Why do exhaust blowouts return poor MPG?

cyborg · 2 Apr 2007 at 14:41

Question in thread title, in my situation the blow out is near the back, and if im working this out right im getting about 18 MPG. Car is a toyota celica GT.

Phil W · 2 Apr 2007 at 14:58

Lack of back pressure usually, well thats what i was told anyway

cyborg · 2 Apr 2007 at 15:02

But why does that matter, i would have thought the lambada (im probably not spelling that right) sensor would take care of it. Its after the centre box/cat before the back box, so its just missing out on the back box.

Phil W · 2 Apr 2007 at 15:04

cyborg said:
But why does that matter, i would have thought the lambada (im probably not spelling that right) sensor would take care of it. Its after the centre box/cat before the back box, so its just missing out on the back box.

But if theres less back pressure, the lambada won't be working as it should be, would it

Durzel · 2 Apr 2007 at 15:09

The change in back pressure would cause the AFR (air/fuel regulator(s) ) to behave differently. Usually this means they run rich, so you use more fuel to cover the same amount of distance.

cyborg · 2 Apr 2007 at 15:10

Im just working this out, well trying to, I think its worse than 18MPG. I've done 59 miles since last filling up, and that was £15 worth (17 litres say?) and its nearly an empty tank again. Isn't that more like 14MPG?

Jonnycoupe · 2 Apr 2007 at 15:17

The leak will suck air in once the engine is loaded up with a decent gas speed in the exhaust.

This then leans the reading the lambda sees, so to compensate the engine fuelling increases. The lambda then sees around stoich BUT the engine is actually overfuelling by a significant amount hence effecting MPG.

Thats only really gonna happen if the leak is upstream of the lambda sensor though.

cyborg · 2 Apr 2007 at 15:41

Using an online calculator im getting 16MPG. May as well be driving an EVO! Yikes.