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

iTrader: (2)

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

iTrader: (2)

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 *****. 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

iTrader: (1)

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..!

FB Tuning

iTrader: (30)

Whereabouts are you based? - I have a set of det cans, and can listen to your motor if you like.

Rob_f

Hi,

The hesitation / timing retardation does initially show itself when going up a hill in higher gears or overtaking in higher gears (the worst time). From then on it's safe mode. This certainly would point to real det as opposed to my phantom det theory. Plus it not being audable until its too late when driving is worrying - i'll re-torque the sensor - thanks Janonius

Fiestaboy - I appreciate the offer of listening with the det cans, I am in the NW - Nr Preston. I can try and make it to you if you're not too far.

Cheers,

Rob

Rob_f

I'm going to change the MAF, i'm optomistic this is the problem.

Will post results.

Rob

jasonius

iTrader: (1)

You can try the Maf , but inevitably you'll be chasing ghosts and that can cost a lot on these cars..!

From what you've said it sounds quite ominous, so I'd get it to someone who knows what they're looking for ASAP..

The offer from FB is exactly what this site should be all about..

Keep us updated..
HTH, J

AshMurc

with respect, not sure i agree... i think the MAF is the most logical, simplest possibility for the symptoms described and for a car of that age (i.e. could be original MAF). I've been speaking to Rob and his symptoms seemed very similar to mine. MAF sensors are bloody pricey so i got on ebay and the problem seems to be sorted... a new one is obviously a better option and to be honest even if it didn't solve it, you will probably need it one day assumeing you keep the car for a while.
I would always look toward componets that can lead to a lean mix, and then concider other symptoms (or the lack of certain symptoms!) such as a high idle speed which might rule out problems with things like fuel delivery, etc, etc. But yes, it is trial and error!

Ash

Rob_f

Hi,

I have already taken it to a professional who only does Subarus. I have a new exhaust and knock sensor to show for it. The guy reasoned that a decat pipe is a good upgrade and a clogged cat is a likely candidate for the knock so try that first, that failing, the knock sensor replacement has confirmed it is knock and not something else (det cans would have been better for this perhaps).

Now I know there is knock I can see several things that can cause it. It is not overboosting, plugs changed, not using excessive oil, so that leaves the MAF, ECU, or some mechanical problem... or something else. MAF is the cheapest of these so i'll try that next I think.

If it is possible to listen to the knock and from that tell whats causing it then that would be my next move.

I'm hopefull about the MAF but not entirely convinced. Got to try something.

MAF hopefully arriving this week so i'll let you know.

Thanks again for all your comments.

Rob

Rob_f

It's not the MAF.

It was expensive to find out but I needed to know for sure.

I am now going to check the voltage on the Lambda sensor to confirm the engine is running lean and then look for leaks in the low pressure air pipes.

If anyone knows the colour of the sensor wire coming out the lambda then that would be helpful (3 wires, 2 are heating wires presumably).

I figure that if the lambda sensor was faulty then it would deliver a lower voltage to the ECU and cause a rich mixture - which wouldn't cause the knock i'm getting.

If the lambda reads lean (beyond the ECU capability to compensate) then it's either too much air or too little fuel, either an air leak or the injector. The low pressure air systm looks complicated though, so links to diagrams etc are very welcome.

Forgive me for the long thread but hopefully it may help others with similar symtoms diagnose the problem before they start spending unnecessarily.

Cheers

Rob

jaytc2003

iTrader: (1)

just read this, mafs on that year and earlier are actually quite reliable units, so I wouldnt have bothered with that. In my opinion if a maf was faulty then its faulty throughout the whole rev range not just at a certain point.

Have you actually done an ecu check by connecting the wires under the dash to see what code comes up when it goes into safe mode?

Rob_f

The code check gives the all clear and the engine light doesn't come on during driving.

Somethings happening that the ECU does not see as a problem (apart from the det which it is all too aware of)

Any ideas? I am new to this so if i'm way out with my current thought processes please post.

Rob

jaytc2003

iTrader: (1)

sorry, I thought on the post it mentioned the check engine light came on.

Okay, your saying the car is going into safe mode with no engine check light?
Is the car cutting out at around 4 - 4.5k rpm? or is it just running badly at that point but will rev higher?

saiklon

Idea was given to you way back. Use det cans and an experienced person to listen to your engine and tell you what it is that the knock sensor is hearing.

Rob_f

Hi,

The car will not cut out but will hesitate initially after an ECU reset while accelerating until the ECU learns to hesitate constantly.

Yes, no engine check light. Earlier in the post I loosened the knock sensor to check this was causing the ECU to react. It was, the engine check light would come on if the knock sensor was loosened too much therefore I know the knock sensor is working, and the engine check light, and that the poor running is because the knock sensor is detecting knock.

Would det cans tell me the cause of the knock? If they just allow someone to confirm there is knock and how severe it is then it's not worth it as I already know I have knock.

I am still learning but I am thinking the causes of knock are: too much pressure in cylinders, too lean a mixture, oil getting in the cylinders, spark plugs too hot.

So I am going to check the mixture.

If I am wrong regarding the det cans let me know.

Cheers

Rob

jaytc2003

iTrader: (1)

knock can also be cause by worn big ends as well.

The thing is with the earlier ecus which you will have (assuming std), when the car detects knock it will retard the timing. The early ecu will not re adjust the timing quickly (if at all) It is normal for a car to hesitate slightly after an ecu reset as the engine is learning its best running parameters.

Have you checked the heatshields on the headers to see if they are loose?

If you car is hesitating, maybe check your coil packs as well as that year of car now they are coming upto 13 yrs old (assuming original) and they have probably began to break down.

Scoobytechnix are not to far away from preston so it may be an idea to pay them a visit

PeeVee

iTrader: (1)

Rob,

Are you sure you are getting knock?

As you have said, if it's that bad that you have no IGN advance then it should be loud enough to hear!

I have driven cars with det that's audible & it is soon sorted by the ECU with little effect on power.

You say above that the ECU goes into safe mode, but later in the thread you indicate that the check engine light is not lit.

If you were in safe mode the lamp would be lit & you would have restricted RPM and only 0.5 bar boost (half the norm)

I have a feeling that you are chasing the wrong thing & you have different problem somewhere. Misfire - Coils/Plugs Maybe... Incorrect Fueling - Coolant Temp Sensor ???

Phil