Cylinder Crank Speed case

I was working on friend's Civic while ago.
We cleaned EGR and throttle body. And when I looked over PGM FI params I noticed something...

But first, let me show you a little theory of operation.
On new Civics we have CYL Crank Speed for all cylinders. Cut from HDS help.

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This parameter shows engine speed change by combustion pressure torque of No.X cylinder.
This is used for misfire detection.

Calculation method:

1. FI ECU will calculate average engine speed (i.e.NE in fig.) and engine speed change (i.e. Δω in fig.) from measured crank plus.
   NOTE: Δω (rad/s) = 2 * 3.14 * ΔNE(rpm) / 60
2. FI ECU will select δω at the crank position when combustion torque will accelerate the rotation. (i.e. Δω_n in fig.)
3. FI ECU will calculate engine speed change about combustion torque from Δω_n. This value will be output from FI ECU to HDS.

Fig:Example of engine speed change at 2000rpm

Engine speed will be increased at combustion because combustion torque to accelerate crank rotation will be happened.Then, this parameter will be positive value if there is no issue in the vehicle.

• Acceleration by combustion toque is weak when this parameter closes to 0.
• Acceleration by combustion toque is strong when this parameter becomes large. Especially, this parameter will show large value when cranking, idling (i.e. low engine speed and unstable engine state) etc.

On the other hand, when misfire is happened, engine speed of the cylinder is lower than average because of no combustion torque. Then, this parameter will be negative value, and show -8 rad/s or near.

NOTE: minimum value of this parameter is -8 rad/s, so this parameter may continue to show -8 rad/s when misfire is happened.

How to use this parameter:

1. Please check this value whether positive or negative. If it is negative value, misfire is happened in the cylinder.
2. Please compare the value with other cylinders. If this value is pretty smaller than other cylinder's one, abnormal combustion is happened in the cylinder. Then, please diagnose possible failures of misfire.
   For example: poor injection flow, poor valve clearance, poor air flow etc
3. Please compare the value with other cylinders. If this value is pretty larger than other cylinder's one, combustion pressure is shortage.
   For example: compression leak etc

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I think that's pretty cool! We can monitor combustion in each cylinders.

Now, look here and tell me if you see something abnormal...

Can you see what I see? :-)

Ok. Check the 2nd cylinder. It contributes less than others, it's purple line below all cyls.

I think that strange. Especially, when I looked into my car.

This is how I expected it to be - all contributes the same force (combustion torq). It looks even like DNA shit. No anomalies. One thing of course is different is that my reading was done with air cond on, so you see how RPM drops on ac clutch engagement and then restores.

I will upload later same readings without AC, but trust me, it will be the same but with lower speed, about 40 rad/s.

So what we did to investigate.

We did nothing that date, we ran out of time. But later we went for valve adjustment to our common mechanic. Before that, he inspected spark plugs - they were fine, no difference between cyls.

Random pic from Internet

When he adjusted the valves, 2nd cylinder indeed was worth. We were happy.

But when we hooked up HDS...

It wasn't DNA like we wanted...

2nd still below, but now 1st goes to top.

Things got worth when we swapped coil and spark plugs with 4th cylinder.

What??? Now 3rd???

This last reading was done on not fully warmed engine (RPM ~760). But that's very strange.

I have few things in mind:

1. I wasn't there while they did valve adjustment, but he is pro and do a lot of Hondas on daily basis.

2. Maybe adjustment wasn't perfect as we see patterns changed.

3. But I can't explain why cyl#3 came with new behaviour after we didn't touched it (swapped 2 and 4).

If you have ideas - shoot.

36,400 km | 2Y 4M