So I'm thinking that as my car approaches 200k, it might be a good idea to inspect the oil cooler lines on my '80 300SD this weekend.

Just a couple of general questions about the lines. Are they under pressure, as indicated by the gauge? Meaning, if I'm running down the road at 3,000 RPM and a line lets go, will I loose my 8 quarts at 3+BAR? Or are the lines/cooler at a lower pressure?

As for the cooler, is it a good idea to replace it, as it's close to 30 years old? Would an aftermarket cooler be more effective?

When I replaced my lines on the SD I took the cooler down to the rad shop and had them pressure test and inspect. They gave me the green light. Yes, Line failures can be a catastrophe.

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The lines are connected to the very same oil filter housing as your oil pressure sender. Flow through the cooler is controlled by a thermostat. If the thermostat is open, the pressure in the cooler supply line should be about the same as indicated on your gauge.

Doesn't take very long to empty oil all over the road. A few seconds is about all you have. Best guess is 20-25.

Oil cooler pressure is always the same as system pressure. Even if the thermostat is completely closed, breaking an oil line will pump all the oil out of the system.

It all depends on how the line breaks. If it blows clean off you will dump all your oil in a short time, no more than a few minutes.

My upper line just got rubbed through by a belt last week. I drove on it for at least 5 miles, maybe more. About half my oil was dropped.

Where's the thermostat, in the filter housing? Is this something that should be changed?

In the filter base, only if your oil cooler does not get hot.

The oil flow is top to bottom so you can tell if its working by feeling the top of the oil cooler (next to the radiator).

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Same for me. About 15 miles. Its not a pretty sight. Very messy.

According to the 617 Engine Manual, the flow through the cooler is from the bottom to the top. Which is not to say that the top of the cooler won't get hot anyway.

Well, never mind then.

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So is the thermostat something that ever needs to be changed?

I was thinking that the oil cooler would get hot just from being next to the radiator.

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83 300SD (Pepe)
03 Golf TDI (Sandy)
Both running on B100

They don't physically touch and there is a lot of air going over them when driving, so it would not get hot if there is no oil flow going on.

I highly advise replacement of the lines, bursting line = dead engine unless you shut it down in those precious few seconds.

Lines: couple hundred $$ Engine.... $$$$$$

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I just found the lower hose leaking at the fitting by the cooler. It looks like the upper hose was already replaced because of the different type of hose material and compression joints at the metal line/hose junctions. It looks like this one.



I'm just going to replace the lower one that is leaking.

What size wrench do I need to hold onto the fitting on the cooler so I don't twist it off while loosening the hose fitting?

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83 300SD (Pepe)
03 Golf TDI (Sandy)
Both running on B100