I have considered this but the leak was very minor if it was indeed leaking. Also, it was lasted tuned in 2014. If the intake was melting that long ago, i'd have expected it to be worse by now - but I am not very familiar with WRXs and the characteristics of how individual parts fail.

Go with getting the MAF sensor tested using a digital multimeter. I would start there, also when you took the manifold off what condition was the gaskets/seals? It could be the gaskets are leaking?

Yeah, am hoping to check out the MAF this weekend.

The gaskets were reused but were fine and showed no signs of leaking when I pressurised the manifold

Ok. I finally got around to checking a couple of things today.

Measured at idle (825rpm)...
Voltage at the MAF:
1.96V

Voltage at ECU post Unichip:
1.85V

So the Unichip does indeed appear to be doing it's job (car has yellow STI injectors so Unichip is reducing injector pulse width by pulling down MAF readings).

BUT.. isn't 1.96V a bit high for IDLE revs?!? Is this conclusive evidence that the MAF is playing up by reading high and thus ECU is increasing fuelling.. or can someone else think of a better theory?

MAF voltage climbed steadily as I raised revs.

I wish I had an Evoscan so I could be recording this while driving and see if the MAF peaks at 5V considerably earlier than it should. It tempts to me wire in an Arduino and program it to data-log.

What voltages do other people's MY98 MAFs read at idle? Anybody in SE QLD have a spare MAF I could test with?

Thanks for your help everyone

A quick Google search told me that idle voltage is around 1.20 to 1.26v for a WRX MAF, so you might be onto something.

I tell you the one I always forget when I do anything intake related - purge can vacuum line. The one at the bottom of the SFB. That being said, you would be running lean not rich if that was unplugged.

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My findings were similar (up to 1.4V being the highest) and I compared with data logs from my other 3 cars and they were all between 0.7V (70mm MAF) and 1.3V (little NA engine so has a tiny MAF) at idle.

Besides, 2V is 40% of the total air mass it can read! At idle?!? That's gotta be way off.. Except if an EGR wasn't recirculating when meant to, I can't think of anything that would actually make the intake legitimately suck more air at idle than normal.. ADM MY98 WRXs don't even have a traditional EGR system, do they?

Will disassemble the MAF sensor to inspect solder joints etc. and make sure it's got a good ground connection and, if that doesn't help, I'll buy another.

Haha! I can understand forgetting that one! Not the trouble in my case but thanks for the suggestion

Ok, so I've found that the MAF's ground is not so great. I've measured a consistent 0.8V on it which is thus telling the ECU that there's more air than there really is entering the engine because the MAF uses it's GND as a 0V reference i.e. MAF sends 1.2V, ECU sees 1.2V+0.8V = 2.0V

Sure, I can just give it a new ground, but are there common grounding points that often fail/corrode on the GC8s? I couldn't see any obvious ones that I would have disturbed when I took the inlet manifold off but I'm all ears! I think I saw a wiring diagram on this forum so I'll have a look at that for ground points tonight

Everything is fixed!

I had previously checked all of the connections but upon finding the bad ground on the MAF, I rechecked the two main connectors near the battery that join the inlet manifold loom to the engine bay loom. When I actually pulled one of the connectors apart and examined the pins on the male plug, I noticed that two of the pins were not as long as the others. I used pliers to pull the pins out further and clipped them back into their normal place. Plugged the male and female connectors back together and went for a drive to find everything is back to normal now.

Glad it was a simple fix, but having an SSMI reader would have greatly assisted me in making the discovery much sooner. I've ordered an FTDI TTL converter and will be making my own cable shortly.

Have promptly done an oil change too to get rid of the old fuel-diluted oil.

Thanks everyone