PeterK821

So I have a 2021 SQ8 and am experiencing a strange drivability problem related to hot weather and using the A/C.

When the weather is hot (90F or above), the vehicle is totally warmed up and hot, and I have the A/C set to LOW all around (synced) with a fan speed of 4 or 5, the vehicle starts delaying shifts, like it would if you were towing 5000lbs or going up a steep incline. This is in normal transmission setting (not sport) and comfort driving mode.

Specifically, I noticed that even with very light throttle from a stop, it won’t shift out of 1st until 15-18mph and won’t come out of 2nd until almost 30mph. Basically it won’t shift until it reaches almost 3000rpm (kind of like sport mode). Even with steady speed on a surface street if I accelerate to, say 25-28mph and hold that speed I’ll be driving along with it buzzing up at nearly 3000rpm and it’ll stay there and won’t upshift. This is annoying…and even more so in a parking lot or said surface street because if you lift off the throttle you immediately get engine braking because it’s still in such a low gear.

If I turn the A/C off or set the cabin temp up near 76 it immediately goes back to normal shifting (where it’s in 4th or even 5th already by the time I’m at 30mph).

I’ve also noticed that under these conditions it won’t go into 8th gear at all until you reach 75mph, where normally it goes until 8th at 55mph or less.

This only seems to happen when the outside temp is hot (again, 90F or above) and the A/C is on LOW with a hot vehicle like I said before. It also sometimes happens if it’s maybe 85F and the vehicle has been driven and “hot soaked” in the sun outside for a bit and then driven again.

Now I know A/C will add some drag to the engine but this seems a bit ridiculous when I’ve never even had a 4 or 6 cylinder engine demonstrate this notable level of behavior with the A/C on before (it’s super obnoxious in stop and go or slow-moving traffic).

Has anyone experienced this?

UrusUser

I’m guessing AC compressor is electric not belt driven anymore (as with most vehicles equipped with start/stop systems), so I doubt that’s putting any load on the engine.

PeterK821

No…I believe it’s a belt-driven variable displacement compressor. Thing is…if the compressor had an issue I’d expect the A/C itself to have issues (such as lack of cooling performance), but it doesn’t.

Michael99

I have a 2020 Q8, not the S or RS, but I experience the same thing you described. I'm not sure why it happens. I just ignore it.

HDCL

If this is a belt driven AC Compressor, from an engineering perspective this makes sense. The compressor works best between a certain RPM range and if it is very hot outside, your vehicle will not cool down fast enough if the engine is not spinning above a certain average RPM. Once the cabin cools down, does the behavior change (go back to normal)?

That being said, it has been pretty hot here in Chicago (95 actual, 105 heat index) and I have not noticed this in my RS Q8.

PeterK821

Do you drive in sport mode or have the transmission in sport mode? Because you probably wouldn’t notice it if you do because that’s how it acts. It winds up to 3000rpm before it’ll shift when this is going on.

The cabin seems to cool just fine even if I’m at idle. And no…the issue remains as long as I have the A/C set on LOW, no matter how cool the inside is if it’s hot out,

And granted, the compressor will pull a load but…I’ve literally never had any vehicle (even 4 cylinder ones) that will act this way — like it’s pulling a 5000lb trailer. Imagine being at a stop sign on a surface road, doing light acceleration to 25mph and it won’t go higher than 2nd gear and is wound up to close to 3000rpm while you’re cruising down the street. And engine braking whenever you let off the throttle because of the low gear. That’s what it’s doing. If I turn off the A/C or if I turn the cabin temp up to 76 or so it stops doing that and goes back to normal (not holding low gears and shifting as expected).

I tried two other SQ8s and couldn’t duplicate it but wanted to see if others had seen this behavior because it’s a real annoying drivability issue when it’s going on and I’ve certainly never seen a normally operating A/C pull so much load that it holds gears like that before. I absolutely wouldn’t expect it in a 500hp V8.

HDCL

I normally do not drive in Sport mode in the city due to traffic. I am about to go out for a few hours and will try to pay attention to this, although the temperatures are much lower today in Chicago. I would have noticed this behavior if it was happening because I do hear and feel how much the shift pattern is different in S mode including a higher idle speed while stopped. If you have a 100% reproducible situation, a trip to the dealer may be in order.

agfours

I have a 2021 SQ8 and get similar behavior after pulling out of the driveway about 2/3 of the time. I keep the car in Dynamic, D (not S), and I have found no correlation to A/C settings - they are usually at 73 degrees on Auto. But the car will stay in first or 2nd, sometimes up to 4k or 5k, but most of the time to 3k before shifting up. I've noticed it only really happens under super light throttle as when I leave the driveway onto the street, it is slightly downhill. Once I start to hit the hill going up a bit, it will shift again. Here's the strange thing - when my wife drives, it doesn't happen, as she seems to use a heavier foot when she leaves the house. I'm sure my neighbors love the exhaust resonator at 5k every time I leave the house...

retom

The compressor in Q8, SQ8, and RSQ8 is belt driven. If you set the air conditioning to maximum cooling with high outside temperature, the compressor works at maximum load to cool the interior of the car as quickly as possible. This consumes a lot of power generated by the engine, which does not deliver full power anyway due to the sucked hot air. Therefore, the software changes the behavior of the engine and transmission so that they operate in the optimal range of revolutions where the power and torque supplied is high enough not to lower the car's performance and to fully drive the air conditioning compressor. This is also due to the fact that the high load on the engine and transmission at low revs is very "unhealthy" for them. IMO, this behavior of the engine and transmission is intentional and there is no any fault in the car systems.

PeterK821

So being an engineer myself and having worked in the automotive space, a properly functioning A/C system should not draw so much power from the engine that the vehicle is screaming along at nearly 3000rpm in second gear while holding a 25mph speed. And it will never stop doing it the entire time until I set the internal temp to 76 or above with 90F outside temp. An A/C compressor running full time will typically draw 5-15hp out of the engine maximum…not half its output.

My old 4 cylinder Honda Accord won’t act that way, neither will my 8-cylinder Hemi Jeep, or any number of vehicles I’ve ever driven. Heck even a base model A4 won’t do it. At most all I’ve ever felt from any vehicle with max A/C on is a slight reduction in power. Not a staggeringly obnoxious delayed shift pattern where the thing holds 1st or 2nd gear on surface roads, engine brakes jerkily when letting off the throttle because if the low gear, and so forth. Driving it in stop and go traffic when it’s hot out is quite an unpleasant experience unless I want a 76 or above cabin temp or I turn off the A/C if I want it to go into 3rd when creeping along instead of hanging in the low gears at 3000rpm.

Anyway, I’m trying to see if others have experienced this as I did drive two other SQ8s and have not been able to make this happen with those vehicles.