Hi Citrus Smoothie,
(I really miss the Silver Streaker though <g>)
Point by point replies:
3. Engine juddering should be related to a misfire condition. Of which the OBDII system should detect it and set a code (at least back on this side of the pond). Just what a Euro spec mixed as a Japanese conversion import will show under those conditions is beyond me.
For Spark plugs to be at fault the wear would have to be severe, well in excess of the 105K mileage replacement interval. However, if something is amiss in the combustion process the spark plugs should be able to tell the tale. "Reading" spark plugs is a complex skill which doesn't briefly translate to the written word. Try a web search.
Removal for reading? Sure. Replacement? Maybe. Another possible money saver here.
5. If the converter is a "standard" adapter that is commonly used over there then turn them loose. However, if this is something unusual then you need to briefly educate them as to where, and what the device is. If the car uses the standard OBDII connector its a simple task to plug in with a scan tool and watch the speed sensor output. If it's output is stable while the speedometer is erratic then the signal is breaking down somewhere in between. Maybe it's the conversion thing-a-ma-jig.
2 & 7. Mixing dino to synthetic has no downside that I can imagine. If you were going the other way and wanting the better protection of full synthetic then a flush would gain you a less diluted oil change. But it's still only a slight advantage. Maintaining the proper oil change interval over the life of the car IMO would be a more significant factor. Save your money here!
HTH!
If you request a diagnosis and report cost of repairs and you got nothing, then back here in the States in general you pay nothing. Unless the costs of the tests that must be done to perform the diagnosis were agreed to in advance. An extreme example would be: If they want to perform a code check when you know the check engine light hasn't been on then refuse the test, nothing will be found.
An hour is a long time labor charge for a first step "blindfolded" diagnosis. Until the car is road tested and the basic visual inspections performed, e.g. transmission fluid level, condition, odor etc. Then something more specific should become apparent that is needed to further isolate the problem before the specific cost to repair can be determined. E.g. the road test seems to show engine irregularities. Next step, check the basic input values to the engine computer (a type of code check).
Obviously I could go on, but I hope you get the gist of what I am trying to communicate. As a customer you need to understand their system of diagnosis and the charges that are associated with it. And understand what you are effectively "buying". If the shop suggest a test that seems out of line then have them explain the reasons. An average driver isn't as technically advanced as is sometimes required for diagnostic procedures and a "good" shop is happy to help you understand the _needed_ expenses.
A good shop should always have an If, Then, Else presentation as the diagnosis narrows. At first there are many "Ifs" with as many possible "Then, Else's". Its this first step that will carry a blanket labor charge.
If your experiencing a clear and readily reproducible problem, then the blanket charge must reveal a clear path toward repairs (which still _may_ require a few different branches), else I would think that the charge should be refused. Basically it demonstrates that they are in over their head. A good shop will waive the fee and return your car.
If the repair shop you have chosen doesn't use this basic approach to diagnosis, repair and customer communication, then you really need to look for a better shop, else (ouch, $$$ <VBG>).
HTH!
