A while back I had a few issues with my 3.2 x300, slight idle wobble, low mpg and sometimes stalled when coming to a stop at traffic lights. I changed the spark plugs to the recommended champions, cleaned the throttle body and changed the air intake temp sensor and coolant temp sensor. Also had a few exhaust holes repaired along with the exhaust manifolds being replaced.

Now all of the above is pretty much resolved except the mpg, I’m still only getting about 16 to 17 mpg on long steady runs of over 100 miles. I’ve had another x300 before and was always getting mid 20’s or higher on the big runs.

My thinking is, the only exhaust holes I may still have is in the down pipes from the exhaust manifolds, these were only ‘roughly repaired’ by a mechanic as he didn’t have time to take off and weld or fix properly.

my question is, could holes in the down pipes be causing this issue or could it be the O2/lambda sensor causing over or under fueling, or the sensor getting a false reading due to the exhaust hole ?

im in the uk so have a couple of questions if I were to replace the downpipes with a second hand or new part.
1-I have the with cat version of the downpipes I assume as I’m in the uk?
2- I know there are only 2 lambda sensors on uk cars? Is it just the one on the downpipes that calculates fueling?
3- will I need the exhaust with the olive type or slip type joint connection to the intermediate cats?
4-on many of the new downpipes there is no heat guard like the original parts, will this matter?

thanks for any help!

 

I’d been chasing the same issue for the last seven months and finally fitted a new o2 sensor on the near side as the mot had ten times the emissions it should have had !

after changing the sensor I now get over 30mpg on a good run and mod twenties round town !

and no more stalling now thank god but I do still get an occasional idle wobble when cold that I haven’t found

I suspect that it’s a vacuum leak or air leak to exhaust above the lamda sensors that seals up once the car is fully warm ,as after getting hot it doesn’t wobble at all at idle and is like a new engine

 

Now that is an impressive MPG and exactly what I need!

when you say the near side sensor, do you mean the one that is in the downpipes?(only one here in the UK as opposed to 2 in the downpipes and 4 in total in other markets?) or the other O2 sensor?

my concern is that the hole in the down pipe may be what’s causing the O2 sensor to get a false reading or it may be a bad sensor, or both?!

I’ve ordered a scan tool that is supposed to be able to read fuel trims on these cars and possibly sensor data, that may give a clue but I do suspect my problem is the same as yours x300exec.

 

One sensor is near side down pipe and one off side down pipe ,mine was near side that had failed 👌🏻

if you shine a torch down the back of engine between fire wall and head you’ll see them both

 

A hole in your exhaust upstream of an O2 sensor will likely produce an excessively rich mixture, leading to poor fuel economy. A hole after the O2 sensors, whilst not ideal, should not affect fuel economy. Basically, a hole allows oxygen IN to the exhaust between pulses, and misleads the O2 sensor into thinking the engine is running lean and needs more fuel.

 

Thankyou!

well I think first I need to find where on the downpipes the hole is, either before or after the Oxygen sensors, then I can repair that before possibly/probably changing the sensor.

Being the idiot I am, do any of you know if the down pipes in the uk are normally or always the slip type connector to the under floor cats or the olive type? And is the heat shied essential as I see new parts most often don’t have it?