Bucking/Jerking - ARGH

[96DiamondVIII]

So for a little bit now, my MM has had a "jerky" feeling at part throttle--it sort of shudders/bucks/feels almost like the trans is doing something funky. Only happens at part throttle. Seems worse on hot days and it's definitely worse when the A/C is on. I can tell it's still doing it with the cruise engaged but it's much harder to detect at that point. Idle doesn't feel rough to me. '03 300B with 114K miles.

So I poked around a bit and decided that I probably needed to replace my spark plugs, as that particular service had slipped my mind and I was over the service interval. (Was about 111K at that point) Also, I previously had a Mark VIII which was doing the same thing (except worse) and new plugs/wires cleared it right up. Did that maybe a month ago, it made the problem lessen somewhat but not go away, and now it's getting worse again. Very frustrating.

So the question is what is the most likely cause and what's my next move. Could it be Torque Converter issues (God I hope not) and should I go for a transmission flush/refill? Fluid still looks nice and red, level is good, if I'm reading the service intervals right that's a 150K mile service but it's probably not a terrible idea to do it early?

EGR valve? I hadn't even thought about it but I saw a post in the Transmissions forum which mentioned that as a possible cause.

Coil boots or coils? The boots looked fine so I re-used the old ones when I replaced the plugs. I have a multimeter so I can test voltage on the coils if that is a possibility.

Something else entirely? Thanks for any light you can shed on this condition which is irritating the hell out of me.

[RF Overlord]

If it's torque converter clutch shudder, you'll only feel it when the TC tries to lock. It'll feel like running over rumble strips. The way to determine if it IS TCC shudder is to keep your foot steady on the gas to keep the shudder happening, and use your left foot to gently tap the brake, just enough to turn on the brake lights. This will cause the TC clutch to disengage and if that's what's causing the shudder it'll stop immediately. The fix is a fluid change.

If the above does NOT work, then you have a misfire...could be a weak coil; I just had that issue with Phoebe last week. #3 COP was weak causing a "bucking" at moderate throttle in 2nd and 3rd.

BTW, that 150k interval is marketing BS and is about 5 times too long. ATF should be changed every 30,000 miles, so you most likely need new fluid regardless.

[JoeBoomz]

Try to test it to see if you can duplicate the behavior at the same speed and throttle position. i.e., does it always happen at light throttle at 45 MPH? If so, it is the TC lockup, and as stated above, fluid changes are the first, easiest thing to try.

[96DiamondVIII]

I've gotten mixed results trying the brake tap trick--seems to stop it sometimes, not others. In either case I'm going to have the trans fluid replaced tomorrow and also check to see if it's throwing a code (CEL isn't on, but I don't suppose that always means much).

[captJ696]

Maybe a misfire, replaced my coils and car runs fine. Seems to be common problem.

[HITEKBALANCER]

My GM supercharged did same thing. No code ended up the #4 coil. It feels like the trans is messed up. Be sure to change the plug or plugs too. Ford can diagnose even if no code. If you don't want to spend $100 on diagnosis change all the coils and plugs.

[RF Overlord]

I've gotten mixed results trying the brake tap trick--seems to stop it sometimes, not others.You may have 2 separate issues...if the problem was TCC shudder alone, then tapping the brake will always stop it.
(CEL isn't on, but I don't suppose that always means much) A weak coil will frequently NOT set a CEL or even leave a pending code. The one on Phoebe didn't...

[babbage]

DPFE can get water in them and this will cause bucking at float. New DPFE/EGR deal costs about $85 If i remember correctly.

In 4th gear at low rpms and you are touching gas to keep speed is where you will notice a bad DPFE.

Good luck.