knock sensor?

[GTIrl]

hi there, does anyone know how a malfunctioning knock sensor would affect the performance of an 8v? and is there a way to test it, or do you just have to switch it out and hope for the best...
the story so far..
my car is intermittently underpowered, backpopping, jerky throttle and noisy like the engine is labouring, also sounds like its fluffing at the exhaust manifold. otherwise (when it runs well), it revs smoothly and is pretty nippy. exhaust sounds loud but fine. so it must be an electrical fault with it being intermittent. but apart from changing all the sensors and everything im stumped. im totally skinted but wondering if it is worth taking it to a vw garage... eeeek!
(timing has been checked, checked again and then rolling roaded and checked again.)
any ideas would be greatfully received

[Hatchet Harry]

right best way to test it is to simply run hte car and tap the block with a hammer... this will cause the car's timing to alter and it will be noticeable... if it doesnt make any diference then its probs shagged...
and it sounds like ur ign timing maybe out still check it via the correct procedure and also check for air leaks.. thiis would cause poor running

[GTIrl]

thanks for info, i'll try the hammer. one other question tho, if the ecu doesnt get info from the knock sensor does it change the ignition timing automatically? or does a nonfuntioning knock sensor just not make any difference at all? sorry if my questions are a bit dim...

[GTIrl]

just tried the hammer trick, i hope i was doing it correctly.. just kinda tapping it on the block? but it made not a parp of difference... and i had my ears out on stalks listening for a change, but nuffin. i guess thats it not working then?

[wellfan]

I had similar intermittent poor running problem, had thought my mixture setting was wrong, however, having re-set it the problem persisited for weeks. Fortunately, I have a spare knock sensor, distributor, air flow meter and coil. I embarked on a process of replacing each of these items. The knock sensor and air flow meter change made no difference, however, on swapping the coil the car returned to its normal healthy state.

[Hatchet Harry]

you do have to give it a whack... (within reason!)

but if nothing is happening then it may be fooked! the coil could well be the cause and for the sake of a few bob it maybe worth changing it anyway...

[GTIrl]

hmm thats interesting about the coil. wish i had all spare parts lying about... i had another go with the hammer, well actually i got my boyfriend to do it cos i thought maybe i wasnt proficient enough in the art of block hammering... and he did tappety tap on it for a bit, no change, and then all of a sudden it changed and sounded like it might stall, and then went back to normal after 10 seconds or so. so now i dont know if its broken or not/ intermittently broken, or what. blinking thing! i will check out the thought on the coil though, what exactly was your bad running symptoms which turned out to be the coil wellfan?

[wellfan]

GTIrl, Looks like my problem has returned. It is either a loose wire somewhere in the vicinity of my coil or I have another broken coil. My symptoms are similar to yours, most pronounced is a misfire (sounds like the car is spitting out unburnt fuel) the car can also be slow to respond to my foot on the throttle. It is definitely an electrical problem in my situation. Might be worth checking all of your connections in the engine bay.

[Judderi]

Might be a long shot but i once had an electrical problem which turned out to be a problem with my front engine mount. As i was flooring it the engine would lift backwards pulling on the wires causing it to misfire. Changed the engine mount and the problem went away. Its worth a look as all you have to do is put y handbrake on tight then rock the car on and off the clutch in 1st gear or reverse. If the engine tries to climb out of the bay then your mounts most likely foooked.

[GTIrl]

wellfan when you blip the accelerator when at idle does it pop pop when coming back down the revs? (when its running badly that is?) and does your brain bleed out of your ears when going up a hill?!
total bummer your problem came back. i was hoping the coil idea might be my saviour. still might be i suppose. all the wires in my engine bay are arranged in a spagetti like fashion, and some had to be re-routed cos they were a bit tight. but there is nothing obviously corroded or bad looking, apart from some of the plastic coating around the knocksensor wire has melted which is why it is suspect. oh for a donor car...
i think my engine mounts are probably not great, but not problematically bad, but ill check it out again, thanks for the suggestions.
 The fault isnt as regular as that unfortunately, sometime i start it up and immediately its rubbish, other times its fine. sometimes like the other day when i was overtaking it suddenly started to work well and it was like a turbo boost!!