QuestionQUESTION: I own a ford 2001 f-150 ext cab 4x4 with the 4.6L v8 and a 5 speed tranny. the truck runs perfect when cold for the first 3-5 miles. When it starts to warm up it develops a miss while accelerating only at low rpms. I have tried to brake while holding the accelerator down and can get it to miss quite regularly, but it will not turn the mil light on or even set a pending code. I have gone through the coil on plug assemblies and even bought a new one and rotated it through all eight cylinders. It didn't make a difference. I changed the fuel filter and tested the fuel pressure at idle and under a load. that all checked out fine. I cleaned the mass air meter and intake air temp sensor and that didn't make any difference. I put in new plugs and still no difference. I am at a loss and looking for any hint as to what is going on. thanks.
ANSWER: Nick, I don't know if anyone has helped you yet. I just started reading some of the questions and wanted to give you some advice. You stated that when you apply the brake, it begins to miss much more than if you did not apply the brake. It also sounds as if the miss might go away after you gain some rpm's. From your description, the first thing I would listen for is a "hissing" sound coming from the brake booster. Especially when you apply the brake. Brake boosters contain a high percentage of your vehicle's vacuum. When they go bad, your vehicle will loose vacuum. At low rpm's your vehicle will run very, very bad. As your vehicle gain's rpm's, it will clear up because there is more air flow around the leak, therefore causing the vehicle to get the air it needs. Also, 9 times out of ten, a vacuum leak will not cause a code to kick off. However, a bad booster could set off an ABS light if you have ABS. Listen for a possible "hissing" noise. If you hear one, replace the part, or re-insert the hose that came off. I hope this helps, Jason p.s. just because your "check engine" light isn't on, doesn't mean there is not a code on the computer. Take it to your local parts store and have them check it.
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QUESTION: I appreciate you getting back to me. The brake has nothing to do with the miss. The only reason I used the brake was to torque the motor. It's a 5 speed so you really can't simulate the miss under load in the garage without smoking the clutch. I also have a code reader and have checked for pending codes. I'm starting to be convinced that there is something wrong with the pcm. Any other ideas? Thanks again.
AnswerNick, I spoke with a friend of mine that is an excellent mechanic, and has worked on many late model Fords. We discussed your situation, and he is convinced that you have a bad ignition coil. Unfortunately, you have eight of them. I know that you have went through them already, but you might want to try doing it the old fashioned way of disconnecting them one at a time until you disconnect one that doesn't change the way the engine is running. Of course, this will only work if you can get the engine to miss regularly at idle. He recommends that you hook up a boat and trailer behind the truck, hold the brake, and stay with it until you get a code. I wish I had a better answer for you. It sounds like you are doing everything the right way. I wouldn't give up on the coils just yet though. Keep me informed, and if I can be of any more help, feel free to ask. Good luck, Jason
