Well, i have been dealing with 11 mpg lateley with my '86 420SEL. It started this when it started getting cold out and my car would not warm up past 60C. I did change the thermostat and oil, and it rund just a little above 80C in all conditions. I thought that ought to solve my bad fuel mileage issue but to no avail. I know a couple things need to be changed. One is my rear shocks. I know that can affect my mileage, but I don't think that would cause 17.5 mpg to go down to 11. Another is maybe my distributor rotor. I have not replaced it yet. Then I was thinking maybe my ignition coil because sometimes if I give it gas it hesitates. This is rare though. I know there are a million things out there that can cause this. I am wondering if anyone has some suggestions. The wires are 2 years old and plugs are say 6 months to a year old.

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1984 300SD Orient Red/ Palomino
1989 560SEC
2016 Mazda 6 6 speed manual
1995 Ford F-150 reg cab 4.9 5speed manual

Fuel economy OR lack of economy is related to engine temp, lamda operation, & driving styles.

Possibly the O2 sensor isn't functioning?

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MERCEDES Benz Master Guild Technician (6 TIMES)
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Retired Moderator

I'll have to check that. Its not too old either, but who knows.

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1984 300SD Orient Red/ Palomino
1989 560SEC
2016 Mazda 6 6 speed manual
1995 Ford F-150 reg cab 4.9 5speed manual

uh, gee, the WRONG plugs, and yes, the new wires, if they are bosch, leaking injectors, leaking fuel distributor, leaking cold star injector, leaking pressure regulator, leaking eha, weak cap and rotor, weak coil and vacuum leaks. i think that covers it. good luck, chuck.

Don't ferget them tars! They lose pressure in the cold.

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Prost!

ok plugs are correct NGK bp5es and wires are oem. I measured the voltage between pins 2 and 3 of the x/11 plug, and got .2 volts. the battery voltage is 13.4. what does that mean? a faulty o2 sensor?

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1984 300SD Orient Red/ Palomino
1989 560SEC
2016 Mazda 6 6 speed manual
1995 Ford F-150 reg cab 4.9 5speed manual

Don't forget another possibility - engine temp sensor.....


-tp

So if the engine is cool, say only running 60C on a cool day never getting past say 75C is it going to run super rich? Then say I take it to my mechanic who leans out my mixture, before I changed the thermostat, is it now going to be super lean after I changed the thermostat? All the while my fuel economy still suffering? He told me that running cool on this car will not effect my fuel economy that bad.

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1984 300SD Orient Red/ Palomino
1989 560SEC
2016 Mazda 6 6 speed manual
1995 Ford F-150 reg cab 4.9 5speed manual