Sorted my problem but was a bit surprised I couldn't find more help here, so just posting this hoping it'll help someone else.


I have 97 Classic UK car with Apexi ECU so behaviour might be slightly different for Subaru ECUs. However overall symptoms will be similar.

I started having a slight misfire under part throttle, actually in 50mph average speed road works. It seemed to go away on wide open throttle to start with.

Checking plugs I found the car to be running very rich, certainly exhaust became very sooty and looking back before this I noticed my HCs were much higher than in previous MOTs

Misfire became steadily worse and more frequent.

Apexi Commander unit shows voltage read out of Lambda sensor and I noticed when spluttereing was happening the voltage seemed to be steady and high about 0.88V, rather than cycling from 0 to 0.76V as it should under normal partial throttle when warm.

My car seemed to be suffering from Lambda sensor failure, so I got a new one. This improved the car considerably but not 100%. I have noticed that the voltage cycling 0 - 0.76V is now much quicker than it was.

I found a post on another forum about the MAF. I removed mine and cleaned it with white spirit and cotton bud and could see it was quite oily, but here's the bit I didn't find on Scoobynet.

It seems MAF problems are usually due to dry joints rather than sensor failure. I removed the wireing loom plug from my sensor not knowing the lid is held on with sealant that makes it look permanently sealed (sharp new blade to cut sealant and small screwdriver or 2 will ge the lid off). The plug is soldered directly onto the sensor PCB and I broke all the connections but no matter, I soldered short wires between the plug & the board, refitted and tested the car.

Hey presto, its all fine again.

So for anyone suffereing with intermittant misfire, certainly on an older car, if you're capable of soldering give that a try, an hours job instead of a new MAF at £75+ I understand you can get dry joints between the top and bottom PCBs, thats a bit more of a fiddly job but still much cheaper than buying a new sensor.

My Lambda sensor was on its way out judging from the increase in speed in its voltage cycling, replacing this simply masked the dodgy MAF. I needed a new one but it wasnt the route of the problem.

The clue is that if the Lambda is the cause check to see if you get misfire with wide open throttle, the lambda is ignored under this condition so can not be the cause of a misfire when you floor it.

It doesnt seem possible to renovate a lambda sensor in a DIY fashion, so you just have to bite the bullet and pay the man.

 

Thanks for this, going to look into it myself. Cheers