Hey guys, just picked up a 13 Taurus PI with the 3.5 Eco, 79k miles. Day 1, I immediately noticed a rough idle but didn't think anything of it. The 2nd day is when I drove it to and from work. On the way home (30 min each way), I was cruising uphill about 40mph at low RPM (1500) and noticed surging and rough acceleration. Once the RPMs climbed above 2K, or when I pressed the gas hard, it smoothed out - until I slowed down and started cruising again. The next day I noticed the issue got worse once the car warmed up, meaning at colder temps, the shuddering wasn't as bad, but it gets pretty bad at operating temps and when the RPM is below 2K.
I took it to a shop who confirmed it had nothing to do with spark, air, or fuel. I even supplied fresh coils and plugs, which I wanted to do anyways. They noticed the PTU was leaking (and a broken motor mount) so they distilled the problem down to the PTU and/or torque converter. The dealership I purchased this car from (located out of state) offered to pay $1,500 for a new PTU, but I'm still looking at a hefty bill if I decide to get both done.
Here are my question and other observations as I'd like to start by replacing just the PTU first:
1. Could a bad PTU make your idle rough and fluctuate when not moving, say 50RPM up an down?
2. Could a bad PTU cause this surging and rough acceleration? (there is a slight grinding sound from the drivetrain)
3. When mashing the gas, the gears shift just fine - would a bad torque converter make every gear gift rough?
Basically I'm asking what exactly may support it being just a bad PTU as I don't exactly mind replacing that, flushing the transmission, and seeing if that fixes the issue.
Is there something definite that points to either PTU or TC? If the PTU is leaking, would that be an obvious culprit?
Thank you!