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I will try to answer all of your new questions related to my previous post:
Regarding Network diagnostic trouble codes, there are actually four types of DTC: Powertrain (P-prefix), Body (B-), Chassis (C-) and Network (U-). Most scan tools can only read Powertrain codes that begin with a P. It can be very helpful to know if other types of codes have been triggered. For example, ABS faults often trigger Chassis codes, and computer communication problems will trigger Network codes. These codes will not always trigger any Powertrain codes, so unless your scan tool can read these other codes, you don't know that other faults exist.
Fuel trims are an important measure because they tell us if the Engine Management System (EMS) is able to properly maintain the air-to-fuel ratio (AFR) within an acceptable range. Not to try to provide a full explanation here (if I even could), but fuel trims represent the degree to which the EMS is adjusting fuel injector pulse duration in order to optimize AFR. A positive fuel trim number indicates that the ECM is adding fuel to compensate for issues such as excessive air, as measured by the oxygen sensors. A negative fuel trim number indicates that the ECM is subtracting fuel to compensate for issues such as insufficient air. Typically, a long-term fuel trim of +/-10% is considered acceptable. If the LTFT exceeds +/- 25%, a lean or rich DTC will be triggered. But if the LTFT is running at more than +/- 10 but less than +/- 25, engine performance, fuel economy and emissions will be adversely affected but no DTC will be triggered. STFT represents real-time adjustments the ECM is making, alternating slightly rich and slightly lean, while LTFT represents an average over time.
If your scan tool or SDD can monitor the throttle position sensor and accelerator pedal position sensor signals, you can test them for smooth changes with no glitches or dead spots as the accelerator pedal is moved toward the floor. There are more sophisticated tests you can perform - search the forums and consult the workshop manual.
Regarding the EVAP system, there is not much to do in the way of maintenance other than to correct leaks or replace valves as necessary. A stuck-open purge valve will allow unmetered fuel vapors to enter the engine. If the purge valve or canister control valve is stuck closed, insufficient purge flow will result. Usually, these problems will trigger one or more DTCs, but not always. The valves can be tested with a 9-volt battery and a vacuum pump (or your mouth, if you don't mind the taste of gasoline).
Thinking about your accelerator pedal moving on its own, it almost sounds like cruise control might be engaging. I'm sure you would know if that was happening, but that seems like an important clue. We need to think more about what could cause that behavior.
+1 on dielectric grease being an insulator that should not be used between electrical connector terminals. It's fine on bayonet light bulb bases where it gets scraped away from the contact points when the bulb is inserted.
Originally Posted by Oliver94
Do the big ground wires need changing alltogether or just a good clean up of the attachment points?
Usually, the engine ground strap can just be removed and the end terminals and contact points on the engine and body thoroughly cleaned with a brass-bristle brush and electronic contact cleaner. On the X308, the bellhousing bolt where the engine end of the ground strap mounts cannot be removed with the downpipe/catalytic converter in place, so I loosen the bolt as far as I can and use a small brass-bristle brush to clean the bolt threads, the underside of the bolt head, the ground strap eyelet terminal, and the bellhousing where the strap connects when the bolt is tightened.
A few times I have found the ground strap to be so badly corroded that the green copper oxide had worked its way up inside the crimp joints of the eyelet terminals. In those cases, I added a second ground strap between the body and one of the lower bellhousing bolts that was removable with the cat in place.
it has been some time since the last update but here are the "findings" or the lack thereof. So maybe this helps someone reading along.
In the beginning of the new year the vehicle has been scheduled for paint correction and a ceramic coating, so I did a couple of things in the meantime while it was out of service.
1) Main connection at the false bulkhead (was pretty obvious)
2) The big ground point at the transmission (the old one was badly corroded)
3) Plugged in the "gearbox ECU" fuse
I strongly suspect #3 to be the culprit. For whatever reason fuse #6 (5A Gearbox ECU) was "out" about 3 to 4mm of the socket (0.15inch in freedom units) so I put it out completey out and back in. Since then, the car has done around 5000km of daily driving without a single issue.
Again, thank you very much for anyone participating and helping me along the way!
the symptom is back after pretty much a year! Only in the wintertime though, so the cold temperature might be a part in the equation and always right after starting the morning commute. Only then it happens.
I guess I was wrong in calling the title "Gearbox fault?" as I did quite a bunch of things on the transmission as well as some other items on the car. Yet the issue is the same and is unfazed by my efforts. The conductor plate with all sensors has been changed plus a full ATF flush with a special cleaner. While all this has made the transmission better and better, then issue must be somewhere else.
Some of you have already pointed out that there might be an issue with the throttle body and/or correlating sensors. I have read all available threads reg. the AJ26 TBs on our cars as well as their exact workings on a repair guide. Now,, my then described faults make more sense as well (self driving, gas pedal moving itself) as this could only be correlated to a malfunction the TB.
Before diving super deep into the repair of a TB or getting a new one, does anyone know how to clean the AJ26 (MY1997) Throttle Body? Can I just open up both sides (PPS left / TPS right) and "lubricate" or clean anything?
Furthermore, there seem to be a lot of repair guides for the AJ27 TB available. The ASI TB rebuild, the gold connector upgrade but I seem to have overlooked anything about the AJ26 TB. Can someone give me clarity on this one?
I'm sorry to hear that you are once again having issues with your beautiful XJR.
Have you had the car scanned by a system that can read the Powertrain, Chassis and Network diagnostic trouble codes? There should be some codes stored which should help narrow down the diagnosis.
I do have the iCarSoft LR V3.0 which does work well with the car. Yet there are no stored codes or any signs of trouble to be detected.
Shame on me, I did manage to keep one piece of information to myself. Once the symptom did arise again (a few weeks back) I did try to diagnose it with the OBD2 to no avail. Then I disconnected the battery and took out the respective fuses for the Engine Management and the Transmission Control Module which did not change the situation and the issue persisted (same day still). I then did disconnect the right side connectors of the Throttle Body where the throttle motor sits (two connectors), did spray contact cleaner inside and blew it out with compressed air. Afterwards, the issue went away and did not come back to this day. Hence why I am quite positive to see the TB and its respective motors/sensors to be the suspect.
12-30-2022, 11:11 AM
