I know these beautiful beasts are not economy vehicles, but my 85 VP suddenly changed from a 16 mpg suburban driving machine to 12 mpg performance. I replaced a split crankcase vent hose (that half-golfball looking hose with the neck sticking out), ran my yearly can of fuel injector cleaner through the system, made sure the intake air connections were tight, lowered the idle speed (which increased about the same time my gas mileage went south) and even filled both tanks with high-test fuel, but no significant improvement in gas mileage. The car is running good and idling acceptably (a bit rough but about the same as always) so I probably should be happy, but am puzzled about what what would produce such a sudden 25% reduction in economy. Does anybody have any ideas about what is going on and what I might do to get my "good" gas mileage back?
Jim

 

Points:


1. Hitest gas does not provide better mileage than "low" test. These engines do fine on 87, "low" test. But, not my call. To each his own.


2. I don't recall the exact way the vent works. But, the split hose might a vacuum leak. Run lean? Better mileage. So, it would seem? But, less power, more throttle, eat more gas.


3. It seems that lost fuel mileage is either due to a change in engine tune. Wide range here. Or it leaks!!!


4. Wild card. Fuel from a different station????


5. One tank to another is but a small sample. Only several can demonstrate a reliable pattern.


Carl

 

Thanks, Carl -- pretty sure the decrease is real after 3 tanks from 2 gas stations, but I don't understand the fuel injection system well enough to intelligently speculate. The engine has always run rich (noticeably sooty exhaust pipe) due to missing coolant thermostat removed by mechanic when we lived in Calif desert (to keep the engine from frequently overheating) and I guess I should put thermostat back in now that we live back east, but that is not a new factor and wouldn't cause this sudden drop. Guess I'll go back to cheaper 87 octane gas & be happy that performance is good; there are worse things than poor fuel economy.

 

This might help

XJ6 Series - EFI Rich Mixture

Cheers
DD

 

Great article - thanks!

 

Wow?


1. Removing the thermostat is a weird way to solve an over heat issue.
Actually, some opine it might do the opposite.


2. Way back when, black soot in a tail pipe denoted a 'rich' fuel mixture.
No longer quite the same. Modern fuel blends just leave black deposits, even if the engine is "on tune".


3. Each of my critters have black stuff on the exhaust ends. But each runs just fine.


4. I am no longer **** on miles per gallon. Although at one time, it was fascinating to watch my Jeep's display. Light throttle, bit numbers. On t a bit, and way down.


Carl