740 with white smoke and erratic running.

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My recent purchase of a 1988 740 saloon seemed to be going well, that is until it started belching white smoke, and the engine became extremely erratic. I managed to limp back to its usual parking spot, and started investigating. My first thought was a blown head gasket. There is no steam coming out of the exhaust, no froth on the oil filler cap, and no bubbles in the coolant expansion tank. The engine oil looked clean as well. I then thought the coughing might be the result of it catching the 'Rona, and I had visions of having to drive around with a mask over the radiator, But I rejected that, and decided that it needed to be triple Vax'd, the trouble is that I only have a Dyson.

On a more serious note, it belched white smoke for a couple of days, and I watched a few youtube videos about it. The guy who negotiated the purchase of the 740 turned up, and I asked him to look at the smoke. Would you believe that, when I started it, there was no white smoke at all. My current thought is that it was a blocked injector, and the smoke was unburnt fuel. Do you guys think this is the probable cause? I bought an ultrasonic cleaner, and I'm considering using this to clean the injectors - do you think this is a good idea? If so, what would be the best fluid to use? Some people suggest diluted fairy liquid, and that would certainly be the easiest for me.