Rough Acceleration, Any Ideas?

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Barney366

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Location
boston
96 Mark VIII, 82900 miles.

The car runs fine overall, but I've noticed this new issue:

The car seems to slightly shudder if I accelerate slowly from about 30mph and above. Once I stop acceleration the shudder stops and she drives smooth. She has no problems maintaining highway speeds.

If I step on it (hard acceleration, very fun) there is no shudder.

From 0-30mph, no shudder, no problems.

No codes, she idles nice and smooth.

Any insight as to what might be causing this?

Thanks in advance for any assistance.
 
I'm not sure on the fluid type as I recently purchased the car, and I don't know its maintenance history. I'll check the levels, maybe have the transmission flushed and changed so I know where it's at.
 
Barney366 said:
I'm not sure on the fluid type as I recently purchased the car, and I don't know its maintenance history. I'll check the levels, maybe have the transmission flushed and changed so I know where it's at.
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If the fluid is clean and full you can add a tube of Dr. Tranny Shudder fix to see if that clears up the issue. It's about $6 shipped off ebay. If the problem goes away then it was a converter shudder issue. At that point, assuming the fluid is good.. You can either leave it as is until it acts up again or you can get ONLY a Mercon V fluid replacement.

From my experiences the shudder happens anywhere from 25-45mph. Does it feeling like your running over rumble strips?
 
Barney366 said:
The speed is about right, and yes it does feel like rumble strips, maybe a little bumpier.
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Yeah, it can get kinda violent. It's best to stay off the throttle when it's shuddering, just ease back into it.

Iirc mine only did it around 40mph. A lot of us have had luck with the Dr. Tranny stuff, it starts working almost instantly. Ford sells a similar friction modifier but it stinks to high heaven from what I've heard and is a bit more costly.
 
Would a transmission flush and replacement with Mercon V resolve the issue, or will I still have to add the Dr. Tranny? Does this indicate an imminent need for some transmission service?
 
Dr. Tranny is amazing stuff. It takes care of torque converter shudder for about 6-8 months before it wears off. If that works it means your tranny fluid's friction properties have been exhausted and needs to be replaced.

If you replace the fluid you shouldn't need the Dr. Tranny stuff. It's more of a band aid until you do replace the fluid. And make sure it's Mercon V. The generic crap they'll try to sell you at the jiffy lube won't be Mercon V. Legit Mercon V is actually pretty pricey and not everyone keeps it on hand.
 
I just got in from work and checked the fluid. The level's OK but the fluid looks spent. Still red, but not bright red like the new stuff, pretty dull. I'll get it flushed and changed tomorrow. Hope that resolves the issue, I'll let you all know.

Thanks for the advice guys!!
 
tap the brake while its accelerating and shuddering.
if it goes away, its the converter lock up clutch shuddering.
dr tranny, or fresh mercon v.
some have done both.
 
Ok, so I had the fluid changed and it didn't help. The mechanic who did the change drove her with his scan tool and he didn't get a code but he did see indications of a misfire. I guess plugs and wires are next. Maybe fuel filter after that. Hopefully not coils.

He said $375 for a tuneup, including new plugs and wires. Does that seem reasonable for the Boston area?
 
Plugs and wires are so easy on these cars I could never imagine paying someone to do that for me but it's your money. If you've got a bunch of it then go for it.
 
soduka said:
Plugs and wires are so easy on these cars I could never imagine paying someone to do that for me but it's your money. If you've got a bunch of it then go for it.
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I planned on doing it myself. Never done it but it looks easy enough. I was just trying to get some input if this guy was OK, price-wise, for things I can't do.
 
If you're in the Boston area there are a few knowledgeable member North of Boston. Plugs and wires should be fairly easy, and not a bad idea to do on a 20 year old car.

Spark Plugs is $9 for 8 autolite 764s on Rockauto, Motorcraft wires are $55. I believe coils are $32 a piece Motorcraft. So you're looking at $120 in parts doing it yourself...fuel filters are about $12....and not too bad to do with a quick disconnect tool, the front passenger fender liner just has to come off half way.
 
Problem solved, Guys!! Thanks!!

I installed new plugs and wires (Motorcraft), and that took care of it. Given the unknown maintenance history on the old girl, I'll defintiely replace the fuel filter.

Any merit to replacing the coils, just to be sure?
 
Well I had the opposite issue once. Slow accel and all was well, hard and it would bog down bad. I had replace the coil packs with new ones so I of course ruled them out. Then the engine ingested water. So while I was installing a different engine I had another Mark to drive. It was never maintained well, so I grabbed the coil packs from the project car and suddenly this Mark had the same issue. I was not impressed since I had even taken it to a dealer and spent $600 for them to be as confused as me.

So I learned even new coil packs may not be trusted and to rule out nothing, ever. Obviously I had no codes and the dealer came up blank too. But it was the coil packs, well one of them. So I put the old ones in and all was well. As for the project car, I converted to COP so no more wires/coil packs anyway.

At least you found the issue though! If you ever suspect the coil packs in the future for some other issue, you could always buy one and swap it in each side at a time. I doubt both would ever go bad.
 
ON MY 95 WITH 128k MILES, I've changed the plug wires 4-5 times. One bad wire will feel like trans problem, and one cop on gen 2 will do the same. Orginal coils still good.
 
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