poor gas mileage/1973 450sl

[zeggate]

I have been driving the car for the last week and I have managed to get only 100 miles out a tank of gas. I popped the hood and found the breather hose from the air cleaner was not connected to the crankcase. How much would this contribute to my poor gas mileage?. I did reconnect it. Thanks

[Tomguy]

Wouldn't affect it at all - it might get some dirt in your engine (nothing major I'd imagine) but have no effects on mileage.

Has this been a frequent problem? First thing to check, assuming ignition is in order (new plugs, wires, rotor and cap, timing and dwell set right) is VAC LEAKS!

There are several places to look for vac leaks:
- Door lock system (from manifold to one-way valve, then to master switch and reserve tank)
- Brake booster (manifold -> 1 way valve -> booster)
- Tranny line (manifold -> tranny)
- Plenum gaskets between manifold halves (8x)
- Injector seals (8x)
- MAP hose (from manifold to MAP sensor)
- Distributor retard (throttle body -> distributor)

These are all the vacuum lines that can affect mileage. Other than the vac retard line, they all connect at the manifold and a leak in any of them will make the MAP think that there is a much higher load on the engine and wider throttle, thus feeding it much more gas.
If the retard line leaks, then it's been set at 5ATDC without a vac - it will not advance in 3rd gear so you lose a ton of highay efficiency.

Also, look at the air temp sensor on your air cleaner's intake horn. If you unplug it while the car is idling (careful of the fan!) you should definetly notice an idle speed change (especially when warm), if not, then it's not connected properly or the sensor is shot.

Finally, check out the throttle position sensor, make sure it actually sends to the ECU when it's at no throttle. If the ECU doesn't know your car is getting 0 throttle, it won't run on idle mix or cut fuel supply during deceleration.

All of these will give bad mileage, and any in combination will cause black smoke under any type of heavier throttle. If you floor it on the highway and it pours out black smoke, you're running WAY too rich.

[MikeTangas]

Are you sure your odometer is working correctly?

I ask because when I noticed I was only getting 6-8 MPG on the 4.5, I also noticed the odometer would stop turning after 15 minutes of driving. Once I noticed that I started keeping a log of guestimated mileage and saw my fuel mileage climb to 10 MPG. Once I got the speedo head rebuilt I was able to track true mileage.