That's my conclusion after this long quiet time in the Forum... it was a little too quiet...

I hate to break the calm, but now I am the one facing a challenge! For the first time ever, the engine isn't running smoothly and is undriveable. Suddenly, I have intermittent power loss, surging, near stalling, sucking, etc. Very inappropriate behaviour.

I did manage to get home that day with the Trans Limp light and a CEL. I think the trans is ok. It was doing its best to shift gears, although harshly. The only code stored was P1108, MAP sensor low output Bank B.

The engine starts and runs perfectly from cold. But after reaching near fully warmed up, I noticed that B bank goes into Open Loop fueling, as the sputter events start. So far I have repaired a probable short circuiting fuel injector wire (4A) with no insulation and broken strands. I then swapped MAP sensors, ignition modules, and ignition coils (one set at a time), and always came up with the same symptoms, with B bank going into Open Loop as the trouble starts.

Just tonight while watching live data I noticed that the B bank O2 sensor #1 stops cycling whenever the intermittent (but frequent) event occurs. Could it finally be burned out? So my next step is to replace the O2 sensors. I have new ones on hand because I have been meaning to replace them for a while!

Postponed preventative maintenance could be the root cause. Will I ever learn a lesson? At least I will have new O2 sensors and can rule that out as a cause. Stay tuned. Thanks for any input.

 

Last time I started the X305 two weeks ago, it threw an HVAC "Err" (although I couldn't remember the correct nerve pinch so I still haven't read it). Streets are a mess with wet leaf litter so I'd rather ride the train than wash the car in the cold.

What are fuel trims doing? When my B bank was stumbling, the B2STFT would rise over the course of ~20 seconds, max out, revert to zero, and repeat.

 

Mine passed its MOT with a couple of minor advisiories. I still haven't driven it or seen it yet. My friend took it to his local MOT centre and they did it. He said it drove really well and the MOT tester remarked that it was better than some cars he sees at first MOT at 3 years old!

Hope you get to the bottom of the issues. Does sound suspiciously like O2 sensors.

 

Trick I learnt with O2 sensor replacement in case of use/not already known: use an O2 sensor removal wrench/socket but tighten a hose clamp around it to stop it flexing and opening up under torque thus chewing up the old sensor nut, causing outbreaks of shouting and the eating of time and effort.

My car is still indoors as I continue to fettle it - doing the rear shocks, fuel injection refresh, got a spare junkyard rear screen and will replace headliner while the old one is out etc....

 

The 2 MAP sensors will read 1.7 volts dc at idle and will increase with engine speed

This will be the middle wire of the MAP connectors with it still connected

If you swap the MAP sensors and the code follows to the other bank then you can conclude a bad MAP sensor and not in the wiring or ECU connector

 

Thanks for the updates fellows! Today I replaced the left front O2 sensor. Got it out with a 7/8" open end, with only about a 1" clearance next to the edge of the oil pan to get the wrench on and tap it loose with a hammer! Everything on this car is "just barely". Oliver that idea for the removal wrench is clever. I don't even have one so thought I would give it a go if I could. Only rounded the edges a bit! Probably benefited by a mostly dry environment here. Service manual says to remove the oil filter. That would be easier access but messy. How much oil would come out with a full oil pan? Usually its about a cups worth after the oil pan is drained.

So I have one new O2 sensor, and the exact same problem and symptoms. I realize that if the bank goes Open Loop, it will ignore the O2 sensors anyway. While running normally, the sensors are cycling like normal. The A bank rear sensor is duff, looks to be stuck at high voltage, but I seriously doubt it is the cause of the intermittent surging. Or would I be seriously wrong?

Parker you are right about the MAP sensors, I installed a small wire on each from pin 2 so I could test the voltage as the engine was running. They read same while normal, different during the surging. The swap showed the same readings. I bought a used 95-99 Subaru MAP sensor on the Bay last week as my first repair attempt, and indeed it registers the same as the Jag OEM. Both are Densos and are identical but with slightly different Denso part numbers and labels. I was wondering about that, as new OEMs are >$100.

I pulled the main fuel pump relay and ran the car on the secondary pump/relay.... the same. No CELs either.

Parker you have also inspired me to get into the PI1 and PI61 for a look-see. Also the ignition and fuel injection relays that live in the neighborhood.

And yes, I had the ECU out yesterday to check for water damage. None, and as new.

 

You can jumper the ignition positive ( on the right engine bay fuse box ) and ignition coil and fuel injector relays between socket 3 and 5 and test run

Suggest use blade type connectors on your heavy gauge jumper wires for better current and secure fit

There is a concern if using both fuel pumps at the starting sequence it may be a over fueling situation if the fuel pressure regulator will bypass enough fuel to maintain the fuel pressure under the target pressure , over pressure could be bad on fuel metering amount of cylinder charge

 

My car, a 1995 XJ12, has over 350,000 kms/225,000 miles on it and still pulls and runs like a train. It uses no oil but fuel is another matter! 🤪

Cheers,
Jeff.

