I've had an ongoing intermittent issue with my engine stumbling slightly. It hapens when cruising and I start going up slight inclines, but doesn't do it every trip so I have had a hard time trying to narrow it down. Most noticable at highway speeds, but definitly does it while in the city. No rhyme or reason, tho it appears only to happen once the engine bay gets warm. This weekend got a chance to be a passenger and monitor obd data. I see anomolies but I'm pretty stumped. Here's what's happening:

1. The stumble feels like when an engine hits the rev limiter, but the car is cruising steady and not downshifting
2. STFT on both banks spikes to -25% for a second (pcm trying to cut fuel)
3. The front O2 sensor current spikes to -0.85 mA (indicating too rich--coasting downhill pegs the O2 current at +1.2mA)
4. The rear O2 voltages both spike at near 0v (indicating lean)
5. MAF, MAP, and throttle reads steady, no spikes in any of those
6. Downshifting during the stumbling doesn't help, actually makes it feel worse (makes sense if the engine is lean surging)
7. Fuel rail pressured drops from 55 to 20 psi during the above spikes, but I presume this is part of the pcm trying to cut fuel.

No check engine light, engine has 116k miles. I've heard several people say to replace O2 sensors at this age, but I'm not convinced they're bad. Both O2 sensors going bad the same way at the same time? I can't think of what else would cause it to spike rich up front only, on both banks equally, except the MAF, map, or fuel pump. Open to any suggestions.

Thanks,
Michael

 

Have you updated the headlamp earths?

The O2 sensors are powered by a relay (R12). This relay is earthed via the G2 earth behind the headlamps. If the earth is intermittent it would cause intermittent power to the O2 sensors

It could also be an early indication of R12 failing.

As both are an easy fix I would address these. Even if its not the issue call it preventative as they will eventually cause issues.

For the relay you need to pull the fusebox apart to replace the relays. I did a post on this but if you haven't upgraded the headlamps earths do that first

Cheers
34by151

 

I have not done anything to the front grounds, but I did check on them a few years ago. I will check again. This car rarely sees rain, never salt, so probably less likely they are the issue. Your mention of the relays piques my interest--I had problems with that underhood fuse box in the past related to the air suspension pump relay not making good contact. That issue was also affected by temperature.

A common electrical fault makes way more sense to me than 2 failing sensors. I'll go look up your post on the relay replacement. Thanks!

I'm just thinking out loud, would a bad ground in the heater circuit really relate to a rich spike only when under moderate loads and no other times? I'm guessing the O2 heater circuit can't cool/heat as quickly as I'm seeing to change the O2 sensor reading. The ground is definitely worth a check either way.

I do see that the ignition relay shares the same G2 ground, and if the ignition coils lost their 12v supply for a moment it would spike my O2 readings rich, which would cause the symptom I'm seeing.

 

It sounds like the earth could be a factor

Upgrade the earths rather than check them so you can be sure.
Install a stainless m8 bolt between G2 & G3 earths.
Move the lugs over to the new bolt.
You can also add a connection between the old and new earth points. I did this on mine

Use the same method for the other side on the G1 earth

I doubt you have failing sensors. Its extremely unlikely for both to spike at the same time. Even more so to do it again. Its more likely that its caused by an external issue.

Also be aware of delays in data if you are using to an elm and an app like torque. The data can be delay by up to a second. Also not all pids update at the same time. Flightmode in SDD will be ok though

Cheers
34by151

 

I wouldn't necessarily exclude the 02 sensors as being faulty - they have been a known common fault.


However - if you're looking at fuel trims, anything over 5 % would indicate a issue, vacuum leak, faulty MAF, etc.


When reading MAF - you should see around 6gm/s at idle, and should go up to about 80 or so when your rev it up. But you need to watch this closely, I have had ( on other models ) where when you rev the engine, the MAF sensor reading is not reflecting that, and still shows 7-8 gm/s when it should by 10 times that.


Do you have a way of looking at Flight Data?


By all means - as these members say to check those grounds, agree with them. This will be quick and easy.


