I have a Jaguar 1999 XJ8 LWB Sovereign. When starting from cold it runs rough for a while and if you rev it then it clears after a while. If you turn the ignition off and restart the car it runs fine. I have already changed the MAF sensor but its still the same. On my diagnostic tool, it is giving me a code of P1646 which mentions oxygen sensor when I search on google. I wondered if this does sound like a problem that a faulty oxygen sensor could cause ?.

 

What is the number of kilometres/miles on the vehicle?

What is the battery voltage across the terminals when measured with a voltmeter with the ignition in the OFF position?

 

It can't get the fuel/air right if the O2 sensor is bad so maybe put in a good one and see if that fixes it. If not, look at fuel trims for clues.

 

If this just started, along with colder temperatures outside, your problem is likely the temperature sensor. If it is bad, it holds the warm sensing from before it failed and is failing to enrich on startup (like not having a choke for us old guys). Once the engine warms up, all's well.

 

I have changed the oil, oil filter and air filter. I have changed the maf and coolant temperature sensor. It still seems to have the same problem and petrol consumption is really bad also. I am just wondering if to change the lambda sensor as well because I have been told this controls petrol and air intake.

 

An O2 sensor does not turn on until the engine almost reaches operating temperature. It then switches quickly from rich to lean to control mixture. My thought is you have a leaking injector/ injectors. O2 sensors are blamed for many reasons but rarely fail. If it were mine I'd run some injector cleaner in the tank for a week. My theory is one? injector drips raw fuel into a cylinder overnight and once started, is burning a rich mixture for a few seconds therefore setting an O2 code. A code reader can monitor this switching voltage from the sensor to verify proper switching of the sensors voltage. The O2 is the last thing I would suspect.

 

@mat32essex, before throwing more parts and money at the problem, check the STFTs and LTFTs to see which cylinder bank is causing the issue.

When you changed the oil, was there petrol mixed in? What DTCs are stored in the system?

 

I'm getting a p1646 ecu 11 error message on my diagnostic tool.

 

 

Sounds like it could be well true because it seems to to hesitate when driving and putting foot down. It seems to be getting worse also. Sometimes it seems like it wants to stall when stopping. Would the relay just not fail completely than slightly ?. Also do you have any idea where I would find the fuel pump relay ?. Many thanks.

 

Here is a link to the X308 Workshop Manual:

https://www.dropbox.com/s/9aid15mx5t..._1998.pdf?dl=0

 

I have to agree that I would think the relay would either work-or-fail.

Here's a pic of the relay location, in the trunk, next to the battery. Simple to replace and worth a try.

 

I reset the ecu by touching the battery terminals together and it was ok for a while till I went to restart it after it was sitting for a little while. It was low on fuel and the fuel gauge was slightly in the red. I put petrol and some petrol system cleaner in it then it seemed ok. I still think there is a issue somewhere still though. Many thanks to everyone for there advice and support I really appreciate it.

 

Hi mat32essex,

Sorry to join your thread late but I'll try to add a few thoughts to the good suggestions you're already received.

According to the Diagnostic Trouble Code Summaries for the 1999 AJ27 engine, the P1646 code does point to a problem with the A Bank/Bank 1 upstream oxygen sensor or its electrical circuit: Your normally-aspirated 1999 vehicle has only one fuel pump, so you do not have a Fuel Pump 2 relay to trigger the version of P1646 that relates to the 1998 supercharged vehicles.




You can download the DTC Summaries for your car at this link:

Jaguar AJ27 DTC Summaries 1999

However, it is unlikely that the O2S issue is the cause of your rough running immediately upon cold starts, because, as alluded to by grantorino62, the oxygen sensors are ignored by the ECM while their heaters are bringing the sensors to operating temperature. O2S operating temp is not directly related to engine operating temp. The O2 sensors usually reach operating temp within 30 to 45 seconds, sooner on vehicles with secondary air injection. During this time, the ECM operates in open loop, meaning it determines fueling based on tables stored in its memory rather than based on feedback from the O2 sensors. Once sufficient time has passed for the O2S heaters to do their job, the ECM goes into closed-loop fueling, meaning that it adjusts fueling dynamically in response to inputs from the O2 sensors, engine coolant temperature sensor (ECTS), mass air flow sensor (MAFS), etc.

I like Jhartz's mention of the ECTS, because failure can lead to both starting problems and increased fuel consumption, but you've already replaced it. Another component that can cause increased fuel consumption is the coolant thermostat. If it fails open, the engine will not reach full operating temperature, so the ECM will continue to apply cold-start fuel enrichment even after the engine reaches full temp.

You don't mention how long it's been since the engine had a good tune-up, but often problems with rough running are due to multiple components worn or operating out of specification. A good tune-up would include at the very least new correct spark plugs properly gapped, air filter, fuel filter, cleaning the ground points in the engine bay including the engine ground strap, and cleaning the throttle body and checking the Air Assist Control (or Close) Valve, which is directly involved in cold-start fueling. See the diagrams on pdf pages 19 and 32, and the explanation on pages 30-32 in this manual:

Jaguar XJ XK Technical Guide / Model Year Update 1999

Besides the P1646, are any other DTCs stored? Have you been able to view the STFTs and LTFTs with your scan tool?

Cheers,

Don

 

There is no other stored codes apart from the one previously mentioned. I don't think my tool can measure these settings it is only a really cheap diagnostic tool. The hesitation seems to be intermittent. If it has this problem then you turn the car off and restart it then the problems normally goes and does not return again which is strange. Is there any way to check if a relay is faulty ?. I do not mind spending money fixing the fault but do not want to spend loads more than is necessary. Could a fuel filter or bad spark plugs cause these issues or could a bad coil cause these problems ?.

 

I wondered if anyone is local to East London or knows someone that has a knowledge of Jaguar X308's and has the diagnostic tools to check it ?. I am willing to travel and pay whatever is wanted for there time.

 

Update!

The car kept stalling the other day. I opened the bonnet while it was running and I pressed the clip on the connector connected to the MAF sensor and the revs changed and it has run perfectly since. So the clip must have been slightly loose as it got worse over time. The mpg has increased greatly also. I appreciate everyone's support and help.

 

By pressing the clip you have moved the connector a bit and the pins inside have established better contact. The pins are probably somewhat oxidised causing poor connection. Unplug the connector and plug it back in some 4-5 times. This will rub-of the oxidisation and the connector should be fine for some time. If the problem returns, clean the male pins using a cotton bud dipped in alcohol (do not use sand paper or a screw driver to rub the pins). If you find any dielectric grease inside the connector, clean it out with a contact cleaner spray.

 

Thanks for your advice. I originally took it off and plugged it back in a few times.