 

By ignition relay are you referring to the ignition coil relay ?

On the inline 6 there are 2 relays that power the ECU and for your different ECU for the V - 12 ................ researching

Have you swapped the bank A and B ignition modules and see if some of the codes swap banks ? but you did see a meter difference on the 2 MAP sensors

Does your live data show a lowering / dropping of the coolant temp to go back into open loop mode ?? this may or may not correlate with your instrument cluster gauge

 

Today I tested again after checking PI1 and PI61, and swapping the Ignition relay and the Fuel Injection relay with identical but lesser-used relays nearby. I didn't find any anomalies, so doubted anything was fixed. I was right. Same problem came up as I nursed the car to the gas station and back. But the work had to be done and it is much cleaner in there now!

Day before, I pulled the main fuel pump relay and ran the car only on the secondary pump and relay. Same problem came up.

Next: I am replacing the O2, Bank 1 Sensor 2 (downstream). Live data shows it is stuck at high reading, as it has been since I first noticed it a week ago. I don't know yet how I am going to reach that sensor, but I must.

My current theory (there have been many) is that this failed sensor is causing the A bank to mis-fuel (even though data always shows Closed Loop). Then the B-bank has to make up for the lack of power from A-bank, confusing the ECU and sending the B-bank into Open Loop Drive fueling. "OL Drive" fueling engages during a high throttle demand. The theory is backed up by the voltage readings from the MAP sensors during failure mode, B-bank showing ~1.1V (more vacuum) and A-bank showing ~1.9V (less vacuum). This is what it feels like too; like only half the engine is powering, as B-bank struggles to keep the engine running.

Coolant temp data is normal. Failure mode happens after 3-4 minutes from cold start, coolant temp 130F or so and rising.

Thanks for the support guys, glad to hear there is also good news out there!

 

130 F is a cool ECT value

The true intake vacuum on each bank should be independent of what happens inside each cylinder after the intake valves are closed

This is not the case on the exhaust side

There are valve / cam angle overlapping effects on higher RPMs but not on the intake side

This points to your different MAP values

I can be totally wrong

There are some clean ups on the O2 sensor wiring

The )2 return wire to the ECU is shielded by a ground terminal

One of the heaters in the O2 sensors may not be getting a sonsor to operating temp

The ECU has dedicated wires for a frame ground

The ECU may have a case ground wire

 

If the O2 sensors don't do it, I'll go for a new CKPS. Its odd how there is very little talk of this part failing on the V12, but it is a must-do-right-now part on the AJ16! I can at least check the connector and measure the resistance for now. It is another possibility due to the temperature dependent failure mode.

 

Once I figured out how to raise the car up enough, replacing the rear O2 sensors was relatively easy. Short ramps under the right side tires, then 4 jacks with blocks (stepping stones) on the left side to get more lift. The left side tires were about 4 inches off the ground. Plus, I'm skinny.

After 15 minutes of perfect running (in the yard) from cold start, all live data reading normal, Closed Loop, coolant temp at 190F, all 4 O2 senors cycling, I went for a drive. Within a mile it started cutting out again. Same problem. Left bank going into Open Loop again.

Now it is on to the CKPS. I will take it out and test the resistance. Then bake it at 200F for ten minutes, and recheck. I doubt it would be the cause. But SOMETHING is failing after coming up to full operating temperature.

I am at a loss for where to go from there. Which is not good to hear myself admit.

 

What are the variables that would change of state from close loop to open loop ?

The only one I can think of is the engine coolant temp

Any why would it be on 1 bank

The CKPS will probably come in around 1300 ohms which would seem to be high but that's a fundamental resistance reading and not a functional

Both sensor faces clean as this is more noticeable as a fault at higher RPMs

 

A change from CL to OL could be caused by cold coolant reading, or high throttle demand (wot). Probably other scenarios too. Compounded by unidentified malfunctions.... seems strange to go OL on one side only.

Staring at the live data one more time, I saw the Bank 1 downstream O2 stuck high again. Graph flat as a board at 1.25V. This is after putting in a new sensor today. There could still be a fault with the wiring to the sensor although I did not find any yet. Earlier that sensor was cycling in a normal way from what I could tell, until the malfunction happened again. So I have to revisit that.

 

All 4 O2 sensor heaters come off the same fuse on the V12 , # 14 / 10 amp right engine bay fuse box,

But it passes through connector Papa Indy 61 before it splices in the 4 paths

It could be corrosion in the connector limiting the current to the heaters for the O2 sensors to read correctly, they themselves have a operating temp

This would not blow the fuse but can be on the edge of badness for one bank to read correctly and the other bank not OK

It would be the White / Green wire in position12

 

Thank you Parker! I just finished cleaning up PI1 and PI61. Also my scan tool tells me that the O2 sensor heater monitor is complete and passes. No trouble with the heater circuits as far as I know.

