2002 3.8L automatic, chugs at 30mph in 3rd gear

[351C Mach1]

Just got a 2002 Mustang 3.8L automatic with 41,000miles. Everything has been fine except when the car is doing 30mph in 3rd gear, the car starts to chug, shaking you forward and backward. It is gentle but very annoying. Tried turning off traction control and overdrive but it still occurs. The rpms seem to remain steady and the engine sounds steady. If I shift into second at 30mph, the chugging goes away. If I get up into overdrive for any period of time, then slow back down to 30mph in 3rd gear, the chugging doesn't occur. No codes on the computer.
Anyone have any ideas?

 

[timeless2]

When did you last change your fuel filter? Do other gears lug as well?

 

[351C Mach1]

This is the only time it chugs. I've been told that it should shift into 3rd at 30mph, but it should stay in 2nd, and that it's chugging because 3rd is too tall for the 1200rpm the engine is turning. If this is the case, why would it be shifting too low?

 

[timeless2]

Go into more detail on the chug.

Does it do it when you aggressive accelerate? Moderate? Easy?

When did you last change the fuel filter?

 

[351C Mach1]

I just bought the car yesterday so I do not know when the the fuel filter was last changed. The car has 41,000 on it, so worst case scenario it's still original. The chugging is a lot like when the rpms are too low for the gear you're in. It's a back and forth motion like the car is speeding up and slowing down very quickly, but it's not violent. Imagine shifting into fourth gear when you're doing 20mph, that kind of a feel. It only occurs if I haven't been in overdrive since starting the car, and only when I'm holding it steady around 30 mph. Let off the gas, it goes away. Really step on it and it accelerates normally.

 

[timeless2]

Preventative maintenance never hurts and it sounds like fuel flow can't keep up at lower, lugging rpm's. When you mash it, the pump flows fine through the restricted filter because of increased demand.
I would start there. You'll be able to tell if it's been replaced or not once you get under there and look around.

If you aren't comfortable, hit up a Firestone or equivalent shop nearby.

 

[351C Mach1]

Continuing the difficult diagnosis: Once the car warms up and has been driven normally(through all the gears), it will run normally at 30 in 3rd gear with no hesitation of chugging. Anyhow, it's under warranty so I'll be taking it to the dealership to get checked out.

 

[timeless2]

Post up when you get the answer, please--I am very curious to know!

 

['13_ImpactBlue_GT]

Might as well get the fluid changed while your at it.

 

[351C Mach1]

Took the car into the Ford dealership and it was the DPFE sensor. They replaced it for free as a part of some Ford Program. Car drives fine now!

 

[timeless2]

Took the car into the Ford dealership and it was the DPFE sensor. They replaced it for free as a part of some Ford Program. Car drives fine now!

Great! Warranties rule, when they actually make a difference, that is.

 

['13_ImpactBlue_GT]

DPFE sensor

 

[351C Mach1]

Differential Pressure Feedback EGR Sensor. It senses the pressure(and therefore the flow) from the EGR valve back into the engine. My sensor was reading EGR Flow Insufficient.

 

[timeless2]

All the more reason to say that EGR's suck.