My dad's Octavia 1.6 petrol has developed a fuelling problem (multipoint injection, same motor as Golf, Audi A3, various Seats etc)
About 10 days ago it started intermittently revving up by itself at idle to 4 - 5,000rpm, gearbox in neutral, foot OFF the pedal. It would shut off on the key, and then refuse to restart straight away.
If left for 15 mins or so it will restart and behave normally for a day or so, then repeat the above.
Only does it when warmed up. Runs fine when cold.
No check engine light, and no fault codes on a Skoda dealer's diagnostics nor an independent garage's.
The independent had a look at the throttle body and said there was some muck in there, but not much. He sprayed some cleaner in and wiped some gunge out, but because he couldn't see any fault codes he could only suggest driving it for a bit and see if a code came up.
I'm thinking it's possibly: a) Idle control valve, b) throttle butterfly position sensor, c) MAF d) Lambda sensor or e) a dodgy connector on any of the above :-/
Any other suggestions / recommendations please?
Edited by Pugugly on 18/09/2008 at 19:38
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Answering my own question :)
It's the throttle potentiometer which is at fault, it's actually twitching the throttle cable as it runs, it can be clearly seen under the bonnet.
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craig
What's the engine code? Beware jumping to conclusions - that may be a [mega-bucks] motorized throttle body, even if it still has a throttle cable as well. [It's a Skoda....]
Stick a scope on the coolant temp sensor and watch for spikes when it does it.
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Thanks Screwloose, it's an AKL engine code.
The rpm at idle pulses and hunts between 1400 and about 1700 rpm. If you raise the rpm a little by moving the cable pulley on the TB, you can actually feel the pulley tugging in your hand in time with the rpm rise and fall.
Now it's funny you mention coolant. The overflow back into the coolant header tank (the sensor is teed onto this pipe) is constantly dribbling hot coolant back into the tank. No signs of head gasket failure (coolant tank doesn't boil over, coolant is clear, no oil traces in it etc.
Would a dodgy coolant temp sensor cause the rpm "hunting?"
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craig
I'm struggling to write this, as my PC is totally HJ-ed by those blasted videos.
You have a motorized throttle body - and a cable. Clean it spotless; erase the learned values and re-adapt the idle settings. [Good scan-tool needed.]
If that doesn't fix it; check the ECT sensor's voltage stability - it certainly can affect the idle if the ECU thinks the engine is at -40 o C for half a second.
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Thanks again Screwloose -- I will pass all this on and let you know what transpires.
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update -- coolant temp sensor replaced (£28 plus a bit of labour) and car running sweetly.
Thanks again for your help, Screwloose.
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