Heat and No-Starts: Myth or Reality?
 

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.

Just thinking out loud:

When the ignition key is turned -- both on and off -- i hear clicking from relays in the engine bay, in the general vicinity of the air filter.

Is it possible that one of these controls or affects the flow of current to the starter, and is now failing (remaining open when it should close, or vice versa)?

Given their location directly over the hot engine, I would not be surprised if one of these, or some similar component, was the source of the heat-related problems.

Then again,

the occurrence of problem does seem to coincide with the time I began to notice the transmission bushing failure.

Hmmm, looks like I'll be talking to myself on this one.

No takers?

SOLVED IT!
 

The culprit was my pushbutton starter wire.

It ran from the battery to the purple ignition wire near the fuse box, and was quite hot when I touched it.

The previous owner had cut the ignition wire as part of an elaborate engine-kill alarm system. (Apparently, he was good friends with McGuyver and the A-Team.) Before I bought the car, he removed the alarm but left all his wires, relays and other crap behind, which I cleaned out a couple of years ago, during which time I had a "happy accident" that necessitated the pushbutton starter in the first place.

As I prepared to replace the existing pushbutton wire with a thicker, insulated wire, I discovered, concealed in a black sleeve of sorts, the other half of the above-mentioned purple ignition wire! I joined them together, turned the key, and the car is back to its normal self; no more pushbutton starts. (But I am already missing it. It was very cool.)

As a test, I let the engine run until it got auxiliary fan hot, then shut it down for a few moments let it get a bit hotter. Held my breath, crossed my fingers, turned the key, and it cranked and started like the happy car I remember.

On to the next problem...