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Old 07-26-2011, 01:25 PM   #1
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Unhappy Misfire under light load

Hi Guys Here is my Problem....I have an F-150 Lariat 4x4, 3v 5.4, 90,000 miles has a misfire between 45 and 50 miles an hour under light acceleration. Once parked the truck smells of unburnt fuel and the #2 (Left side) Cat smells hot and shows white discoloration, right side Cat is unaffected. The truck has intermittent check engine light and showed a #5 and #8 cylinder misfire. I replaced the COPs on the affected cylinders with no resolve. My question;: is there a reason a misfire would surface at 45-50 MPH and what steps could I take to further troubleshoot the misfire? (note: 3V 5.4 Triton does not have an EGR valve). I would appreciate any help on the subject. Thank You
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Old 07-26-2011, 01:33 PM   #2
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What about the plugs for thoses 2 cylinders...90K miles it may be time for plugs.
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Old 07-26-2011, 01:49 PM   #3
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I am going to change the plugs this weekend. What do you make of the misfire only occurring around 45-50mph? Is there something that the engine does at this speed that would change and cause the misfire?
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Old 07-26-2011, 03:17 PM   #4
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Quote:
Originally Posted by Trav View Post
I am going to change the plugs this weekend. What do you make of the misfire only occurring around 45-50mph? Is there something that the engine does at this speed that would change and cause the misfire?
At about that speed the engine is at a point where it kind of lugs just after it up-shifts. I suppose that's probably intended as part of the engine management (you tend to get the best gas mileage when the motor is slightly lugging). More than related to the speed itself, I think that it's just that any problems become more apparent when it's at that point.

I had the same problem in the truck that I bought and also noticed the same to varying degrees in most of those that I drove. Changed a few coils which made it a better. Changed the plugs which made it better. Changed the phasers, adjusters, etc., which made it better still. Now with the motor running as it should, I haven't had any more problems. Think that it's probably just a question of getting things right overall whatever that might be in your particular case.
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Old 07-27-2011, 07:11 AM   #5
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Your symptom coupled with your mileage are indicative of bad plugs. Change ALL the plugs and you should be fine.

I had the same exact symptom with less miles, and a plug & boot change cured it! If you are going to do it yourself, be prepared in case some break.
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Old 07-27-2011, 07:35 AM   #6
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My 07 was misfiring at the same speed. Had 90 k miles at the time. New plugs and it ran like new again. Follow the TSB and get the lisle tool and you should be ok.
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Old 07-27-2011, 05:00 PM   #7
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Trav man change your injectors before it's too late man.
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Old 07-27-2011, 05:29 PM   #8
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I'll start with the plug change and see if new plugs fixes the miss. My Lisle tool just came in the mail today. I am going with the Champion 7989 spark plugs so I wont have to deal with extracting broken plugs again. As for the solvent to soften the carbon build up on the plugs I was going to try Aerokroil. Has anybody use this stuff to loosen there plugs? and it so how good did it work?
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Old 07-27-2011, 07:59 PM   #9
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heres what i did for the carbon problem. took the air filter out and got a funnel with a hose on it to extend inside breather. then i took 3 bottles of the cheap STP injecter cleaner (walmart) and slowly poured them down the engine while revved to about 2000 rpm`s. next i poured a quart of transmission fluid down it. (lots of smoke) drove the truck about 10 miles before i changed the plugs and there was not even a trace of carbon. all 8 plugs came out without a hitch. this is a truck that i bought at an auto auction with 205,000 miles. i have no idea of its service record.
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Old 07-27-2011, 09:21 PM   #10
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After dumping a quart of tranny oil thru the combustion chamber you probably toasted your cats and o2 sensors. Why in the world would do that.

To the op - what year is the truck. 2005s have know injector problems. At 90k definitely time for a plug change. Use carb cleaner for soaking the plugs. Follow the tsb and you should be good.
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