Classic Impreza flat spots

[Lefty 200 Drams]

I've already asked a few questions about classic Imprezas, please bear with me as I learn a bit more about them!

Have a '99 UK Turbo which was running fine. Didn't drive it over the weekend and she seems to have developed a nasty fuelling problem in the 2 days of inactivity. Some really noticable flat spots have appeared - around 3500rpm and again at 5000rpm.

I'm thinking it's maybe just the plugs but are there any other obivous candidates for causing this problem?

It was pulling hard and smoothly on Friday night!

[TVR Tommy]

It could be the MAF on its way out, £70 for a replacement. They're a known weak spot on the later classics. If you have a aftermarket cone filter or an oiled type they are more likely to go wrong. If it's that that’s going wrong it'll be running lean, which is not good on an Impreza.

Symptoms of MAF fault

Sometimes you'll get a slight hunting Idle
Car feels a little faster as it's running lean
Flat spots

[Lefty 200 Drams]

Hmmm, thanks. It does have an IRG air filter (was on the car when I got it, just last week).

[WhoseGeneration]

Yes, a possibility, MAF sensor is considered a consumable on this version, so often folks change them at 50K miles.
It is rumoured that the Subaru replacement is improved and is identified by a green dot on top face.
I replaced mine as a precaution and it had the green dot.

[WreckedGecko]

Beaten to it! I would vote for a MAF sensor all gunked up. sounds almost exaxctly the same as mine. Although I remeber it being a little bit more expensive. Around £100, of course this was a few years ago.

[TVR Tommy]

I guess you mean an ITG filter. Well that's one of the ones with a green thick oil, which destroys MAFS. All a MAF is, is a wire which is hot. The more air that rushes past the cooler it gets. When it's contaminated with oil it doesn't give the right readings to the ECU, so the car will run strangely. When you replace the MAF put a paper filter back in as the benefits of a ITG aren't worth destroying your MAF over. It's gets very expensive when it's been run lean as I'll burn a hole in a piston. I wouldn’t give it any WOT until you get a new MAF installed.

It's easy to replace so DIY friendly. You'll also need a security type star bit (has a hole in the middle).

Edited by TVR Tommy on Monday 17th May 12:16

[Lefty 200 Drams]

I've looked through the big wedge of invoices the previous owner had and the MAF sensor doesn't seem to have been replaced before. I'm leaving the car with a capable motorsports garage for a week tomorrow while I'm on holiday so will get them to put a bog standard filter back on.

And, yes, I meant an ITG filter

I shall also refrain from full throttle until then.

Thanks very much for the advice chaps.

[GravelBen]

Worth cleaning the MAF as well to get the crap off it - CRC make a MAF cleaner product which is just spray-on, evaporate off so avoids touching fragile sensors to clean them.

[rb5er]

Would it not throw up an engine management light?