# How does the ECU detect a misfire?



## djnibler (Aug 5, 2005)

2000 Golf, obviously OBDII. Has a code for cylinder 4 misfire. On my 01 Passat 1.8T, I have a seperate coil pack for each cyl where as on this car it looks like the coil pack is one unit. So how does the ECU determine a misfire and what could cause it other than a bad coil pack? I'm doubting a borderline spark plug wire or spark plug would be detected by the ECU no? Are the coil packs on these cars really 4 separate units that are attached together and can only be purchased as one full unit... so when one goes out this is how the computer detects the misfire? Other ideas? Thanks.


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## tdogg74 (Mar 1, 2002)

*FV-QR*

sensed through the oxygen sensor


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## djnibler (Aug 5, 2005)

*Re: FV-QR (tdogg74)*

Thanks for the reply. How does the Oxygen sensor narrow down the misfire to a single cylinder? I would think the car would need to have one oxygen sensor per cylinder on the exhaust manifold to do that no?



_Modified by djnibler at 6:20 PM 3-18-2009_


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## djnibler (Aug 5, 2005)

*Re: FV-QR (djnibler)*

Anyone?


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## digraph (Jul 23, 1999)

*Re: FV-QR (djnibler)*

guess - the ecu monitors current going thru the coil pack, when there's 0 current it's correlated with which cylinder should be firing (either by cam position, or which injector it's trying to fire). And yeah, somehow the O2 sensor's output figures into this...


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## tdogg74 (Mar 1, 2002)

*FV-QR*

Ok class...what is the function of the O2 sensor? To read oxygen levels in the exhaust, right? If there is a mis fire, then there is raw fuel, thus a rich condition.


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## djnibler (Aug 5, 2005)

*Re: FV-QR (tdogg74)*

Sure, a rich condition could definitely indicate a misfire but if that were the case, I would excpect the code to display "rich" or "lean" condition... I've seen codes like that when I have had vac leaks. But for a single Oxygen sensor to detect which specific cyl is misfiring? I just don't see how that is possible. I'm guessing it has more to do with the coil but that's what I'd like to get a confirmation on from someone who knows for sure.
My 1.8T has seperate coil packs for each cyl. When one went out, it threw a code that there was a misfire in cyl 1. I thought it would be easy for the computer to detect this because the coil for THAT cyl would not be responding in the proper way. But on the 2.0 I'm working on right now, there is only 1 coil pack. It could be that the one coil pack is composed of 4 different packs that are bolted together and the computer has a way of recognizing each one so that when one fails, it knows which one... but that's part of what I'm asking.
As for a fouled plug or bad wires, I don't see how the ECU could detect a specific cylinder misfire based on this.... unless the wire is broken completely and the spark from the coil doesn't ever hit ground. But if the plug is firing or the spark plug wire is shorting out somewhere, I don't see how the computer or coil could detect this at all. Wouldn't a spark plug firing look the same to the ECU and coil as a plug wire arcing on the engine body?


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## digraph (Jul 23, 1999)

*Re: How does the ECU detect a misfire? (djnibler)*


_Quote, originally posted by *djnibler* »_Are the coil packs on these cars really 4 separate units that are attached together and can only be purchased as one full unit... so when one goes out this is how the computer detects the misfire? Other ideas? Thanks.

Oh - No they aren't. AEG+s use the wasted spark system. So you should be able to switch ignition cables 1 for 4 and 2 for 3. The misfire will move cylinders if it's a cable problem or not move cylinders if it's a spark plug or other problem.


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## djnibler (Aug 5, 2005)

*Re: How does the ECU detect a misfire? (digraph)*

Could you expand on the wasted spark system? Are you saying that when one plug fires, they all fire?!?!?!


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## djnibler (Aug 5, 2005)

*Re: How does the ECU detect a misfire? (djnibler)*

Nevermind, I just looked it up.... wow never heard of that before... learn something new every day.
So you are saying the sparks are sent to 1/4 and 2/3 at the same time? That's the pairing? 
If so, then this makes the trouble code even MORE confusing! Without a seperate coil for each cylinder, how on EARTH could the ECU pinpoint the exact cyl the mis is occuring on!!!


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## tdogg74 (Mar 1, 2002)

*FV-QR*


_Quote, originally posted by *djnibler* »_Sure, a rich condition could definitely indicate a misfire but if that were the case, I would excpect the code to display "rich" or "lean" condition... 



no.... thats the function of the short term and long term fuel trims.


