High fuel consumption

[Gaz mr-s]

I have a Zero manifold (usual twin O2's), a Fox silencer & a catless midpipe. The latter was kind of bought on a whim about 3 months ago.
The car goes very well. There has been a lot of O2 sensor & maf-swapping between two cars, as the other one was getting quite a few codes.

So mine has a new Denso maf. It's common for a catless pipe to throw a code, so a spacer is needed for the O2 sensor.  I haven't read into the technicals of that (should have...) but I did get a code, & the spacer stopped it re-occurring.

I'm only getting 27mpg approx.  I may well have to swap back to the o/e cat to prove, but assuming it is the catless pipe causing the problem, WHY is it?

There is no brake-binding or the like... the car is easy to move.

[thetyrant]

Cat has no impact on fuel consumption, post cat 02 is just for checking if cat is working and not used for any fuelling tweaks by ecu.

Most likely suspect is the manifold 02 sensors as they control the closed loop (low speed and cruising) fueling so can have a big impact on mpg.

Do you have Torque app and obd dongle so you can monitor output from 02 sensors?

I'm presuming that mpg is not when thrashing it?  If it is then not far off, but if you get that on long run and steady driving its poor and something not right.

[Ardent]

Condition of air filter?

[Gaz mr-s]

Good shout Jason re air filter....it is pretty mucky.  Whether it's enough to cause problems...?

I do have TP Ian, but not familiar with looking at fuel trims.  I'll need to read up.

[Ardent]

@Gaz mr-s

As a sweeping generalization. The closer the fuel trims are to "0" zero the better.
 But anything below 5 is deemed ok and nowt to worry about given the age of the car.

But still better the closer to 0.

This and part 2 very informative.
https://www.youtube.com/watch?v=5WnM_NsOtd8

Edit
If filter = iffy.
Clean maf as well

[james_ly]

You could also have an exhaust leak?

[Gaz mr-s]

I don't think so. I did get one when the catless pipe went on, but that was sorted about 2 months ago.

[normanh]

Right foot? Being excessive?

Norman

[shnazzle]

I'll try one more time, this always goes down like a lead balloon... 

On our cars, and many other Toyota's, BMWs, Hondas and some French cars the post-cat o2 sensor is used to adjust fueling.
I added sports cat, which lead to a lot of p0420 errors and fuel adjustments (trimming by about -10). I installed a spacer. I've had one p0420 since on a 400 mile journey and fueling is stable between -2 and +6

On Helen's old car, it had severe fueling/running issues. Replaced the bad post-cat o2 sensor and the car was MUCH better. Still shit, but had to do with the piggyback tune.

I think I might get a voltage regulator chip, plug it in, and just prove this for once and for all.

[Gaz mr-s]

Ok Patrick.... "the post-cat o2 sensor is used to adjust fueling."
That contradicts what's been said. The air filter was pretty dirty  (no pipe on the airbox...will need to sort)
O2's have had a clean in petrol & back in. Hopefully with better weather tomorrow I'll get to check the fuel trims checked. 

[Petrus]

Ok Patrick.... "the post-cat o2 sensor is used to adjust fueling."
That contradicts what's been said.

Í thought too that the post cat sensors only serves to check the function of the cat and that the two in the manifold are in a loop to adjust the mixture.
I bow to superior knowledge though.

I have a total decat and a spacer in the pipe for the post cat sensor. No code in several thousand km. and bet the car will do or come damned close to 50 mpg if I´d cruise religiously adhering to the max speeds to Sevilla and back.

[shnazzle]

I'd rather not claim superior knowledge until I've proven it with actual data, but I've done the research and it has been confirmed.
And I've just seen evidence of it so many times across at least 3 cars. 

I'm going to see if I can steal the voltage regulator from my old piggyback ECU assembly, hook an o2 sensor plug to it and set it to 0.45v. Run it for a while and then just blast it to 0.9v and monitor fuel trims

[shnazzle]

Exhibit A, from the official Bosch Automotive Handbook. This book is no joke, it's an industry-standard reference.You cannot view this attachment.

[Gaz mr-s]

I'll add something else into this. A few weeks ago along  with fitting the cat-less pipe I took off the Zero manifold with the O2's still fitted....I was shocked to see how much the sensors protruded into the airflow!

It seems counterproductive paying a lot for a high-flow manifold & having two of the branches with bloomin' great obstructions  in them.  So I've bought two more foulers.  I haven't managed to read what the principles are of them though. O2's measures the Oxygen...the 'foulers' move the sensors further away.

The two new one's have very small holes for the gas to be sampled-through.  What's the theory?

[Petrus]

Good enough for me Patrick.

If I understand correctly it is a slower ´fine tune´ by the post cat sensor.

The ham question I now have is what the consequence of mounting it on a spacer is for the mixture.

[Ardent]

@Gaz mr-s

Not sure if you have looked over at the clean fuel thread.

But curiously both @shnazzle and I have observed a recent dip in mpg.
Mine was in the daily.
Has your mpg just gone off, or been like it for a while?

[Ardent]

Good enough for me Patrick.

If I understand correctly it is a slower ´fine tune´ by the post cat sensor.

Yep.
Slow and subtle. But it does feedback in to the overall calculation.

[Gaz mr-s]

It's kind of hard to say Jason. This is my 3rd '2'.  I got quite a bit over 30mpg with the other two. This one is a pfl but after having for just a few months the gearbox or diff was making nasty noises. That ended up with engine coming out & loads of  work, so prior to the recent regular driving it wqasn't being used.
It has a 6 speed box now.