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968 Throttle Switch?


PCGB Member

Since the car has been idle for a few months (and after trying an "Italian Tune-up")

I have a problem I believe may be down to my throttle switch - Idle is high (around 1400rpm) - squeeze the tube or disconnect the idle stabilisation valve, and the revs drop, So I believe the ISV is working. As I also had a problem with the heater, I thought vacuum leak, but get very measurable vacuum, and the heater problem was fixed by changing the heater fuse which had blown.

Disconnect the Throttle switch, and the revs drop back to normal (around 950rpm) so I suspect that is the culprit. My first question: Should I be able to hear it click when it is in the idle position (as per a 944 S2), second question - is it adjustable at all - it doesn't appear to have the adjustment slots of the 944 S2 one, so my guess is not. Third question - there appear to be cheap, aftermarket ones available, does anyone have any experience of their reliability?

Many thanks,


i have a spare I can post you to test with, but I think your is ISV jammed one way, you should feel it vibrate when your at idle, the isv is controlled by the ECU, you could be getting a bad signal from the blue temp sender or the MAF
i would try unplugging the blue temp sensor and cleaning all contacts on the engine sensors

The switch is not adjustable and does not click like an S2

and yes there are other options, use the Bosch number off the device, or alternatively look on there is an alternative parts list

Many thanks - I hadn't, for whatever reason thought of the ISV failing that way - good point! My first thought was take it off and clean it, as this has only started since the car has been idle however long. Blue temp sensor I have had issues with before, and somewhere I have teh resistance figures, so can test that one. I also have access to a JDS Spanner, which is saying it is fault free as far as what it can detect goes. However... at the moment I have a more serious worry - I thought I had a heat-shield rattle, so hadn't worried too much about it, but now I have had a chance to try and track it down, it sounds worryingly like it is coming from the Variocam unit, at between 1500 and 2000 rpm... I haven't had the cam cover off for about 40k miles - so long enough for things to become worn I guess... Doesn't happen to be a common problem with a simple solution does it?!
I would doubt very much if it’s the vario cam unit, and they are nearly 2k to buy new

i would be looking at the heat shield above the oil filter housing, and the one over the oil level sensor, mine was in two bits

the ISV can get jammed and there is an O ring inside that falls to bits, if you really get stuck you can buy one off a Hyundai Lantra 2.0 they are about £30 on the bay of fleas, instead of the £300 that Porsche want
Yeah I checked the price of the Variocam unit with the OPC, as I was thinking if not too expensive it would save time or leaving the car too long waiting for parts... I was quoted £2.5k. My first thought was the heatshield, and that is why I have initially not worried too much about it. This thread made me think again Variocam, as it does describe the noise very much as I have: The engine came from a Tiptronic, so I have removed the heatshield above the exhaust manifold - that was cracked, but pretty sure the front part is intact - I will have another look, poke it with a screwdriver etc whilst revving it through the range at which it makes the noise, but it does sound more central to the other side of the engine, and midway along the length.

I am thinking the ISV isn't jammed, or disconnecting the throttle switch or ISV wouldn't make a difference would it? For £30 though I don't mind swapping it out to prove it (I must admit I was expecting to be able to remove it and operate it and see whether it was working or not) The throttle switch I should be able to measure the resistance of I guess before I replace a perfectly good component!

Many thanks again for your help here.

The thing with these cars is there is no idle control other than the ISV

its all managed by the ECU, the rattle of the variocam is slightly worrying, but the part is so expensive and very rare second hand

also the post does not say if he bought a new one and that fixed his problem

mine has done 160k and is fine, and a friends has done 220k and is also fine

i do run my cars on thicker oil than required currently 20/50, I think these thin synthetic oils that were not around back in the 80 when these engines were designed are not good for them

i have one noisy tappet that comes and goes, far to much hassle to replace

but I did replace the check valve and spring, ball, seat and o ring a while ago, to try and shut it up but it had no effect
Well thank you for causing me to question myself and go and check the heat shield again - it all feels rock solid, but if I press an extension bar on it through the exhaust manifold I can get the rattle to go away. So glad I double checked before pulling the cam cover off! I did check the cam chain teeth and slipper 32K miles ago according to my records, so it will be due a check again soon, but a few K miles yet hopefully!

I cannot feel any vibration of the ISV when the car is idling (but that could be my insensitive fingers.), however, disconnecting and reconnecting the Throttle switch has a direct effect on idle speed - the speed is steady in both states, but too high with the throttle switch connected. Time to trawl the net and see if I can find some resistance values I guess.

I agree on the oil.. I use 10/40 semi synthetic - Synta in fact... Bombers engine has done at least 250-odd thousand miles on it (don't know what was used before my ownership - now on 393k miles in total on that engine), and oil pressure is still good. Water ways have nigh-on eroded through in places, but otherwise it is still good. Running on LPG is an advantage though - it is always a beautiful golden-brown colour.

Replacing that heat shield is one of the hardest jobs with the engine in the car, it’s held on by the top two bolts for the oil filter housing, loosening those normally causes an oil leak

i replaced the gasket and o rings on my filter housing in the car, took me nearly a whole day for 4 bolts

the heat shield is £200 ish

unplugging the ICV or TPS changes the info the ECU is getting, might be worth cleaning the MAF, use only MAF cleaner, a new MAF is 1k
Yes, I have played that game with the heat shield before... Still happier doing that than finding broken stuff under the cam cover though!

I didn't even know there was such a thing as MAF cleaner! I'll have to look into that.

I did look at the resistance from the TPS, but cannot find information on what the values should be. I also only have a digital multimeter - an analogue one would have shown the sweep over the track and if there are any dodgy bits... Not the first time on the car I have had the desire for an old-school needle rather than digital display... I may have to invest!


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