I have been trying to figure out what happened since this morning. I was driving to work my my series 3 xj6 with the 4.2 L6. The car was running fine until I got it up to about 60 mph (100 kmh) and then it started choking and struggling to maintain speed before dying. The vehicle will start after some hesitation but will only choke for a few seconds and stumble before dying. I think it may be a fuel issue but I am not certain on what exactly it is. Has anyone had anything similar happen? I should also note the car does run a bit rough and smokes until it warms up. I was thinking maybe the cold start injector was over-fueling but im not certain. I also replaced the temp sensors as well

 

have you checked the air bypass screw yet? It is not so easy to maintain perfect. I know that there is a fine YouTube video from livingwithaclassic describing it.

have you checked the vacuum hoses on the engine? There are two crucial ones to my knowledge. 1st the one for the distributor advance. 2nd there is one below the fuel rail into the air inlet. This last one took me weeks to find and my car did not start properly in some cases and stall tight away.

Have you checked the fuel Pump yet? I had a similar case and in the end there was junk in the fuel pump. Use a vice grip on the inline hose before loosing it on the fuel pump. Then you will not spill to much fuel into your trunk. Hit that pump a few times to see if there is clear fuel or some junk coming back out of the input side. A pre filter did not do the trick for me. In the end I had to open the tanks (service door behind the back lights) and drain the fuel from them via the drain plugs. This was a little longer process, because there was a lot of junk in one of them. I used a garden hose to flush that tank until no junk came out of the drain, let it dry, and then used some rust protection in it. Let that cure and then closing all of it again.

hope that my experience may help you.
best
levin