Cross fingers, knock on wood etc, but after 3 more weeks at the dealership the mechanic is confident that he has finally found and fixed the problem!
Try to guess the cause of an intermittent, extremely hard to locate, makes no sense electrical issue. If you said grounding problem you win the prize!
And it turned out to not even be caused by Jeep!
Short version:
When a brake controller was installed by a third party company a few days after we picked up the Jeep, they grounded it to the main grounding bolt.
Apparently at some point in this process, a hair thin gap was introduced between a nut and washer on the bolt. Every so often, and only under full load with the glow plugs and (probably) brake controller on, this gap would arc, causing just the right electrical connection to be made in the glow plug controller wiring to trigger the 1648 code
Long version:
The Liberty had a trailer hitch and wiring on it when we bought it, installed by a local company that is probably one of the biggest and most well known in the area. We needed to upgrade it from a 4 pin to 7 pin connection and install a brake controller, so we took it back to them to do the work. No problems were reported and the brake controller and wiring has been working fine ever since.
8 days later, one day before we were scheduled to take the Jeep back to the dealership anyway for some upholstery work deferred from the purchase, the check engine light came on for the first time. As I noted before, at this point there were 3 trouble codes, not just the 1648.
Six months of work ensued trying to find and fix the problem. During the last 6 weeks we just left the Jeep at the dealership and borrowed it a couple of times when we needed it.
Since the previous post, the mechanic had replaced the glow plug controller module (again), and was tracing wires (again) getting ready to overlay all the wires between the PCM and the harness when he noticed that, under certain high load conditions, the circuit he was tracing was sometimes getting a 1 instead of a 0 (resistance? I'm not clear on this), and that this was related to the code being triggered. He traced it back to the ground post, and under careful examination small 'welded' spots could be seen where the arcing was occurring. He called it the classic 'needle in a haystack' issue.
The grounding wire from the brake controller appeared to be installed correctly and grounded and tightened to a location that seemed reasonable and normal. It was just a fluke that one of the stacked nuts on the grounding bolt bound at just the exact wrong place to introduce this fine gap.
He has been able to reproduce the problem consistently, and after cleaning, reassembling, and tightening the ground bolt and nuts he cannot get the problem to occur again. We got the Jeep back yesterday.
Now for the really great part. We were not even charged for all this work! The service department essentially wrote it off to a 'freak of electricity', 'act of God', or whatever phrase you prefer. They said that under normal circumstances they would probably pursue some compensation from the third party company, but in this case they were unlikely to get anything, probably because it would have been so difficult to make the connection between the CEL and the brake controller.
I don't know about the hard starting problem. I'm assuming it's fixed and an unrelated issue, because nobody has mentioned it in all the time the Jeep has been in the shop.
I'm looking forward to finally being able to enjoy the vehicle I bought 6 months ago!
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Both times the CEL came on (same code, 1648). Also when it happened this morning, the diesel gauge was stuck at the bottom to the extent that it triggered the low fuel warning light, even though I had over half a tank remaining. It gradually climbed up to the correct location over 5 minutes of driving.
