when the heater turns on it is warm for about 30 sec to a minute then blows cool air, the aux pump is working.

are there known issues with heater cores clogging up on these, or something i should look for first? it seems like the vacuum stuff is working right but there just isn't enough volume to keep the car warm inside.

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"NEW" 1984 300D TD needs some love

Just a thought, have you recently changed out your thermostat and also checked the levels of your radiator coolant.

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Collector of Fine Hose Clamps
'77 240D 'AVA' is in heaven now
'80 240D Kanarienvogel
'82 300TD 343k was my daily driver 'ADOLPH' In Surgery for a severe Deer Bite to the Nose
'88 560SL 102k 'White One'

yvairguy - Which car, the 300D? Sounds like the Mono valve located on the firewall right of center.

https://encrypted-tbn2.gstatic.com/images?q=tbn:ANd9GcSIkw_Zvpc4i0Ekyv3nPUWZTR-06_SdxmKve1yRxapKt9zS_Pnz

https://encrypted-tbn1.gstatic.com/images?q=tbn:ANd9GcSN9hSLreNw8qym3MhOvKxUn9mFtu4VWlgaiYo42W9rTkuHuh3O

The black diaphragm rips causing erratic behavior.

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1985 300D, 250k
1980 300SD, 180K

took off for the holiday and didn't have a chance to check anything before i left, got back today and started driving it and it has issues but was nice a warm the whole time, and the front dome light decided it would start working....

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"NEW" 1984 300D TD needs some love

Just as I thought the German Gremlins must have been drinking the Antifreeze, it must have killed them. This is why I asked if you had checked your coolant. I cant explain the light problem.

__________________
Collector of Fine Hose Clamps
'77 240D 'AVA' is in heaven now
'80 240D Kanarienvogel
'82 300TD 343k was my daily driver 'ADOLPH' In Surgery for a severe Deer Bite to the Nose
'88 560SL 102k 'White One'