I have a Subaru Impreza Turbo 2000 (1999). Its done 40K from new and is completely standard. I had started to notice a high rev misfire that occured between 4 and 5000 rpm.
Its fine in first gear but as the speed builds up I can definately feel a slight judder and misfire. The car doesn't get used very much and I have probably done 300 miles in the last year. It definately doesn't drive as good as it used to.
I decided to swap the plugs as a first fix and fitted some NGK Iridium plugs to replace the standard NGK's that were fitted. On removing the original plugs they didn't feel very tight when I undid them and one of them was slightly oily on the hex, colour of the plugs is a light biscuit brown. Anyway fitted the new plugs and torqued them to 15 ft/lbs. Car ticks over fine but if anything the misfire is now slightly worse and the acceleration feels a bit flat lower down. The car was MOT'd today and the tester said it was running a bit rich low down and may have an issue with a potential injector.
Thinking I may try leads or a coil pack next, or maybe the lambda sensor is on its way out. Thought being if the lambda flatlines it may lean it off and hence the issue.
Anyone got any other ideas??

 

Personally over the years find the Iridium plugs troublesome stick to the usual PFR7B or PFR6B if not modded at all.
Gap them a bit tighter if you wish 0.6mm.

 

99/00 cars are notorious for MAF issues...........

 

Just joined the diagnostic wires under the dash and the check engine light flashes constantly which I believe means no codes. Are there any other diagnostic tools I can use? I have a Snapon Ethos but this does not connect to the car.

 

As Alyn says the MAF's are notorious for problems on the V5&V6 cars and they often don't show up codes.

 

Does anyone sell a MAF sensor for a 99 Impreza, seem a bit thin on the ground. Is it best to go genuine or are the cheaper ones ok??

 

Genuine. Alyn at AS Performance 01914103770

 

Cleaned the MAF sensor with some contact spray and it’s made a big difference. Checked the resistance on the plug leads and got 13 kohm on the RH bank and 9 and 12 on the LH bank. Still feel a very very slight misfire but nowhere near as bad as before.

 

I found on mine it was the coil pack. Sounds like a new maf sensor maybe in order. Or keep trying to clean the old one.

 

New genuine MAF sensor fitted and the misfire has gone. Car back to normal now. Many thanks for your help. Why do the MAF sensors fail? Is it lots of low speed running.?

 

Aftermarket induction kits don't help too much vibration.

 

Completely standard car with a known history. Thinking that it may be the low speed running as it’s just moved in and out of storage mostly.

 

That's good to hear. The sensor gets dirty, very sensitive. Also when people run aftermarket filters a lot of them are oil based and clog up the sensor aswell.

 

usually just age/damp etc.

i always had a spare in the glovebox in my old MY99 classic. i only had 1 MAF failure though in 7yrs ownership and that was daily driving on std airbox.