1990 560SEC - starts easily but immediately dies. Turn key back to off, then crank again, starts and stays running fine. Does not happen every time, but perhaps about 75% of the time. Where to start looking?

Thanks!

Very likely a bad fuel pressure accumulator, located by the fuel pumps under the car.

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

Update:

I swapped out the accumulator and fuel filter with new units, no change in the problem.

I've observed that the problem occurs only on a cold engine, so I'm led to suspect that perhaps the cold start valve is not providing adequate fuel during a cold crank.

If it starts the first time, it is not the cold start injector. It could be the check valves in the fuel pumps (2). The check valves are replaceable. The best way to trouble shoot this problem is with a fuel pressure gauge. The system should hold some pressure with with engine off.

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84 500 SEL (307,xxx miles)

Likely an issue with the idle air valve or EHA valve or something not working right. Or possibly the Injection engine temp sensor is failed.
.

__________________
-diesel is not just a fuel, its a way of life-
'15 GLK250 Bluetec 118k - mine - (OC-123,800)
'17 Metris(VITO!) - 37k - wifes (OC-41k)
'09 Sprinter 3500 Winnebago View - 62k (OC - 67k)
'13 ML350 Bluetec - 95k - dad's (OC-98k)
'01 SL500 - 103k(km) - dad's (OC-110,000km)
'16 E400 4matic Sedan - 148k - Brothers (OC-155k)

Typically, it does not actually "start" the first time. I'd say maybe one or two cylinders actually fire, nearly instantly, but it's not enough to get the engine truly running. I have to then turn the key completely "off" and then try again, at which point I'll usually get several revolutions of cranking and then the engine fires and runs fine.

Interestingly, the car sat for a few days this week, I went to start it this morning, and instead of the above-described behavior, it cranked 4-5 revolutions and then fired, and ran fine.

Thanks for the assistance. I may buy a CIS gauge to attempt troubleshooting this before throwing more $ in unnecessary parts at it. I'll clean the idle air valve too.