Missing the Point

April 4, 2014 | By | Reply More

OBD II has spoiled us. When we were kids, we had to find misfires BY OURSELVES! Now drivers have a blinking Malfunction Indicator Lamp and diagnostic trouble codes telling us which cylinder is in trouble. You would think that would make our job easier, but we still need to find the cause.

Ignition might be the first thing you think of as the cause of a misfire, but it's sure not the only thing.

Ignition might be the first thing you think of as the cause of a misfire, but it’s sure not the only thing.

We all have those customers: People who come in for the smallest detail. They very often play 20 questions with you and it seems like they never lost a game in their lives. We understand. They’re giving us their-hard earned money, after all, and they want to make sure the car is going to be fixed right. You probably also have those customers who never come in until the car is on its last legs. Those are easy to diagnose because whatever is broken is so severe a simple inspection will often reveal the problem.

While oil itself will probably not cause the misfire, it does eventually soften the coils' spark plug boots. This can allow the spark to arc down the spark plug hole and be lost.

While oil itself will probably not cause the misfire, it does eventually soften the coils’ spark plug boots. This can allow the spark to arc down the spark plug hole and be lost.

The toughest troubles are the ones that are intermittent. Every so often such-and-such happens, but when you have the car it runs fine. Misfires come to mind. Sometimes the problem is glaring — a “dead” miss that never goes away. Those are usually simple to diagnose.

The other kind is a little harder. Sometimes the car misfires, and sometimes it doesn’t. We can perform the necessary diagnostic tests, but everything may look good at the moment. It helps if we can see and feel the problem. Now one of the problems is misfires itself. The cylinder is misfiring. A misfire is a cylinder that is not contributing as much power as the others. Each cylinder that fires exerts a force on the crankshaft causing it to twist. A misfiring cylinder will not exert as much force as the other cylinders and therefore the crankshaft slows down. A misfire is a misfire, we don’t know what the cause is and OBD-II may tell us which cylinder(s) it is but not why. It is our job to find the cause of the misfires. There are many possible causes of a misfire. We are going to have to perform tests on the engine to isolate the cause.

Testing? What Testing?

From an emissions standpoint, a misfire is about the most dangerous malfunction that can occur in a vehicle. Not only does it pump raw hydrocarbons out of the dead cylinder, but this extra fuel can cause the reaction inside the catalyst to become so hot that it loses its ability to do its job.

So, OBD II regulations state that the instant the management software detects a misfire, it doesn’t just switch the MIL on, it flashes it once per second. The flashing stops only when the vehicle is being operated

 

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Category: Engine Mechanical, the bimmer pub

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