Demystifying misfires
Offering three misfire problems for the price of one, Neil shows how experience and good process will help you get the correct diagnosis
Published: 03 May, 2023
Is it just me who thinks that when most mechanics are presented with a misfire on a petrol engine, they go straight for the ignition system as it’s the ‘most common’ system to cause the problem?
Then, depending on the arrangement, they usually replace the ignition coils, spark plugs and either strike it lucky and get a fix or are still met with a misfire fault. Only then do they start doing some digging. I recently had to deal with three different vehicles, all petrol, and all with misfires. I want to show that there is more to it than just spark plugs, coils and ignition leads.
Running issues
My place of work includes car sales. With stock running low, my boss decided to buy some more cars. This included three vehicles that were described as having “warning lamps illuminated and running issues.” Being a blind sale online you have to take a risk as you cannot view before you buy. You then have to hope the damage is not catastrophic or that it has not been around the houses with no joy and sent to auction to become someone else’s problem. The vehicles purchased with running faults were a Citroen DS3, Dacia Duster and a Nissan Qashqai, all of which had petrol engines.
After a week or two, the vehicles arrived and I set to work on diagnosing each of them. I chose the Citroen first which had the 1.6 EP6C engine fitted, simply because it was the first off the lorry. The car started and ran but had a misfire accompanied by warning lights and messages for a fault. Scanning the car for faults showed a cylinder 4 misfire code. This code doesn’t tell you why it is misfiring, only which cylinder is misfiring. We now a had direction in which to head.
For a petrol engine, the key elements required for correct running are the fuel system, ignition system and mechanical integrity, i.e. good compression. This also must all be happening at the correct time. If we have a misfire one or more of these must be off.
A good habit I have gotten into over the years with misfires is to check the mechanical condition of the engine first in order to ensure it can create compression. If all is ok, only then do I move onto the ignition/fuel side of things.
To do this I first prevented the engine from starting, which can be done in several different ways, and cranked the engine, listening for any irregularities as it was turning over. A trained ear can pick up an issue before any tools need to be used. In this instance, all sounded well but to confirm this I carried out a relative compression test using an oscilloscope. While this can’t physically measure the compression, it can compare each cylinder to the others which confirms if they are even or relative to each other. This is done by placing a current clamp around a battery cable and measuring the current draw from the starter motor in amps. In this way we can visually see the work done by the starter to crank the engine over.
A key thing to remember though is that the scope trace may look good and have even peaks throughout, but if all cylinders are low on compression the waveform will also look good. As I said, we are testing them relative to each other. A good thing to do while carrying out this test is to check the amperage at the top of the peaks and compare it to the battery AH (amp hour). A rough guide is to expect three times the AH of the battery. So, if we have a 60AH battery I would want to see a peak of around 180 amps drawn by the starter upon cranking. This way I know the compression of the engine is in the right ball park. As shown in Fig.1, I have put a thin black line across the top of the peaks where you would measure with the scope software and arrows pointing to two individual compression events to show what a good trace looks like. So, for a 4-cylinder you would count four and for a 6-cylinder, six peaks etc. You can then add in a reference point to work out which cylinder is 1 or 4 or whatever one has an issue. I will explain more about this later in the article, so read on.
Stress test
Back to the Citroen; After confirming mechanical integrity in the form of compression, I then decided to make sure we had a good functioning ignition system. As we had a fault code for cylinder 4, I decided to test that first by carrying out a stress test of the ignition coil. Upon pulling the coil from the engine I noticed it was soaked in engine oil and the rubber coating of the coil had been softened by the oil and was damaged and badly mis-shaped. The coils on this vehicle have three wires, which include a power supply, a ground and a control/trigger/turn-on signal. This vehicle uses four individual pencil coils. Upon testing the coil, it was noted there was no spark whatsoever so it appeared to be faulty. To be certain the wiring side was ok, I removed cylinder 3’s ignition coil and plugged it into the connector of cylinder 4. Having four separate coils allows me to swap their positions to prove system integrity quickly. Attempting a coil test again on cylinder 4 showed no spark and swapping the coil back to cylinder 3 and repeating the test showed the coil to fire. This meant we had proven the fault was somewhere in the supply, ground or control of the coil. Using the oscilloscope, I proceeded to test the power, ground and turn-on signal. The power and ground were good; However the turn-on signal was missing. Next, I tested the wire from the connector on cylinder 4 coil back to the engine control unit. This proved the wire to be good so the fault had to lay within the engine control unit. The evidence would suggest that the coil was damaged due to the oil leak. The damage to the rubber insulation had allowed the high voltage intended for the spark plug to be instead sent to the control unit causing internal damage. A replacement control unit was sourced, cloned and fitted along with new spark plugs, a new ignition coil and gasket for the cam cover. The vehicle then started and ran without fault.
