Need towing advice - managing transmission temp (petrol engine)

[RhythmMachine]

Thanks for the detailed update. Petrol X5s are pretty rare in the UK with the vast majority sold being diesel until it got flagged as ‘demon fuel’ a few years back (think VW Dieselgate time). The torque of the diesel IL6 engines is so useful. I know the diehard V8 fans will stay with their engine symphonies, but the IL6 petrol isn’t bad either, just a bit thirsty

I do miss the effortless torque from the diesel and the 1000km range on a single tank (speaking about my ML320... I don't have direct experience with a diesel X5), but there are so many very expensive problems that result from the emissions equipment that I wasn't willing to take the chance. Repairs are prohibitively expensive once things start to go with the DPF, EGR, DEF and so on, and the delete process seems to be around $4K CAD, which would have been about 30% of what we actually paid for our 35i. Just too risky for me.

[RhythmMachine]

We got out again last weekend with flatter terrain, but I gave it a try with the transmission in automatic sport mode.

Fuel efficiency may be a bit better: I had it in Sport Automatic for the first half of the trip to our campsite and got 17l/100km for the trip there (13.mpg). The temp, though, was significantly higher, getting up to 109 C on my gauge for ATF, which should translate to 101 C for actual transmission temperature.

I switched back into manual shift mode for the remainder of that trip and most of the return and it pretty consistently stayed between 98-103 C on my gauge for ATF (which would result in transmission temp of 91-96 C). Mileage on the return trip was 18l/100 km (13mpg), so still pretty good.

We have an upcoming long highway trip without the trailer, so I'm curious to monitor and see what the temps are like on that to determine how much impact the trailer actually has on the operating temp.

[RhythmMachine]

Another update after a trip without the trailer. Not as many hills on this one, but some high temps on the return (up to 34C, 93F). In lower temps, regular automatic mode, transmission temp was around 90-95C or 194-203F (showed as mid-90's to around 103C on my ATF gauge), with engine oil temp just over 100C (212F) and fuel economy just under 9L/100km (26mpg).

On the return, with higher ambient temps, engine oil was hovering around 115C (239F), with transmission temps in the high 90's C (about 208F). Fuel efficiency was just about exactly the same.

So my conclusion is that the temps that I'm seeing when towing carefully (going into manual mode, getting torque lock-up in sixth gear, and keeping speeds around 100km/h) are pretty much in line with "regular" highway driving. Since expected range seems to be 80-100C and there isn't really cause for concern below 120C, things look good.

What I'm not sure about is how much more headroom I have in the transmission cooling system... if transmission temps in the high 90's (C) are still within the comfortable operating range, there may be additional cooling capacity that isn't being used (if the fan doesn't kick in until higher temps than I've reached, for example), in which case my caution while towing may not be gaining me anything. I'll try to test this further over the summer, but don't want to risk overheating/damaging the transmission in an effort to find out how hot is too hot.

[RhythmMachine]

I wanted to close the loop for anyone referring to this thread down the road... we had another trip in the Alberta foothills in moderate weather (25-30C) last week, and on climbs I was having trouble maintaining a reasonable speed in sixth gear in manual mode, so decided to give it another try in Sport Auto mode.

Coming from the Mercedes diesel as our last tow vehicle, it was a bit shocking hearing the N55 downshift and rev to keep speed (just over 100km/h) on longer or steeper hills, but it was certainly able to keep pace.

While climbing, engine oil temp got to around 115C (about 240F) and ATF fluid temp to just under 110C, meaning that the actual transmission temp would have been about 102C (215F). After climbing, though, it would quite quickly regulate to around 105C (220F) for engine oil temp and 95C (200F) for transmission temp, so both within the acceptable range. Fuel efficiency for the trip was around 20L/100km, so not significantly different than using manual mode, but certainly easier for the driver.

My conclusion after all of this is:

• There may be some benefit to using manual mode and locking in on sixth gear when towing on fairly flat terrain - it will keep wear and tear on the transmission to a minimum

• On trips with more hills, using Sport Auto mode simplifies the driving, allows me to keep the speed up on cruise, and manages the engine and transmission temps adequately, even if there is a slight decrease in fuel efficiency

[X5Sport]

I’m not at all surprised that a petrol engine would downshift as the torque curve is completely different than present in a diesel one. As you will no doubt remember, diesel grunt is all at the bottom end with petrol well up the rev range. That last 6-cyl BMW petrol I drove didn’t come ‘on song’ until well above 3,000rpm, about double that of it’s diesel equivalent.