What can cause knock values to appear?

[nikp]

I have had a decent map put on my car - one that gets no higher than 20 knock on the same setup as my car, bar the intercooler.

However, on my car i am getting very odd readings on the commander. Went out for a 20 minute each way journey. On the way out, the commander hit 75 knock :2eek at zero bar boost under partial throttle, then a bit later it got 45 knock under hard throttle (0.45 bar boost). Then on the way home, the knock got no higher than 30 at any time! So, it's a bit wierd!!

Any thoughts?

[Red and Single]

As far as I understand it, the knock sensors can often go faulty. I think max once said that about half the ones he came accross were no good.

I can get daft reagings like 100 + off throttle :rollin
I figure my map is safe, and unless I get consistant warnings I just ignore the thing :2eek

[Jit]

well the knock sensor is just a microphone so any sort of transmision noise or something rattling around in the bay could trigger knock readings...since the pfc doesnt use knock sensor feedback, it shouldnt really mattr overly

[RAW]

i have random, often high readings too. as you say, Max said it was probably a faulty sensor as it is tuned safely. nothing to worry about!

[nikp]

ah - cool.


since the pfc doesnt use knock sensor feedback, it shouldnt really mattr overly


It shouldn't matter as long as it isn't knock that is!!

Maybe i should have a check out fo the knock sensor, see what condition that is in. Greg (Royston1xx) told me that a sure sign of a dying knock sensor was black gunk coming out of the knock sensor

[Jit]

well yeah as long as its not knock!!! although if it was knock proper, i think your car would be on the back of a tow truck by now?!
My engine was suffering pre-ignition due to a chipped apex seal...this showed up as knock on the pfc but the car also ran like a piece of **** (bucking hesitation etc)....

[nikp]

Yeah, as far as running goes, it's sweet as! Idle is a bit of a pig, currently runs at about 1400, but does fluctuate by +/- 50 or so, but the boost gauge needle is rock steady. Boosts well, drives fine off boost (Really, lovely and smooth in fact). It just couldn't be better. I'm only boosting it to 0.5 bar, and it still feels quick!

[Jit]

:( stoppit.... :( I havent felt that feeling for 4 months now...and it wont be for another month atleast :(

[nikp]

Yeah, but i'm still concerned about the knock values! Think i might have a look at the knock sensor tonight? Does anyone know how much they are new?

[Ricco_04]

I have managed to see in excess of 160 on mine. Been told that the sensor is fine & is picking up other noises.

knock over 120 and the engine would pretty much instantly die so if it hasn't then its something else dodgy!

[ianelvar]

Surely you shouldn't be ignoring knock values, and the safest assumption to make is that there IS knock occuring, rather than assuming that the sensor is faulty, or it's picking up some noise? Especially if the map has not been specifically setup for your car, even though it's from a similar setup. I've done a bit with megasquirts, and one thing they say never to do is use a map from another engine and assume it'll work well. May be a good starting point, but who can say if your fuel pressures are the exactly the same, your base timing is the same, or your injectors are flowing the same etc....? I know you're trying to run the motor in before you can get he mapped, though. Bit of a tricky situation.

Knock sensors are tuned to the kind of frequency that knock produces secifically to minimise false readings. There is also some conditioning that needs to happen to the signal, so I guess it's slightly depenant upon that as well.
I'm not saying it isn't a false reading. But is there any point in not aiming for the safe side of things?

You could try adding a little fuel, and retarding the timng on the part of the map you're having issues with. The areas you've mentioned are ones that could potentially cause problems. Making it a little richer and retarding a degree or two can't really hurt, and the knock sensor should soon tell you if it's made a difference.

Good luck, either way :cheers

[nikp]

You could try adding a little fuel, and retarding the timng on the part of the map you're having issues with


We've already done that in fact. Greg didn't want to add any more fuel for it could just make things worse. Hopefully going to go down to RX motors on sat to have a 'health' check.


and the knock sensor should soon tell you if it's made a difference.


Yes - if the knock sensor is working well, and it's not picking up the right noise from some place else of course! :p

[ianelvar]

Sounds like it's under control. Hope all goes well :cool

[nikp]

Cheers.

Could the knock sensor be affected by bad grounding? I have checked the electrical wiring diagrams, and the sensor is earthed to the block - but there appears to be additional grounding for it to. And there are two wires coming off the sensor. So, is it possible that a grounding issue may be at fault here?

[ianelvar]

Sounds a bit weird with the wires.... Could be a bad earth or a dodgy sensor, seeing as it seems to have been fiddled with. Is it definately an RX7 sensor?

[andysmee]

Another cheap and cheerful way of testing the sensor is to buy a bottle of octane booster and apply it to half a tank of fuel. You should see a resulting reduction in knocking - if so, the sensor is probably working...

[nikp]

I need to clarify the 'additional grounding' comment i made: What i meant was, on the electrical diagram, Mazda seem to have additional grounding along with the ground to the engine. It's all standard stuff.

Which is why it brought me onto that question - could it be a grounding issue - but even if it is, i have no other issues anywhere else! But i was missing the ground wire that goes from the upper inlet manifold to the body connection - i have put that back on now.

[nikp]

Here's a cut out of the section with the knock sensor

[nikp]

Had a check around last night - probably didn't help that the trailing plug on the rear rotor didn't have a firm enough contact with the HT lead, and even worse was actually loose in the hole. I corrected both issue...and for a good amount of time though i had sussed it - didn't get any higher than 24 knock, even under boost. Then it hit 68! :reallymad

So, back to the drawing board. I'm starting to wonder whether it might be the secondary injectors. They're second hand 1200s, and the wiring to the front one is a bit iffy. I know, i know - sort the wiring and get better injectors! But they cost soooooo much! :p

[nikp]

Ok, i have an update for anyone who may, or may not be interested.

Firstly, i have found out that the knock sensor wiring is shielded - presumably to minimise interference from other electrical devices. Now, i have located the wiring for the knock sensor, and it's in bad shape (The wiring on the loom). There are 2 'knicks' in the plastic shroud of the wire, and about 2 inches of the wire are not covered in the shield (Similar to coaxial cable).

Here's a pick of it. The slash represents where the shielding ends, and the 2 circles show where the 2 knicks are (One knick isn't even a knick - it's the plastic shroud missing as it has broken). What options do i have!?

(P.S. The pic is a bit rubbish - the camera focussed on the wrong part!!)