Low fuel economy - Rover 420Si

You should probably be getting low 30's in town (possibly less if you do only short journeys) and mid-high 30s on normal motorway driving, so if your figures are correct there is something wrong with your car.

I don't know much about the specific engine, but things to check are things which affect fuelling - lambda probe, air-flow meter (if fitted?), leaks in inlet manifold/piping. If none of those things are amiss then second order checks are things like sticking brake calipers, dodgy spark plugs, fuel injectors and a compression test.

If you don't have the test equipment, it's probably easiest to take it to a tame mechanic and ask them to check the obvious things - a consumption as high as yours should have a fairly obvious cause.

Start with the simplest thing - check that your air filter is clean!

Regards

Paul

That's a good start. Also, if it is predominately a town car I usually change oil and filter every 6k due to the extra battering with all that stop start motion.

After that, start checking the hoses etc.

PDH

But that shouldn't significantly affect fuel economy, surely? As long as the management system is operating in closed loop, that will give a reduction in higher-rpm power, but the mixture should still be correct, so economy should be broadly similar.

Not if the engine is being starved of air intake, making the burn go rich to a point where the ECU cannot cope. This also applies to things like hoses where leaks can take the operating parameters of the ECU outside of its normal envelope.

PDH

The MEMS system does not have an airflow meter so it cannot compensate for a clogged filter, plus the closed loop system only operates at part throttle cruise and idle conditions.

Ahh. Ummm. At the risk of revealing my utter ignorance: but if there is no air-flow sensor, surely the mixture control is purely derived from the lambda sensor feedback, in which case I don't understand why it doesn't compensate for a clogged filter (ignoring the minimum-fueling limit)

I was assuming that he wasn't driving everywhere at WOT - if he is then 18mpg probably isn't bad. :-)

Thatz great information ... I will get the fuel filter and air filter changed and recheck the fuel consumption.

Thankx for the replies.