Phantom Knock?

Rob_f

Hi,

I've no more hair left to pull out so I hope someone can help.

In brief:

1995 impreza wagon (import) Std apart from decat downpipe

Knock sensor is detecting knock and eventually putting the ECU into 'Safe mode'. This takes about 10 miles from an ECU reset until the ECU has had enough.

-Knock sensor has been checked (well, replaced with 2nd hand one).
-Decat pipe was put on after problem arose (in case clogged cat was the cause)
-No overboost - pressure checked ok.
- Idles ok.

I can't hear any knock, I would guess it would be audable if it was severe enough for the ECU to go into safe mode.

Loosened the knock sensor to render it ineffective and drove around listening for knock. (I realise this is bad but i'm stumped). Drove ok, no detectable knock sounds.

One thing could be related. When revving the engine from the engine bay I can hear a metallic tap / jangle at a particular rev range (approx 3500rpm) as the engine increases revs AND again on the way down. It's only about a 100 rpm range that it happens. Almost like a loose bolt rattling on the exhaust when it reaches a certain frequency.

Oh, fuel is about 100 octane (vpower + booster)

Apologies for the long post but wanted to give as much info as poss.

Any help very welcome.

Rob

Rob_f

Just been for a long run (without knock sensor listening) and it ran great - didn't blow up, no knocking sounds that I could hear. I'm convinced there isn't serious det going on.

Does anyone know of an alternative knock sensor that is more selective wrt actual knock rather than other noises: piston slap, vibration etc?

Rob_f

Does anyone know if the ECU will actively ADVANCE timing from reset values over time if no det is fed back from the knock sensor????

saiklon

My experience from when I had a dead knock sensor was that the engine would advance timing to the max. The ECUs strategy is to keep advancing timing till it detects knock and then back off slightly.

Actually my engine would have knock at anything more than about 0.7bar of boost till I had the knock sensor replaced. although this was on very poor quality fuel.

I would suggest that you get to the bottom of why the knock sensor is detecting knock, you will have more peace of mind. Maybe something loose around the engine block?

Rob_f

Cheers Saiklon, it sounds like leaving the knock sensor 'deaf' will cause the timing to advance and eventually start pounding the pistons. I wonder if too advanced timing is as bad as knock for the engine. I don't know how much longer I can put up with 'Safe mode'.

Either way, I need to find the rattle or a better knock sensor. Hopefully as you say, it will be a loose bolt.

Butty

perhaps something like the KS3?:
http://www.phormula.co.uk/KnockAnalyserPro.aspx

An old knocklink of mine used to pick up noise from a vibrating up-pipe heatshield. This was presumably also picked up by the OEM sensor and retarded the timing.
But perhaps the best spend of money would be to get a mapper to take the car out for an hour and use the various types of kit they employ.
Nick

Rob_f

I think the ECU is too old to be mapped, a new mappable ECU is an option if I don't find the cause.

The tinkly mettalic sound is coming from the drivers side front of the block, possibly underneath so i'll get it on the ramps and have a look. It does sound its possibly comming from the chamber but the exact rev range it happens, on and off power is too weird. It's strange I dont have an engine light on with the knock sensor loose and sitting on a rubber washer.

dan17

hi rob i think i have the same problem as you,i got 95 wrx runs sweet at first but after a short drive check engine light appears then on boost hesitates all over the place,anyone no of a good place to get get a knock sensor replacement hopefully this will solve the problem,the worse thing about it is it allways happens when someone in a starlet or something similar try to blow me away and i just have to duck and hide coz my car dont want to play

Butty

Oops, looks like I finished in mid sentence.
I meant to say that mappers use various knock listening devices when mapping. Ask one to take the car out and confirm with their kit that it is real knock that is being picked up by the ECU - it should only cost an hours labour.
The CEL will only come on for the knock sensor if it is disconnected or returns an out of range signal voltage. As you have it, the ECU doesn't realise the sensor is isolated.

nick

Rob_f

Hi,

The knock sensor is definately reading knock, I have taken it in and although I gather the diagnostics are much more basic on this ECU, it is possible to tell how many degs the ECU is adjusting and at what rpm.

The engine light not being on makes sense now you mention it.

Dan17 - yes, a sheep in wolfs clothing, it's almost tempting to put a sign in the back window explaining why i'm being a pussy. Still goes round corners ok though.

Possible breakthrough... with Butty's help I checked the heatshield and it's making a terrible racket at 3000+rpm. Only held with 2 bolts, the third holding a broked piece of heatshield. If I can just get the damn thing off i'll test it.

I will post back with results later

dan17

i have the same rattle sounds as if its coming from around the cat area under the car dont sound good at all,keep me updated with how you get on with yours rob could well be the same problem

Rob_f

Took the heat shield off, reset the ECU and it's just the same. I suppose it's ruled it out and I can hear other noises better without the heat shield rattling.

I can now hear ticking / tapping sounds around the turbo area, in time with the engine but intermittent and not very loud. This could well be detonation but i'm not too sure what it sounds like. It happens when I give the throttle a little blip.

Can anyone confirm if this is likely to be det and what could be causing it. lean mixture / common faults etc.

Cheers

Rob

saiklon

If the noise you're hearing is not on load i.e. just revving the engine freely, it's not knock. Even without a knock sensor the engine will only knock at moderate boost levels e,g, 0.5bar or more.

I think you should take the advice to have a mapper find out what the noise is.

Rob_f

The noise occurs when blipping the throttle in the engine bay and is under slight load, i.e. the engine is trying to accelerating, but revving freely, i.e. not in gear, and no boost.

Trouble is because it's a classic import the ECU can't be mapped to my knowledge, all I can tell is that the knock sensor is detecting knock and the ECU is taking the appropriate action. The headphones idea would help me hear noise but not sure I could differentiate what is knock and what isn't.

The engine doesn't use excessive oil, not overboosting, new plugs, oil, downpipe, filters. The knock is either very severe or there is another noise.

Cheers for your help so far, can you think of anything, anything, that can go wrong to cause a severe knock: lambda sensor / MAF sensor / pipes leaking or ingess of air into pipes/ common ECU failure symtoms.

Or that makes a similar noise:

Injectors / wastegate contoller / valves / turbo etc.

Otherwise any reputable garages in the NW you can reccomend.

Cheers

Rob

Rob_f

It's also worth mentioning that with the knock sensor disabled (car running normally) I can hear nothing unusual while driving. Would I be able to hear severe knock while driving?? (this was really risky to find out so I hope it helps..)

jasonius

the engine cannot be under load without being in gear (ie revving it on the drive)..!. Your turbo will not be producing any boost in this scenario, check your boost gauge if you have one whilst revving engine in neutral..

Some of the biggest loadings on an engine occur in higher gears at low revs, like trying to pull up a hill in 4th/5th at >2k rpm..!

Also, when hitting peak boost in the higher gears (~3500-5000rpm) at WOT.

You need someone to have a listen with det cans whilst driving on boost..

Without det cans you'll most likely only be able to hear det when it's too late..!

__________________
Whooohoo, I'm a big member..!

fiestaboy