1994-2002 [Subscribe to Daily Digest] |
Cleaning the throttle body might help idle or starting, no effect otherwise.
Engine temp reading OK? Should be at 9 o'clock. A thermostat that is sticking open will have and adverse effect on mpg's. But again, also on highway miles.
To check for compression with a 5spd transmission, put try rolling down a slope with the engine warn, ignition off, so that the engine is turning slowly. You can sense if one cylinder is leaky by the the feel of it. Otherwise get a compression guage.
I had a drastic loss of mpg's. Two issues were found. One was a split in the vacuum line for the absolute pressure sensor. That was my fault from the boost guage line tap. But it would be worth checking for that. Other leaks would have a minor effect or would simply lead to high or irratic idle speeds. The other issue, was with the O2 sensor. The sensor's ground for the signal is via the exhaust system. They found that the resistance from the sensor body to the system ground was a bit high. They backed off the sensor and cleaned off the threads and contact faces and reseated it. I had a code for the sensor when I got to the dealer. Running in limp home mode...
Are you using dyno oil? If so, sticking rings are possible. But that should affect highway miles as well.
Any sign of a brake dragging. On a highway run, coast to a stop, hills are hand for this. Then feel the wheel hubs or disks thru the wheel. Heat indicates a issue.
If you have an old exhaust system, a rusted shifted baffle is possible, but that would show up on highway mpg's.
I replaced my O2 sensor at 102000 miles just to be able to control the time and place. I spiced in an inexpensive 3 wire unit. It is possible that yours is getting old, but your highway miles say different.
Normally folks would notice a problem with there highway miles, as that is a more controller and predicatable situation.
Another issue would be a fouled restrictive catalytic converter, but again your highway mpg's don't indicate that.
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