Couple of months back I had the fuel pump go out on me. I dutifully replaced it and noticed over a couple of weeks that the MPG had fallen by a couple of MPG. Typically it ran around 18-19 MPG for Town and Highway. Did the normal MAFS cleaning - didn't work. Replaced the air filter (it was due), which always brought up the MPG a tad - didn't work. Diligently checked for leaks, but there were none.

So, I settled into accepting the 17+ MPG, but did notice that on the highway the MPG would not improve as well as it had in the past. About three weeks ago it fell to 15 MPG and wouldn't move from there.

After pondering, I decided to replace the battery, which was about five years old. Lo and behold, the MPG came back to a higher figure and now hovers around 18 MPG overall. When I changed the battery, I had the store check the voltage and load test it. Load test was okay, but they did say the voltage was low.

I was wondering if anyone had experienced this and even more puzzling, why did the battery change affect MPG?

Cheers,
Hamish
MY 2001 XK8 - 77k Miles

 

no idea Hamish, absolutely none. sorry.

 

It's not apparent that there should be any relationship between the battery and the mpg. A longshot could be that the battery was somehow so weak it was causing the alternator to run more/harder thus impacting the mileage but I don't think that is correct. Could replacing the fuel pump somehow affect the fuel pressure and could that impact the mileage? Finally, were you checking your mpg based on actual manual calculation or were you reading the onboard computer?

Doug

 

I could verify, but any ecu is going to take into account battery voltage to generate appropriate fuel trims. A good test to see what is going on is to see what your long term fuel trim levels are looking like over a typical drive in order to get some idea into whether it's running overly rich at a given point or across the board. During our development work on the XKR that is running our catalytic converters, we have noticed a lot of regular maintenance items and system soon-to-be-failures that effected the fuel trims on our car from stock, which made our first round of dyno trips a bit bunk - running a car with bad fuel pressure and potentially leaky injectors that was running massive quantities of fuel. Our baseline for the car was done last summer and it made about 3 more hp with the new high flow cats, and was running flat 10:1 AFR above 3850rpm. The baseline it was in the mid 12's. So we're excited to get these other issues with the car resolved to be able to get an idea of what kind of improvements we can expect over a properly functioning stock vehicle. Similar issues to these on a stock car are going to radically effect the mileage of the vehicle.

Do you have any ability to connect it to a OBDII diagnostic unit like an AutoEnginuity?

Jason Griffith
Nameless Performance, Inc.

 

I checked the MPG by both computer and the old fashioned calculator way and the computer was accurate. I did have a couple of codes, but didn't record them. One was pretty inconsequential and the other was "high voltage at fuel pump" or something along those lines. Regardless, they didn't trigger the CEL, hence my decision to change the battery.

Knowing that these fine automotive examples are finnicky with voltage requirements, I was curious if some other system could affect MPG.

Perhaps we'll never know.....

Cheers,
Hamish
MY 2001 XK8 - 77k Miles