Hi all,

Recently I started getting a Gearbox Fault warning on my dash. Turning off the engine for a couple of seconds then back on always fixes this, but lately it seems to happen more often. First thing I tried was to check for error codes with my OBDLink LX scanner, but none were present.

After searching this forum I learned that the issue might very well be a vacuum leak, misfire or other engine related issue. If I'm correct, a vacuum issue would be visible by having overly positive values for short term trim or long term trim percentages. Below are the values I measured at idle rpm:




Are these values cause for concern?

When browsing through the mode $06 test values I did find an entry which was marked failed, TID $56 (potential misfires?). Im not sure how to interpret this, maybe someone can comment on this as well.



I need to add that the engine seems to run great overall. The only thing I notice is that when idling the car shakes a little bit, probably indicating misfires?

Jag S-Type 3.0V6 1999

 

I had the same warning right before my alternator failed. Found out how finicky these cars are when the smart charge system goes haywire. Just my own experience

 

Being an X200, I would be heading for the FAMOUS 90deg vacuam elbow waaaay down in the bottom of the V, towards the rear on the "B" side.

Both mine had that when first into teh fleet.

QUICKY test, NO rocket science involved, not even and OBD reader, pull the vac pipe off the throttle body spigot, cap that spigot, and run the engine.

IF

Nothing changes, ya got other issues,

BUT

If it smooths out, and runs as sweet as a Jag should, problem solved.

This snap shows my finger pointing at the spigot in question, and is so easy, anyone could do it.



Proper fix, 6 hours, many strange words uttered, 4 Inlet gaskets required, and the 1999 are NOT the cheap ones, then many more words, coz that small CHEAP hose is near impossible to fit.

 

First, thanks for taking the time to reply!

Couple of questions:

- Do my measurements suggest a vacuum leak? I thought under 10% would be ok. Wondering how you came to this hose/elbow being the culprit.
- If the issue was with this hose, wouldn't I have very positive trim figures for both banks?
- What does this hose do exactly and can I drive the car with this spigot capped? The engine already runs great, with just very occasional minimal judder being felt, not heard, when running idle. I'm not sure I would notice the difference right away if there was any.

 

OK,

NO idea on your figures, I dont understand all that fancy stuff, just simple diagnosis works for me, as always.

I am "old" and spent may wayward years with the REAL engine, with 12 cylinders, and NO OBD, probably not even invented in them days.

Looks like a Duck, quacks like a Duck, probably a Duck.

That spigot is the vac supply to the PCV system, and goes to a stand pipe at the rear on the "B" head, and that attaches to that 90deg hose, and that attaches to the vent "thingy" bolted to the crankcase.

That 90deg hose collapses, and leaks vacuam, sooooooo, the MAF sensor cannot meter this extra incoming air, due to the leak, via that spigot, as it is AFTER the MAF.

Shuddering, mainly under light throttle, roughish idle, are the prime "feels" that you notice.

Both mine have been capped for 6 years, no issues, BUT, if emission police are in your area, you may be pinged. Again, no idea if my emissions are AWOL, but the car/s run sweet, and economy is awesome., as is the power available, so must be happy.

Next is the coils and spark plugs, followed by the Pre Cat O2 sensors.

 

What DTCs are stored in the system? Post them here. If there are none, check with a different scanner.

The 'Gearbox Fault' message on the instrument cluster is typically displayed due to engine misfire(s) as the TCM protects the gearbox.

As Grant mentions above, the breather elbow under the lower inlet manifold and the IMT o-rings are common failures that can cause misfire conditions or lean running.

Also, check the spark plug wells for oil fouling as that can cause the coil units to short also causing a misfire condition.

 

No error codes stored at all. Tried with OBDWiz, OBDLink and Torque Pro. It reads PID $03, but says there are no codes stored. Guess no thresholds have been exceeded for any error code to come up. Can you recommend any other (freeware) scanner that should definitely get all those codes if there are any?

I disconnected and plugged the spigot Grant was referring to, but noticed no immediate difference. Engine is already running great to my ears, maybe I'll try driving with it plugged for a while and see what TID $56 does. I will check the spark plug wells for oil

 

90 degree hoses in vehicles >10 years old are an Achilles heel, so common on Ford pickups that you can predict the failure modes. Lean OBD codes will eventually show up, but not for 20 to 40 ignition cycles AFTER the pinhole in the hose has occurred. The engine will still run smoothly, hiding the problem. The ECM can compensate for a small vacuum leak at first, but once a % threshold is reached, it'll throw a P0171 or similar code.

 

Ok, so I've been driving with the spigot disconnected and hole plugged, but same issue persists. Because trim values are all quite low, I'm inclined te believe a vacuum leak is not the issue here. Also after all this time not a single fault code has shown up.

So far, if I interpreted everything correctly, the only thing pointing to an issue (OBD wise), is that MODE $06 - TID $56 keeps increasing rapidly when the engine is running. According to this site this is the potential misfire events counter. Unfortunately there are no specific cylinder misfires reported. Really curious what this counter does on other 1st gen X200's V6's. I read somewhere it can just be reporting misfires for cylinder 7 and 8 which are obviously not there so I might be handling a red herring here??

Can anyone help interpret this TID $56 counter and suggest what to do next to troubleshoot this problem? I hate to just start replacing coils.