Just when I thought I could devote all my energy to my XJ-S, my VDP runs richer than before.
My Torque says LTFT -10 ~ -13 at idle, bit less on the run.
Got the feeling it is slowly getting worse.

The air filter is clean, as is the engine bay, inlets etc.
Problem is on both banks, and the various values for both banks are almost, if not complete, synchronized, letting me believe it must be a 'shared' issue.

I re-cleaned my MAF, dried it, and reinstalled, but no change.
New iridium spark plugs, cars run super smooth.

Just checked MAF readings, seems to be some 15-30% higher then indicated:
- Park, idle: 6.6~7.0 (should read 5.1)
- Park 2500rpm: 22.5 (should read 17.0)
- Drive idle: 7.3~7.5 (should read 5.9)
- Drive 1500rpm: 45 (should read 39.3)

Now there is plenty of info on lean running X308's, but running rich is much less documented.

Am I right to focus on a failing MAF sensor, or are there other things to check first?
I am a bit out of ideas.

Eric

 

Eric:
I assume they are generally the same on both banks? I guess it will turn out to be MAF.

 

There are some test you can run to sort it out. Did you have a check engine light or codes set before you discovered the rich condition?
With the engine running at idle and warmed up you can unplug the MAF sensor and read the fuel trims. When the MAF is unplugged the ECU revertss to a default fueling program. If they dropped to a normal range then the MAF is probably bad.
If they still stay rich then I would look at the Fuel Pressure Regulator and insure that you have 45 to 50 psi on the fuel injection system. If it is higher i would replace the regulator and start testing over. High fuel pressure will cause the injectors not to seat correctly.
The MAF also compares engine temperature against air temperature so you might also want to check the coolant temperature sensor.

 

Yes, as indicated, both bank are basically synchronous, and both run rich.

Avern,
No codes, no CEL, I just noticed it on my Torque app over the last period of time.
I had perfect "around zero" values before.

I warmed up the engine, had same readings as yesterday, then disconnected the MAF as u instructed.
Engine did not idle, and threw a LP mode, but kept it running with some help.

LTFT did not move at all.
Then did a hard reset, the STFT went into PLUS 5-15, the LTFT stayed at zero.
Connected everything, cleared codes, did another hard reset, and LTFT went to -5, then slowly crept further to -8 on stationary idle.

I do not see strange things on the Torque app for coolant and air temperature, I do agree you never know, but I don't suspect them as the cause.
I can't test the fuel pressure right now.

One thing I thought, I used the car a lot on relative slow, low rpm, stop and go rides.
It is a long time ago it went free on the highway.
Besides of the MAF readings I got, might this have an effect?

Eric

 

Eric....methinks it's time for an 'Italian Tuneup' ;o))
Give it a good flogging for say 150 miles, then redo all your tests.
If the O2 sensors read OK...it's down to the MAF ;o)

 

Yeah, guess you are right, might be more fun than disconnecting sensors ....
The car runs absolutely perfect, perhaps I should disconnect Torque instead, and just enjoy.

 

I think you have dirty injectors. spitting not spraying.

 

Jim, of course I thought about it, but I wonder if that should not happen on one bank with just one injector?
STFT and LTFT remain identical for both banks in all conditions.

 

They are all dirty. Just to different degree. A can of BG 44 or whatever is best in your market may be all you need.

 

I am still a novice to all this fuel trim stuff, but it seems from your first post that the MAF is over-reporting the incoming air. Am I correct? Would this not cause the computer to over-compensate when adding fuel?

The oxygen sensors would try to correct for the MAF error by adding fuel, right?

I am just speculating but it seems the MAF is a likely suspect for the problem.

 

Harvest,
I think you meant it right, but reversed it when writing down.
Still hope an other expert can confirm it is right or not.

I think it is as follows:
-MAF reports more air then actual going through
-ECU injects too much fuel (based upon the MAF reading)
-O2 sensors measure too rich mixture after combustion
-ECU takes fuel away (hence the MINUS LTFT)

For the moment I tend to focus on a shared issue, like the MAF.

Still don't rule out things like the dirty injectors as suggested by Jim.
The fuel here sucks if left alone for some time, my scooter refuses to start if not used for 3 months.

I used the car rarely in the last months, and just for short distances, and that might fits fuel issues.
Will just take her out for a long run this week ..... if only I had more time

 

That is right Eric. Positive trims add fuel, negative trims subtract fuel.

 

Yes Eric, that's exactly what I was "trying" to say, sorry. I left out the step about the ECU. I should make a rule to never post from my phone.

 

Yeah, I know that you know what u'r talking about, I followed your "lean" problem from the beginning.
Perhaps our cars should mate, with perfect fuel trims as a result .

I blew some dust out of the old lady this afternoon, but the trims remain the same, although the MAF reading on idle came a bit down.

I don't know if it is in any way related, but my MAF is some 15% off on idle from what it should read, and my trims are some 15% off as well.
Perhaps a too simple conclusion to blame the MAF sensor, but what?

I saw a X308 parts car at the local Indy, see if I can exchange the MAF sensor before buying a new one.

 

I have my order for a new MAF sensor ready, but yesterday, while doing a 100km tour, I noticed the O2 2x2 sensor acts slugish.
Not dead, but just up or down once a while depending engine load.
1. Does this mean it is dying a painful dead?
2. Is the O2 2x2 sensor in Torque Pro the bank 2 downstream sensor?
3. How easily or difficult are they to reach?

Eric

 

O2 voltage fluctuation is absolutely normal. I believe the 2x2 you refer to is bank 2 sensor 2 aka the down stream bank 2. They can be a little tricky to access because of limited wrench throw space without the proper tools or by making it really easy by dropping the exhaust after the cats, but it can be done with as little as a 22mm wrench if you've got the will. Don't think you'll need to though

 

Thanks, Greverrr.

The sensor is definitely behaving different then before, and also completely different then the same sensor for bank 1.
I had some screenshots from Torque, but the pictures corrupted when I copied them.

Anyway, if you also think it is the downstream one, I will order a replacement together with the MAF sensor, and struggle with it when it arrives.

Eric

 

How deviated are the voltage numbers at the downstream sensors? Are they within range still? I would suppose they must if you haven't thrown a code. Has the car run through all of its OBD2 cycles since you last cleared a code or did a hard reset out of curiousity?

 

I thought, perhaps mistakenly, that the downstream oxygen sensors had no bearing on fuel trims. My understanding is that they only monitor catalyst efficiency

I don't remember my source, but that's how it was explained to me by a more experienced member in a previous thread.

 

I only have the Torque info. The problematic bank#2 stays either up (.9v) or down (0v), where the bank#1 happily jumps around at any voltage depending engine load.
Where bank#1 changes immediately with accelerator changes, the bank#2 only does so in 5 or 10sec intervals, and then to either max or min.
No codes, and the OBD resets to P1111 after each and every hard reset.
This all is with the old MAF sensor running +15 max, bit lower on highway.

That is also my understanding, and I have not seen any other deviations other than the reading of the O2 sensor itself.
Although too far off, fuel trim is generally equal for both banks in all conditions.