When overhauling a failed engine, especially when overhauling engine vibration at idle or excessive exhaust emissions, service personnel often encounter troubleshooting tools that indicate that engine has not fired, and we must have a proper understanding of this. After an engine misfire, high concentrations of hydrocarbons enter exhaust system, resulting in excessive exhaust emissions. High concentrations of hydrocarbons also increase temperature of three-way catalytic converter and in severe cases three-way catalytic converter can be damaged. Let's take a series of Toyota vehicles as an example to analyze engine misfire failure.
(1) Engine Misfire Control
To avoid excessive exhaust emissions and thermal damage to three-way catalytic converter, engine control module uses a crankshaft position sensor to monitor engine speed deviation to detect misfires, and a camshaft position sensor to detect cylinder misfires. When frequency of engine misfires exceeds a threshold and could result in excessive emissions, engine control unit starts counting number of engine misfires.
(2) Engine Misfire Classification
Engine misfires can be simply divided into two situations: one is a complete misfire, that is, no combustion, and other is a partial misfire, that is, combustion is unstable. OBD II defines three engine misfire levels A, B and C.
① Type A misfires are most serious misfires that are close to three-way catalytic converter failure. When detected, engine failure warning light flashes to remind driver to repair immediately and store DTC and data frame according to logic of first trip. The engine diagnostic system distinguishes type A misfires by counting number of misfires per 200 engine revolutions.
② In type B misfire, emission of harmful substances in exhaust gas increases by more than 1.5 times.
③ Type C misfires are mildest type of misfires and cause vehicle exhaust emissions to be substandard.
The engine diagnostic system distinguishes between type B and type C misfires by counting number of engine misfires per 1000 revolutions. If type B and type C misfires occur continuously for two cycles, engine control unit will store a fault code. and Check Engine light comes on. An engine with the correct air-fuel ratio, sufficient ignition, and good mechanical condition will not misfire, and if there is a problem in any of these areas, combustion will stop prematurely, resulting in a misfire.
(3) Engine Misfire Overhaul
In process of correcting misfires, special attention should be paid to 3 points, namely, cylinder pressure, ignition and fuel injection.
① Cylinder pressure: It can be easily measured with a cylinder pressure gauge, so I will not go into details, but please note that change in valve spring stiffness and camshaft wear are difficult to detect when measuring cylinder pressure. Also check if intake air volume is sufficient (air leakage or carbon deposits on valve).
②Ignition: Sometimes when troubleshooting a misfire, a problem cannot be detected by just reading engine data stream, and an oscilloscope is required to make further decisions. Factors to consider when igniting include timing of ignition, whether spark plug is operating normally, whether resistance value of high voltage line is within standard range, and whether ignition coil (power line and signal line) is operating normally. connected in a virtual connection) and whether engine control unit is operating normally (including signals provided by CAN data). It is recommended to use instrumental detection as much as possible (Fig. 1). For example, an oscilloscope can be used to check timing of camshaft sensor and crankshaft position sensor, which can analyze presence of misfires and ignition accuracy. valve timing and valve timing; use an oscilloscope to test ignition coil. At same time, it can analyze quality of spark plug; use an oscilloscope to check connection of data lines between control units; use a megger to detect resistance of spark plug, use a multimeter to determine resistance of high voltage line.
③ Fuel injection: first check fuel injection pulse width, ignition timing and oxygen sensor working status through data stream; timing, and then check timing between fuel injector and oxygen sensor (cleaner spray). into air intake at idle, check replacement of fuel injector and oxygen sensor) and finally check individual fuel injector waveform, analyze health of fuel injector and duration of fuel injection time (compared to standard waveform), and finally, brand must be considered gasoline, knock sensor and three-way catalytic converter.
Many service stations like to use replacement parts method to check for misfires. Although this method is not recommended by author, it can sometimes also fix problems quickly. If replacement method is used for verification, following replacement methods are recommended. For example, if cylinder 1 is misfiring, we canWe can replace cylinder 2 line with cylinder 1 and cylinder 3 fuel injector with it, and spark plug can be replaced with 4th cylinder so that we can test car after replacement until malfunction reappears. If fault becomes a cylinder 2 misfire, it's a cylinder line problem, if it's a cylinder 3 misfire, it's an injector problem, and so on. If fault is not reported, consider a mechanical or other component fault.
Actually, troubleshooting engine misfires is not difficult. There are usually many faults in ignition system, and sometimes fault points also appear in mechanical aspect or electronic engine control aspect. Below, author combines two specific failure cases to briefly explain persistence of misfire failures.
Failure Phenomenon: 2003 Toyota Dabawang Multi-Purpose Vehicle Equipped with 2AZ-FE Engine. A user reported that engine shakes at idle after starting vehicle in morning.
Inspection and analysis: Leave car at factory for inspection. After an overnight stay, engine shakes at idle when starting car next morning. Use Toyota intelligent detector to detect lack of engine trouble code storage; check engine data stream and find that 2 cylinders are misfiring. After replacing ignition coils of 2nd and 3rd cylinders, misfire message in 2nd cylinder disappeared, and misfire appeared in 3rd cylinder.
The author's analysis is that ignition coil of 2-cylinder engine does not work well in cold state. After replacement of ignition coil of 2-cylinder engine, idle vibration phenomenon disappears.
Malfunction phenomenon: 2006 Toyota Corolla sedan equipped with 1ZZ-FE engine. A user reported that engine fault light is on and engine vibrates at idle.
Inspection and analysis: use Toyota intelligent detector to detect, extract engine fault code as P0304 (detected that 4th cylinder of engine is not working), check engine data frame out of fire: engine speed 724r / min, total number of ignitions 391 times, 4-cylinder misfiring 4 times.
Basic engine check performed but no problems found. Ignition coils, spark plugs and fuel injectors were replaced but engine was still misfiring. There is a suspicion of a malfunction of engine intake and exhaust system, pressure in cylinder is measured The pressure in cylinder 1, cylinder 2 and cylinder 3 is close to 1.4 MPa, and pressure in cylinder 4 is 1.2 MPa, although this is 0.2 MPa lower than other three cylinders, but also within normal limits.
Finally, it was decided to additionally check intake and exhaust system of engine using a gas analyzer. In order to meet stringent emission regulations and achieve zero emissions of harmful gases such as HC, CO and NOx, a modern engine management system must always control actual air-fuel ratio to a theoretical air-fuel ratio of 14.7:1. (i.e. excess air ratio is always 1). It is measured that exhaust gas composition of vehicle engine at idle is: CO2 is 9.94%, O2 is 8.09%, HC is 596×10-6, CO is 0.784%, NOx is 0×10-6, and excess air coefficient 1.5.
When engine speed stabilizes at 3000 rpm, excess air ratio is close to 1, indicating that control of actual air-fuel ratio by engine management system at high speed is basically normal. The author suspects that when engine is running, air enters cylinder that has not passed through air mass meter, due to which mixture at idle becomes too lean, so that excess air ratio is greater than 1. Remove valve cover and check that valve timing is normal. After checking intake and exhaust valve clearance, I found that one of two exhaust valves of a 4-cylinder engine has no clearance and is always on camshaft, due to which valves do not close, and there is always an air leak. The engine's cylinder head was disassembled and it was found that exhaust valve seat, which was always leaking, had sagged.
Repair a sagging 4-cylinder exhaust valve seat and replace 2-inlet, 2-row, 4-valve and all 16-valve 4-cylinder head seals and repair after reinstallation. .