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Technical discussions specific to 1964-1967, 1968-1970, and 1971-1973 Classic Mustang. Discuss all tech related to in-line six cylinder and V8 powered Vintage Mustangs here.

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Unread 08-16-2010   #1 (permalink)
CrazyRider_66 is offline Rookie

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Default 1966 Mustang 200 6 Cyl; Automatic Transmission Shudders @ Acceleration

Hello allfordmustangs,

We require your assistance once more .

It would seem our straight-six 1966 Mustang Coupe was at the end of a productive cycle when we bought it at the end of 2008. Since then, we've had to replace the shocks, the rear end (and associated mounting hardware), the entire brake system (thank God for kits), and various other bits and pieces. Needless to say, Johns Mustang in Houston has profited.

New problem. My little brother (it's his car) was driving it home late Saturday night when the accelerator refused to return to the idle position. The way he tells it, he was forced to stop the car, pop the hood, and push it back down in a stop-and-go journey home. Nevermind he could have bent over and pulled the peddle up, or far more preferable, called me for a tow. I'm just relaying what I was told at 3 am when he pulled in. All kinds of alarm bells were ringing, I can tell you.

I checked it the next morning and quickly isolated the cause to the accelerator return spring, and replaced it from an O'Reilly's return-spring kit late this past evening. The old one was probably original, and I genuinely expected it to crumble when I pulled it out, it was so brittle.

Now you know the lead-up to the present problem, and perhaps already have suspicions. My little brother had some difficulty with it immediately, so I had to test drive it. After putting a couple miles on it, checking shift positions, starting and stopping, etc, the symptoms are thus:

When accelerating slowly everything is okay, all the way up to high gear as well as the higher RPM's (no gauge; by ear), so I decided the engine wasn't the issue. However, accelerating at an average or greater amount from a dead stop causes the vehicle to stall/buck/shudder and the engine *nearly* dies. When accelerating "hard" while already going a high speed (already in 2nd gear, for instance, and on the way to 3rd), the engine seems to rev a little higher without transferring any additional power to the wheels. I had thought it was a shifting problem with the first-to-second gear until that happened, thinking my brother had shifted from neutral to drive at a high RPM or something, but with this new high speed behavior and the absence of any outright grinding noises my theory evolved (see below).

I didn't dare "force" it to continue whenever it happened, and instead idled and rolled to a gentle stop while the engine recovered. This dead stop acceleration behavior remains consistent across all 3 forward shift positions (dot, circle, & L). Since it was night, I did not test fast acceleration in reverse.

I would consider myself mechanically inclined, but I have not checked how an automatic transmission works. I have the shop manual for the car, but it's too late in the day and the automatic transmission section is substantial all by itself. My standing (ignorant) theory is that, since the general shuddering/halting/odd behavior does not appear to be specific to any gear, the problem has to do with something between them and the engine.

Any help or advice would be appreciated. Any solution has gotta be something we can do ourselves, no shops can be involved. The local mechanics are uniformly corrupt where they aren't incompetent. (Maintenance on half a dozen family vehicles, avoiding repeats at bad shops, quickly brought us through the entire selection and left us bitter.)
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Unread 08-16-2010   #2 (permalink)
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Check the tranny fluid, should be bright red not brownish and smelling burnt. You can change the filter/fluid yourself easily enough, but I suspect it's rebuild time. While you can do it yourself, not rocket science, not for the faint of heart. Pulling the tranny yourself will save a good bit. My local guy does them for $450 if you carry it in.
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Unread 08-16-2010   #3 (permalink)
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If that included parts and labor, and I was convinced I was getting my moneys worth, that wouldn't be an issue. Unfortunately, I already know for a fact the local Galveston shops don't rate that. Even though I am a transmission amateur, the only person who'd give it any TLC during the rebuild is me. We're talking about mechanics that will lie to your face about what they did or didn't do in the presence of evidence to the contrary. The only problem I have doing it myself is the lack of exotic tools, like torque wrenches, but I can probably rent that, or borrow it from someone we know.

