The 1984 300DT appears to have an intermittent coolant leak. The leak appears to be coming from somewhere near the rear of the engine. It isn't leaking much. Is there a method for tracking down intermittent coolant leaks?

You could pressurize the cooling system? Look for leaks?

Check out hoses in that area, maybe one of them has sprung a little leak.

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82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k

Probably the house going from the heater core, through the firewall, into the engine block on the rear of the engine at the driver side. You might have to get the car up to temp and leave tyre engine on and look under the hood at the hose to see if there is a stream of coolant coming out.

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1990 190E 3.0L

OK, I'm going to try that (start the engine, get it hot and look around using a high powered work light). The leak started about two months prior and then stopped so I didn't worry about it. Now, all of a sudden, it is back again. I'm keeping a close eye on the coolant reservoir to make sure it doesn't get low.

Around two years ago I replaced the radiator and all of the cooling hoses. I have a Prestone back flush kit installed on one of the heater hoses; also had one installed on the other car and it sprang a leak (the plastic "T" fitting CRACKED - luck would have it I had just gotten home and noticed the problem - good thing it hadn't happened while traveling); as a result I had to replace several heater hoses on the 560SEL so I'm wondering if this might be the same problem?

Yeah, those things are evil. Don't ever leave one installed in your cooling system. You can always add it in there with an extra piece of hose if you have to do a flush.

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1991 F250 super-cab 7.3 IDI. (rebuilt by me) Banks Sidewinder turbo, hydroboost brakes, new IP and injectors.
2003 S430 - 107K
1983 300SD - Tanoshii - mostly restored ~400K+.
1983 300SD - Good interior. Engine finally tamed ~250K.
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Many parts stores will lend you a pressure tester. It's way better to investigate coolant leaks when it's cool and quiet.