I have a 1990 350 SDL with only 57300 original miles. When the recirculate switch is off the smell will eventually become unbearable.
Any advice

Raw diesel or exhaust?

Raw diesel - check the trunk area on passenger side if wet look at seal on tank cap and vent line. Under hood look at injector lines from IP and look for weeping on injector return lines. You may have to park in dry area and look for drops of fuel to get a general area to investigate.

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Could be as simple as having dirty carpets. One problem with diesel is that all that spilled fuel near the pump at the gas station can get on your shoes then makes its way into the car and into the carpet.

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Current:
2014 VW Tiguan SEL 4Motion 43,000 miles.

2016 Hyundai Santa Fe Sport (wife's).

Past:
2006 Jetta TDI 135,970 miles. Sold Nov. '13.
1995 E-320 Special Edition. 220,200 miles. Sold Sept. '07.
1987 190-E 16 valve. 153,000 miles. Sold Feb. '06.
1980 300-D 225,000 miles. Donated to the National Kidney Foundation.
1980 240-D manual, 297,500 miles. Totaled by inattentive driver.

Check under hood, look at the looped fuel return lines. Is the cloth braid wet, saturated, and possible dripping? These lines are rubber with cloth braid, and when they age, they will weep and leak. When they do, the odor will get into the cab.

If they are leaking, a one-meter length of replacement line at your dealer is under $15, and will take about 20 minutes of your time to replace.

Best Regards,
Jim

For what it is worth I had a similar experience, and my car is GAS. I really thought I smelled diesel. I had just changed out the CAT converter but I couldn't figure out the connection. It turns out that the CATCO (crapco) unit had a malformed flange where it mates to the manifold and it was allowing exhaust to leak. And when the engine was cold especially this leakage would really smell like diesel. I took the whole thing off again (not fun) and reworked the flange and now the smell is gone. Long story short, it could be an exhaust leak.

Mike

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1998 C230 330,000 miles (currently dead of second failed EIS, yours will fail too, turning you into the dealer's personal human cash machine)
1988 F150 144,000 miles (leaks all the colors of the rainbow)
Previous stars: 1981 Brava 210,000 miles, 1978 128 150,000 miles, 1977 B200 Van 175,000 miles, 1972 Vega (great, if rusty, car), 1972 Celica, 1986.5 Supra