Step-by-step rebuild instructions w/pictures are available online, so combining that with whatever instructions come with the rebuild kit, as well as my shop manual, I'm confident I can do a standard rebuild right. I am sure rebuilding is a wise course of action regardless of the specific problem, and I intend to move forward with that mindset. I dare say it seems exciting.

However, I need to know what's wrong. Is my problem likely just burned out clutches that the rebuild kit will replace? I don't want to rebuild it, miss the cause of the problem, and conceivably ruin a rebuild kit. Speaking of rebuild kits, is there anything else I should aim to replace that is not included in this kit? Is there any reason for me to believe the Torque Converter (for instance) will need replacement?

If it matters, we are not aiming to make a street racer out of it. At this point in time our only aim is to repair the vehicle to reasonable standards (factory, or better/modern), not make a competition show car.

Thanks.
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Unread 08-16-2010   #4 (permalink)
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The acceleration problem is most likely not transmission related but it does sound like the apply piston seals or clutches may be going bad too. It sounds like timing or the carburetors accelerator pump may be your biggest issue.
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Unread 08-16-2010   #5 (permalink)
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I wonder if I should try and record the problem in action.

We had no trouble with either the engine or transmission before the accelerator return spring failed. I'm willing to believe a transmission problem if my brother was messing with it at high rpm, but the idea of one problem leading to 3+ others would be...infuriating.

What if I kept the car in neutral and revved the engine? Does it have to be under strain and run smoothly in order to disprove any problems with the carburetor, or can I disprove it just as well in neutral?
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Unread 08-16-2010   #6 (permalink)
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New tests performed just now.

1.) Staying in neutral, the engine did nothing unusual. I tested both gentle acceleration, as well as hard.

2.) Reverse operates normally. Both hard and gentle acceleration yielded no unusual behavior.

3.) Forward average/hard acceleration continues the "abrupt lockup" behavior with the engine nearly dieing.

My amateur conclusion: the lack of any unusual behavior while reversing indicates that the issues with moving forward are unrelated to the engine/carb, and evidently exclusive to forward-drive non-engine components.
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Unread 02-24-2011   #7 (permalink)
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Issue is still present. Anyone give me a hand?

I went to the local muffler shop just the other day and I couldn't get her up the ramp to the lift without her spazzing out. I had to reverse and catch speed to make it up. One of the customers who had heard the issue himself went on about it probably wasn't the tranny cause it didn't sound like grinding teeth. What could it be?
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Unread 02-25-2011   #8 (permalink)
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Hi:

I'm in Houston and recently converted the family's original '65 six to a V-8. However, I learned about cars on the 200 6. Frankly, in my opinion it's a lousy engine and if anyone asks about hopping it up or converting to a V-8 I say convert, but from the symptoms it sounds like an ignition problem. Do the basics: Are plugs clean (these engines burn oil like it was gas)? Are points set properly, not pitted? Check the ignition wires--consider changing them one by one to see if one is defective (that is, put in a new wire for number 1 cylinder. Test. If problem still perists, try that new wire in the number 2 position, etc.). Check rotor and, especially, distributor cap (micro cracks--just replace it--the car was with us in England and replacing to cap solved a similar problem even though it appeared fine.) A terrific shop here in Houston is Ford Specialties (713-464-8704). The owners know me: Chris Kennedy (mention my name). Excellent, PRACTICAL mechanics. You're right about Galveston, as others have told me.

/s/ Chris Kennedy
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Unread 02-26-2011   #9 (permalink)
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OK, since it operates under load in reverse you are probably experiencing failure of the Forward clutches. It is time for a rebuild.
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Unread 03-23-2011   #10 (permalink)
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Thank you all for the suggestions. Issue discovered; one of the motor mounts was snapped in half. The engine would shake loose under any stress.

Incredible right?
Runs like a dream now.
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