Poor Fuel economy --> please help troubleshoot
 

Greetings,

Recently acquired a 1984 300D (164k miles) with an automatic transmission. Mileage in mixed driving is ~15mpg. Performance seems fine although I don't have a direct recent comparison to make.

I'm set to change the fuel filter and have read that the valve adjustment may improve the fuel consumption. But, I find it hard to imagine those two things will have a drastic effect. I would appreciate some recommendations for what to check in order of impact/importance.

Other thoughts include sticky brakes as the car does not roll when idling in drive--is this strange? At 62mph I am seeing ~2700 RPM.

Is there a thread or forum or resource for mods for improving fuel economy?

Thanks!
yem

Check your odometer against a GPS to see if it is slipping. This is a common problem as these cars age.

Agreed, my SD shows .7 mile when in reality I traveled 1 mile.

brakes will indeed drastically affect mileage.
as will valve adjustment, and fuel and air filters and alignment.
all together along with your odo being worn, would easily be 10mpg..

brakes?
 

thanks. does an automatic transmission car typically roll when in drive at idle? is 2700rpm at 100kph (62mph) sound right?

on a flat surface. usually. up a hill, to an extent. at 60M/h I have about 2000RPM but I have the 603. Not sure on the 617's RPMS. Someone else can chime in on that

expected rpm
 

this is a screenshot from the factory shop manual. seems i've either got a different rear end or a bad tach or ???

Go to different places for fuel.... Keep track of a tank... A switch in fuel can make a big difference...

You have a 3.07 differential in your car if you have a US spec 300D Turbo. IIRC 62 MPH is 3000 RPM.

For the Brakes you can jack up one Wheel off of the ground at at time and spin the Wheel to see the amount of effort it takes to turn them.

Also have someone (with the Engine running) apply the Brakes and release them and rotate the Wheel and see if they the Brakes free up properly.

Look at the air filter, diesels run well with clogged air filters but get bad fuel economy.

Also, are you running any kind of additive? Not a bad idea to run some sort of cleaner though there.

air filter
 

thanks. had already changed the air filter before the last tank. it is an STP filter (with a lot less folds than the mann). i have ordered a k&n from peachparts.

correct tire size?

rear end data
 

thanks. where can i find the right specs if not from the shop manual?

that shop manual is not for the 84 model. it's for pre81...