Hi guys, I need some advice to figure out the cause of a really bad-running XJR6.

The car is a late 1996 German-market model, with 130,000km on the clock.

Issue:
1) Car idles roughly, misfiring, from cold. OBD data shows low O2 sensor voltage, and fuel trim stuck at a high 24.2
--> If it start driving, then the car will constantly stall during deceleration

2) If I unplug, and then re-connect, the connector to the MAF, then the car idles/ drives a lot smoother.
--> However, now the car runs extremely rich - fuel-trims at maximum negative values (-25%!), and black smoke from the exhaust during acceleration.

What I have done so far, which hasn't resolved the issue:
1) Removed and cleaned throttle body (a hell of a job...)
2) Replaced both O2 sensors
3) Tried a new MAF (World Car Parts aftermarket version)
4) Replaced Coolant Temp Sensor
5) Replaced Crankshaft Positioning Sesnor
6) Replaced Fuel-pressure regulator
7) Changed to new RC12YC plugs (multiple-times, since the plugs would get fouled by rich-running...)
8) Replaced Catalytic-convertor
9) Changed to 6 new Lucas coils
10) Did a home-made smoke-test, without finding any vacuum leaks

What I haven't done, but am contemplating:
- Changing the ECU

I've attached a couple of videos with the OBD Data:

1st two videos - Idle jumping/ rough running when car has just been started and is in Open-Loop.

https://1drv.ms/v/s!Ahcfy_avWBXciSn65xLrUVe9R-_Q

https://1drv.ms/v/s!Ahcfy_avWBXciSPDPW0JFPROPZXy

3rd Video - Rough running/ misfiring once in Closed-Loop, Fuel-trims "stuck" at + 24.2% at idle, low O2 sensor voltages.
In the 2nd half of the vid, I raised revs to about 2000 rpm - slight decrease in positive fuel-trims.

https://1drv.ms/v/s!Ahcfy_avWBXciSqNSrVDAAB_wBrL

4th & 5th Videos - Smooth idling, once I have unplugged and re-plugged the MAF sensor connector. Engine now running extremely rich (STFT stuck at -25%), maxxed-out O2 sensor voltage.
I raised revs again to around 2000 rpm - no misfires, smooth, but still very rich.

https://1drv.ms/v/s!Ahcfy_avWBXciSgMo0e3w3i8VAP3

https://1drv.ms/v/s!Ahcfy_avWBXciSuopcTbQgFER5G9

Any help would be MUCH appreciated, as there aren't any workshops that are willing to help here in Braunschweig!

 

When you pull the connector on the MAF the ECU since it is missing this information reverts to different set of maps that keeps the car running but not at a optimum . The MAF feedback sensor wire Green / Pink should read 1.2 volts DC at correct idle and climb toward but not to 5.0 on throttle up . Some guy very experienced in the matter who's name rhymes with the letter " V " came up with a suggestion of twisting the blade / pins on the connector slightly for better pin contact .

The other important sensor is the throttle position sensor and it should read 0.60 + or - 0.02 volts DC on the feedback wire Green / Yellow at correct idle and climb toward but not to 5.0 on throttle up .

You can remove some of the insulation on the wires as they come over the fail rail for easier access .

Before you source and change the ECU inspect the ECU connectors for common corrosion and keep in mind there are many part #s for the ECU depending on the part of the world .

Vacuum leak check ; There should be a list but I found the line missing from the underside of the intake manifold to the round EVAP valve under the aircleaner box .

Check and clean all of the grounds in the engine compartment as there are many including the rear lower bolt on the ECU as this is a case ground

Exhaust leaks before the O2 sensors ?

Now let me watch your movies

 

You can see the numbers more clearly if you go full screen on the video and pause

1st movie :

Misfire , maybe fuel injector cleaning ( binding injector slides ) and fuel filter change

MAF should be around 3.2 grams / second at idle for a 3.2 liter engine , not all readers see this correctly

IAT should be higher at 12 C / 54 F unless it was very cool that day

Idle speed a little high

ECT at 20 C / 68 F

Is this an example of open loop where the STFT is not considered and not calculated as well as O2 sensors ?

2nd movie :

Idle correct

Is the ECT too low and still in open loop ?

3rd movie :

MAF still off

ECT correctly responding

Changing difference in O2 sensors . Are the O2 sensors hunting and the data points just captures this ?

STFT changing differences between bank 1 and 2

On throttle close the STFT goes to 0.0

4th movie :

MAF correct if multiplied by 100

O2 sensors stay together

5th movie :

IAT more correct

Both O2 sensors stay together in good range but went bad together later

Both STFT stay together

Editing complete , does anyone see something I don't ?

Your MAF reading responds correctly to the throttle position and maybe if you multiplied the MAF by a factor of 100 it would be correct .

 

 

@ Lady Penelope - I did replace the catalyst recently, would an exhaust leak between the manifold and Cat cause such a huge issue with idling and misfire? I'm going to look into this a little further then.

@ Vee - If I swap the O2 sensor connectors around to test if they are mixed up, and they aren't, would that cause an issue where I would need a O2 sensor orientation? The problem is the nearest Jag repair center with the right SW to do the orientation is about 400km away from where I live, so I wouldn't want to trigger than inadvertently.

Regarding the Camshaft Position Sensor, is there any way to test or diagnose this?

For the Coils, the car's behavior didnt change when I swapped the new ones in. I tested both the old and new ones at night to look for any lightshow, but didn't see any.

