fuel vs ignition misfires

[josh]

I read an interesting article last month that detailed how to use fuel trims and downstream o2s to determine if an intermittent misfire was the result of a lack of fuel or lack of spark. I attached a link to the article to give credit to the author. Intermittent misfire are often difficult to diagnose and I do primarily diagnostic work so this was extremely helpful. Perhaps this is common knowledge and I have been living in the dark but this was completely new information to me so after I read the article, I went out to 2 cars I had in the shop and tried to duplicate the conditions. To summarize the article, when dealing with intermittent misfires, if the misfire is fuel related like clogged or shorted injectors, low fuel pressure, mad issues, etc, the downstream o2 sensor will indicate extremely lean (low voltage) and the long term fuel trim will be very high (PCM commanding additional fuel to be added). The reason for this is there is less or no hydrocarbons coming out of the effected cylinder(s) so the catalitic converter does not use much oxygen to chemically convert the hydrocarbons into water vapor and carbon dioxide. An ignition misfire by contrast causes long term fuel trim to be lower (usually less than 15) and the downstream o2 sensor to read richer (higher voltage). This is because all the unburnt fuel dumps lots of hydrocarbons into the exhaust so the catalitic converter has to work in overdrive, using lots of oxygen to clean the hydrocarbon rich exhaust. This makes sense when I thought about it but I had to see it for myself. The first vehicle I tried this on was a 2001 Toyota tundra with a 4.7l v8. I unplugged an injector, looked at scan tool data, then reconnected it and unplugged a coil and again viewed the live data. The results were disappointing because both components being unplugged caused the PCM to go into open loop so fuel trims did not react. I tried this same test on a 1999 Toyota corolla and the vehicle behaved exactly as the article described. With an injector unplugged, total fuel trim went to 30 and my post converter o2 went very lean. With the coil unplugged, the downstream went rich but my total fuel trim only went to 18. Taken with a grain of salt, this piece of information makes diagnosing intermittent misfires easier. I think part of the reason the tundra went open loop is because it uses a coil on plug design that incorporates an ignition feedback signal so the PCM immediately knew that coil wasn’t firing.

http://m.searchautoparts.com/motorage/drivability/using-fuel-trims-and-downstream-o2s-analyze-intermittent-misfire-concerns?
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