Logging a fault
Next was the Dacia Duster which also had a 1.6 petrol engine and was logging a fault for misfires on cylinder 3. Again, it didn’t tell us why, only which cylinder. This is commonly done by the crankshaft position sensor measuring the variations in the speed of the crankshaft. Following the same process as before, I checked compression first which was good followed by ignition which also was good. However, upon testing the fuel side of things I found injector 3 was not working correctly. It still didn’t look correct when looking at its waveform using an oscilloscope. Carrying out some more checks, I found its internal resistance to be wrong compared to the 16 ohms of the other three. As with the coils on the Citroen, when you have multiple components which are the same you can use them for a known good and then use that data to test your component in question. On this injector, the resistance was close to 0 ohms, indicating it was shorted internally. A new injector was fitted, the system retested and we now had a fully functioning engine.
Cause for concern
Finally, the Nissan Qashqai was looked at which had a 1.2 GDI petrol engine. Unlike the other two vehicles, this uses direct injection. This presents a further cause for concern as these engines are becoming more and more common for misfires caused by excessive carbon build-up in the inlet ports and on the back of the inlet valves. This is due to the fuel now being injected directly into the cylinder. Previously on manifold injection the fuel would clean the valves on its way into the cylinder. The carbon build-up restricts the path for air to enter the cylinder and this is very important for GDI with its multiple different running modes. However, I will leave the subject of GDI at that as it could make its own article.
This vehicle also had a cylinder 3 misfire fault code stored. Upon carrying out my misfire process, the engine could be heard to have an inconsistent cranking speed, indicating a problem.
Carrying out a relative compression test showed we did indeed have a problem and no compression was present on a cylinder. To verify it was actually cylinder 3 I connected a second channel on the scope to the ignition coil trigger for cylinder 1. Checking technical data showed the firing order to be 1-3-4-2 so if the missing peak was the next after the firing line for cylinder 1 it would indeed pin down cylinder 3 as the cause of the misfire. As can be seen in Fig.2, the green line indicated when cylinder 1 coil fires and the next peak of compression is missing so it was indeed cylinder 3. The next question was why was there no compression on that cylinder? At this point we had two choices; We either stripped the engine down until we identified the cause or we use a pressure pulse sensor so that we could measure pressure pulses in the inlet, exhaust and crankcase to find where the cylinder pressure was escaping to. This would be much like a cylinder leak-down test only we can do it cranking the engine and without having to set the valves at specific positions and introduce compressed air to find where it is leaking. So, it is much faster. In fact, if we had multiple pulse sensors we could evaluate the entire engine in one cranking capture with an oscilloscope, but I am not so fortunate.
Conclusion
I went for the latter and connected my one and only pulse sensor to a third channel and one by one captured an inlet, exhaust and crank case pressure waveform against relative compression and the coil trigger for cylinder 1. Then using a cylinder over-lay chart, I could see what each valve should be doing at what time and compare it to my waveforms to find where the problem was. Now to be brutally honest this takes a lot of time and practice to master, and I still find myself needing some assistance. So, after reaching out to a few fellow techs for their opinion we concluded that there was an exhaust valve issue on cylinder 3. Again, this subject could fill its own article so I won’t get into the nitty gritty of how and why but we had an answer where the compression was going.
Yes, a cylinder leak-down test would have shown this, but not all running issues are obvious and some require the engine to turn to reveal themselves which a leak-down test will just not show. Now this job so far was still in-house, but if it was a retail job I could tell the customer exactly what the problem was without removing much more than an engine cover or dipstick/pipe to put my pulse sensor in to measure if required. This would allow them to decide if they wanted to continue with the repair or for them to take the vehicle away. This would not be so easy if the cylinder head was removed from the engine.
The cylinder head was then removed and one of the exhaust valves was found to be incomplete. Please refer to Fig.3. This backed up the diagnosis made before the engine was even dismantled, proving how accurate and powerful this form of testing can be. All three cars in fact prove how complex the causes of a misfire can be, and how fitting a new coil or spark plugs/leads would not have fixed any of the vehicles in this article. If you carry out logical testing and gather data which drives your next move, you will always find the cause of the fault.