Regarding the Bank 1 downstream sensor... I disconnected the yellow 8-pin harness (PI74) next to the ECU which is for both rear O2s, connected the scan tool again with engine off, and it STILL shows B1 downstream at 1.25V!! This is while the other three show 0V. Now, how is that possible?

So I took out the ECU again. I carefully loosened the boards to peek inside. I spent some time visually inspecting everything, and all looks perfect. No defects seen.

Then I went to trace the ECU inputs from the O2 sensors. There are 4 pins in a row that bring the inputs into the ECU, one for each O2. It turns out that the first stop for each input is a resistor:



See the 4 resistors in the middle: R201, R203, R205, R208, green green gold bands. I believe that color code is 500k ohms. Well guess whose resistor seems to be in failure mode? R203, the resistor for the Blue feed wire coming from the Bank 1 downstream sensor. It is giving a strange resistance reading where it starts at around 30k-50k ohms and very slowly increases (maybe due to the current?). Anyway, that is much below the 500k it is supposed to show. Is this why the ECU thinks the voltage from Bank 1 downstream is still 1.25V steady, and not zero (with engine off)? Possibly!

Two of the other sensors test at about 550k ohms (~correct), and R208 shows 1.5M ohms (way high) for the Bank 1 upstream sensor. The upstream sensor seems to work properly on the scan tool, but who knows, it could be contributing to Bank 1's problems.

Edit: the bands on the resistors might be green, green, "faded" yellow. That would more correctly identify it as 550k ohms. Still learning those codes!

I think those resistors need to be replaced. Now how will i accomplish that?

 

Happy New Year, fellows. Some time has passed, and I have been working on the XJ12 whenever I can, in between work and the holidays. I am at the end of my checklist, and nearly the end of my patience. The same exact problem persists. It has been frustrating. I should know how to fix this!

IRT the above post regarding the ECU: I decided not to play with it anymore, I didn't want to give the car an accidental lobotomy. Though it acts like it had one already. The 4 new O2 sensors seem to work properly whenever the car is running properly (first few minutes from cold).

In review, the symptoms: Start-up from cold is normal. Idle, throttle and transmission operations are normal at first, until somewhere between 2 and 10 minutes into warm-up, engine speed begins to dip intermittently and repeatedly. It nearly stalls, but recovers. If driving, there is an extreme lack of power and the car is essentially unsafe to drive in this state. Following a hot start, the malfunction begins right away. It is clearly temperature related.

During the failure mode, the OBD scanner shows "Fuel System 2" (which I take to mean B bank) going into Open Loop "drive" fueling. "Fuel System 1" stays in Closed Loop fueling. I have on several occasions got a P1107 (not 1108), showing the MAP pressure in B bank to be low. That indicates B bank is working harder, creating more vacuum than A bank. I tend to associate all this information with the A bank somehow losing its fire.

Here is all that I have checked and/or replaced:

Repaired damaged wire to injector 4A, it was probably shorting;
Installed a used MAP sensor on the left side, then swapped them A/B;
Swapped ignition modules A/B;
Swapped ignition coil packs A/B;
Checked the ECU for water intrusion, visually inspected interior;
Cleaned and checked the PI1 and PI61 connectors;
Swapped the ignition coil and fuel injection main relays with lesser-used relays nearby;
ADDED: Ran the engine on the auxiliary fuel pump (by disabling the primary);
Replaced all 4 O2 sensors with new Bosch;
Replaced the CKPS with a new OEM;
Replaced all 12 spark plugs (they were quite old) with original NGKs;
Replaced the Engine Speed Sensor with the one from the CKPS position.

I tested the engine after every single check and repair operation, and the exact same problem came back every single time.

Tonight after finishing the ESS I listened to the fuel injectors during the malfunction (using a big screwdriver to the ear), and they all seem to be operating (clicking) even during the dips in RPM. I also listened to the timing chain and valve covers, and all seems to be running smoothly with no unusual noise.

No misfire codes. There is no smell of fuel anywhere when the engine sputters, and the spark plugs did not show much sooty evidence of over-rich fouling.

I've got a lot of sweat equity in this car. So what's next? Other than shopping for a new car.....

 

I wonder if you have a blocked or kinked fuel line somewhere? This is a real head scratcher without being near the car. Fuel pump?

 

G’day Gents,

The kinked fuel line idea is worth checking. I changed out my fuel pumps some time ago and when I went to remove the fuel tank I couldn’t get the short fuel line from the filter to the tank to release as the metal fitting was buckled and the rubber hose part was kinked.

Luckily I could undo the fuel line from the filter and remove the fuel tank with the short fuel line still attached. It seemed to me that a previous attempt was made to remove tank without disconnecting the fuel line and damaged it.

Of course a new line is unobtainable so I reshaped the metal fitting to its correct shape and rehosed the line with high pressure FI hose and a couple of hose clamps. The new fuel line has been on the car for a few years now without incident.

The reason I changed the fuel pumps was due to hesitating and sluggish acceleration when the second pump should have kicked in. It was the kinked fuel line which was the problem.

Cheers,
Jeff.