When you do the compression check. Especially if you pull the injectors to do it. Number them. Then when putting them all back in switch the number two injector with one other.

Thats if the compression test does not indicate something. See if the problem has moved. In your case I am not suspecting an injector or injection pump as there is smoke with the injector feed on and no smoke with it off.

Also have a look at the head casting numbers as it was a replacement during the rebuild. Is it a number 22? You will also want to look and inspect the valve train for the number two cylinder. But do the compression check first.

If the compression check shows that cylinder low especially because it is a 3.5. You will have to do a leak down test. Low rate leakdown could be a bent connecting rod. They apparently already changed out two rods in this engine. The reason why may be unknown or the last owner may know.

Not potentially the happiest of situations. At least it is fairly easily diagnosed. Personally I would take this car out of service until it is totally diagnosed. Driving on a bent rod may do additional damage.

Strange the guy that repaired the engine 40k ago did not change all the rods if he found two of them comprimised. There is also of course a chance my line of thinking is wrong. I hope so in this case.

For example if air was escaping around the glow plug. Or head gasket. Or a rocker arm was not doing it's job.