3.0 Duratech DOHC Bucking Problem

http://images.ford-trucks.com/forums/images/smilies2/mad0228.gifSomeone please help! I have a 2002 Mercury Sable that bucks (or misses) while going up small hills at around 35 mph. I have tried everything... I think. It started when it had about 40K miles on it and now has over 80K. The engine has always run very smooth and quiet and is very responsive. When it first happened, I replaced the one-piece coilpack and that fixed it for about three months when the problem returned. The coilpack had obvious cracks in the epoxy on the under side. Once again, I replaced the coil pack - although, I couldn't see any cracks on this one. The problem was fixed once again for about 13 months and then came back. I replaced the coilpack a third time - this time adding a heat shield underneath it to shield it from the heat coming from the exhaust manifold. That lasted for one month and the problem returned. Refusing to believe that the coil pack went out so soon, I replaced all the spark plugs and wires with new OEM MC parts. I also replaced the PCV valve, intake manifold gaskets, EGR gasket, and thoroughly cleaned the manifold/EGR pipes, throttle body and MAF sensor. I also checked the EGR functionality and it was working perfectly. After doing all of this, I took the car (http://www.ford-trucks.com/forums/1006019-3-0-duratech-dohc-bucking-problem.html#) for a test drive and it was exactly the same - as though I hadn't done anything!!! Frustrated, I replaced the coilpack again, but the problem did not go away this time! The engine still runs smooth and quiet, does not hesitate and has plenty of power, but when I go up slight hills at about 35 mph; it bucks. At my wits end, I replaced the coilpack again - no change. Occasionally, it logs a P0301 code, but only when it bucks real bad. I have tried swapping plugs around to different locations, but no change. My wife has driven the car that way ever since. Anyone know what's causing this? I will be eternally greatful for the fix. Thanks in advance!http://images.ford-trucks.com/forums/images/smilies/confused0082.gif

Next time it happens, turn off the overdrive. This will test if it is a transmission problem and not a engine problem. I had a similar problem with the engine coil pack but later had a problem with the overdrive rapidly kicking in and out that seemed like it was the coil pack problem and had me perplexed when I thought I had everything fixed in the engine. I never found out what the trans problem was, just took it out of overdrive on the two hills at 35 -40 mph where it would occur. Might have been a sensor related to speed is my guess.

Next time it happens, turn off the overdrive. This will test if it is a transmission problem and not a engine problem. I had a similar problem with the engine coil pack but later had a problem with the overdrive rapidly kicking in and out that seemed like it was the coil pack problem and had me perplexed when I thought I had everything fixed in the engine. I never found out what the trans problem was, just took it out of overdrive on the two hills at 35 -40 mph where it would occur. Might have been a sensor related to speed is my guess.
I have not heard this idea yet at other forums where I have posted this problem... Thank you. I will have my wife try this and will be back to post the results.:biggrin:
PS: My wife just told me that sometimes the car bucks while sitting at signal lights, while not moving. I will still try your idea though.

Nope, that wasn't it. Taking it out of OD didn't seem to change anything, but thank you anyway. Any more ideas, anyone?:confused: