I think it's been half a year of chasing the lean codes in my 3.5. Unfortunately almost every day, on quite random basis, the car goes into Restricted Performance mode. And gets back to normal after some 5-10 minutes.

Here's what I know:
- after the car goes into Restricted Performance, codes P0171 and P0174 pop up on my reader. Meaning Both banks being too lean.
- I have checked the car with smoke machine and didn't find any leaks. Taken apart and put back the intake a number of times. Checked and flexed the resonator too as it's often mentioned to be the leaking point.
- I am 90% sure the lean condition isn't caused by air leak because, as I checked, even if I unplug vacuum hoses it doesn't cause the said codes
- Changed the sparkplugs
- Changed the MAF sensor. Ordered theoretically a Denso but looks like a knock off. Seemingly works. Codes appear on both old MAF and new in same frequency.
- I have no other error codes, however sometimes before the Restricted Performance kicks in there is a little hiccup - like a misfire. No misfire codes ever though.
- O2 sensors seem to be doing fine by the numbers on icarsoft
- Fuel pressure is at 387 kPa on idle which I believe is normal
- I left the car at a Land Rover specialist as it was the closest I could get to a Jag specialist (there aren't any where I live - Tenerife, Canary Islands). The car stood there for over a month and he found nothing and gave it back to me with a throttle body error as he fiddled with position sensor. Fixed it, not going back.
- BEST PART is if I reset the car by touching battery - to + I get around a week of peace. After 5-7 days the RESTRICTED PERFORMANCE is back.

Any help in solving this mystery will be really appreciated.

 

I think you mean P0171 and P0174. (P0172 is RICH bank 1)

 

Thanks @motorcarman , yeah I do mean 0171 an 0174, can't seem to find an edit button. The document you attached suggests clamps if I understand correctly. I checked and adjusted them, like, 20 times by now. But I will look at them again.

 

Other things to try

1. see what the fuel trims are doing, especially at idle.

2. look at freeze frame data

 

​​​​​​
So my STFT are within -1% +2% range on both banks on idle.

However my LTFT are both at 19.61% which I guess means that it's adding some fuel. I do get an occasional burst of white smoke from cat burning fuel.

But does that narrow the search in any way?

 

Something is a miss: LTFT shows adding fuel; but codes thrown are lean?! I say one or more sensors are 'lying'! Could it be a (temporary/periodically/partially?) clogged injector(s)? As far as I understand how misfire warning works it has to happen more than just once to set the code, if it happens once in a while the code is not set, at least the one you can read with Icarsoft. You need SDD or similar, to see that.
The above could be reasonable hypothesis for 'hiccups':

• injector is clogged
• 02 sensor tells ECM to give more fuel
• Fuel trim you see is a read out from ECM command
• Occasionally blocked injector opens up and engine gets truly 19%FT. this will cause misfire and 'hiccup'. But then injector is blocked again.

As I understand, I can be wrong, STFT is adjustment to LTFT, so STFT shows low adjustment.
Keep in mind: I am not an OBDII specialist, all above is a conjecture. I am sure some members who are experienced in this particular field will weigh in

 

Disconnect your MAF and look at the LTFT and STFT. They should default to a normal value which will make the car run really badly if you have a leak. Since both banks are equally bad it has to be a sensor value that both use. Caused by leak or bad sensor/connector/wiring.
Since you say your O2's are happy it should really be pointing to a large air leak after the MAF. When you disconnect a vacuum line you should see a change in the LTFT or/and STFT.

 

So an odd thing had happened and I'm afraid to talk about it not to jinx it. I did 2 things the other day.
1. Reading about clogged injectors I poured Liqui Moly injector cleaner into the tank. Started driving. Nothing spectacular happened right away.
2. At about half a tank of fuel I was going through my garage looking for something and I found a Toyota MAF sensor that I once bought. It's the one that was supposed to be identical to Jaguar one. I remembered that when I tried to plug it in all I got was a Christmas tree on the dashboard so dumped it in drawer. Now I decided to give it one more try.

