I have a 1981 Lincoln Continental with only 30,000 miles on it. About a year ago I was told that I had a carbon build up. I could stop at a light or slow down to turn a corner and the engine would stall. After putting in injector additive while filling the gas tank, I have had no more trouble. How often should I put the additive in?
Let's go ahead on the premise that the fuel injector additive worked. I looked at several different brands to see what they recommend as far as how often to use. Most don't specify, but others suggest about every 2,500 miles. To make it easy to remember I would add it to a tank of gas every time you change the oil.
I'd like to invest in that brand of injector cleaner. In general, all the different carburetor cleaners and fuel additives are by no means a miracle cure. They do help keep fuel delivery systems clean and some may get a little carbon off the back of the intake valves, but usually when the contamination is to the point of causing severe driving symptoms, the cleaners rarely help.
Your older Lincoln with 30,000 miles may be an exception to the rule. Perhaps varnish accumulating in the injectors slowed fuel delivery at idle enough to cause a "deceleration stall", assuming the engine started right back up each time. Then adding the cleaner to the fuel tank cleared up the varnish just enough to prevent the stall. However, there are many other possible causes to this type problem. Some basic checks that should be made are fuel pressure, spark quality and the idle speed control system. If the stalling should reappear, it will require further diagnosis.
In reference to a carbon buildup, it's true that low mileage older engines do tend to accumulate excessive carbon on the back of valves and the top of pistons. Normally, symptoms from this condition would be rough running, lack of power or noise (pinging, knocking). More commonly, carbon builds up on the back of the throttle plate inside the throttle body assembly causing a stall condition, and injector cleaner in the tank would have no affect.