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Strange issue with knock in certain rpm range

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Got a bit of a head scratcher guys.

(Engine is a 200cc Toyota VVTI engine, 11:1 compression non turbo)

I've found that my engine is knocking, between say 2500rpm and 3500rpm at motorway cruising sort of load.

At the region it's knocking it's currently running ~20-25 deg timing, and has done for quite some time without issues.

In fact I even advanced the timing a whole lot further than this a bit prior, without any issues.

I recently found a problem with one of my coilpacks and fixed that, and changed the plugs as well.

But it's still knocking in that region, if I let the knock control pull timing it out pulls timing right back to almost nothing.

Yet at the rpms just above and below that range, it doesnt knock with the normal amount of timing.

Any thoughts?

I'm thinking it sounds like I've probably got some physical problems with the motor.

But I cant think what it could be, or why it would only knock around that rpm range.

A partially blocked injector perhaps? Or hopefully not a broken ringland or something from a few over revs haha.

This is happening around the rpm range that VVTI starts to advance the cam, I've normally had it set to roughly 20 degrees advance at that load/rpm combo.But I've tried adjusting it up and down and doesnt seem to help... And previously it hasnt caused issue.

Maybe I should get a compression test done.

The car otherwise runs fine. I will pull plugs again after work today and see if there's any evidence of anything weird going on, maybe double check valve clearances.

Any other suggestions? I've been thinking maybe it's a bad batch of fuel, but I've refilled the tank since this problem has started from a different petrol station/company. Both with 98 octane.

Does the knock go away with less timing? Are you actually hearing it yourself or is it "reported knock"?

I can hear it myself, its definitely there.

When it pulls the timing right back to something like 1-2 degrees BTDC the knocking would go away. That was while I had an issue with one of the coils though, havent tested pulling timing right back that far again.

I'm thinking it must be something physically an issue with the engine, as previously there has been no issue with knock at identical settings.

AFRs showing a steady 14.7 and the engine seems to run well enough otherwise.

Ahhh sorted now I think.

Looks as though it might be VVTI pulley rattle.

I had 3000rpm and low load advancing the cam by only a few degrees off its base position. which I think makes it clatter around.

I've tried setting the advance to ~13 deg past base position, including the lower load areas where previously I'd had it turned off, so there was a transition point. It seems like it's helped.

Haha awesome. I know it sounds like a likely story, but I was honestly going to ask if you have ruled that kind of thing out - which is why I asked if you heard it and if timing affected it. I've had that with a car I tuned as well, sounded a LOT like but not *exactly* likely knock... maybe a bit less sharp and abrupt (that's relative) but ws definitely enough to raise alarm bells. They changed the VCT mechanism and it went away.

It's the kind of thing which is unnerving when tuning someone elses car - going... "This sounds like knock, but I reckon it's not knock. I think there's something weird going on with your VCT setup". and sending them off to spend money to fix it. Huge sigh of relief ensued when that sorted it!

Haha!

Yeah I thought that the VVTI angle could contributing to actual knock happening, but hadnt given any thought to the idea of a pulley rattle making the noise.

Which now makes a lot of sense.

However I'm relieved that it now seems a lot less likely to be an issue with big end bearings or whatever.

Thinking about it though, I've had a ticking sort of noise for a long time at idle that I've always attributed to maybe valve clerances or similar, but I could never find what was causing it.

Maybe it's been the pulley causing that too, I'll have to pull the covers off and have a listen.

Was it the pulley itself or the solenoid or oil supply to the system? They run a gauze for filtering the oil before the solenoid and this can clog.

I replaced the pulley with a brand new one and it fixed the issue.

Interestingly the previous pulley was more or less jamming at a minimum of 12 degrees advance (and has done since forever)

And then new pulley goes all the way down to zero degrees advance. (and doesnt rattle, haha)

Knocking noise is now gone with standard ignition timing.

We usually reply within 12hrs (often sooner)

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