VW Polo 1.2 2004 - Misfire, interesting question #
Posted by Pete Mutlow on March 6, 2014, 7:39 pm
Year of Manufacture *: 2004 Engine Size and Code *: 1.2 ( Optional ) Scanner Used: ANY ( Optional) Fault Codes: P301/2/3
.....Or not such an interesting question. I went to the subject vehicle with a misfire on number 3 cylinder (3 cylinder model). The owner had replaced plugs, coil and injector on No 3 cylinder to cure misfire code P303 but to no avail. The vehicle also drops into limp when the EML illuminates. Now the archives are awash with this subject so I was well prepared with an insight to the problem. For those not in the know, it is caused by an engine problem normally burnt or poor seating valves and the management system will switch off the injector to the affected cylinder.
My question is this: How does the ECU know that there is a mechanical problem with the affected cylinder? Archives point in the general direction of the crank sensor but I would like to know if this is so and how can the sensor determine a misfire on a particular cylinder?
the ecu knows through the crank sensor's output depending on each cylinder's speed. if one of the cylinders has low compression it's speed is slower than the others. this is easy to see with a scope
As above, or to quote a piece of text written by Tom Denton in his rather excellent book: Advanced Automotive Fault Diagnosis.
"Misfire monitor - When an engine endures a period of misfire, at best tailpipe emissions will increase and at worst catalyst damage and even destruction can occur. When misfire occurs, the unburned fuel and air is discharged direct to the exhaust system where it passes directly through the catalyst. Subsequent normal combustion events can combust this air/fuel charge in something akin to a bellows effect, which causes catalyst temperatures to rise considerably. Catalyst damage failure thresholds are package specific but are in the region of 1000°C. The catalyst itself is a very expensive service item whether replaced by the customer or the manufacturer under warranty. The misfire monitor is responsible for determining when misfire has occurred, calculating the rate of engine misfire and then initiating some kind of protective action in order to prevent catalyst damage. The misfire monitor is in operation continuously within a ‘calibrateable’ engine speed/load window defined by the legislation. The USA requires misfire monitoring throughout the revs range but European legislation requires monitoring only up to 4500 rev/min. The crankshaft sensor generates a signal as the wheel rotates and the microprocessor processes this signal to determine the angular acceleration of the crankshaft produced by each engine cylinder when a firing event occurs. When a misfire occurs the crankshaft decelerates and a cam position sensor identifies the cylinder that misfired. Processing of the signal from the crank position sensor is not straightforward. A considerable amount of post-processing takes place to filter the signal and disable monitoring in unfavourable conditions. The misfire monitor must learn and cater for the differences in manufacturing tolerances of the crankshaft wheel and so has a specific sub-algorithm for learning these differences and allowing for them when calculating the angular acceleration of the crankshaft. These correction factors are calculated during deceleration, with the injectors switched off. They should be re-learned following driveline component changes such as flywheel, torque converter, crankshaft sensor, etc. The misfire monitor must be able to detect two types of misfire: ● Type A misfire; ● Type B misfire. A type A misfire is defined as that rate of misfire, which causes catalyst damage. When this occurs the MI will flash at a rate of 1 Hz and is allowed to stop flashing should the misfire disappear. The MI will stay on steady state should the misfire re-occur on a subsequent drive and the engine operating conditions are ‘similar’ i.e. engine speed is within 375 rev/min, engine load is within 20%, and the engine’s warm-up status is the same as that under which the malfunction was first detected (and no new malfunctions have been detected). The rate of misfire that will cause catalyst damage varies as a function of engine speed and load. Misfire rates in the region of 45% are required to damage a catalyst at neutral idle whilst at 80% engine load and 4000 rev/min misfire rates in the region of only 5% are needed. A type B misfire is defined as that rate of misfire that will cause the tailpipe emissions to exceed legislated levels. This varies from vehicle to vehicle and is dependent upon catalyst package. MI operation is the same as for standard MI DTCs."
Thanks both to you for that....most informative. This was alluded to in one of the archived posts and I found it difficult to get my head around that the crank sensor can discern the tiniest amount of difference of crankshaft rotational speed by one mis-firing cylinder.
As a footnote - if all cylinders are providing low effort, misfire monitoring can't discern a poor cylinder where all cylinders are weak. (Valve timing, bore washed etc...) Something I have seen on a 1.2 Polo, all compressions were equally feeble, thus no difference in effort at each cylinder & no misfire logging.
Thanks Jon for that. Taking the question further down the line, would it therefore be a fair test to check the compression values of a diesel or petrol engine by scoping the Crank Sensor? At the moment, I use a compression tester on petrol engines or amps clamp on a diesel. I am wary of compression testing on a petrol engine fitted with coil packs for fear of damaging the ignition system by removing the coil pack.
As I understand Removing the low voltage connection on coil should avoid and damage concern. It is open circuit of HT side that may cause damage due to high voltages.
Ideally disconnect injectors as well to prevent fuel injection.
on March 6, 2014, 8:25 pm, in reply to "