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How do EECs detect cylinder misfires?

Post by cgrey8 » Thu May 23, 2024 6:27 am

I've been watching a lot of Youtube/Tiktok videos about Mechanics and how they diagnose issues with modern day vehicles. As I watch these vids, I realize just how out of date my knowledge and tooling for of modern day vehicle-diagnostics is.

But on an episode of Dave's Auto Center, he was diagnosing a 2002 V6 Taurus which intrigued me since that's far closer to the era of vehicle controls I'm more familiar with, however I suspect that vehicle is not controlled by an EEC-V. If I had to guess, it's one of the Oaks.

In this case, the problem was found to be a bad injector on the misfiring cylinder. The injector had gone open circuit and obviously wasn't firing. However the codes indicated a misfire, not an open circuit injector. This suggests the EEC was not capable of monitoring the current-flow of each injector otherwise this would've been an EASY recognition for the computer to throw a code for a specific injector.

If it isn't monitoring injector current flow, maybe it's monitoring the coil pack current flow to each coil. But with a misfiring injector, you'd still get a current flow to the coil. And being this is a coil pack, the misfiring cylinder's coil is shared with another cylinder. And the computer indicated no issue with the shared cylinder. In the video, Dave connected an oscilloscope to the coil to show its current use. He pointed out the tell-tale distinct difference in the current flow pattern between a correctly firing cylinder and one that's running lean (as would be the case with no fuel). And while we can see that characteristic on an oscilloscope, I'm not convinced EECs of this era have the sophistication to monitor and analyze current flow patterns at the speed an oscilloscope can AND recognize the very subtle differences between a normal and lean-firing cylinder. That would require resolution in the microseconds which produces a LOT of data that EECs of this era just wouldn't have the memory or processing power to actually make use of. So even if the EEC is capable of current-monitoring the coils, it isn't monitoring them to this detail.

So the question is, how is the EEC determining misfire?

One possibility I can come up with is it see's rotational velocity changes while monitoring the crankshaft position and it can see a rhythmic reduction in velocity on the misfiring cylinder's power stroke.

The other is it's seeing a rhythmic lean spike on the HEGO on the misfiring cylinder's exhaust stroke.

But again, these are just guesses. So I thought I'd pose the question here and get other people's thoughts on how this is done...preferably from those that have seen the code doing it.

The other thing that was a surprise to Dave and informative to me is when an EEC detects a misfire, it INTENTIONALLY turns off the injector for 30 sec or so, then turns it back on to try to see if the problem is resolved. Dave indicated newer EECs do this to save the CAT, but he wasn't aware of that being a control-strategy on EECs as early as 2002...but clearly that's what was happening. I'm also guessing this has the added benefit of not washing the walls with fuel that isn't being burned. In the case of a bad injector, that wouldn't matter. But I can see if the problem was an ignition problem how that might be a reasonable thing to do.

So when did all this get introduced? Did EEC-IV/V ever have this as a misfire response (turning off the injectors on a detected misfire condition)?
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Re: How do EECs detect cylinder misfires?

Post by Paulie » Thu May 23, 2024 11:23 am

cgrey8 wrote: Thu May 23, 2024 6:27 am

One possibility I can come up with is it see's rotational velocity changes while monitoring the crankshaft position and it can see a rhythmic reduction in velocity on the misfiring cylinder's power stroke.
This is correct. The EEC watches the speed of crankshaft and can determine a misfire on a cylinder. All EECs with a functional OBD2 port have this capability. This rolled out with EECV in 1994.
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Re: How do EECs detect cylinder misfires?

Post by Sattech200 » Fri Jun 07, 2024 10:39 am

Would this also be how the Power Balance function is getting its data?

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Re: How do EECs detect cylinder misfires?

Post by TobyAdam » Wed Jun 12, 2024 5:00 am

It's fascinating to see how diagnostic techniques have evolved over time! Modern EECs have significantly more sophisticated algorithms and monitoring capabilities compared to earlier models like the one in the 2002 Taurus. The rotational velocity and HEGO monitoring theories make sense, highlighting the blend of mechanical intuition and advanced electronics in vehicle diagnostics.

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