The higher in elevation you go, the less oxygen (lower atmospheric pressure) so the computer injects less fuel to keep the mixture right. Back in the day, cars sold in Colorado had smaller jets in the carburetors to compensate for the altitude. Occasionally, people visiting from sea level would have their cars stall out on some of the higher mountain roads because the fuel mixture became too rich.
The trade off is you also lose power the higher you go, I believe it is 3% drop per 1000 feet in elevation gain. That means if you live in Denver, your car has 15% less power than at sea level. At the Eisenhower Tunnel on I-70, you are down to 2/3 the power you would have at sea level.
Let us assume this is age related. Any guesses on what components I should pay special attention to correct?
Note: I do not have mpg computer readout. I fill tank to top every time I get gas, squeezing the handle three time to really top out tank (not recommended) and record in spread sheet the gallons and miles. Have been doing this for last 11 years (purchased in 2004 w/60K).
Some here have suggested it was my driving habit other feel the mpg was within range, I disagree. Where yes the up and down hill, wet roads, dry etc. will and did make difference, this does not explain difference I saw. Fact return leg out of SA TX and last leg up to Denver were both ascending, in addition leg to Denver was rainy wet roads for 70% of time. This last leg should have taken more torque/HP and should have been the worst mpg, with resistance of water/rain and pushing up hill.
I've always keep my LC at top notch condition end to end doing all FMS maintenance timely. I've only taken on one other road trip out of COLO before this trip, which was to LV NV. The mpg was as expected on that trip. Only differences were 6 years older and 40K more miles (120K on odometer then) and I used a K&N filter back then.
BudgetLC: I agree the CPU leans gas mixture at higher altitude, and doesn't it also adjust timing. But wouldn't I need less torque/HP to achieve same speed at lower altitude as I'd have more available power. When climbing up I-70 to Eisenhower I need to keep speed up pressing the gas pedal more and more as power noticeable drops, this is an mpg killer.
I'm thinking the computer or sensor input like o2s, mass air flow or something downstream like injectors are not adjusting fast enough. Note: I found (about two years ago) after developing the dreaded vibration at stop lights/sign, that resetting the CPU by disconnecting battery, each year (in the fall as gas change to winter blend) was key to eliminating the vibration. I've read it takes about three tanks for CPU to adjust to changes in gas grade, but I did expect real time adjust for altitude by the CPU system.
I expected better mpg at lower altitude when setting cruise control at a constant speed. As I'd need less HP or have more power available for same speed. Note: I noticed less power one time when I punched it to pass at a lower altitude, this concerned me. MPG is rated at sea level IIRC and I've always expected slightly lower mpg at higher altitude. I understand the CPU is to correct the mixtures accordingly and timing, as we once had to do by changing jets and timing. Could it be the CPU is taking to long to adjust for some reason?
Drops in mpg always concerns me as it can indicated tune up is needed, it is the main reason I watch mpg. My LC has always had all FSM done timely, and was in top condition for this trip. I suspect ageing component(s), but which one(s) can I check, replace or repair to correct this? My biggest concerns are: mixture too rich on fuel side washing cylinder walls or too lean running hot (which temp gage will not show in the LC).
Let us assume this is age related. Any guesses on what components I should pay special attention to test & correct this issue?