- Klarius: engine maintenance and service fluids
Klarius Products' range of automotive maintenance products has grown with the addition of new engine fluids. Klarius engine flush is designed to purge sludge and carbon from the engine prior to adding new oil. Suitable for the oil systems of either petrol or diesel cars, the flush helps to stabilise cylinder compression, extends the service life of exhaust devices and reduces emissions while lowering fuel and oil consumption. Petrol System Cleaner removes varnish and gum from the fuel system. When added to a half full tank, the fluid clears EGR valves and tackles carbon deposits. Diesel System Cleaner is used in the same manner, helping to clean injectors, compression rings and valve seats. Completing the range is DPF Regeneration Aid, which is formulated to allow soot burn off inside the device at lower temperatures. This is useful for diesel vehicles that only conduct short journeys where high regeneration temperatures aren't typically reached, it keeps the DPF filter clear to prevent clogging and power loss.
www.klarius.eu
- Surely a thermostat is a thermostat?
Dayco is urging garages to make sure they look to use the highest possible quality thermostats when replacement is required.
- Engine management: Past and future
I have long accepted that nothing stands still for long in this industry. Just when you think you have a grasp of the subject something is sure to upset it. Nothing illustrates this more than powertrain diagnostics. Initially this was called fuel injection, and later became engine management. Now I’m afraid it’s even more complex.
I find myself fortunate to have been there at the beginning; Bosch l Jetronic, a 25 pin ECU with if I recall correctly, only 13 pins occupied. No serial diagnostics, no specific tools. So why was I fortunate? Consider my reflection on diagnostics back in the late 1970s and see if they are still applicable today.
Firstly, you had to understand what the system had to achieve, what components it had at its disposal, what role they played and how they interacted within that system.
The next challenge was measurement values; what to expect under a variety of conditions, and what equipment was required to access this information. This all seems so straightforward now, but in those days it was a little like Columbus sailing across the ocean. He knew it was wet, he needed a boat, he knew which way west was, despite this being blasphemy in the eyes of the Pope, and so set off without a clue as to what was out there.
Hardly a logical diagnostic process, however I was writing the rule book and did understand the meaning of the words test don’t guess. So, what’s changed that undermines these basic principles?
Acessibility
With even the most basic of vehicles now relying on a level of technology that makes accessibility almost impossible, OE manufacturers totally forbid any intrusion within the wiring loom and I am sure this explains the why design and manufacture precludes access as a high priority. However, we are brave, and have the Starship Enterprise at our disposal for our journey of discovery.
The problem is one of integration. Systems don’t function in isolation any more, and Columbus now has to map the Americas and Australia at the same time. In order to conduct an accurate assessment of a function it must be in its natural environment and be observed when functioning normally.
Complexity
This is not restricted to a physical state. It also includes software, algorithms, and predictive response, correction or adaptive action. Systems now change their mode of operation based on environmental influences, affected by a very wide range of changing influences. Cylinder select or dynamic stability comes to mind. The driver selects an option from a long list of choices, engine, transmission, and chassis. I used to say that for a function to occur it must have a command followed by response. In today’s world, the command may be a software decision followed by a constantly changing response, stratified and homogenous fuelling, infinitely changing camshaft timing and variable valve lift to name a few.
Test options
Manufacturers are driven by non-intrusive process dictated by guided diagnostics. Pre-determined test plans more often or not end with a pass or fail result, foregoing any data reveal. Is this due to a control of process and cost, or a mistrust in their techs? Actual evaluation of circuits, voltage, current or complex profile is getting ever more difficult. Attachment of gauges in order to measure pressure and flow is often restricted by sealed transit hoses or internal ducting within castings. Serial data has become so much, more powerful and trustworthy, however it does not and will not replace the functions available from an oscilloscope. Specialist mechanical tools and assembly techniques prohibit casual examination, due to cost or the ever more common single fitment parts.
Data extraction
This may lie in a multitude of directions; Physical extraction, camshaft timing, fuel quantity per stroke via the serial port or fuel pressure rise time via the scope. We are forced to monitor not just a physical value, but not how the PCM is adjusting or adapting a value. How do we know the parameters of operation when VMs are removing more and more data in favour of the pass-fail flags from a software automated test profile?
SENT
Rieve gauche, no not a walk along the Left Bank, but a completely new protocol for data and diagnostic transmission. SENT has been developed specifically for automotive applications, rather than being a black-market hooky copy from other engineering developments. SENT stands for single edge nibble transmission, and is a uni-directional out-only data line to the PCM. SENT is essentially a serial interface, used predominantly with throttle position, air mass and temperatures. The basic unit of time is the tick, with a minimum data unit nibble. 0Data transmission speeds over fast or slow channels, where bitrate can also vary: 1xtick= 3us. In essence it is very similar to a single channel can transmission, where the function includes synchronisation, calibration, CRC and checksum.