There was no Christmas tree. But the Restricted Performance popped after some 5 minutes. Oh well. Was worth a try I thought and erased the error code.

Now here's the interesting part. It's been a week since I did that and I drove the car everyday without Resticted Performance.
Checked the fuel trims. One day LTFT dropped to +10% on both banks. The other day +16%. And next day back to around +10%.

Could it be a clogged injector unclogged a bit?
Or was the knock off MAF sensor doing as bad job as the old one and suddenly Toyota saved the day?

I'll keep investigating but sure would rather do that without RP.

 

After a week of happiness the codes came back and they decided to live here now - every time the car goes idle the positive trim goes to 19.61% which I think is just end of scale on my reader and God knows where they really are.

I decided to check what you suggested. Interestingly absolutely nothing happened when I unplugged the maf sensor. The ltft was at the same high level and the engine ran just as before - smooth. As if the MAF sensor didn't make any difference at all if it was there or not.

​​​
​

 

It would sound like the signal from the MAF is not getting to the ECM. You should have a code thrown for that even before disconnecting it. Could be dirty connector or wiring issue. Also it appears as though the fuelling is being controlled by the O2 sensors. The change in fuelling indicate a small vacuum leak. As you slowly increase the RPM the STFT and or the LTFT would decrease fuelling as the vacuum leak becomes less significant at higher RPM/

 

Hi Canarian,

Your symptoms do seem to indicate a small unmetered air leak between the MAFS and the intake manifold gaskets, in the EVAP system, or in the exhaust system before the upstream O2S's.

The fact that your STFTs are so close to ideal (at least at idle) suggests your MAFS could be OK, but it would be worth monitoring its signal for awhile to see if it is responding properly. A general rule of thumb is that the MAFS signal in grams/second will be approximately equal to, or a little greater than, the displacement of the engine in liters. So for a 3.5L engine you're looking for maybe 4 g/s at idle, and the signal should gradually increase with engine speed. I believe there is a chart in the workshop manual or the engine training manual that shows typical MAFS readings at various engine speeds, but I couldn't find it in a brief search in my library.

The reason I believe your leak is relatively small is that it is taking about a week for your LTFTs to reach +25%, which, if I recall correctly, is the threshold that triggers the lean codes.

The reason the ECM is adding fuel to the mixture is that the O2S's are reporting excess air in the exhaust gasses, so the ECM adds fuel in an attempt to maintain stoichiometry, or about 14.7:1 Air Fuel Ratio at idle, which results in all the fuel and oxygen being used during combustion.

These engines have numerous potential leak points that do not all communicate with one another, so when using a smoke machine the smoke must be injected into several places to test as many of the potential leak points as possible. The first thing I do is look for oil seeping from the VVT solenoid seals, the oil filler neck and cap, breather hose O-rings, etc. If oil can get out, air can get in under engine vacuum. Small cracks or pinholes are also very common in the accordion pleats of the main air intake pipe.

For some tips on smoke testing, see these posts:

https://www.jaguarforums.com/forum/x...2/#post2783963

https://www.jaguarforums.com/forum/s...5/#post2597722

Cheers,

Don

 

I would be quite concerned and looking or the cause of my LTFT being +19 at idle and disconnecting the MAF not making a difference.

 

If I recall correctly, on the 32-bit Denso EMS, if the MAFS is disconnected the ECM defaults to a stored map. Someone may correct me.

 

Yes. That is what I expected. However his LTFT was the same whether connected or not!

 

I think that the ECM runs in open loop with the MAFS disconnected, so it may not track or update LTFT.

 

You're misunderstanding. It's adding fuel to try to compensate for a lean condition.

 

Yes.

If you stay parked but rev to 2500-3000 do the LTFTs drop a lot?
If yes, classic signs of an air leak.
If no, something else.