 

In the processes of mounting the downpipe to the manifold I discovered one of the studs had deformed the donut gasket seat on the pipe to where it was leaking . The leak can pull in fresh O2 between exhaust pulses going down the pipes biasing the O2 sensors seeing unburned O2 not being consumed to make the ECU to go rich . The same can happen with a cracked manifold . This condition would decrease with higher RPMs . Not a fun job to inspect for this and the gasket is easily available as a common part # X I got from Vee I'll have to check if I still have the package left .

The Camshaft position sensor code is P0340 and mine cleared out after connector clean and code reset at the local parts store . It is a hall effect sensor so it test differently then the inductor Crankshaft sensor which should read about 1300 ohms as a fundamental reading and not the actual functional reading of a pulse wave .

Someone yesterday found some metal debris on the face of hie Crankshaft sensor , cleaned , and resolved his issue of cutting out a high RPMs .

Will go through your movie data again later but I did notice a difference in O2 sensors then it changed back to even , sensor connectors clean as well as the return signal wire shield grounds on the rear engine firewall ?

 

See responses in bold.

 

I agree with the fresh air mixing into the exhaust, but it would not create this kind of problem, at least not to the severity Ali is experiencing. I've had two cracked exhaust manifolds, one pretty bad, and I couldn't feel it in the car, my mileage took a dip, but not even that was monumental. This dude has a severe malfunction/misreading going on...from a cold engine no less. Exhaust leaks could only create a problem once the car has warmed up enough to go closed loop. When cold, the ECU is ignoring the O2 sensors, so exhaust cannot be a factor.

 

It's not unknown for there to be water coming into the ECU. It's in the passengers side footwell, behind the outer kickpanel, it's worth taking a look at the connections and making sure there is no corrosion.

When you tried the second MAF was it one specifically for the XJR? The XJ6 one will massively overfuel an XJR engine.

Regarding the O2 sensors, you can trace the wires back into the engine harness and look at the colours to determine if you have them connected properly.

 

The obvious sign that O2 sensors are connected the wrong way around is that STFT goes Max +ve on one bank and Max -ve on the other. You don't have that so that is not your problem.

 

Went through the films again and updated observations .

Vee , I agree with your amount of biasing of the O2 sensors with a exhaust leak .

Jagboi64 , I agree with the incorrect MAF sensor part # if present .

 

 

The TPS responds in the correct direction . I don't think I have ever seen a 0.0 % reading on the mechanical closed stop . It may not get to that closed mechanical stop because of a weak return spring or gummed up TB so you may twist the butterfly by hand and see what you read .

Is the TPS on 0.60 volts + or - 0.02 on the wire discussed earlier on post # 2 as a second reference / test ? Mine was at 0.63

 

Hi Ali,

You've already received some excellent ideas and questions, but your mystery is quite compelling, so forgive me if I think out loud with a tired brain.

First of all, was the onset of these symptoms sudden or gradual?

You have reported no diagnostic trouble codes (DTCs). I assume that if any codes were flagged you would have reported them here?

Another possibly helpful clue would be to know if any of the Inspection/Maintenance (I/M) monitors are not completing. I notice your scanner has an I/M button. Will it tell you which, if any, I/M monitors are incomplete?

Thinking about your rich running, some possible but more difficult-to-detect air leak points include the carbon canister purge valve or secondary air injection system (if equipped), a stuck-open EGR valve, or a misbehaving Idle Air Control Valve (IACV).

Have you checked your fuel pressure? Have you changed the fuel filter? Have you checked the spray patterns of your injectors? Any chance the fuel return line is kinked or blocked?

Have you checked the supply voltage to the ignition coils? I'm wondering if weak spark could lead to unburnt fuel in the exhaust?

Ditto to Parker's suggestion of cleaning all the engine bay grounds, especially all that are referenced by the ECM.

I'm not very familiar with the X306 Eaton M90, but could a problem with the supercharger factor into these symptoms? Say, if the S/C was supplying reduced boost due to a slipping belt or pulley, a broken flexible coupler, a stuck-open bypass valve or loss of vacuum control to close the bypass valve, or an air leak somewhere in the supercharger plumbing? Is the intercooler clean and is the electric water pump working properly?

Regarding the camshaft position sensor (CMP or CMPS), it is only referenced at startup to identify cylinder 1 compression TDC within 2/3 of one full revolution of the engine. Once the engine starts, the ECM references the crankshaft position sensor (CKPS) for ignition timing and "CMPS input is not required by the ECM." "The ECM tests the CMPS and CKPS by cross-checking the output of the two sensors." The CMPS section hast two apparently contradictory statements: "Without the CMPS sequencing input, the ECM would attempt engine start by trial and error, firing each cylinder in sequence; several engine revolutions might be required for successful engine start." However, the same section of the manual states, "The engine will not run with a CKPS fault." Since some of our members have reported that if cranked long enough, the AJ16 will eventually start and run with the CMPS disconnected, the latter statement appears to be incorrect. I suppose a problem with the CMPS could theoretically get the engine to start with incorrect ignition timing, it would have to be 180 degrees off, since the CKPS also knows cylinder 1 TDC but doesn't know the difference between compression and exhaust strokes.

Cheers,

Don

 

 

I had a dud World Car Parts MAF. They sent me a free replacement , so worth letting them know (that was earlier this year).
Now I buy used MAFs when I see them on eBay...gotta have a spare