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Old 07-26-2009, 12:47   #1 (permalink)
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99 3.0 misfire under load after tuneup

I used all Motorcraft parts. Changed plugs, wires and air filter. After it's all done the truck runs, idles and revs great. Once I start moving the speedometer pegs to the max and the truck misfires and hesitates under any load at all.
No DTC's either. That sure would help a lot if it would give me one...
It wasn't doing this before the tuneup.
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Old 07-26-2009, 14:02   #2 (permalink)
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Re: 99 3.0 misfire under load after tuneup

I pulled all of the spark plugs and they are a little sooty, which is telling me the truck is running pig rich. I double checked the MAF, intake pipe, and vacuum lines. I couldn't see any obvious issues. I even sprayed around the vacuum lines to see if I could find any leaks that way. Can't find anything.

I don't know what happened but I hope it's something stupid.
The worst part is I need to have this fixed by tomorrow evening!
My dad has to sell his 2007 F-150 for financial reasons and I'm going to loan this truck to him.

Normally I would have plenty of time to fix it since it's an extra vehicle.

If someone can help me out I'll be happy to send some (or beverage of your choice) money!
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Old 07-26-2009, 14:45   #3 (permalink)
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Re: 99 3.0 misfire under load after tuneup

FIGURED IT OUT!!!!!
It was the freaking new G.E.M module I got from the dealer! It was causing both the misfire and speedometer problem.
Put the original one back in and it runs like a champ.
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Old 08-01-2009, 06:18   #4 (permalink)
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Re: 99 3.0 misfire under load after tuneup

I'd bet the GEM was telling the ignition that the truck was traveling too fast, and was cutting off power to slow you down... First thought was a VSS going flaky, as it will goof up a speedo, OD, and shifts.
tom
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