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Discussion Starter · #1 ·
Hey all - newbie here. Got an annoying problem I'd appreciate some advice on:

1) Have an intermittent knock - the worst sort because it comes & goes and of course, is never there when the shop is looking for it...

Symptoms: knock occurs (intermittently) at any/all throttle applications (i.e., sometimes idle, sometimes accelerating, sometimes cruising, sometimes closed throttle) Weird thing is - no perceptible problems with APC/Turbo - performance is just as when new, no better, no worse - but accompanied with this stomach-turning knock.

Other:
-always use premium fuel, infact, when I found some 94 octane, the knocking got worse!
-replaced knock sensor
-replaced APC solenoid
-new (correct) plugs
-very recent tune-up (timing, etc. by Saab dealer)
-correct oil
-no noise from timing chain or any other unusual noises
-no disconnected vacuum lines/wires/etc...

Has anyone experienced this? Can anyone offer any advice? The intermittent nature of this - and the fact that performance is not affected is driving me nuts! Thanks in advance for advice!!
 

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Discussion Starter · #3 ·
RE:
Is the knock frequency related to engine speed or road speed?

**independent of both

Is it on straight roads or bends (if so, left or right)?

**independent of both

Is it a regular knock or an irregular rattle?

**'regular' engine knock (if it can be called regular?!?)
 

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Could it be a worn multi-drive belt / mdb tensioner?

When my tensioner wasn't performing properly it caused wear in the belt, a frayed bit of the belt caused a knock / slap sound, but because the tensioer was working with varying degress of effectiveness it was only heard intermittently.
 

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Ahhhh, Are we talking about an actual knocking noise here or the engine 'pinking'? (which is detected by the 'knock' sensor). If it's the latter my father had the same intermittent problem on a '96 2.3t the cure for which I will try to remember/ask him if that's the problem you're having?
 

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Discussion Starter · #7 ·
Hey all: Thanks for your advice so far...

Mutungi: can't differentiate left or right side of engine...the knock i am talking about is definitely engine (spark) knock...and so the sound is coming from the heart (literally as well as physically) of the engine.

Gassy: thanks for the suggestion on the tensioner...but my knock is definitely engine knock...the exact knock the that *new* knock sensor *should* be picking up!?!

Dave: would definitely love to hear what the solution was for your Dad's 2.3T...may be the same problem/cure???

Cheers-
J
 

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Whatever the "Knock" is it certainly isn't detonation, no engine will suffer detonation on the overrun or at idle. Detonation normally only occurs under heavy load conditions when combustion pressures are high but can also be a result of excess ignition advance, something we have no control over as it is set dynamically by the ECU.
Could be a fault with the ECU or one of the sensors.
 

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You said at the top of the thread that the noise wasn't related to engine speed or road speed. If that is the case, then it won't be a 'moving' part - more likely suspension related or something loose in the engine compartment.

However, as you think its engine knock it may be engine related. If it's loud, it could be mechanical bits hitting.

Stuck valve touching piston?

I've also had a top ring break up (not on a Saab) and work its way into the combustion space. The knocking was the small bits of broken ring being hammered into the piston top. Noise stopped when it was in far enough!! There was also a slightly quieter time when very small bits fluttered around before being blown out through the valve. The noise happened at random intervals and lasted up to several seconds. You could then wait a few minutes or several days for the next bit to break off. Took a while before we realised the nature of an 'intermittent' fault.
 
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