Car: 1989 260E
172,000 miles
Pushbutton Starter

In the past few months, I have been experiencing occasional starting problems. Specifically, when the car gets hot it will not crank. I'll turn the key, push the starter button, and nothing happens.

I have read many threads here which suggest it could be a bad starter solenoid, or perhaps a worn shifter bushing. While these are both valid and applicable to my circumstances -- at least as far as the worn bushing is concerned -- I am not satisfied they are the root of the problem, since recent experiences with the car keep raising the one central question:

How does Heat factor into the no-crank problem?

The car always starts when cold, suggesting a faulty starter, perhaps with insulation breaking down and causing heat-induced resistance. However, I have observed in recent days that if I reverse my ignition sequence, the car will crank no matter how hot it is.

Normally, I turn the key and push the starter button. The car may or may not start, depending on its temperature. However, if I push the starter button first, and then turn the key, it always starts no matter what the temperature is. The starter therefore cannot be faulty or else it would not crank in this reverse sequence, given the high heat conditions. Here is what I have observed after some experimentation:

When car is Cold:
1. Turn Key & push starter button
2. Car always starts.

When car is Warm/Hot (coolant between 80 and 90):
1. Turn Key & push starter button
2. Car usually starts.

When car is Hot (coolant between 90 and 100):
1. Turn Key & push starter button
2. Car may or may not start. It's a crap-shoot.

When car is very Hot (coolant over 100):
1. Turn Key & push starter button
2. Car never starts

When car is very Hot (coolant over 100) -- Reverse Ignition Sequence:
1. Push starter button & turn key
2. Car always starts

3. Shut off car then Turn Key & push starter button
4. Car usually starts (which is perplexing, since it should not start at all.)

5. Shut off car again then Turn Key & push starter button again
6. Car never starts

7. Remove key for about a minute, push starter button, turn key
8. Car always starts

9. Shut off car Turn Key & push starter button again
10. Car never starts

From this analysis, I would conclude the problem is in the key-activated ignition circuitry, in which something is blocking the current to the starter when the engine is hot.

There are a lot of tech and non-tech geniuses here, and I hope one or more you can help shed some additional light on this.

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black