Hi Rossco and everyone that chimed in,
Thanks for the heads up. Sorry for being inexperienced but how should I reset the TPS and how do I measure the volts to determine if indeed 0.5 volts is being sent to the ecu at idle?
Any advice is much appreciated.
Cheers

So I’ve checked the TPS voltage. Have a look at Alternate’s great post here to see how to do it: http://www.perth-wrx.com/vb/mechanic...on-sensor.html. It’s not clear from the P1 Supplement instructions I was looking at, but I did this without the engine running (ignition on obviously), but he indicates the manual says it should be off.

With engine running, the voltage is higher and quite erratic too, which I find weird but must be linked to some duty cycle I guess?
Engine off, I had 0.52 V throttle closed, 4.12 throttle wide open. P1 Specs say 0.5V is standard for closed, with 0.2 V to 0.8 V acceptable. Wide open should be 4.0 V to 4.6 V. The return to closed gave consistently 0.52 V although I could get 0.49 to 0.54 on occasions, blipping the throttle plate open then closed by hand. Being an inquisitive person and figuring I could always put it back, I thought I’d try sliding the TPS with the engine running. It clearly affects the idle speed, but also seems to bring in the IACV valve into play and at one point I managed to bring on the CEL light and the RPM sat at 1,000. Presumably this is the ECU going into limp mode and playing it safe. I’m afraid I didn’t bother checking the code as it cleared when I moved the TPS back to mid range. I ‘judged’ what was the smoothest idle at the lowest rpm (couldn’t get under 850 rpm), switched off the engine and checked the voltage: bang on 0.5V! (It had drifted back to 0.52 after a test drive).

My take-home from this is my TPS is fine, though seeing how sensitive the whole idle system is to it, definitely something to consider as a failure point. While I was fiddling with the engine running, the idle was only changing when I moved the TPS, or very slightly just afterwards. The symptoms I had at the start of this thread seemed like at least two systems working at cross purposes with a bit of a lag. Moving the TPS here was just like opening and closing an old fashioned idle screw. I guess it would depend on exactly how the resistor fails.
Anyway, after fiddling, it drives just like before. Maybe very slightly smoother at idle, but probably just in my head.

What sort of rpm variation are you seeing bteoh, and over what sort of timescale?

PS
Don’t forget to tighten the TPS locking screws again if you do loosen them. And you’ll need a good quality long shaft Phillips screwdriver and some care or those screw heads are going to strip. The lower screw is hard to get to – I guided in the screwdriver head using an arm under the intercooler, then once seated in the screw head, took my time to get a good grip on the handle to break the screw 'seat'. “Do, or do not. There is no try!” Because once you mash the screw head it’s going to be a pig to get to it properly because space is tight. It seems the TPS pivots on the lower screw, both screws need to be just loose, not lost! Of course, if you enjoy taking the intercooler off, go for your life (and please pm me so I can get you to do mine next time)!

It never occurred to me before that you might want to get the TPS off to clean that area of the throttle body. I reckon it must seat directly onto the throttle plate spindle so I can't see it holding any crud there that could affect anything - unlike the IACV and associated air passages. But does anyone know about that?

As I said, Alternate has a great post on here about how to actually check the voltage. A dress-makers pin through the harness wire (white and red) worked a treat with alligator clips on the DVM wires. I disconnected the TPS plug and harness to get a better angle to get the pin in, then reconnected the plug. I left the pin in for test drives but it had an insulated head and the shaft was nowhere near any metal, be careful with your wiring folks! I earthed the DVM to the dimple on the top of the throttle housing – I’ve checked before that is a good earth with negligible resistance between it and the negative battery post. Once you're done, remove the pin!

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WRXGC82000,

That is an excellent post and pics. Will definitely try to sort out the voltage this weekend and hopefully get 0.5v at idle. The other post says to place the pin into the white wire but looking at your photo, it looks like it wasn't the white wire on yours?
Cheers

it could still be your Iacv, I have one here you can try and see if that fixes it.