Engine Bogs Down at Highway Speeds

[ChristineX5]

New member here, thank you for your knowledge and experience.

2008 X5 3.0si (6 cylinder N52) 110,000 miles

We just ran a trailer full of goods up North to our cottage in a brand new Haulmark enclosed 7x14 tandem axle trailer, 2050 pounds empty. (I had just installed a Curt hitch rated at 5,000 lbs, along with the BMW wiring kit. Then I had it coded and all the lights worked perfect).

Not knowing how well an X5 6 cylinder (N52) would pull, I carefully weighed the load as each item came aboard (bathroom scale). Quit at a total of 3,500 to be on safe side and our first trip being 1550 miles.

Amazed with how well it pulled the trailer! Smooth and straight, no issues whatsoever, had plenty of power. Plenty! Easily ran at 70 MPH. Transmission would stay in regular full automatic, and only occasionally would downshift such as going through the mountains. Running 2200-2300 RPM in 6th (top) gear. Starting off sometimes after getting gas, etc., I would start off in manual shifting, to keep the RPMs and torque up. Seemed to like that treatment. All went well the full 1550 miles going up... except no heater!!! And headed from Florida to Wisconsin! (Turned out to be the heater valve under the hood, drivers side, bought a used one on Ebay for $85 and all was well).

But...as soon as we started back to Florida, pulling the EMPTY trailer (but still 2,050 pounds) it felt on the highway like we had a full, huge load and then some! No power up on the highway. Starting off, the rig felt as peppy as always. Going up the on ramps, perfectly fine. But hit 65 and the vehicle was reluctant to engage 6th. And when you pushed on the gas pedal, really push far ---absolutely nothing--- just bogg down!

We ran almost the whole way home in manual 5th gear at 3,000- 3,100 RPM. Sounded smooth and correct, even dare a say purring. No fault codes, no silent fault codes, we disconnected the trailer brake controller (BMW brand new connected to the BMW harness), thinking brakes may be dragging. Thought we got a tank of bad gas. Put in a bottle of Heet and a bottle of octane booster, and always bought top brand name premium all the way home. No help. Just had no power past 60 on the highway??? And boy...did it eat the gas!!! Ideas Please?

Vehicle feels and runs fine even on highway now with no trailer. We have another load to go North in summer. I thought I might jack up the trailer and manually spin each tire to see that they are free??? Always checked the bearing by feeling the hubs along the way. Bearings were always totally cold. Thanks.