Since this is where JagV8 hangs out

In particular on the 4.0L/SC ...

What might be the implications of the LTFT staying at +10 pct at all times, while the STFT sits at -7 pct on startup and drifts to 0 when warm? There are no pending codes other than P1111. The observations were all made at idle.

And on a less technical note, any opinions on the Obdwiz/Touchscan Pro add-on?

 

L O L

I'm not unwilling to follow a link to another forum, though the cars vary a bit and that can affect OBD.

Case in point: earlier S-Types (pre 2003MY USA) have pre-cat (upstream) HO2S sensors which are the "old" type, and are the same type as just the post-cat (downstream) sensors on all the S-Types (ignoring diesels).

The new type are WB (wideband) aka linear aka UEGO (daft name). They cost more but provide a signal that effectively indicates how much O2 (oxygen) is in the exhaust gas whereas the older type just say rich or lean and switch fairly quickly between those 2 readings.

The old type mean the PCM has trouble doing exact fuelling, which you want for lowest emissions.

So... the car here is, I think, that kind of sensor. (If it has total 4 then they're probably all the same kind.)

BTW, P1111 is good as it means the various OBD monitors ran OK. Granted, could be ages ago so doesn't rule out a problem now.

Mostly it's best to ignore STFT but it will likely switch around a lot because the way the PCM has to try to get fuelling right with the older type O2 sensor is to go a bit too lean and then a bit too rich as often as it can - it's a (computerised) control system with awkward feedback so it'll overshoot a bit in each direction. It's the STFTs that show what it's doing.

LTFT +10% means the PCM is adding quite a bit of fuel. I like nearer 0, but say 5% or less.

On many cars you won't get a code till say 25% (+25 would give one code and -25 another code).

If it's +10, engine warm, at idle, what happens if you rev for a few seconds to 2500rpm or so? If the LTFTs head much nearer 0 then you almost for sure have an air leak.

However, it could also be (for example) a MAF that's not reading properly.

On a V engine you usually have 2 trims (well, 4 if you include the short ones). If they're very similar and move (or stay) alike with revs then you're normally looking for a cause that's in common. If only one is odd then you're looking at something not shared by the banks (though you don't know which bank the fault is on).

Sorry, not used obdwiz or touchscan. Anyone put anything in the OBD tools thread?

BTW, Steve (xjrguy) put a good thread about OBD, O2s & lean codes etc with a quiz... er, somewhere. Well worth a study and try to do the quiz.

 

After some delay occasioned by the developer of ObdWiz making the foolish and very strange assumption that software assets should reside in the user temp directory ...

The vehicle is a 2001 North American spec XJR, or in the UK a 2000 reg, with the 4.0L SC engine.

The readings are:

* When the engine has idled for some time, it likes to go to -7 percent on the stft. On short idles like stop signs, it will tend to be around 0.

The stft doesn't flicker up and down as some texts might suggest, but the 16 bit Denso ECM on this model only returns about 6 PID's per second with the obdlink.

The ECM gets reset regularly due to battery disconnects for maintenance, but the last one was a couple of weeks ago, and the P1111 ready code is shown.

The pdf by xjrguy was the first thing I reviewed after buying the obdlink

 

Up to some date, probably at least 2001, the generic OBD is (via DLC pin 7) ISO 9141. That's 10.4k bits per second including framing etc, so 6 packets a second overall is about right. The pre-cat O2s would switch about 1 per second so you'd maybe have to read nothing but a single item (say left pre-cat O2). I don't know for sure how many O2s there are on that car, however! It looks like it has the WB ones pre-cat and the others post-cat. OBDWiz should support them regardless, as even the WBs have been around over 10 years.

 

+10 isn't something to worry about, but best is always to make 2 major readings, one is at idle (which you have done), and the other should be done whilst driving (50 or 60 mph). Make sure that the ltft values stabilize, so also try to find a good stretch of road where you can drive a constant speed for a while. This will tell much more than just 1 test at idle.

PS The AJ27 engine (so SC engine from 2000,) does have the wide band O2 sensors (pre cat).

 

Thanks. I forgot to mention that so far I would not be too concerned. It doesn't look like an air leak.

What I like to do is watch the waveforms of the post-cat sensors to make sure they're similar and NOT switching much. In practice, though, over here we have emissions tests every year as part of the MoT so you get figures annually which are true ones (tailpipe). You can readily pay hardly anything just to get some readings without the full MoT if you're struggling with OBD / the actual tool. (I haven't needed to do that with the STR but I did with a previous car.)

If there are no apparent problems and once you've tried what avos suggests I'd just look at the LTFT once or twice a year to check they're not getting worse.

 

Thanks to both of you for your answers.

The major test at highway speeds will need to wait until the September long weekend. On the last highway run, the need for GPS battery life won out over using the scanner.

If it makes any difference, the vehicle is usually filled with regular rather than premium.

Some reading on other sites suggests that one cause might be a dirty MAF. So, I will try monitoring just the MAF, clean it, and repeat the monitoring looking for dropouts. That would be the third cleaning in 10K.

The same goes for monitoring just the upstream O2 sensors to make sure that the voltages take a good healthy swing.

Does that sound reasonable?

 

It’s a long time ago I played with MAF sensors, but iirc cleaning them will also change their readings, so it isn't abnormal that you get a deviation.

But first do the high speed test that will say much more together with your idle test.

Have no experience with regular fuel, never tried it ;-)