QOTW #73 – OBD reader to monitor misfire counts on a V12

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Welcome to Bamford Rose and another question of the week. This week it’s about V12 engines and misfire detection. Here’s the question from one of our own comments section on another YouTube video of ours. Asking, ‘What is a misfire type A and misfire type B, acceptable reading?’ Previously, I’ve discussed using a fault code checker to check for basic DTCP code errors. And for misfire this would be P0300 for general engine misfire. And then P0301, all the way up to 12 for cylinder specific misfire the same as this chart that I flip up now.

So, the idea is if you have the check emissions light come on and you plug your fault code reader in. And normally it’s a fault code to do with the lambda sensor or something that is not really critical to the engine running healthily. But in the odd instance where the fault code is for something real and on V12 it’s misfire. Then understanding that the fault code is for something serious like misfire is a way that you can work out whether or not you need to get someone to look at the car for the illumination of that check engine light or you can just reset and ignore it or monitor it .We all know that’s a good thing to do if you own a V12 engine car. Because we do not want the effects of misfire, which is inevitably going to be a broken-up catalyst. Because the engine will most likely ingest that debris and fail the engine.

This question is going down deeper one more level which is, if you do have a misfire there are two different types of misfire category. And with an intelligent fault code reader, what is an acceptable level of misfire counts for each category. There is what is called type A misfire and this is misfire which is deemed serious enough to fail the catalyst. Type A misfire is monitored over a 200-engine revolution cycle. And during those 200 cycles if misfire level is severe enough to damage the catalyst, then the type A misfire counter will increase.

Type B misfire is deemed non-catalyst damaging and is measured over 1000 cycles. Where again the rate of misfire in that thousand cycles is compared against a predetermined threshold. And if that threshold is exceeded, the counter for type B misfire would increase. With a handheld OBD reader the reading for type A and type B misfires, will continually change as the engine is run. Meaning if you started from cold and drove for say five minutes pulled over and then decided to plug your OBD scan tool in and then go off on a drive. Look at the type A and type B misfire rates. It’s going to update fairly real time. Meaning that initial cold drive that you did has probably been overwritten. Which means that because you weren’t monitoring real time the misfire count at that particular point in the drive, could mean that you’ve missed some misfire events.

Because the figures in type A and type B misfire will continually evolve. Means unfortunately, it’s very unreliable to periodically monitor those figures and determine whether you do or do not have misfire. We can look at this AMDs screenshot of the real-time misfire count. And we can see that cylinder number 11 is reporting some serious misfires. Yet because this was recorded over quite a long time span, even AMDs is showing no misfire counts for misfire type A. It’s also very unwise to rely on the figure from misfire type A or misfire type B. Because misfire type A whilst it has the classification of a catalyst damaging misfire. The damaging aspect of that relates to the catalyst’s efficiency to convert gas. It does not mean the catalyst would break up.

Although we all take the environment extremely seriously and we don’t want polluting engines misfiring engines putting out excess pollutants. We’re not really interested in being told that we’ve got a misfire rate, which may harm the catalyst and harm it to the extent that its gas conversion becomes less efficient. We’re really only interested in being told that a misfire is seriously enough to cause the catalyst to break up. And we want to protect against that, because the engine is most likely going to ingest the debris causing a very expensive engine rebuild.

So, because type A misfire isn’t telling us we’re at the point of catalyst breakup and because misfire type A continually revolves and resets. Means that periodic checking of misfire type A is not really that helpful to us. And misfire type B is of no real help to man nor beast. Misfire in nature being sporadic erratic, actually means that no count of misfire is acceptable. And there are only two real ways to check for that. First is to learn the misfire profile correction and the onboard diagnostic system tells you when it’s misfiring when it’s reached that type A threshold. It’ll put an emissions check engine light on and the p code will tell you what cylinder is.

The next would be a point of service with a specialist looking at the real-time misfire count under the conditions that the engine is most susceptible to misfire at. Which would be low engine speed, low to increasing load. We make very many videos about catalyst failure, because the consequences can be so catastrophic in terms of engine rebuild cost. But never ever have we had one of our own customers suffer catalyst failure. This is always a new car to us that we see to repair. This is because each annual service, there is very close attention to detail relating to misfire detection.

That AMDs misfire monitor needs running at idle and at that low speed, low to increase in low condition to try and promote a misfire. The cars that have come to us after failing the catalyst have misfired. And in some instances, there never was a check emissions check engine light come on the dashboard before the driver knew there was a problem with the engine or catalyst. That was possible because of two reasons. One would be that the car left whoever serviced it without a misfire correction learned. So, the car was never in a fit state to tell you about a misfire or most likely is this very important subtlety.

The check engine light isn’t there as a helpful friend to tell us when something is wrong with the car. It’s illuminated to inform us that something is amiss which would affect the emissions performance of the car. And here’s the subtlety. Before the misfire type A threshold is reached, remembering that this threshold has been set to determine when the catalyst might be damaged. But that damage means conversion efficiency. Before that threshold is reached, if you ran for a prolonged state in that misfiring condition actual physical catalyst damage would occur.

Automakers never want to illuminate that check engine light if they don’t have to. And that means that it’s only for legislation reasons that they will put that light on, because they are forced to. Regrettably in this instance, mechanical physical breakup of the catalyst will occur before the type A misfire light comes on to indicate that its conversion efficiency might be put at risk due to the misfire level. And it’s very easy to take a cruise down the motorway and not feel this misfire as a driver. So, let’s keep the math really simple. If you needed 120 newton meters torque to propel the car down the motorway at 70 miles an hour, that’s obviously 10 newton meters per cylinder.

And a quite badly misfiring cylinder would fluctuate between producing 5 and 10 newton meters torque. When all other cylinders are firing correctly at their 10 newton meters torque, therefore giving 110 newton meters torque. If you have one cylinder which is fluctuating between the engine making in total 115 and 120 newton meters talk, there is no way that you’re going to detect that as a driver. So unfortunately, to answer this question and drilling down into the misfire another level by looking at type A and type B misfire is not really robust. Detailed check at service with AMDs and driving the car in the areas where the engine is going to exhibit misfire is the best prevention.

The ultimate prevention is just to remove the primary catalysts and the engine could misfire forever down the motorway and there will be no onward effect. As any car ages, misfire is inevitable. It’s difficult to put a time frame on it, but I would say any V12 engine that is over 10 years old needs close attention to detail at service as to whether a misfire is present or not.  Easiest check that you can do for yourself would be periodically find a nice long gradient. Get engine speed up to about 50 miles an hour, have it in something like fourth gear. You’ll probably be doing something like 2000 RPM and just slowly, slowly increase the throttle pedal. And you’re checking for any shuttering in the chassis.

If you do that together with the car on board diagnostic system set up correctly to report misfire and should you get a check emissions light. You plug in and check it’s not P300 code, then interim each annual service that should be good enough to make sure you don’t suffer this problem.

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