Vx - Does knock detection cause spark misfire?

[antus]

Depending on what you are looking at for the knock, its possible its normal. I am not close familiar with VX but I know it has a high and low octane spark table and uses knock detection to figure out where it needs to be between the two. It'll gradually push the limit to increase spark to get the advantage if you fill up with higher octane fuel, and it'll pull back if it sees knock until it goes away, or it hits the minimum in the low octane spark table. It could be this function happening normally, and it could be real knock and the pcm doing the right thing depending on the quality of fuel. Unless you know for sure you have a problem with the false knock detection, I would not touch that part of it. One day when your tank is low it might be worth putting some premium 98 in it and see if it makes a difference.

Having said that noise in an old engine can trigger false knock. But the knock system in these PCMs is tuned to the frequency of expected knock in that engine, so I do not think transmission vibration would likely cause it as that'd be a very different frequency sound, though it is not impossible.

[JohnDee68]

Thanks antus,

I had forgotten about the knock detection being used for fuel/timing table adjustments. Maybe I am overthinking it, or just hoping to find a fault to resolve the issues.
Keeping in mind I don't hear or feel any knock in the engine. The knock in the log files is only relatively brief and does seem to be on acceleration. I haven't compared the speed/rpm to see if it is at gear changes. Though thinking about it, if it was the transmission causing knock detection. I would also be experiencing it when driving over a pot hole or corrugated dirt roads. Which could pretty much disable a car when driving on some SA roads.

Here is three sections of what I am seeing, is it pretty normal then and should I not worry about it?

I see that in Cruise Control, the car goes in to Open Loop mode, the knock retard is actively changing and the AR is up around the 15-17. is that normal? It seems logical to me to be the case.

Knock1.jpg (111.11 KiB) Viewed 736 times

Knock2.jpg (104.11 KiB) Viewed 736 times

Knock3.jpg (99.25 KiB) Viewed 736 times

Thanks.

[immortality]

What fuel, what engine/mods? Plugs/gap?

[JohnDee68]

What fuel, what engine/mods? Plugs/gap?

Unleaded 91.
No mods, just stock. 3.8L Vx series 2.
Plug gap as per service manual 1.5mm. Plugs recently new from Mace Engineering with a set of coils also. Leads are less than 6 months old.

[The1]

hows your engine mounts? These can cause issues with vibration and knock detection, also non oil filled ones can do it to. A lumpy Cam or noisy valvetrain can also do it.

There is settings to numb the knock aggression down a bit. Ignore time from memory.

[JohnDee68]

hows your engine mounts? These can cause issues with vibration and knock detection, also non oil filled ones can do it to. A lumpy Cam or noisy valvetrain can also do it.

There is settings to numb the knock aggression down a bit. Ignore time from memory.

Thanks The1,

Engine mounts, good idea. I will recheck the mounts again tomorrow just to be 100% sure. But I think they are still ok. They were replace new in 2017 with the motor fitment and still looked physically fine when I checked them a week or two ago. But cant hurt to recheck using some leverage on them.

I'm not aware of it having any special cams or anything other than stock parts when I rebuilt it. The only thing I could never find info on during the rebuild, was what the crank tooth sprocket was meant to align with out of the 2 or 3 positions on the sprocket. I placed it on the zero mark. which seemed to align all valves correctly when the timing chain was fitted.
The engine seems rather stable stationary, except a little noticeable self adjusting which I see in the log files as fuel/air issues. Which I also link to the smell from the exhaust. I think it is a rich smell?

I did a 6000km trip in March this year which I later found out I had some injectors which physically have a higher (about 55% more) flow rate than my originals which are now back in the car. So it was running severely rich over that whole trip which would have sooted up the whole exhaust system. So as the Cat would likely be coated in soot like the exhaust outlet is. It could be a smell I am getting that is just because the cat cant remove the other fumes it is meant to now? I have no experience personally with what it would smell like if the cat is just sooted up but functions fine as far as flow through being ok.

I think it was immortality that said a drive in lean (cruise) for a while should help to clean out any soot build up in the cat. I am yet to do such a run. I am hoping i can do it this week sometime as now the fuel logs are a lot better than they were even a month ago. I calculated around 12L/100 since the refill today. So, i still feel there is a little bit left in a tune or the O2 sensors that might bring it back in the 8 L/100km range economy again.

A thought just then about the running rich for such a long time. Maybe the pistons have also become coated in excessive carbon build-up? Maybe enough to hold excess fuel which could then cause a slight miss balance in combustion under certain conditions ? Is that a possible concept worth me trying to check out?

Thanks again,

[Charlescrown]

The carbon that builds up on a piston is caused by burning oil not a rich mixture. Also look over the engine for anthing loose like an alternator that coulld produce a knock like noise. To check the knock circuit disconnect the knock sensor and go for a drive and it should then log a code.

[JohnDee68]

The carbon that builds up on a piston is caused by burning oil not a rich mixture. Also look over the engine for anthing loose like an alternator that coulld produce a knock like noise. To check the knock circuit disconnect the knock sensor and go for a drive and it should then log a code.

Thanks Charlescrown,

Well it isn't burning any oil (other than usual amounts via the PCV system I guess), which is a good thing as it shouldn't then have carbon deposits on the pistons. I will do another slower check over the motor for lose bits today when i check the engine mounts. It is actually a very nice clean engine and in good condition compared to many, even some o the newer cars friends have.

What benefit of it logging a code for the knock sensors will that be for ?

[The1]

when it runs rich for so long it'l kill the o2's quick.

[JohnDee68]

when it runs rich for so long it'l kill the o2's quick.

Thanks,
Yeah, I have a feeling that it all comes back to a set o injectors at the cause of this whole saga of events.

I am hoping that once I can confirm the knock sensors aren't actually an issue with possibly causing phantom retardation of the timing during acceleration. Then the de-carbonating of the Cat will happen in a short time once the cars running right.

One can only hope.