Hi everyone,

Had a long motorway journey the weekend, which confirmed my suspicions that the economy has gone out the window.
Total - 212 miles
Avg Speed - 38mph
Avg MPG - 39mpg

Motorway leg (80mph out of traffic)
Total - 148 miles
Avg speed - 56mph
Avg MPG - 40mpg

Now I used to get somewhere around the 50's-60's so whats the big idea?
It's had two tanks of Redex in the last 2 months prior to MOT.

What can I start cleaning and testing?

For better economy, assuming that your car has no faults from sensors (MAP sensor etc), if it's a gasoline engine change the spark plugs if they are old. Also think about cleaning the injectors (i think it's better to pay a mechanic to do this). For diesel engines i have no idea. If all the things above are in good contition you can also check your tires if they are worn, and your suspensions too as they can increase your consumption.

Does the temperature reach 88C (around half way on the gauge).

If the thermostat is knckered the beast will run at 60C (1/3 on the gauge) so will be over-fuelling (on "choke" in the old money) to make up for a cold engine.

Does it idle okay and accelerate through 2000rpm without any hiccups or sluggishness? If the lambda heater circuit has died you'll have a rough idle until transition through to "above idle" rpm. If the lambda itself has stopped working then it's the other way round. You get a cough as you transition through the switch-over point.

MAF and EGR issues usually result in poor power delivery.

Spark plugs or coils won't affect economy to the extent you've suffered. The beast would be missing on the affected cylinder. It's more likely to be a "fuelling" issue more than a "burning" issue..

You could have a duff injector but you can only measure this by testing the through-flow of each injector separately. You would see more soot in the affected cylinder though (same as running "rich") so have a look at the individual spark plugs to see if any are particularly black.

Also.. presume your air filter is not blocked.. but if the poor economy just appeared out of the blue, it's not a dirty/blocked filter causing it.


Ralf S.