89 F150 Ford engine problem

I have 89 F150 with 300 6 cylinder eng. Just recently the eng started loping constantly when idleing. Tach goes from 500 t0 900 rpm's constantly. The exhaust is black which tells me it is getting to much gas.
What controls this or what could have gone wrong all of a sudden? Truck has 50k actual miles. Thank you for any information. clemjr
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On Mon, 14 Aug 2006 21:38:49 -0400, "clemjr"

Probably a failed fuel pressure regulator. That vintage had a problem with them. Pull the vacuum hose. If it has fuel in it, there is no need to go further until it is replaced. No fuel in hose does not mean regulator is OK.
Lugnut
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