Help with some god awful gas mileage on my mercedes!


I have a 1986 560 SEL Mercedes. I know it has a big engine, but recently in the last month I can literally see the gas gage move on a 20 mile trip! Alot! I replaced the spark plugs, put in fuel injector cleaner, but it feels so powerless from 0-20. It cruzes great but I feel there’s something sucking the gas out. What happen to the head jerking pep I use to have when I hit the gas? Is it the gas?, tune up? timing? fuel injectors? Carb? Please send your ideas and I may have to try them all. Appreciate your responses! Steve.


This kind of problem generally requires hands on troubleshooting. If I remember correctly, this is an overhead cam V8. I’d look at valve timing to make sure the timing chain hasn’t jumped a tooth. Then I’d look for fuel leaks, maybe at the base of the carburetor or the fuel pump. The fuel pump could be leaking fuel through the diaphragm right into the engine oil, so check for a gas smell.

Check ignition timing, vacuum and mechanical advances. Check plugs and wires. Check valve lash if it is adjustable.


One more thing, the Oxygen sensor can cause this also, there should be a test port in the engine bay somewhere.


Going by a dashboard gauge is no way to judge fuel economy. You need to fill it up, set the tripmeter, and refill in a few hundred miles; preferably on a road trip.
A dashboard gauge can change the way it reads due to electrical resistance in the wiring circuit changing; whether it’s a wire connector, gauge problem, or a tank sending unit developing wear.

If the car has developed a general sluggishness (without bucking and jerking), then I would suspect either a partially clogged catalytic converter or transmission slippage (assuming it’s automatic of course), although a failing fuel pump could possibly cause it.
The converter can be checked with the use of a vacuum gauge.

You could check transmission slippage by placing the selector in LOW and performing a stall test. Set the park brake and hold the foot brake firmly. Try to rev the engine. The engine should stall out around 2000 rpm or so and not rev any higher.
Wait a minute then repeat this procedure in SECOND.
Wait a minute then repeat in DRIVE; not OVERDRIVE.

The engine should stall out each time around 2000 rpm at the most. If it revs up to 2500 or whatever, then the trans may be on the way out. Only perform this test ONCE and wait a minute between each test to allow the fluid to cool momentarily.
Hope that helps anyway.


Good suggestions here too, but let me clarify the term stall. In this case, it doesn’t mean the engine stalls and dies. It just means that that is the highest speed the engine will turn. In other words, it won’t go past 2000 rpm. Most cars have a stall speed somewhere between 1500 and 2000 rpm.


Good point. I should have been clearer on that word.