Looses power at 55 mph

Most likely it is the Throttle Position Sensor (TPS). Since 55 mph is a relatively constant speed for many drivers, the TPS get a work spot at that location and causes signal drop out to the ECM, resulting in a jerking feeling almost like a high speed intermittent engine misfire.

Distance= 503 miles.

503 ÷ 55 = 9.145 hours.

Now this is IF you could maintain 55 mph constantly AND didn't have ANY stops, i.e. fuel, bathroom, food, etc.

Hope this helps.

cattalatic converter is going bad

u may have a sensor or computer issue--it shud be scanned--the fuel injector pump is the heart of a diesel-probly needs a tune, rebuild or new one--pick up a haynes manual for maintenance details or go chilton online

Have your Cat Converter checked, ASAP.

These cars are funky and this is normal...gauges on all of the earlier cars are for general reference...checking the oil level manually while the car is running and warmed up is the most proper way to do this...generally speaking, your pressure should drop as the car warms up, but still reach 5 bar on full accelleration (928's have the same gauge). The oil level gauge will actually increase as the car warms up...the difference can be dramatic.

How does it perform acceleration wise up to 55 mph? If good, then I'd have exhaust checked for restriction as well, as in plugged catalytic converter.

Check your drive shaft u-joints..Number one cause.Have a great night!!!

I’m happy to assist further over the phone at https://www.6ya.com/expert/joe_a3369546f8067589

the car may be in need of a tune up, spark plugs, ignition wires, fuel filter, air filter..