My ford f 150 with a 300 inline six is idling rough but with run, runs good at high rpm but is running rich

My first suspect is a faulty O2 sensor causing the computer to make the engine run rich (too much fuel). This would account for the fouled plugs and rough running. I'm assuming the exhaust smoke is also quite black. This condition will also, unfortunately, quickly ruin the catalytic convertors.

Was the fuel drained out and flushed ? The engine coolant temperature sensor may be bad and reading too low causing an overly rich mixture .

The 335i and 335xi come with the exceptional twin-turbocharged version of the 3.0-liter inline six. It makes 300 horsepower and 300 lb-ft of torque, and is offered with the same transmissions as the base engine.--------------------------------------------------------------------
mini vci

If exhaust is rich, you may have excessive fuel getting dumped into cylinder(s). A sicking or leaking fuel injector can cause this. How many miles are on the engine? Improperly seating valves can cause this also. Or if timing is incorrect.

go to auto zone,its free ck.throtle senser

For this types of problem,there are few help inks to troubleshoot the problem.Click the link below to go through the troubleshooting procedure to resolve the issues :----Idles rough and quits when gas pedal is pressed less? http://technoanswers.blogspot.in/2012/01/1999-ford-f-150-idles-rough-and-quits.html-----------van idles rough? http://howtobyme.blogspot.in/2012/01/2004-ford-freestar-van-idles-rough.html
--------------runs lean and idles rough? http://technoanswers.blogspot.in/2012/02/ford-runs-lean-and-idles-rough.html
-----------Car still runs Rough after tune up? http://howtobyme.blogspot.com/2011/12/car-still-runs-rough-after-tune-up.html
------------Idle flair is noticed on car? Rough idle? car will not idle? Erratic idle speed? http://technoanswers.blogspot.in/2012/02/idle-flair-is-noticed-on-car-rough-idle.html
----------High rpm in neutral? Inconsistant high RPM in neutral? http://technoanswers.blogspot.in/2012/02/high-rpm-in-neutral-inconsistant-high.html
------------Engine Backfires? http://technoanswers.blogspot.in/2012/02/jeep-backfires.html----------- Nissan stalls and bucks on Acceleration?

http://technoanswers.blogspot.in/2012/02/nissan-pathfinder-stalls-and-bucks-on.html

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Nissan Pathfinder Backfires? http://technoanswers.blogspot.in/2012/02/nissan-pathfinder-backfires.html
---------How to adjust rpm? http://schematicsdiagram.blogspot.com/2011/12/how-to-adjust-rpm-on-2009-dodge-charger.html
------------Engine cuts out at 3000 RPM? http://schematicsdiagram.blogspot.com/2011/12/engine-cuts-out-at-3000-rpm.html
----------This will help.Thanks.

have you given it a tune up latley? is it carb ot throttle body. check distributer cap inside for corrosion or moisture. fuel filter another possibility

As you know, you have one of two things going on. it is running too rich or too lean. Run it hard for 15 minutes pull the plugs and check for black, wet looking soot = rich. Brownish, whitish dry looking soot = lean. Be sure your vacuum advance on the carb is hooked up to the proper port. Check the full advance of the timing with a light to be sure you have proper advance during rev up. You want 22 degs. full advance. If you have a pull choke try to close the choke to see if that will level out the hi end. If that works you are running to rich. adjust as necessary to lean it out. Too big a carb can also be your problem. For the best carb to engine size double the cubic inch of you engine. If your engine is a 290 ci you max size carb should be approx 550 cfm, Now if you engine has a hi rpm cam profile you have to factor in needed cfm at hi rpm. Thats why such and engine for racing will not have good bottom end or low rpm power. Sounds like you have just the opposite going on. Which can still be caused by a over sized carb. Too much fuel not enough air or room in the engine to burn it.

Have you checked the air cleaner? .The computer should be read to see what problems are indicated. It doesn't seem that you are getting very good gas mileage. Black smoke means high fuel use!

Sounds like your automatic choke is giving you problems. Check to make sure the choke is operating correctly. If it ins not backing off as the engine gets to operating temp it will cause a rich condition and excessive black smoke out the tailpipe and extremely poor fuel mileage.