If you're able to get access to SDD - there might be some Flight Data recorded, and have a look at the various sensors, specially coolant temp, MAF, Vehicle Speed ( VSS ), RPM, etc....


I had an S-Type do this to me, we found several issues that all contributed. When checking fuel trims, spiking around 10-15%. Sprayed intake down, found leaks causing fuel trims to change. Fixed leaks, recheck all was good. Car came back about 2 weeks later, same complaint. This time flight data was present, and noticed that when the fault was logged, it showed the car was on the highway, but MAF was only reading 8 gm/s, which is around idle.


Check the grounds first by headlights - see if that does anything, let us know what happens.

 

Checked the G2 ground last night, there is no problem there. I'm also thinking there's nothing wrong with the ignition relay R7 since this issue is completely dependent on slight uphill loads while cruising.

I did watch the MAF data graph while the problem was occurring. While cruising flat road at 70-75, was reading around 38 g/s, when going slightly uphill the MAF smoothly increased to about 40 g/s, no spikes or dropouts. This is actually a new OEM MAF sensor. I had replaced that last year because the old one was diagnosed bad by the dealer. (I did try swapping the old MAF back in, it is unrelated to this issue).

Earlier on with this stumble issue, I found if I reset the battery the issue would go away for a few days. If I have to throw away money on a guess, I think I will try the upstream O2 sensors. They are old and if the PCM is learning a bad fuel trim block it's not going to get better.

My LTFTs are +6% B1 and +3% B2 while cruising on the highway. I believe idle LTFT is about the same, if not slightly closer to zero (will have to double check). So they're compensating for a lean condition, but I'm thinking nothing too extreme for this age/mileage.

I do have SDD installed on a Netbook, but the battery in that is shot. Right now I'm relying on a Creader6 for live data (limited PIDs). I know I should be using the better tool... before I wildly throw money at this I will either fix the Netbook or install a VM on my laptop and get my friend to drive me around with it to recreate the issue.

 

Looking at the live pids will give you false readings as they update/change in sequence so they are not in sync with each other.

You need to use the flight data to keep it in sync

Also I rad in another thread you reported getting pinging. Was this the case before the stumble started?

How old are the cats? Take a read of this thread https://www.jaguarforums.com/forum/x...solved-137070/

I would still upgrade the earths even if you dont think its the issue. Call it preventive as they will cause you problems at some point. I also would not rule out an intermittent issue here.

Cheers
34by151

 

You could use the power cord for the laptop, run datalogger in SDD monitoring the 02 sensors from cold engine start till engine operating temp. You might be able to see one sensor not quite mirroring against the other bank of same position - upstreams should look similar, downstreams should look similar, if not same.

 

The pinging has happened as long as I'v had the car--not at WOT but only at light throttles. It's a very light ping, nothing that makes me think the engine is going to be damaged.

I'm not looking at multiple PIDs at one time. The way the Creader6 works is limited, so I can only watch STFT1 or MAF or whatever at a time. What I was doing was looking at a single value while the car was driving, I'd feel the engine stumble (as a passenger) and then see a spike in the value. I'd then switch to another PID and see what it did at the next stumble. Whether that spike occurred before, during, or after the stumble I don't know. The graphing function doesn't respond that quickly.

Read that entire thread about Cambo's XJR... clogged cats make sense with poor WOT performance. My car works fine at WOT. In fact, on the rare occasion it's hesitating badly and surging I can kick it down hard and it will take right off. My surging is only occurring at low RPMs and light loads.

I did pick up a clue from his diagnostics--when my stumble is ocurring I do see the fuel rail pressure PID drop down to 20 psi. If it's true that the PCM should keep the fuel pressure at 55 PSI at all times, then that is a smoking gun right there. Is the fuel pump accessible under the rear seat of this car?

 

Yep it is - pull up the rear seat cushion, you'll see 2 big black blanking plugs. There is a special tool that you will need as well, might be available aftermarket though.

If you have an XJR - early models had pumps on both sides of tank. XJ N/A 4.2 had 1 pump. Later XJR's went to one pump - higher volume.