How am I to challenge the authenticity of data? For example, sensor error may come from power or ground discrepancies, range error, environment influences, calibration error or simply a genuine condition fault. Its design is of course intended to provide an autonomous diagnostic platform via the serial port, excluding any assessment by the techs.
Full circle
What does this mean for the industry? I suspect it will go full circle back to the 1970s, when part swapping was the norm for Christopher Columbus frauds.
- 888... Lucky for some
With this month’s focus in Aftermarket on cooling, I thought a look at how technology has affected one of the oldest systems of the internal combustion engine. For illustration, I have chosen the Volkswagen Auto Group’s en888 engine, built in Mexico, Hungary and China hence the 888 insignia; It is their lucky number.
Its one of Audi’s high-performance variants. Its fitted in my Seat Cupra 2ltr, producing 400bhp with stock mechanicals. So, what are the benefits of advanced cooling systems? Heat derived from combustion, transferred by conduction and convection into cooling and the environment is in effect wasted energy. Controlling and where necessary containing it improves efficiency, not forgetting reductions in emission pollution.
Efforts
They have made stringent efforts in the mechanical design of the 888 to achieve savings in efficiency. Reducing engine weight, minimising internal friction, increasing power and torque, current with fuel economy initiatives.
The cylinder block wall is reduced from 3.5mm to 3.00mm. Internal friction is reduced with smaller main bearing journals, revised timing chain design, incorporating a dual pressure lubricating system. The balance shaft has roller bearings, piston cooling jets further improve thermal stability. The jets have PCM mapped control, while extra oil cooling is provided adjacent the filter housing, close to the activation solenoid and twin oil pressure sensors.
The engine can theoretically reach Lambda 1 from cold within 20-30 seconds.
Further technical innovations include reduced oil level, reduced tension force in the auxiliary chain mechanism, down shifting achieved with variable valve lift and twin scroll direct mount turbo design.
Advances
You will now appreciate that it is no longer possible to separate mechanical design, power delivery, emissions, and all-round efficiency, treating cooling as an afterthought.
Take the cylinder block design, which possibly has the biggest advances reserved within the cylinder head and coolant control module (water pump). The exhaust manifold is housed completely within the cylinder head casting. This ensures very effective conductance of heat. The emphasis is now on increase, maintain, reduce, thanks to an advanced dual valve PCM controlled coolant control module. The module is mounted at the rear of the engine block, belt-driven with a cooling fan to keep the belt cool.
By manipulating the two rotary valves, flow and temperature can be effectively controlled within very carefully controlled limits. The rotary valves are manipulated by a PWM 1000hz motor with SENT position feedback (single edge nibble transmission), a method used by the latest air mass meters.
Heat transfer into and from the turbo is much more efficient due partly to the direct mount and integrated cooling galleries surrounding the exhaust tracts.
The piston to wall clearance has been increased, with a special coating on the piston thrust side complimenting a direct gudgeon pin to rod contact, the DLC coating removes the need for a bearing bush.
The cylinder head porting incorporates ignition sequence separation, thus ensuring preceding exhaust pulses do not impede the energy from the current. This in combination with advanced turbine design further improves torque range and downshifting. Cooling control priority is applied to the occupants, then the transmission, further reducing frictional losses.
Complexity
Although not directly related to the cooling system, a dual injection system is fitted with its main function being emission reduction. Cold start is provided with three direct injection events, followed by port injection warm up. These systems do not run in tandem. Two thirds of the load range is controlled by port injection, with full load above 4,000 rpm delivered by induction stroke direct fuel delivery.
From a practical point of view, previous low-tech tasks like replacing coolant components and bleeding now requires electronic support through the serial interface. Using the correct antifreeze is now essential if premature corrosion is to be avoided. As a warning, capillary coolant invasion within wiring looms is well known in some French and GM vehicles, as some of you will be aware.
It is also worth mentioning that Volkswagen has modified the software controlling cooling in some of their diesel vehicles as part of the emission recall programme.
Predictably due to their complexity, I can foresee cooling systems being neglected during routine servicing , so expect to see faults as these systems age in the pre-owned market.
- Schaeffler: Thermal Management Module
The job of a modern water pump is so much more than circulating coolant through the engine cooling system. Advances in technology have led to the water pump having a direct effect on CO2 by controlling coolant flow around the turbo, gearbox and intercooler, as well as the battery pack inside hybrid vehicles.
