So I jump in the truck yesterday (2001 F150), 56 degrees outside, start it and get a nice smoooooth idle..until I put it in gear. As soon as I put it in gear it starts to sputter and knock so hard that I can see the hood of the truck shaking. Take it out gear and it goes back to smooth idle. Put it in drive...rough...feels like it's missing hard. I drive around the block once because I'm about to go on a 120 mile round trip and want to make sure that if anything happens it happens near the house. I figure a trip around the block will also give me more info on what the symptoms are. I hit the gas pretty hard to see if that changes anything...it makes the truck shake and shudder harder. Back to the house, pop the hood, look for any obvious signs of trauma...fluids where they shouldn't be, vac lines unhooked...can't find anything amiss. I get under the truck and inspect things. Start the truck again, pretty nasty smell is noticed...nothing else. I try driving it again and it's fine. Runs great all the way there.

On the way home after about 3 hours of the truck cooling off, I start it up and it drives great.

I get home and park it. 4 hours later I need to run to the store and now it gives me the rough idle treatment even in park. Couldn't take the truck to the store.

I go out 10 minutes ago to troubleshoot it and it starts just fine...no problems..nice smooth idle.

I just changed the plugs about a month ago so that shouldn't be an issue.

Any ideas?

 

Did your check engine light flash? Check for a loose plug wire.

 

Sounds simple but check your vacuum lines. My truck recently hada rough idle similar to what you are describing and it was a vacuum line that had a crack in it. Replaced the line and problem solved.

 

I'm going to have to check all of that tomorrow. The engine light came on today.

 

check the elbow vac line that is between the upper and lower intakes. it is a pain to get to but both my dads truck and mine had the same prob. cost about 12 dollars at ford. dad worked on his for two months replaced about 250 in sensors to find out it was that hose