Weird "bucking" issue with my 2001 Grand Marquis

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Apr 9, 2008
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17,184
Location
Central NY
This is my 2001 GRAND Marquis. 4.6L. 162K miles.

When it's between 15 and 30 degrees outside, moreso during snow, I have this weird "bucking" issue with the car. I'm worried that one of these days it's going to break the flexplate when it happens and it has caused me to lose traction driving int he snow when it happens.

It's hard to describe the condition that it happens, but it's super repeatable.

About 30 seconds after decreasing gas pedal input, the car will have one or a series of 3 or 4 big "bucks".

Example is - getting on the highway, then letting off once I'm at speed. Or if I'm going 65 on the highway and come to a hill, I let in a bit. Then once the road levels out, I let off. 30 seconds later the car will buck violently. I've had passengers think I hit something in the road or a giant pothole.

It's obvious the engine stops running for a split second. You can hear it in the exhaust. It doesn't do it when it's around 0F or above 25F.

The reason the buck is so violent when it happens is the torque converter is still locked up because I'm still on the throttle. If I touch the brake pedal lightly to unlock the torque converter, I can still feel it and you can hear the engine really lose RPMs, but then it recovers. Without too big of an event.

My first thought was some sort of vacuum controlled fuel regulator was sticking, but these cars don't have vacuum fuel regulators .. or any sort of mechanucal regulators at all.

My next thought was EGR valve was sticky or leaking. Maybe when I let off the throttle, the vacuum was trying to open it for part throttle cruise and it opened too much ... over the winter I replaced the EGR tube due to rust and put on a new Motorcraft EGR valve and DFPE sensor. Continues to do it.

It feels like in a manual if you let off and on the throttle violently. Except there's no springs in the torque converter to absorb the shock load.
 
I've experience some pretty violent torque converter shudder in a 4.6/4r70w of that vintage that was cured by a fluid change with Mercon V and some lubeguard red. That one should have a torque converter drain so you can get 10+ qts out of it. Does it only happen in o/d, or stop if you downshift?

I may be way off, but only thing I've experienced like that.
 
I've experience some pretty violent torque converter shudder in a 4.6/4r70w of that vintage that was cured by a fluid change with Mercon V and some lubeguard red. That one should have a torque converter drain so you can get 10+ qts out of it. Does it only happen in o/d, or stop if you downshift?

I may be way off, but only thing I've experienced like that.
Happens with overdrive locked out as well. And with my foot lightly on the brake to unlock the torque converter.

I've had an older Ford that shuddered but this doesn't feel like that. It's just once or twice then it's fine until I let off a bit on the throttle.

I do transmission drain/fills on it every 15000 miles with Mercon V.
 
I'm guessing a coil or connector at a coil. Wouldn't hurt to check them all

They aren't brand new now, but it's done it the entire time I've had the car - the first thing I did when I bought it was 8 brand new Motorcraft coils and motorcraft spark plugs. Connectors are still good.

Ignition coils are unfortunately a maintenance item on a 2v modular. Meanwhile my 2000 Cherokee is at 216000 on the original coilpack.
 
Change the coils and plugs.

I have now learned that when there is arc tracing on the ceramic of the plugs, you cant just clean it off with a rag and cleaner, it comes back on the new coil boot. I simply replace all of them with APP104 plugs and new coils.
 
I dunno, that'd be a heckuva ignition event.

Are these TBW? I'd wonder about a butterfly motor doing something weird, much like popping the throttle on a manual as you say.

But if it's cable, forget that theory!!
 
I`ve had something similar but completely on another car, on mine it was a bad electronic throttle body that fooled MAF sensor. Throttle body sent a bit to much air at idle and that confused MAF and my had this "bucking" symptom when i let of the gas. Perhaps that could be a lead on your car as well.
 
Check for codes. It could be a misfire and set a code to a particular cylinder or direct you in the right direction for troubleshooting. Diagnose and don't start firing the parts canon.
 
Do you have a scanner that can see live data?

My friend has an Elantra that would randomly run rough, no CEL but I hooked up my Bluetooth scanner and we could see one of the cylinders misfiring, it was a coil that was arcing out the side.

Also maybe do a smoke test, don’t these modular engines have intake manifold vacuum leaks sometimes?
 
I do have 8 new coils I can put on it and see what happens. It is a cable throttle.

I wonder if it could be the IACv or something doing weird stuff?

The temps are going up so I may not be able to recreate the problem. Once it's above 30 it won't do it.

No codes - set or pending.
 
When outside air temperature is the main condition of note, I wonder if it could be a fueling issue caused by a misbehaving temperature sensor? If a sensor is sending wonky data under certain conditions, it could cause one helluva stumble.
It might be worth doing some driving while monitoring sensor read, with an eye toward a really wacko reading.
 
It feels like in a manual if you let off and on the throttle violently. Except there's no springs in the torque converter to absorb the shock load.

There are damper springs on the converter clutch piston very similar to damper spring on a clutch disc. The converter in my Camaro has a solid billet piston & TCC engagement is every bit as firm as the 1-2 shift.

My gut says misfire but could be TCC shudder. Pull all the plugs & coils & check for moisture & carbon tracking on the plugs.
 
Pulled a few of the plugs and nothing looked out of the ordinary. It did do this when I put them in 40K miles ago, however.

I'm definitely not consuming any oil. Reapplied dielectric grease and torqued them to 25 ft/lbs so I don't launch them.

This morning it was 30 degrees out so it wouldn't do it
 
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