Bogus misfire claims?

NoQuarter

Major
Nov 24, 2017
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Indiana, USA
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Z4 35is, 535xi, X5 35i
We all read misfire threads nearly every day. Something I am trying to figure out is with my own understanding of what is a misfire.

Now, I know what a misfire is. What I don't understand is the "I got a misfire but no codes" statements. "I clearly have a misfire" or "My car misfires at idle then it goes away", yet they are not accompanied by misfire codes.

I don't think a poorly running engine, or rough idle automatically equals an undocumented misfire. Just because my car is running like Sh$t, do I get to call it a misfire every-damn-time?

Or, do I need to change my perception and we all do indeed suffer from legit misfires the ECU doesn't care about?

What do you think?
 

Cheezy

Lieutenant
Nov 7, 2016
609
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Toledo Ohio
We all read misfire threads nearly every day. Something I am trying to figure out is with my own understanding of what is a misfire.

Now, I know what a misfire is. What I don't understand is the "I got a misfire but no codes" statements. "I clearly have a misfire" or "My car misfires at idle then it goes away", yet they are not accompanied by misfire codes.

I don't think a poorly running engine, or rough idle automatically equals an undocumented misfire. Just because my car is running like Sh$t, do I get to call it a misfire every-damn-time?

Or, do I need to change my perception and we all do indeed suffer from legit misfires the ECU doesn't care about?

What do you think?
Im pretty sure the dme needs a misfire to happen a certain amount of times in a periodof time in order to recognize it. Most people find it easy to diagnose a misfire because after you feel it the first time it's a pretty unforgettable feeling, like when people claim they can feel a car pulling timing. Im not that cool yet but its legitimate. Misfires without codes can also be recognized in logs, rpm drops and afr spikes etc. These cars are finicky as hell, if you have too much vibration at a certian rpm it could trigger a knock sensor and read a false misfire thats not actually there, and possibly "turn off" that cylinder. So yes i believe these cars suffer from undetected misfires, as well as detecting false misfires
 
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NoQuarter

Major
Nov 24, 2017
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Indiana, USA
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Z4 35is, 535xi, X5 35i
I know the feeling you are talking about and always think - oh a misfire. Then nothing comes of it as far as codes, or the cylinder being shut down and having to restart the engine, So I guess I experience the same thing.

Must have some BMW tech blood in me - "If there isn't a code, then nothing is wrong!" ;)

That said, I still feel like there are plenty of people on the forums reading about misfires and suddenly they use that term to describe everything. The same people I guess who end nearly every problem description with "Could it be a fuse?".
 

doublespaces

General of the Army
Oct 18, 2016
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You can hear a car misfire, in fact our cars misfire regularly, so saying your car is misfiring isn't really abnormal IMO. Look at top fuel for example, they misfire a ton, that's why you often see fuel spewing out the headers, only to light back up a moment later. Sometimes the misfire is momentary and other times its prolonged to a point that a code appears. If we got a CEL/SES every time the car misfired, then we would have one every time we drove the car.

As you know, this is why there is money in ignition mods, not all combustion events are equal. Some are less complete burns as others, and in the case of a misfire, the ignition may never happen.

So yes, some people are probably experiencing these misfires, but I don't know the threshold very well for what triggers a misfire code. The only time that happens to me in a noticeable way without a code, is when my fuel runs out and my cylinder goes lean, in which case I immediately get off it. Usually no code, but definitely happened and impacted my run.

As far as detecting timing pulls, you can hear it on the dyno sometimes. The tone of the exhaust changes slightly and you get a different pattern in the rhythm of the pull. I think a lot of it just depends on how bad of a timing pull it is, multiple cylinders, etc.
 

Tuppidsay

Corporal
Aug 3, 2017
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Seattle, WA
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2008 535i
The obd2 standard started the mandatory tracking of misfire events and the requirements are updated as the years go on in the form of LEV, SULEV, PZEV etc. Whats important on those standards is whats called a "catalyst damaging misfire" or a misfire that could damage the catalytic converter. The europeans are especially sensitive to the issue and that's why most euro cars kill fuel injection if "cat damaging misfire" is detected. Which is why when we misfire badly we often have to shut the engine off and start it back up to run normally again. Misfires are detected by rpm variance at the crank shaft sensor. Each car/computer/manufacture has their own presets of what flags as a misfire and how many need to happen in a cycle to register a misfire code. You can see this on several vehicle makes that actually show you misfire counters on a scanner. You can watch the misfires register as they happen. And even sometimes see misfires register without feeling anything. Some cars are just more gung ho about throwing the CEL than others. But that doesn't mean the computer doesn't see every misfire. Theres even what are called partial misfires. Which is when combustion happens but due to several possible reasons that cylinder doesn't contribute as much torque at the crankshaft as it should. Ford's later cars have a cylinder contribution/misfire graphing function in the scanner that allows you to see misfires and partials. BMW has the same function in what they call "smooth running values" . At the end of the day the computer has to see it and it has to happen enough times per combustion cycles to flag a CEL.

Clear as mud?

A little more technical on "smooth running Values" https://www.meeknet.co.uk/E38/E38_Misfire_Detection.pdf
 
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