Dirty air filters

[iwaters]

Quite often on the Internet in various forums I have seen people put down the cause of low MPG or rich running to a dirty or clogged air filter.

This is an old wives tale surely? All fuel injected cars have some means of determining the air flow, MAF or MAP etc. and then inject the correct amount of fuel based on that reading. As the sensor is after the air cleaner then the engine must be aware of the reduced air flow and will inject less petrol?

Have I missed something?

 

[timtheenchanter]

I know where you are coming from, but IMO clogged air filter means less power, so you end up with a heavier right foot to get the same "go"

it might be my imagination, but my car is less responsive when the oil is due for a change...

 

[nickso]

i wish i could find it but i read somewhere that up to 40% of airflow is lost in just one year. probably pays to change it annually no matter what the service schedule is.

i often remove mine and clout the loose stuff out, whether that makes any real difference i don't know.

 

[gregers]

i think its more to do with the cars ecu it determines the ammount of fuel/air mixture that is needed to run at optimum spec,if any 1 is given false readings then the ecu has to compensate.

this is the reason why there are service intervals,to make sure everything runs properly.

 

[Avocet]

I think it's (largely) and old wive's tale - and for the reasons the OP cites. In addition, a blocked (or partially blocked) air filter will almost certainly flow plenty enough air at low engine speeds and throttle openings. When the manufacturer determines the required size of air filter, he will do so based on the maximum amount that could be drawn through it. That usually means the most powerful model in the range at full power. ON TOP of that, he'll make an allowance for dirt obstructing part of the element. Usually they stipulate more frequent filter changes for vehicles used in unusually dusty conditions. If your handbook tells you to replace it every 12,000 miles, you can probably guess that the air filter specification is a bit marginal. If it says every 36,000 miles, there's probably a lot of "spare" area in it. If you check in the parts books at Halfords and find that (say) the 2 litre GTI version takes the same filter element as the 1.2 economy version, you can be pretty sure that the filter in the economy version is way over-sized and won't need to be replaced as often. Either way, the symptoms of a clogged air filter are usually lack of power at full throttle / full revs, because that's when it will need to flow the most air.

 

[TicklyT]

A blocked air filter could certainly cause rich running and low MPG problem on an engine with a carburetter, and there are still a few left on the roads. It would probably have little, if any, effect on a modern injection system, unless the mass air flow is determined by measuring the pressure drop through a venturi (which is roughly how a carburetter worked anyway). Even then, a lambda sensor in the exhaust gases would probably attempt to correct the rich mixture.