Removal

1. 
   
   Detach the rear seat cushion.

Special Tool(s)



310-146

Remover/Installer, Fuel Pump Module/Fuel Transfer Pump Locking Ring









Removal
WARNING:
Place the vehicle in a quarantined area and arrange "No Smoking/Petrol Fumes" signs about the vehicle.


NOTE:
Federal market vehicles, refer to step 12.

1. NOTE:
   All vehicles.
   
   
   Open the fuel tank filler pipe flap.
2. Disconnect the battery ground cable.
   For additional information, refer to: Battery Disconnect and Connect (414-01 Battery, Mounting and Cables, General Procedures).
3. Detach the fuel tank filler pipe cap.
4. Release the pressure in the fuel system.
   For additional information, refer to: Fuel System Pressure Release - 2.7L V6 - TdV6 (310-00 Fuel System - General Information, General Procedures).
5. Drain the fuel system.
   For additional information, refer to: Fuel Tank Draining - VIN Range: G45704->H99999 (310-00 Fuel System - General Information, General Procedures).
6. Remove the rear seat cushion.
   For additional information, refer to: Rear Seat Cushion (501-10 Seating, Removal and Installation).
7. 
   
   Remove the scuff plate trim panel.
8. 
   
   Detach and reposition the floor covering.
9. 
   
   Remove the floor aperture cover.
10. 
    
    Disconnect the fuel pump module electrical connector.
11. 
    
    Disconnect the fuel pump module quick release coupling.
12. NOTE:
    Federal market vehicles only.
    
    
    
    
    Remove the fuel pump module locking ring.
13. NOTE:
    Non federal market vehicles only.
    
    
    
    
    Using the special tool, remove the fuel pump module locking ring.
14. NOTE:
    All vehicles.
    
    
    
    
    Disconnect the fuel transfer module transfer feed tube.
15. 
    
    Disconnect the left-hand fuel sender gauge electrical connector.
16. 
    
    Disconnect the fuel transfer module transfer return tube.
17. CAUTION:
    Make sure no damage occurs to the fuel level float.
    
    
    
    
    Remove the fuel pump module.

Installation
WARNING:
Place the vehicle in a quarantined area and arrange "No Smoking/Petrol Fumes" signs about the vehicle.


NOTE:
Federal market vehicles, refer to step 6.

1. CAUTION:
   On vehicles with supercharger make sure the arrow on the fuel pump module and the 'SC' marker on the fuel tank are aligned.
   
   
   NOTE:
   All vehicles.
   
   
   
   
   Install the fuel pump module.
2. 
   
   Connect the fuel transfer module transfer return tube.
3. 
   
   Connect the left-hand fuel sender gauge electrical connector.
4. 
   
   Connect the fuel transfer module transfer feed tube.
5. NOTE:
   Non federal market vehicles only.
   
   
   
   
   Using the special tool, install the fuel pump module locking ring.
6. NOTE:
   Federal market vehicles only.
   
   
   
   
   Install the fuel pump module locking ring.
7. NOTE:
   All vehicles.
   
   
   
   
   Connect the fuel pump module quick release coupling.
8. 
   
   Connect the fuel pump module electrical connector.
9. 
   
   Install the floor aperture cover.
10. 
    
    Attach the floor covering.
11. 
    
    Install the scuff plate trim panel.
12. Install the rear seat cushion.
    For additional information, refer to: Rear Seat Cushion (501-10 Seating, Removal and Installation).
13. Attach the fuel tank filler pipe cap.
14. Connect the battery ground cable.
    For additional information, refer to: Battery Connect (414-01 Battery, Mounting and Cables, General Procedures).
15. Close the fuel tank filler pipe flap.

 

Just remember the PID values are relative pressure not actual pressure.

It is normal to drop and to peak as high as 70 when the TC kicks in.
Cambo did a good job of explaining this in that thread.
The short version is the PID reading is quite different to the reading from an actual gauge on the fuel rail.

