I have a 99 F-150 4x4 4.6l V8 (manual tran), it has 83,000 miles and it just recently started shuttering and vibrating really bad when i try to accelerate. it's fine accelerating if i go "snail speed" and it rides alright once i stop accelerating (unless im trying to go 65 MPH or above). I took it in to get diagnostics run on it and it came back: #7 cylinder misfiring. As soon as the shop cleared the code i picked it up and it ran fine for 2 weeks even though no mntc. had been done. It started vibrating again yesterday, no check engine light yet, like last time, but i'm driving it very carefully. Its the twin coil block set-up not the COP. Any advise would be awesome, It's been for sale for a month now and there's no way i'd feel comfortable selling it the way it's running. I hear horror stories of spark plugs blowing out after being replaced, what the cheapest and best route for me to fix this issue.

Thanks :

 

Welcome to the forum

Usually it's either a plug or plug wore breaking down. I'll bet it has a stored code, even though the light isn't on.

 

Spark plugs only blow out if you don't properly torque them down (I don't personally torque them / just go by feeling) Their are specs. somewhere on this site, but I can look them up for you if needed. To throw a code (misfire codes are light emitting and are generally never just a stored code with no light) To throw a code you have to misfire a set amount of times in a specified time frame set, if you have a vivration when accelerating under load I would suspect it's a misfire, best thing to do to get the code to set is to keep your foot into it while it's misfiring (you'll see the check engine light 'CEL" flashing, eventually it will set the code, then you can go to a parts store and have the code checked) 99' w/ 83k is due for a tune up! (spec is 100k but date wise you are due, and Ford is starting to put specs @ 60k, I am not familiar with 99' F-150 as what the plugs usually make it to mileage wise but have seen many Ford products due at 80k or so. Another thing is you may have multiple misfires if the plugs are overdue and will be harder to throw a code (sometimes you'll get a P0300 = randon misfire or P0316 = misfire on start up.) If you've ever done a tune up you'll do just fine with your truck, I'd just pull a plug and check the gap... if you buy plugs make sure to check / set the gap as they arn't always set when you bought them (I have had to fix some peoples mistakes that have done tune ups and were unaware of that and the gap ended up being too much or too little and they couldn't figure out what was wrong) Bosch seems to not do well in Ford's.

 

Thanks guys... i took it into midas and they said its probably just the plug wires, i bought wires and changed em' out last night (it was a real pain in the ***) and she fired up great. Thanks for your help, this forum is awesome.