Hi, I wonder if anyone has an opinion on these symptoms.. recently I’ve noticed my XJR 4.0 sc 2001 has been really slow to get out of first gear, not until about 25mph and others slower than usual. I also recently had “restricted performance “ and the MIL warning light. So I scanned her OBD. I get P0172 and P0175 bank 1&2 fuel too rich… she idles fine.
I am wondering if there’s any way a sensor on the transmission is failing but would it cause rich mixture?
could it just be the electrical plug to the harness which causes this ?
my OBD scan says CAT and CCM is “inc” which I’m sure means incomplete test… I hope the cat is ok! 😺
I am not sure so help most appreciated!!
cheers !

 

Hello, your XJR need a job gearbox... like replace conductor plate,filter and new ATF. Hope that will get your cat on the road !!!

 

I have found that the most common things for P0172/P0175 are faulty MAFS and faulty fuel pressure regulator.

There might be other problems but check those first along with a STFT and LTFT.

 

Thanks, the MAF is quite new. The gearbox had a full change and service in 2021…what is STFT etc?

 

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

When the ECM reaches 25% it runs out of control so it flags a P0172/P0175.

You can watch the Fuel Trims with more advanced diagnostic scanners.

 

Julia, is this MAF you have installed an oem Denso or an aftermarket one? I’m not saying this is your issue but, I’ve ran into loads of A/M meters for these engines that are bad. I’ve replaced 2 A/M MAF sensors in the past 2 weeks in customers cars. One was an 02’ XK8 and the other was an 03’ XJ8. If you have the previous MAF sensor still, maybe clean it with a spray solvent and blow it out real good preferably with compressed air or a hair drier.

 

Thanks motor car man, always most appreciated. Yes the answer is … from UK specialist
I took it off and cleaned it again nevertheless.
what people are overlooking here is that she struggles to get out of first, or indeed sluggish on other gears. Although no other errors come up on OBD apart from rich mixture P0172/5.
Now I went on a 9 mile run and checked cruise control - all good. That means auto box speed sensor and conductor plate are ok, right?

I checked ATF when I had clearly been using the gearbox fully and she was really hot.

seeing as always she behaves badly in dense traffic or hold ups I’m thinking ATF is by passing the connection seal of the plug and or messing the signal to the ecu.

I dipped the ATF.

came up on the dipper 1/4 “ below the minimum
for 80 deg or at 9 lit. so I’m guessing ATF had seeped out the electrical plug and I need to get the plug replacement.

my main concern is would a low ATF cause her to deliver richer fuel??

Help appreciated
cheers

 

If there are no P07xxx or P17xxx DTCs, then I would discount any TCM faults and forget the gearbox as faulty.(you need an OBDII reader for TCM faults to be sure)

Any P0171/174 or P0172/175 DTCs will trigger RESTRICTED PERFORMANCE warning.(engine will NOT BE HAPPY)

The CAN bus relays info from the INSTPK, ECM TCM and ABS at high speed.

Parameters decide if the RESTRICTED PERFORMANCE is warranted.

Take care of the RICH DTCs before worrying about the gearbox.

 

1. No there are no gearbox faults seen….
2. would low oil create lag, and result in extra fuel?

 

I had the same problem back in 2013 with my previous. Problems went away after I cleaned the throttle bore and plate. Might also try running a can of a good fuel system cleaner through (I like Gumout's look-alike to SeaFoam).

 

thanks Jim, however i did that only a few months ago as i believed the butterfly was sticking. its strange that she doesn't seem to get out of first gear until 25mph and both banks are rich at the same time....

 

no.

things to check...hows your battery?

Maf may be new, but also worth cleaning with specific spray cleaner.

Some people have found that squeezing the contact on the MAF connector (harness side) to make better contact with the maf may help as well.

 

Fuel trims....
There could be an unmetered air leak, if the fault doesn't lie with the fuel pressure regulator as Bob has suggested.
Either the top rubber seal onto the throttle body or the one it's sat on - if it's been off recently, which the top one has as you've cleaned the plate.
Puffing easy start or brake cleaner lightly around inlet connections finds leaks quickly.
Check throttle cable for play and do a reset of the cable if you find slack, some have found as little as 60% actuation, you can watch this on your OBD reader and reset to as close as 99%
Then do a hard reset and start the car and let it idle for 10 minutes for throttle recalibration, then see how your fuel trims are.

 

These are rich codes, not lean so wouldnt be an air leak

 

yeah ok - thanks for responses... what puzzles me is the lag of the gearbox. I am reluctant to top more oil into her gearbox in case i overfill it. I have booked her into my local garage with a new connector plug to be fitted and see if there is somehow something from that which is skewing the operation and the mixture.....

 

Many cars need the fuelling to be right in order to get gear changes right. Ignore the gearbox till you fix the rich codes.

 

Ok thanks jag v8 I’ll bear that in mind. I didn’t know that.

 

If the engine has a fault, the gearbox is instructed to compensate and if the gearbox has a fault, the engine compensates with lower output!!!!!(all part of the the CAN bus)

 

When cold she seems to change up ok.. but when hot, particularly when in traffic or uphill, stays in first until around 25 … still the MIL on, still restricted performance. I’m so curious to see if a rich mixture somehow affects the transmission. Sorry I don’t understand can bus?

 

Was going to send you a PM that if you don’t figure it out I have a transmission from a 2001 XJR for sale then saw your in the UK. I’ll give you a great price if you pay shipping.