2003 X5 4.4 (E53) with 137,000, had the entire cooling system replaced just under 100,000 miles, by dealer using BMW parts. This included new Radiator, all Hoses, Water Pump and Thermostat.

Unfortunately, Interior (smaller) Radiator Fan was not properly tightened on spindle and within 24 hrs, spun off while driving, ripping up entire interior of engine compartment, denting engine hood lid requiring body repair and the complete replacement of all damaged cooling and other engine compartment components (at dealer cost and with extremely humble apologies for poor QC work). The repair was very professionally handled, with no questions or problems.

At 135,000 miles, the transmission locked and failed. The transmission was completely replaced by the dealer, with a BMW core swap.

The Transmission now either fails to upshift underway, holding in gear at 30-45 mph, with shift point rising to 4300 rpm, from normal 3000-3200 point), or sometimes momentarily downshifts at both local and highway speeds when underway. This happens just for a second, then immediately recovers (i.e. traveling at steady 60 mph - shifts down for 1 second, then resumes original gear with no change in gas pedal pressure).

Fault Code showed Thermostat frozen "closed", so dealer replaced both main Thermostat and both transmission Heat Sensor Hoses, in the hope that this would correct the problem. This $1,000 repair had no effect at all.

Dealer now recommends replacement of the Transmission Heat Exchanger, due to the possibility of a coolant blockage, thus conceivably raising the trans fluid temp. They cannot determine whether the trans is overheating, there is no movement of the dashboard Engine Temperature Gauge (locked Dead Center). This remedy appears to clearly be a "fingers-crossed" solution, since there is no indication of any Fault Codes for the problem.

Anyone have any input or experience on this issue?