Mellow Yellow's Poor Mileage

Keeps me busy. 

I am trying to sort out what seems to be lower than average mileage for this model/year.

"Maybe I should get a life..."

What I know

This is a 1984 1.9L auto transmission with 75,000 miles on the odo; 11mpg city, 15.5 mpg highway; one tank of gas gets me about a 240 (390km) mile range on the highway, sea level driving or mountains same thing. 90% of my driving is highway.

Associated Information

  • Odo is accurate per GPS and highway "speedometer test" signs
  • I drive at or below posted speed limit as confirmed by GPS and roadside "your speed is" radar trailers with big digital display
  • I am not a leadfoot. My speedometer reads about 12% high so I'm always thinking I'm going faster than I really am
  • No special gasoline brand preferred
  • Tires are 185R14 aired per sticker. Based on tire wear, alignment seems to be just fine.
  • Engine passes California emissions
  • No sign of coolant loss
  • Engine temp reads normal
  • No sign of fuel leaks
  • no canoes on top or trailers behind

Things I have tested and seem good:

  • Cylinder compression test results: 135, 155, 155, 155 (#1, #2, #3, #4)
  • Not Running Rich: O2 sensor indicates that mixture fluctuates rapidly between rich and low once ECU switches to closed-loop mode as indicated on oscilloscope and on Ken Lewis O2 monitor gadget mounted on dashboard. A new O2 sensor was installed 10/2007, made no difference to mileage. Plugs and tailpipe don't indicate rich mixture.
  • Throttle Idle Position Switch works. I have an LED lamp wired directly across the switch, mounted in the cockpit and when foot is off the gas, the lamp goes dark, which means that the switch is closed. Since the switch gets its 5V from the ECU, the ECU is seeing the signal.
  • Loaner ECU does not improve mileage.
  • Timing tested and idle, centrifugal advance, vacuum advance, and vacuum retard all seem to be within spec. (Note:
  • New rotor, cap, plug wires, coil "just because."
  • Fuel Pressure measures correctly per Bentley's.
  • Temp I and Temp II sensors measure correctly at ECU connector.
  • Intake manifold vacuum is good: 11'' Hg. Bentley's does not directly tell us what normal manifold vacuum should be, but it can be inferred from several of the test procedures in the manual, i.e., ignition settings p. 28.30, which specify maximum vacuum of 360 mbar (27 cm Hg = 10 - 11'' Hg), which is presumably what to expect when the engine is idling. I measured the vac on a second Vanagon in good condition and got the same reading.
  • Brakes are not dragging: checked temps with IR remote-reading thermometer after checking out temperatures of anything that would stay still because it's such a cool toy. 
  • The Torque Converter might be slipping a bit. Tested per Bentleys, the engine stalled out at 2830, depending on the test, as indicated on a Tenma digital engine tester (voltmeter, dwell, rpm, etc.)

Comparisons -- see

Theories to be tested:

  • Exhaust leak? A leak somewhere between the engine and the O2 sensor could cause the engine to run rich to compensate for the leaner mixture measured by the sensor.
  • Notes to self:
    9.20.07, round trip to Laguna, 16.7mpg;
    11.19.07 prior to wheel alignment, timing advanced 13.7mpg city.
    11.29.07: post wheel alignment, city 15.6, Laguna, 17.9 mpg.