Knock Sensors and shifter clunking

[blue thunder]

I actually have two questions. First I got an engine code #28 left hand knock sensor and am ordering both sensors Monday and gathering needed tools. What do these sensors do and are they're symptoms in performance if one or both are defective. Also I noticed tonight by accident with the key turned on but with engine off (dash lights have to be on) when I touched the brake they're was a click/clunking noise coming from inside the shifter compartment. Is something loose? Or is this normal? If I reshifted out of park and back into park holding shifter firm the clunking stopped. As soon as I let go the noise would come back. Again noise occurs only when tapping brake. No noise when actually driving car. Any feedback appreciated.

Mike

[ItsPeteReally]

Originally Posted by blue thunder

What do these sensors do and are they're symptoms in performance if one or both are defective.

Essentially the knock sensors are little contact microphones which listen for knocking sounds. Knocking (or pinging, or pinking) is caused by violent pre-ignition of the fuel-air mixture in the cylinders which is a bad thing as it puts high shock loads on the pistons, bearings, etc. and robs the engine of power too.

One of the causes of pre-ignition (but only one of the causes) is, not surprisingly, firing the spark plug too soon by applying too much ignition timing advance for the actual conditions. These conditions include throttle opening, engine load, engine temperature, fuel octane rating, engine revs, etc. etc. etc. (there are loads of them).

The engine's ECU is pre-programmed to apply a certain amount of ignition advance, but occasionally, especially if you are running on low-octane fuel, it can apply a little too much and cause pre-ignition. The ECU 'listens' to the knock sensors and will back-off the ignition advance to stop it happening, not only that, but the ECU 'learns' not to apply that much advance in the future too.

So if you have a faulty sensor it can either:

• Think that the engine is knocking when it isn't. The ECU then retards the ignition timing more and more in a futile attempt to stop the knocking. Retarded ignition reduces engine power and fuel economy, but at least it's a relatively safe condition.

• Never discover that the engine is knocking, which reduces engine power and fuel economy and risks damaging your engine too.

While you replace your knock sensors disconnect the car battery. Whilst you are swearing and cursing and skinning your knuckles swapping the sensors out, the ECU will gradually forget the possibly incorrect ignition timings it has learnt from the faulty sensors; so that, when you re-start the car, everything will be factory-fresh.

As for the clunking noises, I'd hazard a guess that it's just the shifter interlock that you Americans have, and that all the noises are perfectly normal.

[svxfiles]

This link might help.
http://www.subaru-svx.net/photos/use...svxfiles|23592

[blue thunder]

Thanks Pete for the quick response and in depth information. I've been doing a lot of minor fixes lately. Hose replacement and other sensors. I like these inexpensive repairs compared to what could go wrong. Yeah, I will make sure I have all the tools listed before starting. Thanks a lot.


Mike

[blackbolt94red]

I want to thank Pete also, for his clear explaination, as it appears my knock sensor code is happening too. For me, its the check engine light coming on, so after deciphering the code its one or the other (can't remember right now)knock sensor. This might sound dumb but, is there two of them because "knocking" can occur in either bank of cylinders?

It appears that after blowing out the mouse nests (2), one on each sensor, the check engine light stays off.

And yes, Earl, the five garage cats got some 'splaining to do', as dezi would say.

bruce

[ItsPeteReally]

Originally Posted by blackbolt94red

This might sound dumb but, is there two of them because "knocking" can occur in either bank of cylinders?
bruce

It's not a dumb question at all. The simple answer is a straightforward yes.

Then the questions start to get harder. Does the ECU have to detect knocking in both cylinder banks before it retards the timing?

I don't know the answer to this one, but a moment's thought might lead you to believe that knocking caused by over-advanced ignition should affect both sensors equally.

So, if only one sensor detects a knock, would the engineers at Subaru have programmed the ECU to retard the ignition, or not?