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

Rob Bell · 14th August 2018, 13:24

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.

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

roverbarmy · 14th August 2018, 14:10

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

Rob Bell · 14th August 2018, 16:57

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?

roverbarmy · 14th August 2018, 18:42

Quote:

Originally Posted by Rob Bell

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.

T-Cut · 15th August 2018, 09:07

Quote:

saw temperatures of up to 42.5 Celcius

Intercooler not cooling?
TC

Avulon · 15th August 2018, 10:21

Quote:

Originally Posted by Rob Bell

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

14th August 2018, 13:24	#1
Rob Bell Gets stuck in ZT-T CDTi + Join Date: Nov 2006 Location: London Posts: 561 Thanks: 7 Thanked 10 Times in 7 Posts	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. 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

14th August 2018, 14:10	#2
roverbarmy This is my second home Roverless + 1.7D Sportage Join Date: Aug 2014 Location: East Norfolk Posts: 7,050 Thanks: 729 Thanked 2,021 Times in 1,449 Posts	Oxygen content decreases as altitude increases. "Older" engines needed to be adjusted on the mixtures to allow for it. __________________ Oil in my veins!

14th August 2018, 16:57	#3
Rob Bell Gets stuck in ZT-T CDTi + Join Date: Nov 2006 Location: London Posts: 561 Thanks: 7 Thanked 10 Times in 7 Posts	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?