OBDII and misfiring Q's?

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JTK

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On an OBDII equipped vehicle, what component (or components) actually do the catching or detecting of random, multiple or specific cylinder misfires? Is it the oxygen sensors? Thanks fellas! Joel
 
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you are probably correct. i would guess that the o2 sensor would see a moment in time where the exhaust goes lean as a misfired puff of air goes by from a cylinder that didnt fire. you can see this on a o2 sensor scanner as a rymthic dip in the afr. i think that, they programme into the computer how far away the oxygen sensors are, so that the computer can calculate which cylinder is misfiring based on which cylinder is currently suppose to be firing. i cant see any other way that the computer can tell which cylinder is misfiring. of course, all this is only my guess.
 
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Ford (and some other makes) vehicles use the crankshaft position sensor to catch misfires, by sensing the deceleration of the crankshaft that occurs with a misfire. Apparently, false trips on manual transmission vehicles being driven on rough roads (like gravel) were a big problem early on, but they obviously got around that.
 
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The late 90s Dodge trucks use the crank position sensor to detct misfire. It's not a perfect method but RPM fluctuations make the sensor not signal the ECM precisely when the ECM expects it. When that happens it throws a code for engine misfire
 

JTK

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Brian, one of my vehicles must work similar to the ecotec 2.2. I’m *lucky* to have the GM/Delphi ion sensing ignition system on my 2002 isuzu 3.2L [Roll Eyes] http://delphi.com/manufacturers/auto/powertrain/gas/ignsys/ionized/ (all 2000-2004 isuzu 3.2/3.5 have it). Probably a great system, but if you run any spark plug other than a denso K16PR-P11 you get plagued by random misfires. One of these advanced ignition system must be required to detect specific cyl misfires, while it makes sense CPS or O2 sensors could pick up random or multiple cyl misfires!? Thanks Joel [ November 06, 2005, 08:03 AM: Message edited by: JTK ]
 
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The O2 sensors, typically on the exhaust, one for each bank of cylinders, evidently sometimes also around the catalytic converters (?). Misfiring will usually create an O2 rich exhaust as it wasn't consumed by a proper burn, so the sensor will usually lean the engine out. Now you can start with some interesting cascading failures, as the leaner engine isn't running right but there is still one or more cylinders with a misfire so the exhaust is getting pretty dirty. It can start clogging up the O2 sensors, where they start by responding slower, and the dirty exhaust is also getting sucked back into the intake thru the EGR. Now you're set to start clogging the EGR valve and an EGR feedback sensor if you have one, and maybe an Air Control Temperature sensor if it's installed downstream of the EGR port in the throttle body. Now you also start getting the throttle body and plate dirty, or maybe just dirty more quickly, all of which makes for rougher running, a dirtier exhaust, slower or misfuntioning O2 sensors, etc., etc., and eventually maybe the converter starts getting clogged, the rings start gunking up, you start burning more oil, which really makes for a dirtier exhaust. Now you have an exhaust consisting of partially burned oil and fuel mixed with soot, coating the engine intake thru exhaust, and it's also blowing by the rings and getting into the crankcase. The much dirtier oil is getting sucked into, you guessed it, the intake thru the PCV valve, and it's competing with the dirty exhaust on which can clog the engine up the quickest. The lesson is change the plugs once in awhile, even if they're hextuple plutonium plugs advertised to last for the next two cars that you'll own, as they really won't last that long :^) Plugs are cheap, as plug wires, relatively speaking. Change your O2 sensors once in a great while too. It's the automotive universe version of the butterfly effect, but I guess we should call it the 'spark plug effect' :^)
 
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When there is a misfire, raw fuel and vapor arrive at the catalytic converter where it is burned up. This causes the converter to overheat. This is the main reason for premature catalytic converter failure. The excess heat causes the interior of the converter to break apart. The chunks block the exhaust causing a noticeable loss of power. Of all the OBDII codes, a misfire code can end up being the most expensive to ignore.
 
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As far as I know the 2.2L Ecotec isn't sensitive to the brand of sparkplug used. The crankshaft position sensor can detect specific cylinder misfires.
 

JTK

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"The crankshaft position sensor can detect specific cylinder misfires." [Duh!] Now that I think about it, I guess it would! What ever RPM fluctuation it picks up per degree of rotation would pinpoint the cyl. Thanks Joel
 
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