So...You can push start a 300D with a manual trans in reverse...
 

I had this problem last year and thought I had it solved. It's 99* out here and I drove to the store. When I came back out, the car wouldn't start.

Something about when it's hot out it doesn't like starting on it's own power. Today the battery read 12.7V, the engine coolant was right at 80 (both when I turned it off and tried to start it), and (of course) I was parked with my nose up hill.

Last year, I upgraded the alternator to a 115 amp and upgraded all the main power wires, and that didn't help. I tried a different battery with the same outcome. I do by the way have the right battery in there...I knew it would get asked. Thinking it was the starter I swapped it out, and didn't have any problems for the rest of the year. Any ideas on what is wrong? I don't want to go through another summer push starting my car. What an embarrassment.

I have let cars roll backwards and bumped the engine to start it.

Just be sure you have the tranny in reverse!

you can push start any car whether it has a manual or automatic tranny

When was the last time you adjusted your valves?

About 10k ago. I am reluctant to accuse the valves due to the fact that the engine wouldn't even start turning by using the starter. A push start works like a charm.

I have a similar problem when hot outside and I think it it is the solenoid. But haven't gotten around to replacing the starter because it is a pain in the arse to accomplish.

So does make a "click" sound when the problem occurs? If so, try just turning the key to on and release a little and on again until it turns over. My issue almost always start when I do that.
Or try jumping the two outer connectors in junction box in front the battery to see if it cranks over. I keep a Hazet 2769-3 valve spring keeper wrench for this purpose (perfect sized for jumping the terminals) in my glove compartment.

If you are successful in any of those procedures, then it's got to be the solenoid (even though you changed out the starter). Or maybe the ignition switch is full of goo and disrupting the current flow when hot.

Yeah, it's just a light click, then nothing. I should add that I always wait for the glowplugs to time out. The aux fan is usually on and it winds down when I am trying to crank it.

I will try your way and see if I can get anywhere. Thanks.

Sorry wasn't reading very closely, my bad, I agree with the solenoid.

Have you tried jumping terminals 1 and 3 in the plastic junction box on the right front inner fender in front of the battery? It bypasses most of the ignition wiring and feeds current directly to the solenoid.

Also, its always a good idea to check your battery cables including the attachment of the ground wire to the body and the one which connects the engine to the chassis.

I always push start in 2nd, I have had to do it a couple times on the coupe when I left the lights on. my door switch needs replaced.

I assume you've ruled out the NSS??

I have the exact same problem on my SD. After driving for a while in the heat, I get somewhere, shut off, do whatever, return to the car within about 10- 30 minutes and all I get is a "click" and no crank.

I will typically retry the starting about 10 times, then just pop into neutral, let it roll a few inches, jam into park and try again. Almost every time it will start within 2 more attempts.

I really believe it is the solenoid, but just don't have the time to do anything about it. I've been doing this fix of mine for 3 years and not having any problems, so I'm not going to make it a priority fix.

Since you're only doing something which could effect the NSS, I'd put my money there. Have you tried jumping directly to the solenoid?

I missed the original thread subject. hot start issues is almost always the starter telling you it is about to die. the hot condition makes the resistance in the starter go up causing it to lose function until cold, eventually it wont come back even cold. If you dont want to change it now, I would carry a spare and some tools. if you here a click, it is not the NSS, a bad NSS would electrically isolate the solenoid if it was bad, so no click