Low MPG after changing post cat Lambda

[con1981]

Hiya fella's changed my post cat lambda sensor last week having had the mil light come on and having had the problem diagnosed by a garage

I bought a cheap code reader andcleared the fault code ( didn't make a note of it cos i thought it would just come back if the problem persisted).
I now have poor mpg. I was getting 38-40mpg but am now only getting 33mpg. No fault codes are stored however the inspection and maintenance (I/M) programme tells me cat and OS2 "not ready".

I understand there may be a particular drive cycle i need to follow?

any ideas as to how I get the cat and OS2 up and running? I do 40miles a day minimum and I would have thought these would have sorted by now.

 

[Amos91]

Post cat lambda doesn't influence fueling so shouldn't be causing the mpg issue.

 

[Diagro]

Yo OP.

Were you actually getting 40MPG or are you going by what your OBC was telling you?

My OBC told me i was getting 42MPG, but at the pump was more 27Mpg, I replaced the Lambda and its now on Average 32-33Mpg and the OBC tells me 33-35.

 

[con1981]

I'm going off the on board computer. However the lambda sensors affect fuelling hence the drop. My code reader is telling me that it would not pass an emissions as it has not done a full loop. So both cat and oxygen sensor is showing no ready. These need to be ready and they are not

 

[jesus]

Have you had the car idling for a while to test it whilst fitting? Or something similar? Mine's dropped (only slightly) after fitting the pre-cat lambda but I had it running for about 10 minutes at different times during fitting. Try resetting the trip computer, driving as normal and seeing what MPG it comes up with.

EDIT: This has nothing to do with the post directly above though. Just a thought!

 

[con1981]

Yo OP.

Were you actually getting 40MPG or are you going by what your OBC was telling you?

My OBC told me i was getting 42MPG, but at the pump was more 27Mpg, I replaced the Lambda and its now on Average 32-33Mpg and the OBC tells me 33-35.

Definitely worth a thought. I have put a full tank in when I fill up I'll calculate it to see what I'm really getting.

Cheers

 

[con1981]

Have you had the car idling for a while to test it whilst fitting? Or something similar? Mine's dropped (only slightly) after fitting the pre-cat lambda but I had it running for about 10 minutes at different times during fitting. Try resetting the trip computer, driving as normal and seeing what MPG it comes up with.

EDIT: This has nothing to do with the post directly above though. Just a thought!

Didn't run the car whilst fitting. Replaced the sensor and switched it on to clear the codes and that was it. I didn't run the car with the sensor unplugged.

I've heard there are sometimes particular drive cycles that need to be followed to get the oxygen sensor to give a reading dependent on make but I can't find one for renault

 

[con1981]

http://www.aa1car.com/library/obd_monitor_not_ready.htm

Here's the link

 

[con1981]

Bump

any suggestions fellas as to why I can't get the inspection management system to switch on?

Plan for this weekend will be to swap the two lambda sensors round and see if that makes any difference.

 

[leew88]

the postcat (downstream) lambda only tells you if your cat is working correctly-if you unplug it the car would run the same

the upstream controls fuel trim-id be looking at that and/or cat

 

[con1981]

Well filled up today having had the on board computer say I'm getting 31.7 mpg worked it out on a manual calculation and I'm actually getting 32.8. So not far out.

Tried removing the up stream lambda today and failed. This thing is welded in! Began to round the nut off. So thought this was a good time to stop. I think it is going to need some serious heat to get the shitter out. So I'm going to wait until I have a spare lambda before i do this as I'm frightened I'm going to break it in the process.

 

[jesus]

Well filled up today having had the on board computer say I'm getting 31.7 mpg worked it out on a manual calculation and I'm actually getting 32.8. So not far out.

Tried removing the up stream lambda today and failed. This thing is welded in! Began to round the nut off. So thought this was a good time to stop. I think it is going to need some serious heat to get the shitter out. So I'm going to wait until I have a spare lambda before i do this as I'm frightened I'm going to break it in the process.

Do it straight after a long/hard drive. It may be warm enough to remove then!

 

[con1981]

Do it straight after a long/hard drive. It may be warm enough to remove then!

Will do mate-the post cat lambda came out easy can't believe how much of a b**ch this was has been. I've got a small blow torch but it just didn't cut the mustard- couldn't even get the the thing to glow

 

[dann2707]

I got my paint stripper heat gun on mine and it came straight off after fighting with it for 15 mins when it was cold!

Blow torch should work wonders mate!!

 

[jesus]

I used a lot of WD40 after leaving the engine running for a bit on mine. It was pretty much welded in too but eventually came out using a massive spanner.

 

[con1981]

I used a lot of WD40 after leaving the engine running for a bit on mine. It was pretty much welded in too but eventually came out using a massive spanner.

Will give this a bash next weekend. Cheers fellas