Fuel Pre-Heater on w124 300D

[sjh]

Greetings,

The w124, at least 90 - 93 have a diesel fuel pre-heater. It's also present on, at least some, of the w126 from the 80s. I do not recall it on my w123.

There seem to be two components. On the driver side of the block, below the IP is a thermostat. It diverts fuel that is below 25 C to a heat exchanger. If the fuel is already warmer than 25 C it is not heated. I have two questions -

1. I find the thermostat easily enough. Where is the heat exchanger?

2. Why have a thermostat? Why not always pre-heat the fuel?

Every time I post questions and receive numerous replies that are accurate, helpful and pleasant I again think what a great resource this place is.

I've posted some drawings of the components. They are for a 6 cylinder diesel but are still relevant.

The Good Doctor

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'90 300D (now missing the BFS)
'99 E430
'99 SLK230
'81 300D
'75 240D