From an AutoTap email newsletter I get
My last email promised an interesting topic for this week. I think the misfire detection feature is one of the most impressive and useful parts of the OBDII system. I'll describe how it works in a minute, first let me tell you about the experience I had with my car just about a month ago. (This is a pretty long email, if you want to skip to how-it-works, just read the last few paragraphs).
Returning from a late dinner, 25 miles from home on a very cold night, my check engine light turned on ('98 Chevy, 3.1L w/64,000 miles). At the same time the idle was rough, and power was down. The check engine light was even flashing for a little bit (not good!), then went to steady-on.
Being an engineering-type (my wife might substitute the word "geek") and an AutoTap-addict, I always have my Palm PDA with me and keep my AutoTap in the car. So - I plugged in AutoTap and read the codes. There were three misfire codes, but the most telling was the following:
P0304 Cylinder 4 misfire detected
Now that I knew the problem, I also knew my light was flashing was because driving with a misfire can wreck the catalytic converter. The right thing to do here would be to call a tow-truck. But, I didn't. We drove home very gently. I was able to use AutoTap to monitor the live misfire counts in cylinder four and was able to find some "sweet-spots" where the misfire seemed to mostly go away. I really don't know if that was effective, but it made me feel better. A couple miles into the drive, the MIL stopped blinking and turned steady-on.
The next morning I headed straight to the garage. I'm going to go through this part quickly because I still want to tell you how the misfire detection system works.
I hooked AutoTap back up (the PC version this time, I keep an old laptop in the garage for just such an occasion). I configured the screen to show real-time misfire counts on all cylinders. Cylinder four definitely showed up as the culprit. Now I need to decide if it's an ignition or fuel problem (I ruled out a compression problem on purely on the basis of thinking positive). I swapped spark plugs and plug wires and coils with a different cylinder. With each swap AutoTap still showed #4 as the problem. That confirmed it wasn't a spark problem.
The next day I paid to have the car hooked up to a professional fuel-injection cleaning system. Magically, all my problems were solved (funny how money can do that). No more misfires, and the tech tells me he sees a lot of this. The pintle-valve in the injector gets clogged with varnish from fuel deposits. As preventative maintenance, he recommended using an injector cleaner additive each time I change the oil. Sounds like good advice. I should add that when I brought in the car I told the mechanic about the ignition system tests I had done. He agreed that he didn't need to do any further diagnosis and hooked it up to the cleaning system. I suspect that saved me close to an hour of shop time.
for about three weeks. Then the problem reoccurs. AutoTap confirms it's the same cylinder. I do a little research and decide to change out the fuel injector. Those professional cleanings aren't terribly cheap and I don't want to do this once a month. I pick up a new fuel injector and o-rings for under $100.
I definitely underestimated how much work it was to change an injector on this car. There was a LOT of stuff in the way. Quite a few hours later, my new injector is in and everything's buttoned back up (no parts left, the measure of success in my garage). I start the car. Seems like its running smoothly. Check AutoTap for misfires - cylinder four is looking good. I pull up the DTC page of AutoTap and click the magic (and very satisfying) "Clear DTCs" button and the light turns off. I use my Palm a few times over the next few days to check for Pending DTC's and watch cylinder four. Success!
This was a pretty involved project on this car. A professional tech could have done it faster than me, but there's no way they could have gotten it done in less than three hours, not including any diagnostic work. I never did check what the repair charge would have been, but I suspect I saved a few hundred dollars on this one.
Now - How it works. Here's what I find amazing about the OBDII misfire detection system. Not only did it tell me about a misfire, but it pinpointed exactly which cylinder had the problem. That's a huge timesaver! How? Read on!
The computer monitors crankshaft position. It needs to know this for a number of reasons so this sensor is nothing new. What's clever is that it uses this sensor to measure how much the crankshaft accelerates every time a cylinder fires. So if a cylinder isn't pulling its weight, the computer can tell which one was supposed to be firing at that time and reports that info back through the OBDII port. That it can do this at any RPM and load impresses the heck out of me.
If the computer detects enough misfires in a cycle, it turns on the light. It even does some clever things to detect if it's just a bumpy road causing the crankshaft to accelerate erratically. There are some other types of misfire detection systems, but I understand that this method is the most common. The system is also quite sensitive. It can detect a misfire long before you'll feel it as a drivability problem. Slight misfires can cause a Pending DTC (remember that email?). If they re-occur it turns on the light. Severe misfires cause the light to flash because it could mean that unburned fuel is getting to the catalytic converter (damaging it very quickly). Any misfire means your car is polluting.
That's it for this week's (long) email. I'm getting lots of great suggestions for more topics. Next week I'll share a reader's story of how to diagnose a bad coolant temperature sensor.
Copyright 2003 B&B Electronics Manufacturing Co.