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Old 05-23-2019, 12:54 AM
Desert Panther's Avatar
Join Date: Oct 2011
Location: Avra Valley, Arizona
Posts: 204
Question 85 SD diesel exhaust fumes mystery

1985 300 SD in well-cared-for shape. Car runs fine, no complaints with that. But when cruising at any speed with any or all of the windows or sunroof open, the smell of diesel exhaust comes into the car. This doesnt happen with the windows and sunroof closed, so I know it doesnt have an exhaust leak.

It is mind boggling (to me, anyway) that at 70 MPH on the freeway with all the windows closed but the sunroof open, that the exhaust fumes somehow travels up over the rear bumper & tail light panel, forward over the trunk, rear windshield, back section of roof and finally into the cabin! How is this possible?!?!

What would you suggest to remedy this issue, I am completely stumped.

NOTE: Prior to my ownership, somebody had replaced or re-welded the rearmost section of exhaust pipe and it isnt 100% aligned perfectly, but its really close. I cant imagine that would cause this issue, but I've been wrong before. Can anybody knowledgeable with these cars or this general situation, chime in here & enlighten me?

2007 Dodge Ram 3500 Cummins Turbo Diesel 4x4
2002 CLK 430 Cabrio - moneypit project car
1994 GMC S-15 pickup 4.3 5MT
1985 300 SD
1978 300 CD
1962 220-S Fintail - awaiting restoration
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Old 05-23-2019, 02:43 AM
Registered User
Join Date: Oct 2013
Location: New Zealand
Posts: 508
Originally Posted by Desert Panther View Post
But when cruising at any speed with any or all of the windows or sunroof open, the smell of diesel exhaust comes into the car. This doesnt happen with the windows and sunroof closed, so I know it doesnt have an exhaust leak.

That's interesting logic.....have you actually checked? You probably won't smell exhaust with everything shut if the leak is rear of the intakes below the windscreen.
1978 300D, 373,000km 617.912, 711.113 5 speed, 7.5mm superpump, HX30W turbo...not yet legal
1984 240TD>300TD 121,000 miles, *sold*
1977 250 parts car
1977 280> 300D - 500,000km+
1981 200>240D
1975 FJ45>HJ45

1999 Hyundai Lantra 1.6
1980s Lansing Bagnall FOER 5.2 Forklift (the Mk2 engine hoist)

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Old 05-23-2019, 02:43 AM
vwnate1's Avatar
Diesel Dandy
Join Date: Nov 2005
Location: Sunny So. Cal. !
Posts: 5,519

Look at the rear bumper mount surrounds ~ whey they're damaged / missing or loose, the exhaust fumes get sucked into the trunk thence the passenger compartment ~ look at the carpet on the parcel try around the first aid kit ~ there's usually some amount of exhaust smoke staining it .
1982 240D stripper 380,000 miles
1984 300CD KEEPER ! 481,XXX miles
1984 Euro 300TD Fully optioned SWMBO's
1974 350SLC 4 speed SOLD & missed
Kooky Kommie Ural Motocycles (3)
BMW Moto R60/6 Barn Find, 8,000miles
1959 VW #113 DeLuxe Beetle, 36hp engine, stock
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Old 05-23-2019, 08:27 AM
t walgamuth's Avatar
Join Date: Apr 2005
Location: Lafayette Indiana
Posts: 35,946
Check your connections under the the down pipe to manifold connection and especially around and in the flex pipe. Leaks there are hard to find sometimes.
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.
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Old 05-23-2019, 09:04 AM
oldsinner111's Avatar
once a happy man
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Location: bristol,tn
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wish I could smell it again,really liked my KFC oil,burning in the motor
plant, and animal life matter. human's had their chance.
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Old 05-23-2019, 09:24 AM
tyl604's Avatar
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Join Date: Feb 2008
Location: Atlanta, GA
Posts: 3,363
When that happened to me in my 300SD, it was the plastic vent pipe that fits into the top of the air cleaner with a rubber plug; had to replace the plug and all is well.

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