Something was nagging me about my response to your thread. I always try to use the most effective simple tests to establish faults when possible. Although they require some thought to recall or design sometimes. To absolutly eliminate exhaust fumes have a buddy put something over the tailpipe with engine running. Carefully examine your complete exhaust system while it is under heavy back pressure. If this does not locate a problem have a second buddy stop the engine and start it again several times if necesary for you to totally inspect your complete system with tailpipe partially obstructed. When restarting you get more smoke and this helps locate the site of leakage if present. Could even be a crack in the manifold or poor manifold gasket for example. Also liked the gentlemans ideal of taking a battery powered home carbon monoxide checker into the car with you. A positive test with one would also be conclusive. Or conversly a negative one would clear the exhaust. Do not know but suspect upon reflection that a heater core leak would not make you woosey. It also was no joke about also keeping the heater motor running to pressurize the cab. If that lessons the effects you are experiencing it should point you in the right direction. Discount what the garages did a little as the majority of their employees do not own diesels and are not very familar with the difference in effects and smell. Also one of my pre purchase tests on an old diesel is to turn on the heater blower and see what condition it is in. If extreme bearing wear present you pretty well know what the previous owner was doing. To give you an ideal of how bad some garages are but not all. I purchased a 240d in new jersey. Started to drive home about a thousand miles and smelt exhaust fumes. Went to four muffler shops to no avail. All it turned out to be when I got home was a loose header pipe to manifold connection. The blower motor at top speed kept the fumes out of the cab during the trip home. Found it really hard to believe the shops did not locate it. Just required the bolts to be retightened. Suspect someone might have loosened them to change motor mounts as the mounts were relatively new and forgot or neglected to tighten them after. Guess the only buisiness those muffler shops were interested in was complete system changes. Just glad not to be one of their regular customers. The other thing I noticed was there were no customers at those shops. At home the good shop always is busy and the local midas franchise always pretty well dead like the four shops I stopped at in new jersey.