Coucherias said:

Just a quick update, the engine goes back and forth between running a bit rough and running smoothly. My driving behavior doesn't seem to have any bearing on how it runs. Unplugging the HP sensor doesn't seem to make a difference.

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Looks like injector number 4, then. You might want to get them all refurbished for balance.

No doubt you're right guineafowl, that'll be the next thing I look at. One of my neighbors is the local go-to guy for all things mechanical. I won't ask him to do the work but I reckon he can make a recommendation as to where to take it. I'll keep you posted.

update - A local garage removed the injectors and sent them in for testing, and the results showed the injectors needed replacing. The injection pump showed low pressure as well apparently but the garage is going to install the new injectors then retest.

Coucherias said:

The injection pump showed low pressure as well apparently

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How did they test the high pressure pump without good injectors fitted?

They weren't being categoric. When I dropped the car off last week they hooked it up to their diagnostic computer with me there and saw 10 errors returned, among them low rail pressure. And contrary to my conclusion after pulling the wires one by one, it wasn't just the fourth cylinder misfiring. This was too many errors to be meaningful other than it showed there were problems. Given that and my description of the problem, injector issues seemed to be a good place to start and indeed one or more - "they" - were found defective. This is all I know right now but I will know more on Monday. Once they are replaced they will be in a better position to work their way down the list of possible additional problems.

FWIW, after making the appointment with the garage two weeks ago I let the car sit rather than drive it in this condition. Then last Monday when I started it up to take to the garage it was idling so smoothly that I considered cancelling the appointent, and this lasted the whole time I was runnng errands prior to dropping the car off. But then on the way over to the garage it started misfiring at idle again.

A leaking injector can return a low rail pressure, sometimes giving an accompanying code. Once all the injectors are working correctly, the rail pressure should be normal again.

Boy wouldn't that be a relief. Replacing the injectors is one thing but I was starting to have visions of € signs drifting upwards like smoke from the opened bonnet.

Update -- I got the car back yesterday from the garage and it's running great. They replaced the injectors, then found the engine though running well lacked power. Some poking around uncovered air leaks in two of the three little air lines, one of which apparently leads directly to the turbo, meaning the turbo was not getting enough air to work properly. The mechanic said it looked to him like the holes had been there for a long time. Now everything seems in order, running smoothly and with a healthier sound I had forgotten hearing, little if any smoke at idle and little to none when driving. This last is huge because it's been smoking for two or three years, maybe longer.

As always, thank you one and all for your input, it helped get through the rough patches.