Hey guys! Hoping you can help me out with my '01 Tribute. It's been in the family since it was new, and I just bought it from my Dad about a year ago. 155k on the odo. Well-maintained throughout its life.

I needed a new IAC a couple months ago, the car wouldn't idle at all, it just stalled out unless you kept your foot on the gas, so I'm pretty sure that actually was the issue at that point. Replaced the IAC myself and everything seemed fine.
Then I started to get a shudder when the car wanted to downshift out of O/D on the highway. If I give it more gas it always downshifts, just seems to do it when it slows a bit and I keep my foot steady or just barely give it more throttle. This is definitely getting worse with time. It shudders and kind of 'bucks' like it wants to shift but it can't.

Now it is idling rough again. Hasn't stalled yet but it seems to be getting worse. The engine seems fine when driving or revving up, so I'm thinking vacuum issue? I've been reading a lot about coils going bad as well, but wouldn't it run rough all the time then, not just at idle? The Check Engine light hasn't come on once - but could I still get error codes possibly if I take it to AutoZone and use their reader?
I'm wondering if the two issues are related, since this has been such a reliable car up until now it seems like too much of a coincidence.
Thanks so much in advance for your help!

 

My 02 F-150 did what you described and it was a coil gone bad. Get the codes read and see what is going on...

Trucky911

 

I *think* you are describing a misfire under load. I would check the ignition system and the COPs as noted above. When you are in top gear, and want to accelerate, the plugs have their hardest time firing from what I have determined. That could appear as a shudder. You could try de-activating the OD to cause a downshift manually, and see if the shudder goes away.
tom

 

You guys were spot on. I can't believe I thought something was wrong with the transmission after driving the car now that's it's fixed. I guess it happened slowly over time and I sort of lost the feel for where the shift points were.

Check Engine light came on on the way home last night, diagnostics showed a misfire in cyl#4. New spark plug and a new coil, and she runs like new! Man I got lucky that it was one of the ones in the front! Took 5 minutes to change it out.
Thanks again for your help!