How does an engine detect a specific cylinder misfire?

Question

I have a cyl. #4 misfire. It has spark. Their is electrical current to the injector. If a cylinder has spark and fuel how else can the computer tell that #4 is misfiring. The O2 sensor is common to all three cylinders in the rear bank. -It is 99 GM 3.8L V6-
Any thoughts would be appreciated.

Answer 1

cylinder 4 misfire means bad plug, bad wire, bad injector, bad coil, or very low compression on that cylinder. Any decent shop can determine what is causing your misfire for about an hours worth of labor, sometimes it's easier to have it PROPERLY diagnosed instead of just guessing at it.

Answer 2

The engine detects a misfire by watching the timing of the crank sensor pulses. It senses which cylinder is misfiring by watching the cam sensor position at the time of misfire. Just because that is the only misfire code does not necessarily mean it is your only misfire. Start with the basics, if your ignition wires are original, replace them. G.M. wires dont hold up well past 100k. You say that you have power to the injector, but it also needs a ground pulse from the computer just like an ignition coil does to turn it on and off. Injectors can go bad but not that often. 95% of all misfires are usually due to bad plugs , wires, corrosion on your coil towers, intake leaks(this will occur at idle) or coolant getting into the cylinder. Do not change your O2 sensor. An oxygen sensor is in no way reactive enough to detect a misfire. It can cause multiple misfires by delivering the wrong info to the computer for fuel trim mapping but that will cause multiple misfires and other obvious running problems. An oxygen sensor will switch your fuel trim from rich to lean an average of 20 to 50 times per second. These are called cross counts The oxygen sensor is also used to check E.G.R, operation on many cars. It cannot tell if a cylinder is misfiring.

Answer 3

It amazes me that no one answered the question. When my plugs went bad, I had various misfire codes and O2 codes. The O2 codes are easy to understand, but just how does the engine KNOW that a particular cylinder missed at, say, 2500 rpm? Are the misfire codes REALLY an accurate indication of a miss?

Answer 4

The computer monitors RPMs and O2 sensor readings to determine a misfire. If a cylinder misfires, it isn't contributing to engine power. The computer knows when it ordered the plug to fire and watches for a dip in RPMs that would signify that cylinder isn't pulling its weight. A corresponding change in O2 readings then confirms the misfire.

Rick

I'm a retired ASE Master/L-1 Technician. I still keep current with the latest automotive technology. Visit my blog for cool articles and TSB's: http://free-auto-repair-advice.blogspot.com

Answer 5

Certainly a compression and leak down test would tell the condition of the #4 cyl. compression. If there is no compression it may be due to bad valves, rings, or cam lobes worn off mechanical repairs must be done. Oh by the way, you have a cast iron block.

Answer 6

Travster,that's a great question I have wondered myself.None of the answers address your question with specific information.
The computer software probably looks for a specific set of inputs to determine the bad cylinder,similar to the method used to flag a credit card account with unusual charges.

Answer 7

the computer does this through sensors it knows when a cylinder is firing and when its not,that's sort of what makes newer cars easier to work on in one way,on the other hand if you don't have the right tools to work with they can be harder to work on,but all this is diagnosed by the computer over a 100 times per second.change the plugs in it and it should be ok,good luck on it.

Answer 8

had 3and 4 misfire on a 3,8 ford. changed coil,plugs and wires. now shows only #3 misfire but only after it warms up

Answer 9

Crankshaft position sensor possibly.

Answer 10

check the plugs for fouling, if not fouled, may have bad compression, check seals...GOOD LUCK! that's an aluminum block.