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Hi,
I went through something very similar with my 1993 240 I picked up in 2017 from Reno Nevada.
It would start and run and die is some random amount of mileage or times, at first.
It seemed like fuel starvation as well.
But it got slowly worse to where the car was not dependable.
I traced it to a problem, I thought, to be a dead in-tank pump, thinking it was an intermittent issue before dying. I put in a new one as the old one was definitely dead.
I restarted the car and it seemed to work just fine and then it shut off. Just like with the turn of a key.
This made me rethink ignition so it could be either a CPS, going funky, or the electronic relay that fires the ignition coil per the ICU’s instructions.
On a 240 it’s located behind the battery that often gets neglected because of that reason alone.
So I had a new CPS and changed it. It looked original, so why not, as they do fail in mysterious ways from mine own others experiences on the BB.
I tired it up again and it ran about thirty seconds again and it just turned off.
So I started thinking about that hidden relay.
I got in there and saw something I totally forgot about from about five years ago.
I treat all relay sockets to a maintenance bath of anti corrosion salve. Once every ten years is for my coastal climate but I try to keep them indoors too.
If car is kept outside in the weather doing it more often is advisable.
When I got the car it a broken connector to that relay. Its spring lock was not there I think and one side was missing.
I didn’t have one of those in any stash on a wiring harness ei, then!
So I wire tied it together by slotting the aluminum base to get the wires ties to fit under it and still have contact with the cars body for cooling.
But since that time, I picked up one complete wiring harness with a spare engine from a JY 1990 model.
So I unpinned them both, wire by wire, noting colors and placement. I tossed the old connector.
Somewhere in its lifetime something petroleum, I surmise, got on the plastic.
It the housing was very brittle and a chunk was missing.
What it could have been is still a mystery. It’s the only one I have seen bad.
I started the car again but it hit that repetitive 30 seconds again! I said ok, I will abandon that relay and put on another spare replacement from my stash.
Fired it up again. It kept on running and running?
I said wow, that was it.
So far all has been just fine for the last year and a half!
I’m sure glad too.
The next thing for me would have been to change the ICU. I hate to point to any computer boards.
Patience and using my parts department came through for me again.
I marked the relay as questionable and kept it but I’m too lazy to put in back in to check it.
“If it ain’t broke don’t fix it” is getting to be this old man’s mantra.
Maybe, if I ever need to replace a battery 🤭. Nah! Don’t mess with success.
I will suggest you put a timing light or tachometer on the ignition wires to see if either quits while the engine still coasts down.
I think that’s what gave me some insight along the way.
It is tricky to say how well it works but while grasping around for straws, pay it some mind for finding a flower. 🤭
But the biggest thing I notice was the TIME factor to the shut downs.
Call it an instinct or an arm fitted with a wrist watch, a habit of wisdom gained.
A feeling that was like the ECU goes through a “warm up” sequence series of a program lines with attached protocols.
It checks itself for sensor inputs like rpm, O2, coolant temp or AMM.
If one or more does not send correct feed back or does it not compare, precisely, it will shuts down.
Maybe a safety thing … like saying “don’t drive me!”
The “limping” mode is another well known computer routine.
I have read that the CPS signals are closely watched to be in a certain wave shape and strength so as not to use some erroneous interference.
Good luck and let patience wear it down to find that gremlin.
Phil
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