The rail is a rock steady 55psi wher the pid jumps around

The pid value is the rail pressure relative to manifold pressure. This will vary a lot on supercharged cars, no so much on NS cars.

Cheers
34by151

 

Darn. Thought I was onto something there. I'll have to look at this closer--if the PID for rail pressure doesn't drop off under light load when it's not stumbling, but it does drop off when it is stumbling I think I have an answer.

I'm thinking fuel issues make sense since my issue is showing up on both banks. Here's my thoughts based on what I'm seeing, tell me if I'm way off base here:

Assuming an intermittently bad fuel pressure sensor, rail pressure PID drops to 20psi even though fuel pressure is not actually falling off, which causes PCM to raise fuel pump PWM.

1. Both front O2s see a huge rich spike (-0.8 mA indicated on both front O2s)
2. PCM tries to cut fuel injection PWM (-25% STFT both banks)
3. Rear O2s see this cut of fuel (0.1v both rear O2 sensors)
4. Sensor starts reading normally and everything corrects

I would have to see whether the fuel pressure dip occurs before or after the front O2s seeing the rich spike. But I don't see a failing fuel pump inadvertently raising the pressure, I could see it dropping out. The fact that both front O2s see a huge rich spike leads me to think something is commanding it high unnecessarily.

Since the fuel pressure sensor bases its reading also on manifold vacuum, that might explain why it's affected by slight uphill changes.

 

The only real way to know is to attach gauges to the fuel pressure and manifold pressure.

I think its most likely an electrical issue but cant rule out the intake/exhaust systems either at this point.

Can you replicate it with the front of the car raised on stands or ramps?
If you can tilt the car and force the condition it will be a lot easier to find

Cheers
34by151

 

Sadly I can't even replicate it on the same drive cycle consistantly. Last weekend I drove 300 miles on the highway with no problem whatsoever. I can go a week of daily driving w no issues. Then another time on a 25 mile drive it stumbled frequently. The only consistantly thing is that it happens under light loads. No rhyme or reason related to temp or operating time that I can figure. I actually wish this happened consistantly, it would be far easier to fix, and would probably even set a CEL.

 

On the off chance have you replaced the TPS at any point?

I had to do mine about a year ago as the car kept going into limp mode.
For about 2 years before that I had the occasional stumble and once a shutdown.
Never could find out why (no codes) till it failed completly

Might be worth doing the TPS if you haven't done it yet.
Just another thing that will fail at some point anyway and is worth it from a preventive point of view as well

Cheers
34by151

 

The TPS that's part of the throttle body? Mine is original. It's quite possible that is getting worn out. I can't see all the TPS PIDs (I think I can only see TPS A, commanded TPS, and APP) with my small tool, I need to get the SDD working in a flight mode to see the missing items.

Are there still companies rebuilding these?

 

You can just purchase a new sensor, they are quite cheep and common
It is fitted to the throttle body with 5 point torx bits
It is a simple job, remove the throttle body and bolt a new one in. You will need a new throttle body gasket though as they are one time use gaskets

The sensor itself is just a dual variable resistor
It outputs "A" and "B" signals to the ecu
The ECU commands the throttle body motor to move and the TPS lets the ECU know when its in the correct position

If the ECU gets conflicting info from the TPS "A" and "B" it can produce anything from a stumble, engine shutdown, or limp mode. It just depends on the signals being returned.

When I tested my old TPS it had a dead spot in "A" and "B" This caused my limp mode
I suspect that the dead spots were only on one not both for a while causing my random issues.

As yours is original I'd replace it now before it gives issues, and it will eventually. If it does not cure the stumble you have avoided a limp mode latter on.

Cheers
34by151

 

Who sells the TPS by itself now? I thought it required the TB to be replaced as a unit, or rebuilt by a specialist.

 

You will find heaps on ebay
Just search jaguar tps or DENSO 198500-3300

The TPS is the same on the XJ,S and X type

Cheers
34by151

 

Good grief. I remember my TPS went out on my old 04 XJ8 back in 2009 and the throttle bodies were still super expensive. It makes me shudder to think how easily and cheaply that could be fixed today.