Not quite sure how to describe this, but here goes. About six weeks ago I started up my 99 S70 T5 to go home after work & when I started driving I noticed that when I depressed& took my foot off the gas pedal the car would jerk. It did this in all gears and at any speed all the way home(about 30 minutes). It accelerated and decelerated just fine after the initial jerk. I also noticed that at certain RPMs when driving at a constant speed the car was very jerky. It was as if it did not know whether to speed up or slow down. I never did try to set the cruise. I had to go out again about 2 hours after getting home and everything was fine, no problems at all. I did not have any warning lights come on during this episode. No more problems until 2 weeks ago when the check engine light came on. I immediately checked the gas cap even though it had been about 5 days since the last fill up and it was on properly. I took the car into my local garage and they told me the error code was P4055 but could not diagnose the problem for me, other than it was not the fuel cap. They reset the code and sent me on my merry way. Then 4 days later on my home from workthe jerking problem was back, exactly the same as before. This time I had to go out 10 minutes after I got home and the problem was gone again. This past Sunday my Check Engine Light came back on.
Is it possible that these two problems are related? or are they seperate issues? Any ideas would be appreciated before I make the dreaded call to the dealer.
Thanks
Joe

 

Usually a P0455 is related to a vacuum leak, carefully inspect all the vacuum hoses for loose connections or cracks in the hoses. There should be a sticker under the hood with a schematic of the vacuum hoses.