Accelerating hard at, let?s say, slightly above the motorway speed limit when the revs drop from around 4000 rpm to 3000rpm (like a gear change up, it?s an auto) and the acceleration pretty much stops.

No noises, no warnings on the dash, just less power. The car cruises at motorway speed OK.

Car seems to drive OK up to about 3000rpm then very little more. My bet would be that the big turbo just isn?t working.

If the car computer was refusing to actuate it surely the iDrive would give me some kind of warning?

As the big turbo was probably at full boost when it croaked I doubt it could be a stuck gate or valve?

The car is 5 years old and just out of warranty.

Suggestions of what to check would be most welcome before I have to concede and take it to the dealer. I haven?t got any fault code reading equipment, but I would be prepared to fix most things myself.

 

Fuel filter? I'd think a turbo not working would throw all kinds of errors.

 

It was quite a sudden switch from being alright to not being.

Fuel filter was changed 7000 miles ago (according to the service history).

I'm wondering if it's a vacuum pipe failure - they all look a bit old.

 

I'd be inclined to agree with the line of invesitgation around a pipe either failing or perhaps being dislodged. At least it's a cost fee first step in ellimination process.

Other intake and MAF issues etc would tend to be more prevalent across the entire rev range. Again though, some of those symptoms can be verified with temporary sensor disconnects etc.

 

Does sound like the turbo isnt actuating, I had this will my old Audi following a bad service at the dealership. They replaced the fuel filter and disconnected the the vaccum hose for the turbo in the process. Looking back it still makes my blood boil, the general manager accussed me of all sorts and denied that his guys could have screwed up.

Check out your hoses, my Audi wouldn't go above 50mph and the max RPM was similar to yours.

Good luck and keep us posted.

 

Update:

Next morning the car's perfect and drives fine.

In the evening gave it some beans to see if the fault re-occurred - it did! Flat out on motorway OK to a speed "a bit over the limit" pulling fine then hits some kind of limit...serious power loss. Manual mode on gearbox no change. Foot to floor in M3, rev limits at about 3000 rpm won?t accelerate further, car stays at about 70mph. If drive gently can make progress but well down on power (in safe mode?).

Pull over, turn off engine and restart immediately. Perfect, full power, floor it and it goes like stink. Fine for rest of journey.

I now suspect sensor tripping car into ?safe mode?. Need to read fault codes to find out what - but I haven't a reader (yet).

Faint whiff of diesel under bonnet ? intake manifold leak?

 

Sounds like limp mode to me. Where abouts are you, I have a code reader.

 

Bristol, but I can see you're in Kent. Thanks for the offer.

 

Hi,

I had a very similar problem with my 535d last month, took it to the dealer who diagnosed a split main turbo vacum pipe. had to wait 2 days for the part to arrive but while i waited for the new part to be fixed the car drove just like you describe, full power then nothing just revs. Worth checking if not already done so as dealer stated this is not an uncommon problem after a number of yrs.

Stu,

 

Interesting - thanks.

I've had a good look at all the technical data and there are a lot of vacuum pipes controlling the various turbo flaps. Most of them look a bit perished. Trouble is that most of them are hard to look at.

Any idea how much the diagnosis, part and repair cost?