Make sure that you've used a code reader to find out what codes the car is throwing. While trying to clear codes on my '90 300e, two of the codes pointed to a bad coolant temp sensor (it's the black headed one with the four prongs). I initially thought that the plug that attaches to it might have been attached wrong so I rotated the plug 90 degrees. While test driving it, it wouldn't start when warm. If the sensor is defective, it seems to confuse the engine management controller and shuts the car down.... or so it seems.

The proper way to test this sensor is to test the resistance of two poles diagonally across from each other(like an X) to make sure that the values are the same. This sensor actually has two thermistors that have poles opposite each other. In this way it doesn't matter which way the plug is attached. This sensor seems to be critical feedback for the engine management system. The readings bounce around so make sure you test it a couple of times to be sure. On my bad one, there was one pole that was bad so the readings wouldn't pair up.

By the way, to install it you'll have to unbolt the valve cover and tilt it up by a couple of inches in order to have enough clearance for the 7/8's deep socket you'll need to remove and install.

Hope it helps.

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03 CLK430 Cabrio 211K Pweter Silver/ Oyster Leather
90 300SEL 214k Pearl Black/ Alto Grey/Black Leather

Prior: 85 190E, 88 300TE, 89 300TE, 90 300E 2.6, 90 300TE, 92 300E 2.6, 91 and 93 300SL, 87 Ferrari Mondial Cabrio