300E engine dies on hot days

I have a 300E that will run all day long as long as it is moving. On a hot day with the airconditioner running, it will die withing about 5 min if sitting in a fast food line (half-fast). If there is a traffic delay
where the car is sitting idling for about 5 min, it dies. The aux fan comes on and the temperatur gage indicates normal temp.
The car will crank, but will not start. Sometimes it *might* sputter, but that is about it. If I open the hood and let it sit for 10-30 minutes, it will start just fine and run all day long again (until stuck in traffic)
I have confirmed and can duplicate the problem. I can drive around, come back home, and let it sit in the street idling. After just a short while it will die every time. It will not start until it has cooled. So, if I try a 'supposed' fix, I can verify if it is truly a fix or not.
Any thoughts?
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I would try the fuel filter first... then fuel pump relay.
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Your description sounds like a classic vapor lock but could also be due to a heat related failure of an electrical component. If the fuel system pressure is below specification the fuel will be prone to vaporize in the condition that you describe. An intermittent electrical component can be a devil to find - often a "let's try this and if it doesn't fix it we'll try that" approach - all with your money. Something like the ignition coil, for instance.
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T.G. Lambach wrote:

in this day of semiconductor ignition. I had thought about a vapor lock, but then why did it start happening and also happening with consistancy.
I was concerned about the "let's try this" type of solution. That is why I tried to see if I could duplicate the problem every time. If something is changed, it is very quick to discover if the change was the solution.
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Sounds silly - but you cold try an ice pack on selected electrical components and see when the engine continues running. I'd start with the coil.
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When the engine bay heats up (from slow driving, idling, hot weather), a failing/failed ignition coil will abruptly and consistently stop providing spark until it cools down. Once cooled, the car again starts and runs, until the engine bay gets hot again. The buildup of engine bay heat from the engine itself, the hot weather, the slow driving usually makes a bad coil react that way, failing when hot, working again when cooled.
Try changing out the ignition coil.

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Mmm... never thought about that... sounds very plausible.
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I figured the same thing about the heat buildup inside the engine compartment. However, my first thought was something electonic (semiconductor). The Ignition coil is nothing more than a high voltage transformer. I can't think of how heat would make it fail. The heat can not affect the windings, and I can't think how the heat will affect the magnetic flux of the core. Have you, or anyone else, had experience with this. I would hate to pay ~$100 if it isn't the cause.
Rugbyguy wrote:

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Because it pushed me to the edge of sanity in a 1969 280S that kept stalling in heavy traffic, midway in intersections, on creeping freeway commutes, during hot days, with the A/C on and the engine bay building up heat. It also happened the same way on an outboard engine on a powerboat I owned; after a hard power run, then stopping to an idle, the engine would die in five minutes as if the ignition had been shut off. It would not start for 20 to 30 minutes until the coil had cooled down. It was consistently, repeatably, reliably failing the same way every time.
In both cases, in my 280S and in my power boat, the problem was resolved with a new ignition coil.

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Hmm well I had a similar overheating problem in my 1991 300E, would happen on hot days or heavy traffic. After checking the electrics, my mechanic replaced the black box under the hood with another spare (they are VERY expensive). That didn't solve the problem -- it was the wiring harness attached to it that was intermittent -- replacing it worked.
Afterwards we also found out that one of the engine fans had been incorrectly wired and was spinning in reverse, not cooling the engine at all.
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Lumpy wrote:

the insulation is damaged.

My engine is not overheating. The temp gage is in the normal area. The problem is heat buildup in the engine compartment that is causing some component to overheat.
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