Jaguar Forum banner
1 - 1 of 1 Posts

· Registered
2 Posts
Discussion Starter · #1 · (Edited)
Hello all,

From a seemingly and otherwise happy owner of 2004 X-Type 3.0l V6 petrol AWD manual (Estate/SW) with DSC :).

For the last couple of years I've been suffering from the intermittent check engine light due to the logged P0121 fault (with "Engine Systems Fault" and "DSC Not Available" written out), throwing car into restricted limp home mode. This is not my daily car, so I tried living with it and tracking down this nightmare of an issue (as expensive it would be for a garage specialist) by myself instead.

Read all the related forum posts, done all the tests, TP sensor reports equal resistance readings over the both signal wires, across the 4-pin connector there are no shorts to ground or to the power wire (also reading desired 4.7V on it), no open or crackling circuits between it and the engine ECU hundred-pin connector, battery being 60% SoH (state of health) tested with advanced battery tester as "good condition", good voltage with and without alternator feed.

Yet the car is randomly going into this limp home mode: sometimes after an hour of travelling, sometimes it lasts fine throughout even the whole long roadtrip, yet other times I cannot even reach the shops.

When the failure happens, the only remedy is to clear the fault and unplug-plug the throttle-body valve/butterfly 2-pin connector (that's when it probably resets/moves it a few degrees to and fro, and the TPS sensor is again "liked" by the engine ECU until the next time failure happens again).

I did find the TPS connector receiving hole (of pin #1 -- ground) to be quite a bit loose, I could stick a thick needle without nearly any resistance (other 3 holes were tighter). So I took the connector apart and carefully bent the copper platelets inwards, making it a snug fit for the TPS ground pin.

Afterwards sadly I was still getting the P0121 logged again. I'm at a loss. I'm driving with the OBD2 reader plugged in and measuring TPS position in live graphs (using the Throttle Lite android app) during the journey -- no observable malfunctions there, but wasn't looking into them all the time, neither just before the fault popped again:

The only observation (in addition to that I'm going slightly mad:)) which I started noticing during the last two of such incidents is that the engine ECU would throw the check engine light during a sharp right turn, like turning into a petrol station, or at higher speeds at a protected junction. I hope it's not placebo that I'm observing.

Which brings me to the next point: I cannot fuel up the car normally for the last 6 months. The fuelling gun just clicks off. So the only way to do it is to trickle down for many minutes on end (feeling like an EV owner while standing there for 15mins at the petrol pump!:))

I read here and here that it's the ECU that decides if there is a malfunction in the system, and then logs an EVAP fault. I never seen any related fault logged (neither pendingly, nor permanently) via the OBD2 (but due to my cheap Bluetooth reader it may not be visible if it's either Jaguar-specific, or logged by modules other than the engine ECU(?)).
Next I will inspect the EVAP valves for blockages (bottom of page 5 of that bulletin -> then the flowchart P-1.d, page 15) and try to blow them with compressed air.

Could it be that I'm at a double-whammy situation, where ECU detects fuel sloshing at some sharp turns and invalidates the whole data in relation to TPS and EVAP, due to a petrol vapour blockage somewhere?

I'm throwing everything out there by now, as it's been dragging on for too long. Sorry for the lengthy post.
I wouldn't want to buy an aftermarket TPS (I know they exist, and even some pre-owned Denso 198500-3300 units), due to the egg/oval-shaped slotted mount holes, therefore by disturbing its tamper-proof screws and warranty-white-paint, I might not re-seat the replacement part exactly where the old one was, throwing the calib out of whack.

Replacing the whole TB is only last resort, as that's uneconomical (new unit is towards a thousand of moneys, and pre-loved ones might be just as faulty..)

Hoping on your input, much appreciated!
1 - 1 of 1 Posts