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## SAATR (Oct 3, 2008)

*Re: How does the ECU detect a misfire? (djnibler)*

Some vehicles detect specific cylinder misfires by variations in the crank rotational speed. Misfires cause the crankshaft to accelerate or decelerate erratically, making a shorter or longer "tooth" in the squaretooth waveform generated by the CPS as the crankshaft spins. This can be compared against the pulses generated when the other cylinders fire, or from a predetermined pulsewidth @ RPM known by the ECM. Either way, you can detect specific cylinder misfires. This is one method, and I don't know if it's used by VW but it can be utilized to determine which cylinder is generating the misfire.


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## djnibler (Aug 5, 2005)

*Re: How does the ECU detect a misfire? (SAATR)*

Very interesting, that does make sense and I could see how the computer could detect that through the engine speed sensor. Sadly, it doesn't go a long way to help a mechanic determine what could be throwing this code though... shoot! Could be anything really.... anything with the fuel system or electrical system. 
Guess I will start by replacing some of the common components that should be replaced around the 120k mark like the coil, wires, spark plugs and if those fail to fix the problem, will move on to MAP and start testing fuel pressure and injectors etc.


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## digraph (Jul 23, 1999)

*Re: How does the ECU detect a misfire? (djnibler)*

You got me interested in this. I think I figured out how Motronic knows where to tell you the misfire is...
There's 1 knock sensor on cylinder's 1 & 2, and another knock sensor on cylinder's 3 & 4. So basically there should be a pattern because the cylinders fire in the same order. Let's guess it's 1 - 2 - 3 -4, so the ECU should see:
KS1 * * * *
KS2 * * * *
(hmm my drawing gets messed up, anyway the ECU should see 2 pulses from KS1, then two pulses from KS2, rinse lather and repeat)
When one pulse is missing, you might get a misfire condition and you should be able to tell which cylinder it missed on. Like I mentioned earlier I'm sure it's correlated with CPS, O2 sensor, MAF, and when it fires the coil pack.

_Modified by digraph at 11:24 AM 3-19-2009_


_Modified by digraph at 11:25 AM 3-19-2009_


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## AgentAl (Dec 26, 2006)

*Re: How does the ECU detect a misfire? (digraph)*

cliff notes 
*step 1-* misfire occurs, oxygen doesnt combine with fuel
*step 2-* oxygen then flies by the oxygen sensor
*step 3-* the ECM uses the crankshaft position sensor(engine speed) to detect crankshaft acceleration at each ignition pulse, correlated against the camshaft position sensor (engine position) it can be determined which cylinder is missing. this is also how the ECM can tell the difference between misfire and a lean condition. if crankshaft acceleration remains constant then more fuel is added and this is where the LT/ST fuel trim numbers start to change.
*step 4-* this is the step where the typical vortexer will start throwing parts in an attempt to fix said misfire.
[\cliff notes]
lets face facts folks. if there is a code for cyl 3 misfire then it has to be something that is specific to cyl 3. the wire, plug, injector, or an internal engine problem with cyl 3. if the coil goes out (see waist spark explanation above) then cyl 3's companion cyl will go out too. 
if there are codes for ALL cyl then it cant be anything that is specific to one cyl. well i should say it is very unlikley. look at things like fuel pressure, grounds, maf, o2, any one thing that is used by ALL the cyl.
you will almost always get 'random misfire' code when you get a cyl specific code so just ignore it unless it is the only code present. this code is thrown after the ECM determines there is a misfire but BEFORE it figures out where.
some systems will shut off fuel to the affected cylinder and the check engine light will stop blinking. engine damage will not occur if you drive with the check engine light blinking. that catalyst will melt but the engine will be fine, unless the car burns to the ground because of the fire under the car. 
fuel + 800* catalyst = fire
knock sensors are not involved in anything here. they are tuned to generate voltage when a certain frequency sound is made by the engine. the sound is not made by lack of combustion. based on the generation of voltage the ECM will start to pull timing until said voltage generation goes away.
[\rant]


_Modified by AgentAl at 7:37 PM 3-19-2009_


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## dieselwagen (Jun 7, 2008)

*Re: How does the ECU detect a misfire? (AgentAl)*

SAATR and AgentAl, knows what they r talking about
the ECU uses the crankshaft position sensor on most OBD2 vehicles to detect misfires.


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## onewiper (Aug 29, 2002)

The crankshaft position sensor is so accurate that it is used on some vehicles to perform a compression test based on crank acceleration during engine cranking. You don't even have to get dirty.


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