Rick's Free Auto Repair Advice

P0300 P0301 P0302 P0303 P0304 P0305 P0306 P0307 P0308 P0309 P0310 P0311 P0312

Fix code P0300 P0301 P0302 P0303 P0304 P0305 P0306 P0307 P0308 P0309 P0310 P0311 P0312

Pinpointing cylinder misfire codes

People ask me about misfire codes after they’ve replaced all the spark plug wires, coil, and plugs and they still have the code. Here’s a quick explanation of how a pro would attack the problem.

First, if you’re getting a P0300 Random/Multiple Cylinder Misfire Detected code, that would indicate a universal problem that applies to all cylinders. In that case, start by checking fuel supply issues like poor fuel pressure caused by a bad pump, restricted fuel filter, or a defective fuel pressure regulator. If all those check out, look for a major vacuum leak or EGR valve leak. EGR is recalculating exhaust gas back into the intake manifold. If it isn’t operating properly and letting too much exhaust in, it will lean out the air fuel mixture to all cylinders. So check those things first. If you’re still seeing multiple misfire codes, it’s time to examine the timing chain or belt to see if the valve train is out of time with the crankshaft. That would cause poor valve sealing. Of course, in an abused engine, or one with a ton of miles, there’s always the possibility of poor cylinder compression or leaking valves. But that usually doesn’t cause misfires on all cylinders. So let’s take a look at individual cylinder misfire codes.

The thing to keep in mind about cylinder misfire codes is that the computer really doesn’t know whether the spark made it to the spark plug or not. All it knows is that a cylinder or isn’t contributing it’s fair share of power. How does the computer detect this? Easy. It looks at the crankshaft position sensor and calculates its rate of rotation. It knows when it fired the coil for a particular cylinder and it knows when it fired the fuel injector. So it looks to make sure that the crankshaft position sensor reflects that cylinder’s contribution to power. If the computer notices a SLOWDOWN in the RATE of the crankshaft rotation, it calculates backwards to see which cylinder isn’t doing it’s job.

Like I said earlier, the computer doesn’t know whether the spark actually occurred, so it’s mistake to always assume that a misfire means bad ignition. A misfire can certainly be caused by faulty ignition. But it can also be caused by poor fuel mixture—too much air or too little gas. Let’s look at the possibilities.

In a “too-lean” mixture situation, there’s too little gas present to make a good long fire. The computer could be commanding a lean mixture due to a dirty MAF sensor, a bad oxygen sensor, or a leaking fuel injector. In those cases, the computer is trying to throttle back the gas to correct for what it saw previously as a too rich mixture. Or, the lean mixture could be caused by the proper amount of gas AND a vacuum leak. So, the computer thinks it got the mixture right, but the vacuum leak screwed things up. Whatever the cause of the too-lean mixture, the result is always the same—there’s too little gas (in relation to the amount of air) and the flame goes out too soon due to lack of fuel. Yup, the computer calls that a misfire.

In a too-rich situation, the flame STILLS goes out to soon, only this time it’s flames out because there’s not oxygen to keep the fire going. Yup, that’s a misfire too.

So you have to think in broader terms than just ignition when it comes to misfires.

Ask yourself these questions:

Is the cylinder getting fuel (fuel pressure good)?
Is the injector clogged (too little gas)?
Is there a vacuum leak near that cylinder?
Is the EGR valve leaking?
Is the cylinder getting enough fuel flow (restricted fuel filter—may have pressure, but not enough volume)
Is an injector leaking (causing a rich misfire)
Is the compression good? (valves seating, timing right)

This is where a scan tool with live data can really help. By reading the “fuel trim” numbers, you can tell whether the computer is trying to compensate for a lean or rich condition. Unfortunately, the scan tools that provide that kind of information are pricey. Plan on spending at least $400 for a pro scan tool.


© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

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