I have noticed lately that my HJ60 is producing some grey smoke. I can only see it at night when someone is behind me and thier headlights illuminate the smoke. I can barely see it at idle, but it is noticable at about 50% throttle up until about 40mph (by then the turbulance from the truck is dispersing it). Is this an acceptable amount? What should I check/change ro remedy this? It still pulls as strong as ever and gets ~21mpg mixed driving.