Hello and good afternoon,
Thank you for allowing me to assist you, my name is Joe and I will be happy to assist you with your concern today.
More info is needed such as what takes place if you attempt to push the throttle past that 1600 rpm threshold? Does it have any response such as goes up then down to 1600rpm, boggs, has no reaction/response at all?
Next you would want to check fuel pressure during the problem, check compression, check if missing on any cylinders, if they are turbo engines check if turbo is making boost and for exhaust/intake leaks.
These are all things to start with so we have a little more info to go on.
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I'd be happy to answer any other questions you have even if not related.