300 prob
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Joined: Feb 2003
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300 prob
Well i have a f-150 with an efi 300. the problem is it runs rich when it throttles down. It shoots black smoke and unburned fuel from the tailpipe, it gets horrid gas milage, and runs like crap overall. I just replaced the o2 sensor, and it still shoots unburned fuell and black smoke out the tailpipe, it runs better i think,but its my uncles truck and i need to get it fixed for him. What else could cause the engine to run like this? I was thnking the injecters are bad and just dumping fuel in when i let off the gas. But other than that im drawing a blank.
Senior User
Joined: Sep 2003
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From: Houston TX
Have you tried pulling any codes from the puter? It almost sounds like it isn't working....
Could be a number of things. A bad temp sensor could cause it to run rich when warm. Are you going into closed loop? Sounds like it's running in the base warmup mode. "open loop". Kind of one of those, gotta see it deals...MK
Could be a number of things. A bad temp sensor could cause it to run rich when warm. Are you going into closed loop? Sounds like it's running in the base warmup mode. "open loop". Kind of one of those, gotta see it deals...MK
Senior User
Joined: Aug 2003
Posts: 125
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What year?
So the symptom is misfiring on decellaration .
This sounds like a dashpot problem on the older carb models. The Throttle position sensor replaces the function of the dashpot on fuel injected models. You can check it with a digital voltmeter. With the ignition key on just move the throttle through its range and observe any voltage drops that would indicate a malfunction. The engine will tend to run rough at specific throttle points. Thats the telltale sign of a TPS gone bad.
Early Ford throttle position sensors were troublesome.
So the symptom is misfiring on decellaration .
This sounds like a dashpot problem on the older carb models. The Throttle position sensor replaces the function of the dashpot on fuel injected models. You can check it with a digital voltmeter. With the ignition key on just move the throttle through its range and observe any voltage drops that would indicate a malfunction. The engine will tend to run rough at specific throttle points. Thats the telltale sign of a TPS gone bad.
Early Ford throttle position sensors were troublesome.
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