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cylinder misfire sensors...?

3K views 6 replies 5 participants last post by  ShowStop 
#1 ·
Lately I've been getting a rash of p0301's, but I've run on 3 cylinders before and the truck isn't acting like it's missing at all. Is it common at all for the sensors to screw up (unlike any of the others in these things :rolleyes:) and throw codes when the cylinders are actually firing just fine?

Thanks!
 
#3 ·
There are no misfire sensors on your motor.

When the ECU charges and then fires the coil, the ECU monitors the electricity it uses to charge the coil.

Because the resistance across the plug is different when it burns the fuel in the cylinder than it is when the fuel doesn't burn, the ECU can detect misfire.

The check engine light comes on when some pre-determined number of misfires happens.

You may not be able to notice random misses on one cylinder.

It doesn't have to be a dead miss all the time to trigger the light.

Your best bet is to look at the plug, plug wires, and whatever else (I didn't notice what motor you are running) is connected to spark.

After that air leaks and clogged injectors might have something to do with it.

Let us know what you find!
 
#4 ·
In a sense there are misfire sensors. The ECM determines misfire conditions from data received from the crank and cam position sensors. It looks for miniscule fluctuations in RPM relative to the expected "power" stroke of each piston individually. If it occurs randomly on multiple cylinders a PO300 code is set. If the condition is limited to a specific cylinder/s the last digit of the code will correspond to the cylinder number/s (in the engines firing order). It has nothing to do with any aspect of firing the plugs.

That being said, a good man with an O-scope can determine relative rich/lean conditions by watching the waveform immediately following the coil firing spike known as "spark burn line".

So, with a PO301 we are talking about the #1 cylinder only. It is likely that it is in fact intermittently missing but not be enough to "feel" at all.

Start with a compression test and spark plug condition. The compression test will eliminate the really bad problems. This will leave ignition parts and fuel system things limited to that cylinder.
 
#5 ·
Update, in case anyone reads this while searching for similar issues :)rofl:)

It was a broken spark plug wire. Strangely, when the engine was rebuilt years ago, the mechanic switched wires 1 and 4 at the sending unit. So when the computer was giving me p0301's I was checking cylinder 1 for issues, when in fact it was cylinder 4 that was misfiring.

Yesterday I replaced my plug wires (with sweet blue NGK leads that make my under-hood look super "trick"). When I got to #4, only most of the lead came out of the head. The wire was broken off inside the long plastic "spout" that fits down inside the valve cover. Luckily that came out fairly easily, I connected all the wires into their correct corresponding plugs (everything's numbered, go figure) and the CEL hasn't been back since.

Interestingly, this also seems to have (fingers crossed) fixed my O2 sensor issue (behind the cat) which was throwing p0141 codes for the longest time. My only concern with this is that the O2 sensor was acting up long before I was getting misfire codes. I'm hoping they really are related and everything is jiving once again.
 
#6 ·
welp, turns out I didn't fix it by replacing that spark plug wire. This is pissing me off. It misfires a lot when it's cold, but once it's warmed up it runs fine.

All this considered so far, I wouldn't have such a problem with it, but what's throwing me is that I'm getting cylinder 1 codes regardless of having wires 1 and 4 swapped or otherwise. If it's actually cyl 1 that has the problem, shouldn't it give me p0304's if the wires are crossed?
 
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