Hot weather + high altitude power loss below 1800rpm (CDTi)
 

Hi Guys,

Just returned from holiday driving across France, through the Pyrenees and then across Northern Spain. Fantastic drive - the ZTT is such the perfect family wagon for these kind of trips, as you know :)

So the problem: usually the car goes extremely well. Pretty seamless between low revs with a fairly subtle but noticeable transition when the turbo comes on song. It's had a 160 re-map (it was originally a 116) which certainly helps, plus an EGR delete. But in the recent hot weather, the car felt more sluggish (it was loaded 4-up, with camping gear etc. mind!) but ran okay. However, things got significantly worse in the mountains: the poor engine was staggeringly sluggish below 1800 rpm. In fact - and I've not ever come across this before - I'd find that the car would decelerate if you put your foot down!

Too much fuel, insufficient air I would think. :shrug:

Dropping a gear or two and getting the engine above 1800 rpm would rescue the situation.

The problem certainly got better on returning down to sea level, but was still present in the heat (saw temperatures of up to 42.5 Celcius on the external thermometer in central Northern Spain - thank goodness for effective air conditioning!). The power loss was not quite as extreme as at 2,000 metres, but certainly not normal.

Interestingly, now the temperatures have fallen down to the mid 20s, the car feels much more normal (and it isn't carrying the camping gear any more!)

On interrogating the ECU, no recorded faults. The MAF appears to be functioning - not sure whether the numbers are appropriate or not (I don't know their "normal range" values) - but if the MAF is at fault, it probably is not bad enough to cause major problems under most circumstances.

The injector trim settings reveal that injector 1 is being trimmed quite differently to the others - so that's a problem, but I doubt the problem causing the appalling off-boost performance.

The intercooler to turbo hose has been replaced with a good silicone item some while ago (the original rubber hose had had it). I haven't looked at the intercooler O-rings - but they were replaced just a couple of years ago - and the engine performance now, at UK temperatures and near sea-level altitude, is really not bad...

So in summary poor off-boast performance at altitude and in heat with a distinct feel that the car was possibly over-fueling (although no observable smoke in the rear-view mirror).

Any ideas? I am pretty sure that this is not normal! Cheers :)

Oxygen content decreases as altitude increases. "Older" engines needed to be adjusted on the mixtures to allow for it. :shrug:

Thanks RoverBarmy. Is this something that the Rover 75 and MG ZT really need? Surely that (air density) should be accounted for by the MAF?

That's why I said "older" ( ie carburettor) vehicles. Sorry for the confusion.;)

Intercooler not cooling?
TC

... That would be the MAF which may be on it's way to failure. Disconnect it and see if things improve.

Thanks guys - yes, I was wondering about intercooler function - but as the turbo did boost and the problem was more off-boost lethargy, unless the thing is partially blocked (or blockage else where in the inlet tract), I don't think that this can be it.

I do think that the MAF is the likely (and usual) culprit. I'll make a note of the MAF figures - I don't know what they should be. Live data provides both measured MAF and "requested" MAF. I'll see whether I can get these numbers and post it here both at idle, and on load.

Shame I didn't have my diagnostic kit on holiday - I suspect that the problem would be much more obvious in live data when the running problem was so evident!

Ambient pressure air at 40-odd Centrigrade contains a lot less oxygen, per litre anyway, than the engine's designed for. Perhaps that's why it didn't run so well without the turbocharging?


TC

High altitude air is less dense, and so is hot air as well so probably a double whammy when the engine was running normally aspirated, the fuelling was probably adjusted to compensate and so the loss of power.

When the turbo comes into effect then this obviously allows more air to be compressed and so this would reduce the effect of "thin" air - the intercooler, cools the air down to increase its density but likely that with very hot ambient temperatures then there may not be enough of a temperature differential to be able to cool the air down enough.

The fact that everything has returned to normal back at sea-level and normal temperatures would suggest nothing to worry about.

Hi there,

I have the same problem with my ZT cdti, everytime i go to the snow, altitude greater than 1500m it's impossible to ride below the turbo range, there's no power....
Bosch MAF changed last year, what could be?