As part of my ongoing preventative maintenance, I replaced the two upstream o2 sensors this weekend. I used OEM Denso parts from Rockauto which were around $90 a piece. I also needed to source a 22mm wrench to loosen/tighten the sensors. The hardest part of the job actually was disconnecting the electrical connectors. The plastic becomes brittle over time and it took some time to loosen their connection.

I put the front of the car up on ramps and shimmied my way under. The sensors are very visible, not hidden in any way. Everything was very straightforward.

I don't know yet whether I'll see any real difference in fuel economy, but my idle vibrations have subsided a bit, so it was worth it just for that.

Old on left, new on right:

 

I've got a pair in my '100k tune up' box too. o2 sensors are designed that as they age (die), they read richer and richer (better using more fuel than running lean and blowing up).

If original, I expect to see a mpg or two come back.

Someone has pointed out on the forum that the jag actually uses the rear sensors to tweak fueling. That's a first for me seeing an ecu that does that - typically they just read the rear sensor to make sure the cats are working (as long as there is a differential in the oxygen reading, it knows the cat is working). I wonder if that tweaking by the ecu is simply for emissions though, and not a tuning factor per se.

 

Interesting. I was under the impression that just the upstreams had an effect on engine management.

 

The upstream sensors are wide-band and the downstream are narrow-band
The primary sensor that governs the STFT is the upstream one
The LTFT is generated by the ECU using the STFT data
The downstream sensors monitor that the cats are in range.

Most learning ECU's also use the downstream in conjunction with the upstream during warm up to heat the cats quicker. They also provide a fine trim for the upstream sensor data, the course trim being the upstream sensor

I have read in jag docs about the warmup function but cant place where. I think the tuel trim primer doc mentions the downstram trim but would need to re read it. I cant place reading it in any jag docs though

Cheers
34by151

 

Good info 5134, I didn't know that they were wide band sensors. (I'm used to them being much larger; dyno wide bands/tuning).

Is LTFT and STFT long term fuel trim and short term fuel trim?

 

STFT = Short Term Fuel Trim
LTFT = Long Term Fuel Trim

Its really worth reading the "fuel trim primer" pdf as you will learn a lot but may need to read it a few times till you get what its talking about

I have a car mount bracket for my android phone. When i put the phone in it goes to troque automatically and starts the main gauge screen.
On this I have the 4 fuel trims, Lambda (upstream sensors) and AFR (Downstream sensors), Oil pressure and Water temp

Touque is also setup to alarm when some of the ecu parameters go out of range such as the IAT, Water Temp, oil pressure ect. This not only gets around some of our guage issues (water temp) but lets me know as soon as things go wrong and before damage can occur

Getting to know the fuel trims is the most important measure of your engine health and will lead you to the source of most faults. It will also tell you before these faults happen and when you have fixed them

The point of all of this is spotting when the trims begin to go up
Normal (very healthy) is close to zero, meaning the ECU is running as per the factory map, OK is under 8%, bad is under 15%, over 25 is a mil alert and restricted performance message

Lets say you have a very slight vac leak but nothing to trigger an ecu alert
The trims will be above 5-8% or more telling you to look for a leak but also causing you to use more fuel. Now this might be plugs/coils ect but if a vac leak gets less under high load. So if you watch the trims while you punch the throttle. Wahtch the trims and if you see the trims come back closer to zero you have a vac leak. Vac reduces less under load and the ecu knows and expects this, so the leak gets less under load and the trims go closer to normal. If the trims get worse you have something like a plug/coil type issue

Now this is simplified and reading the "fuel trim primer" will give you a lot more understanding but you get the idea

To give you a real world example after a recent oil change my trims rose from 0.8 up to 1.5%, not much and most will say under 8% is normal. Putting 2 and 2 together I stopped, wiped the o-ring and reseated the dipstick. Result after a few km's trims back to 0.8%. Why, a very very small vac leak from the dipstick probably was just some grit on the oring or tube.

This gives you an example of how to keep things really healty so when you do have an issue begin its easy to identify and fix rather than when something happens (trims maxed out at 25%) you have the actual fault + lots of little ones to diagnose and confuse the issue. If you are paying a mechanic this equals a much bigger bill. If you are not yet generating a fault you are losing power and using a lot more fuel both good reasons to keep on top of things.

Cheers
34by151

 

For Touque read Torque (there's also Torque Pro).

