1991 300D - Revs @ hiway speed?
 

Dusting off after a 2.5 year absence. I am currently looking at a 1991 300D and am curious what rpm the engine turns when traveling 75mph on the highway. What can of mpg can I expect at that speed? I'd be buying off ebay from Florida and don't have the chance to test-drive it first.

While we're at it - is there anything I should be aware of? My last 124 was an E420 and I got to replace the engine wiring harness. Is there anything I need to know about?

IIRC it has 2.65 gears so it’ll be more relaxed that W123 Diesels. Not as relaxed as an E420 with 2.24 gears. Early 2.5s had accelerated wear of the cam sprocket (and chain?). Many need a head gasket by 150-200K mikes. The oil supply to the head breaches the gasket and leaks into #1 cylinder. Not a difficult DIY.

Sixto
98 E320s sedan and wagon

IIRC my '91 runs around 2700 rpm on the highway. I'll check this morning on the way to work and post back.

I commute 70 miles/day running at 60-65 mph (state roads, not freeways) and I'm getting 32+ mpg. Best tank so far was 34. Worst was 22, but that was prior to replacing the fuel filters... I'd replace those first.

Other than keeping your eyes open for the potential issue sixto mentioned, I don't think the w124/602 diesels have any endemic problems. I had to replace a flex disc, thermostat and the serpentine belt tensioner/arm/spring/pulley within the first two weeks I owned it.

The fuel warmer on the passenger side of the engine seems to be a common problem. I bypassed it (after it dumped a gallon or two of diesel in my driveway).

See picture

Thank you everyone!

65 mph = 2600 rpm
75 mph = 2800 rpm