Comment by huehehue
2 years ago
Oh man, I'm currently fighting a problem with a mid-70s coupe that's driving me equally batty.
Randomly during longer trips, the car will just die for no discernible reason. It's the Car of Theseus at this point with how much I've replaced, but the issue persists, and the nature of these intermittent problems makes debugging a nightmare. More puzzling still that the car starts up fine after a short nap.
I'm sure you've tried a bunch, so my comments might be talking into the void.
I had a similar issue that turned out to be a slightly loose battery connection. While the battery clamp was making contact with the battery terminal, I didn't tighten it enough and it made poor contact.
Is it correlated to temperature at all? If it is, I wouldn't be shocked if something, like a relay, is building up heat and increasing resistance to the point of operating incorrectly. A short nap might give that component enough time to cool off.
I had a Toyota pickup that would die randomly, sometimes at highway speeds. Although in those cases, the engine would restart after a second so it felt like you hit a brick wall then kept going. A friend suggested a bad fuel pump. OK, replace the pump and filter (it was cheap and an easy R/R) but no change in behavior.
After a while, I correlated the problem to very high humidity: usually happened during heavy fog or rain. So, it's probably an ignition problem, right? Replace spark plugs. Nope. Distributor cap/rotor. Nope. High performance plug wires. Nope.
Drove me nuts for about two years. Then one day I'm in my garage looking for something and I move my timing light out of the way. Hmmm, didn't think about that...
After two years, problem turned out to be timing slightly out of spec. Fixed in five minutes!
Cracked fuel line? Might let air into the system in certain operating points; might make the fuel pump unhappy (refuses to prime) in some situations. I figured mine out when it finally broke completely: All the other issues went away after replacing it.
Assuming there are not a lot of electronics on a 70s car, this sounds like a fuel issue.
What brand model is it?
Mercedes 450SLC
Wiring harness issue. Reseat any and all connectors from the battery to the distribution box (there may be two of those depending on the model year) and reseat all of your fuses, if any of them go in too easy then use a small screwdriver to force the contacts to be closer together (or slightly twist the fuse tab, not the most elegant solution but sometimes those contacts in the fuse box are so far recessed that you can't get at them). Good luck!