Acronyms below

 

Wow, that was very helpful. I must admit I never thought to implement Torque in this way, but rather as a gauge suplement. This type of proactive engine health monitoring is intriguing though it requires some foreknowledge. It think reading (and re-reading) the fuel trim primer just went a few notches up in my priorities.

The examples you used explained the value of this type of monitoring inn plain and simple terms that show its valur


?

 

For the record I have and use Torque Pro, I also have some of the plugins but the only one I really use is the track recorder

This takes video from the phone with the gauges overlayed on the screen

It makes sense to read the "fuel trim primer" a few times over
It has some examples in it and some practical tests for your understanding

I expect you will have questions after that so it would be best to start a thread on the trims where we can discuss it

Also my engine varys slightly from the OEM map (using trims) due to intake (cadofy) and cat converter (100 cell cats) changes. I havent yet fitted the S/C pulley upgrade that is in the toolbox but this will alter the trims a bit as well due to the extra boost.

Its also worth, while you are getting to know the ecu and how it works, having the 4 trims, boost or manifold psi, and open/closed loop

When you mash the throttle you will switch back to open loop. Its worth getting in your head how this works and how it relates to the o2 sensors. BTW Trims are different in open and closed loop closed loop

Cheers
34by151

 

When I did my new 200 cell cats and header pipe upgrades

I replaced both upper and lower 02 sensors also got the Denso's from Rockauto also did map and maf sensors plugs and a few other things it had 165,000 k,s on the clock

After the high flow cats where fitted I kept getting fault codes saying catalyst inefficiency problems so had the car remapped on the dyno I am pretty happy with how she goes now LOL

Rule of thumb when you replace cats replace 02 sensors as well they do wear out and they do end up running rich
Oh and they are a bastaxx to get out that's for sure

 

Doc,

I used mincats on the downstream sensors for my 100 cell cats

Using the minicat means the downstream sensor is almost useless in terms of detecting the cat efficiency

These also work for a decat. I used the 100 cell because it almost as good as a decat without the heavy fines. It a massive fine for removal of the cat but not much if you fail emissions

As for fitting mine was easy. I posted pics of the install at the time

Cheers
34by151

 

I just did 2 upstream 02's on the 2004 XJR and the ce went away now I am getting a surging issue at idle and while cruising, seems down on power too.

 

Also forgot to add I did a hard reset while I was dong the 02 sensors. I had the battery off all day before I did the job. I did do the pedal reset to no avail. It just moves 50-100 rpm at any speed. Serious vibes to when i first start the car. All I did was swap 02 sensors and disconnect battery very annoying.

 

Old thread but very professional. So I thought I may post my problems here.
I got error massages for both 02 up sensors and bought a set of four to replace them.
I couldn't get them out so I turned it into the shop and they replaced them and it passed the German mot.
But the car doesn't run well.
When idling it very Very little changes rpm in a constant sinus like up and down. Like it looses a bit and then throttles back up. Very minor but I notice.
When accelerating normally with a bit more force it does the same. I can't accelerate constantly but I'm 'swinging' more less more less..
Ann suggestions on what's wrong here?
Before this I just had an idle that got worse over a year and sometimes at around 100km/h only with the a on. I lost torque shortly sometimes when giving little gas.

 

You need to have the vehicle scanned for codes first

 

It was 1646..but after replacement there are no more codes and it passed the MOT

 

@Vasco Pridat What year car do you have?
Is oscilation, about 200rpm, while waming up or constant all the time?
I had similar experience during warm up, but it ALMOST gone away after chaging transmission fluid, and some other transmission maintenance: Now that oscilation under load (driving on flat road with constant very little throttle, no acceleration) is barely noticable and present only on COLD starts in the morning untill coolant reaches half way normal operating range.
It seems like coinsidental: what does transmission fluid has with engine RPM. However there were some reports, you can search for that in this forum yourself, that some early models 2004, for instance had some kind inconsistensy with torque converter. Some members reported that TCM flash fixed issues, some had no effect. My car had TCM flash under previouse ownership, there is sticker on A-pilar on driver side.
I am not vorried about it: it is so small that the only way you can notice it if you watch tach needle and only during warmup 3-4 min in summer and in cold part of the year car is not moveing out of garage!

 

I have a 2001 4.0..
It has only started since I put the new ox sens in. Hm

 

This part of the forum is for 2004 on XJ8's you need to post in X308 section.