Since the end of last summer my Subaru Impreza 2000 Turbo had the problem that it sounded like a diesel taxi when the engine was cold.

The sound was a real worry when it first started, it sounds rattly and very diesel like. Revving above 3,000 rpm made it go away and after the engine had run for five minutes it was just about gone.

Everyone that I spoke to, both online and at my Subaru dealer said the same thing. They all said it was caused by piston slap, which is apparently occurs when one or more of the pistons become slightly oval and bang on the cylinder walls until the piston expands when it warms up.

It appears this is a common issue on older Impreza engines but that it isn’t anything to worry about as long as it goes away when the engine is warm.

I did have one symptom that didn’t fit the piston slap case. Very occasionally, when using engine braking from 70mph+ down to zero, the diesel sound would reappear and go away when the engine was revved.

I had my 105,000 mile service the other week and asked them to take a proper look at the sound (I left the car parked round the corner from the dealer the night before, so that I could take it in with a cold engine).

Much to surprise I got a phone call later that day to say that it wasn’t piston slap after all, which brought on a brief moment of panic and thoughts of engine rebuilds. I needn’t have worried though, the problem was a faulty timing belt tensioner, which was banging against the engine block.

The tensioner had been fitted at my 90,000 mile service less than 12 months ago (when the timing belt was replaced), so it was replaced under parts warranty.

So my Scooby now sounds like it should again, no more taxi impressions every morning.

So, enough about C#, today we are on to cars.

My year 2000 Scooby (Subaru Impreza) has been suffering from the symptoms that I seems that most of them have at some point in their lives. The symptons are inconsitent delivery of power, with flat spots in acceleration (feels like you have stepped off of the gas suddenly) and surging.

It seems that there is a long list of things that can cause these symptoms (or very similar symptoms), that include:

  • failing lambda (oxygen sensors)
  • failing HT (high tension) leads
  • failing crank angle sensor
  • failing or dirty MAF (mass air flow) sensor
  • air leaks in various pipes

The most common one appears to be the last one, the MAF sensor (which measures the amount of air going into the engine). When the problems developed on my car I therefore checked the MAF sensor first.

When I pulled my MAF sensor it looked spotless, it was clean and I thought it was almost brand new. I ruled it out as a cause and plugged it back in.

Over the next month the symptoms got worse and my local Subaru dealer had a couple of failed attempts at diagnosing the problem (the ECU wasn’t reporting any fault codes, which didn’t help).

They finally fixed it this week, after having the car for three days to give them time to track it down. The main cause of the problem? Yes, you’ve guessed it, it was the MAF sensor failing.

So if I had just replaced my MAF sensor when the problems started (£60 and 10 minutes work) I could have saved myself £200 in labour costs.

So, next time something like this happens I’ll think carefully about just replacing the MAF sensor before looking at other things. Some Scooby owners replace their MAF sensor every year as a preventative measure (it seems that the MAF sensor on the “classic” Impreza is a fragile item), not sure I’ll go quite that far.