire, its pretty easy to figure out the cause, most mechanics should be able to figure it out.
Blackbird
Did the report from the dealer say how they deduced that compression is low? If not, ask them, or take the car somewhere else for a second opinion. If the dealer refusees to share the information, I would suspect something is amiss.
“You will NOT get a misfire code if a spark plug has failed.”
Huh?
I’d say that 90% of the vehicles that come into my shop with misfire code(s) are caused by one or more worn/bad spark plugs.
Tester
When a plug fails but the coil is actually firing and generating current the computer sees nothing wrong aside from a rich condition. I have only seen codes when the coil does not function at all. Basically I am under the impression that the computer has no way to monitor the spark plug itself…it only monitors the coil and crank speed.
Is this incorrect? I’ve used this theory to diagnose an awful lot of misfires, seems to hold true. Let me know. Wont be the first time I have to re look up how the computer monitors or knows something is awry. I know for certain it monitors crank speed and the coil internals. I never thought the computer knows whether the plug has fired or not…I don’t think it does know anything about the plug status itself…unless that manifests in the coils internals voltage wise.
I’ve repaired more misfires than I can count with 100% success so I have to be doing something correct. Do you have new information on this?
Blackbird
If a cylinder misfires because of a worn/bad spark plug this causes the crankshaft’s rotation to slow down. The computer detects the slow down in the crankshaft rotation thru the crankshaft position sensor.The computer will then set a misfire code indicating which cylinder(s) are misfiring.
Tester
never thought the computer knows whether the plug has fired or not…I don’t think it does know anything about the plug status itself…unless that manifests in the coils internals voltage wise.
A misfire is a misfire is a misfire. Any number of things can cause a cylinder not to fire properly. Some cars monitor coil primary circuit and spark discharge, some do not. But codes don’t tell us what to fix. They just tell us something is wrong somewhere.
I just did a quick brush up on another document and it says OBD2 will post a code for when the spark does not jump the gap. I can absolutely swear on a stack of bibles that several misfire conditions I’ve repaired posted no code what so ever on these vehicles. I forget what they were it was a long while ago.
I’m all for codes the more the merrier when in troubleshooting a misfire. Those 3 or 4 vehicles that didn’t post a code all had worn plugs. Which steered me to think the computer cant monitor the spark plug itself and only the coil … Oh well…who cares…they were repaired and are still running now.
I have to look things up all the time anymore…wont be the first or last time. I need to look up all the possible problems with O2 sensors at the moment…time to take a peek at the misfire codes as well. yay
Blackbird
This explains it real well.
http://www.autocarepro.com/tech-feature-detecting-misfires-in-obd-ii-engines/
Tester
Okeedokee… Thx I have like 4 similar pages open at this time for O2 and now misfire…haha…whats a 5th page? Always good to brush up from time to time.
Here is a question for you… Is the OBD2 system of todays cars lets say 05’ and up…more powerful that the systems of 96’? We certainly have more computing power now… I get all sorts of vehicles with OBD2. To me it feels like it has improved of late quite a bit. Are there flavors of OBD2 or did the computing just get stronger yielding more accurate results?
Thanks for the info…Im brushing up on a nice rainy day…perfect.
Blackbird
Funny you sent the same document I was going to send to you in the second post. That was the first document I opened to brush up on. The first doc you referenced only skimmed the subject. The second one does explain it much better.
Blackbird.