MPG on the highway lower, in the city unchanged?

[pasadena_commut]

Can anybody think of a reason why the MPG on the highway might not be as good as it used to be, but MPG in the city is unchanged?

When I first bought this 2003 automatic (used) it would routinely do 30 MPG on the highway. Since I mostly drove in the city, where it would return 21-22 MPG like clockwork, even a little bit of highway driving would bump up the MPG for that tank noticeably. However in the last few weeks I have driven about 10% of my miles on the highway, and if anything, the overall fuel economy there is now lower than in city driving. The highway sections consist of one long steady run at 70 mph in very light traffic, after the car is already warmed up, where it really should be doing well. This is followed by about a mile on city streets and then it is parked for the night.

Actually I think the highway MPG is not just worse than it was, but worse than it is in the city. I have changed my driving in the city to save fuel (mostly this is timing lights and avoiding the brakes, so lower speeds and a lot more coasting) which so far gave a peak tank of 27 mpg and a couple at 24 mpg. But with the highway driving added, I'm down to 22 mpg again! There is no evidence of dragging brakes (hot hubs, pulling to one side), no CELs, no funny odors or noises, no difference in power. I'm mystified. Could the change in city driving style have reprogrammed the ECU to be inefficient on the highway???

Here's the problem, in pure city driving 24 mpg or better was observed for the last three tanks, but in 240 miles at 24 miles highway, 216 city, only 22 mpg was seen.

216 miles/24 mpg = 9 gallons
240 miles/22 mpg = 10.9 gallons
10.9 -9 = 1.9 gallons burned on (or because of???) the highway
24/1.9 = 12.6 mpg highway (!)

Even if we assume that city driving efficiency has fallen as well and is back down to 22 mpg, that still gives a result of only 22 mpg on the highway.

It may be time to change the plugs, coil packs, and wires, and hope that fixes whatever this is. Somehow, I don't think it will though. So, is there anything that would make the highway driving mpg suck but not affect lower velocity fuel consumption?

[kura808]

i read somewhere that the ecu adjusts to ur style of driving. if its the ecu, maybe u should reset it? try driving on the highway more if it dont work haha

btw how many miles r u at? might be time for routine maintenance. check ur tires too just in case.

[pasadena_commut]

I still keep seeing this MPG drop, but my interpretation has changed.

It now looks more like the MPG on the highway is OK, but MPG falls off terribly for the subsequent city driving, and then recovers. I think this because in routine city driving the car hits 1/4 tank at 80 miles, plus or minus a mile or two. However, a couple of times recently after highway driving I have filled up very soon after, and seen MPG for the just completed tank go up (as expected for highway driving), but the next 1/4 tank of city driving only returned 60 miles. The in city MPG seems to recover after that, with the next two quarters of the tank lasting the usual 80 miles. In rough numbers, if 80 miles at 22 mpg is 3.6 gallons, then 60/3.6 is 16.7 MPG, which is a 25% drop for the exact same driving conditions. And that assumes that the low MPG state was maintained for the entire 60 miles, it may well be that the MPG drop was even more dramatic, and the state is not maintained very long. (For instance, the first 20 miles at 11.2 MPG followed by 40 at 22 MPG).

The valve cover gasket (which was leaking around one of the plugs), plugs and wires have been changed since the first post, and the car passed a smog inspection with no problems. No CELs. The car runs normally, except for these brief MPG dives. No smoke, stalls, or anything else untoward.

What the heck could this be?

The ECU shouldn't reset that dramatically after only 10 to 20 miles on the highway, nor should it return to its original state within 60 miles of city driving. Are there any valves or switches that are at different positions on the highway than they are in the city, and that when stuck in the highway position might cause the observed MPG drop?

[rustychops41]

Are you using the A/C now with the warmer summer weather?

[pasadena_commut]

Are you using the A/C now with the warmer summer weather?

No. Oddly enough for S. California, it has been overcast and cool for the last three weeks solid, which includes the latest observation of this effect. Neither the compressor nor the heat has been on during this entire period.