I've got an intermittent power loss problem. Does anyone have any advice on how I can diagnose this please:

Car is a 90bhp TDi Octavia I, 130k miles, I've owned it for many years and so far so good.

An intermittent power loss problem has started. Car will initially drive just fine, but when putting the engine under sustained load (eg climbing my way up a Pennine Hill) when 2/3 the way up the hill, a marked power loss suddenly happens. So one minute I'm confortably going uphill at 60mph, then a few moments later engine although sounding normal, won't let me pass 40mph.

It's not the fuel filter. Ive had that problem before, I've just changed the filter again, but it made no difference (indeed symptoms are slightly different then.... blocked fuel filter leads to misfiring, which isn't happening now).

Only other clue is that for the first time I've noticed a ratting sound from under the bonnet. Suppose I brake hard from 60 to slow. Engine and car slow down rapidly. I can hear a rattling pump-like sound from the engine bay. It too slows down, but rate of change is different to that of engine/road speed (rattle takes a couple of seconds longer to fade that the duration of the deceleration of the car/engine).

So my thoughts are turning to whether its

i) The Turbo ... I guess expensive.... anyone know what sort of money I'm looking at?

ii) The ancillary bits of the turbo (waste gate or whatever)

iii) A defective intake/exhaust sensor that is confusing the engine management.

Fault is intermittent. I can drive for 20 miles without a problem (so engine long since running at normal temperature). Its just when I go for sustained power on a hill climb that it has problems after a while.

i had this although going on the flat, wouldn't go past 70, turned out sticking vains in the turbo. i thought it could be expensive but gave it a few tanks of diesel red-ex done about 3k since, nothing, hasn't happened since!!

you may or may not have a VNT turbo so it could be the sticky vanes problem. You need to get the car checked for error codes really, look in the diagnostics forum and see if anyone with VCDS is near you.

I can't shed any light on your power problem, but is it possible that a failing alternator pulley could give the "rattling pump-like sound from the engine bay"? during deceleration the alternator pulley should "free wheel" and will continue to do this as it slows down to the same speed as the pulley is being driven at.

Just thinking out loud! but the noise might not be related to your power problem.

Changing the coolant temp sensor seems to have cured my power loss (limp mode) problem, but this power loss occurred randomly although always under acceleration.

you may or may not have a VNT turbo so it could be the sticky vanes problem. You need to get the car checked for error codes really, look in the diagnostics forum and see if anyone with VCDS is near you.

Something I just don't know: WIll the diagnostics tool show an error code just if the fault is present there-and-then, or does it remember if a sensor has had an error recently?

In other words, since this is an intermittent fault, will the diagnostics help if at the time I plug it in, everything is working properly?

there should be a fault code stored in the ecu even if it is an intermittant problem. IIRC you may get a warning light that comes on and then goes out but the code will still be stored until it is cleared using VCDS or similar. An intermittant fault will be listed as such when you scan the car.

Where in the country are you?

If the problem is the sticky VNT vanes (seems most common on gently driven 110s, but is known on VNT 90s) there should be an error code with a narrative along the lines of "turbo over-boost detected".

fwiw I have never seen a light on the dash when mine overboosts and goes into limp mode, I do get a error code logged in the ECU however.

CONCLUSION: Just thought I'd post the final outcome on this for anyone interested.

I think the root cause was taking in some contaminated fuel. What I didn't mention at the start is that this happened shortly (30 miles or so) after filling up with diesel.

About half a tank of fuel later, the problem went away and never returned.

I did get the ECU codes read, and they showed no faults (either recorded in memory, or whilst the car was being driven briskly).

It's happened to me before - just about managed to get home, called the AA, and the car started fine and ran perfectly from then on!!!

Glad you got it sorted...

I had this problem on my old '55 Leon 110 TDI (driven gently!). I looked it up on the interweb and thought it was new turbo time, but after a while it seemed to sort itself out too, so maybe it was the samea s you - bad fuel.

More than likely it is sticky turbo vanes, I ensure I get over 3500-4000 rpm a few times in each journey (once the engine is warm) and then seems to keep the stuttering pretty much at bay.

Can anyone tell or show me what you mean by sticky turbo vains please ?

Can anyone tell or show me what you mean by sticky turbo vains please ?

Link: VAG VNT sticky vanes

I've been having the exact same problem with my 2011 Octavia 1.9 TDI.  Sudden lack of acceleration after driving for about 15-20mins on the Motorway.  It happened after filling up with fuel too (BP).  Only happened twice but enough for me not to trust it for our holiday with kids and dog in the car.  The though that it could be bad fuel does set my mind to rest after being told it could be the EGR valve. 

I think i'll let the fuel run down and fill up with something decent before starting to replace parts!  Thanks