Fuel Consumption

[t-ford_man]

I bought my '68 280 SL last fall but now it's spring in Canada and I am preparing for the summer driving.  Yesterday, I took the car out for the first time and noticed evidence of over-rich mixture on acceleration ie. black smoke (car warmed up).  The mileage is poor as well, so this seems to tie in.  I'd be interested to know if I should tackle problem this my self and where to start.  As I said above, I'm new to this vehicle but am a fair mechanic on more conventional (carburetted) vehicles.  Suggestions?

Carl Luttrell

Carl Luttrell

[TheEngineer]

Carl, these cars get poor fuel milage compared to modern vehicles because the system doesn't measure air density. Expect 16 to 19 mpg. The injection pump can be adjusted in many ways, refer to the shop manual. All cylinders must be uniform in compression, or leak down, because the pump delivers an equal quantity of fuel to each cylinder. If you have one or more cylinders not at full compression, you will run rich on those. The spray pattern of the injectors is important. Take it easy with pump adjustments: Make only one click at the time and test again. The pump can be tested for uniform delivery using a bench. If you want to drive this car, please make yourself familiar with it and remember that all the mechanics who knew the car forty years ago, are retired now.

[Cees Klumper]

quote:Originally posted by theengineer

If you want to drive this car, please make yourself familiar with it and remember that all the mechanics who knew the car forty years ago, are retired now.

That's true, although there are some mechanics that picked up from the initial ones as the cars 'matured' through the years, and some of them are here (Joe, Dan, Al and I am sure I am forgetting several others)!

It could very well be that your car has not been driven all that much in the year(s) before you purchased it. Speaking from experience, sorting it out can take some doing, but in the end you will prevail and have a car that drives clean and strong. And consumes plenty of gasoline by today's standards. But ... no smoke, good acceleration, faultless starts (unless you have the "starts hard when warm" -48% of our cars seem to have this condition- or "starts hard when cold" -another 48%-) and thousands upon thousands of trouble-free miles. In fact, my car only left me stranded once, in the first year that I bought it, now 8 years ago and I've covered some 30 thousand miles since without a hitch (pun intended).

Areas to check:
- ignition
- fuel delivery (rusty deposits in the system, blocked (return) lines)
- fuel injection (linkages settings, injectors, mixture setting, cold start systems)
- general condition of the engine

Good luck, use the archives and ask questions!

Cees ("Case") Klumper in Amsterdam
'69 white 280 SL automatic

[ja17]

Hello Carl,

These injection systems tend to wear "rich".  In other words they begin to run a little richer as time and miles accumulate. You must go through all the basic tune-up and linkage adjustments before you tamper with the fuel injection. Don't expect too much fuel economy. 20-24 mpg on the road and 16-20 mpg around town should be about normal.

Just hang around this list and will get you through your difficulties!

Joe Alexander
Blacklick, Ohio

[al_lieffring]

hey Carl

Besns that I'm one of them experts, I'd  say check to see if the cold start thermostat is closing completly when the motor is warmed up.
Remove the small air filter on the injection pump. When the motor is up to temperature there should be no air being drawn in through there. It is possible that after long storage that the air slide could be stuck open, holding the mixture controll in the rich cold start position.

Al

66 230sl
113-042-10-014715
904/396 blue, Ivory Tex
condition: not-as-rusty-as-before-bucket

[TheEngineer]

That's a very good point made by Al: A good way to check for airflow is with one of those smoke sticks the air conditioning shops sell or use a piece of audio tape. It will detect even a small amount of air flow. On my car I found that the small air filter (p/n A 000 075 11 32) was broken; i.e. the paper element inside had disintegrated. Hard to see, and you can make one from a Fram filter, using the existing shell. Read the plugs immediately after a run on the freeway. Check the thermostat: The engine should reach operating temp within a couple of miles.

'69 280SL,Signal Red,Automatic,retired engineer, West-Seattle,WA

[al_lieffring]

quote:Originally posted by theengineer

A good way to check for airflow is with one of those smoke sticks the air conditioning shops sell

'69 280SL,Signal Red,Automatic,retired engineer, West-Seattle,WA

Most mechanics of the era would have used a lit cigarette to check for air flow,I never smoked though. Now it is impossible to find a smoker to borrow a lighter from.

Al

66 230sl
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