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post #1 of 12 (permalink) Old 12-07-2016, 08:26 PM Thread Starter
 
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snake oil before rebuild?

Hey all just thinking out loud, my th350 in my lemans is giving up the ghost I think. I can drive it for about 10-15 min it shifts a little hard but it hits all three gears, once its warmed up it however the fun begins, it stays in first till it i lift on the throttle then it catches second. third gear is a crap shoot but I don't expect it to find the gear, once it cools off for the day same story with the shifting. Needless to say its deadlined until resolved. The question, has anyone had success with trans oil additives? if so is it worth a shot just to say i did it before blowing it apart to rebuild? History of the trans is been sitting for a few years, as far as i know its never been messed with, exception of me swapping out the vacuum modulator, fluid is red but a little dark. think snake oil can bandage it till I can get it to a shop?

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post #2 of 12 (permalink) Old 12-08-2016, 01:50 PM Thread Starter
 
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Well I rechecked everything pumped a qt out of the trans, added Lucas Trans slip stop stuff, and what do you know it now finds all 3 gears shifts smoother not silky but I'll take it. Still have to lift a little to let it find 3rd, hope this bought me a little time but for now the car is drivable, just thought I'd share my story to anyone, like mysel, that wasn't sure.
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post #3 of 12 (permalink) Old 12-11-2016, 08:26 AM
 
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thanks for the results
on Lucas, I really didn't think it would help your tranny. You probably have a trans rebuild in your near future though. I recently (May 2015) had my PX-69 TH400 rebuilt at a local shop for $700, including a B&M shift improvement kit. Of course for that price I had to remove and replace, just dropped it off at shop.
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post #4 of 12 (permalink) Old 12-11-2016, 10:39 AM
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Garage
You might try just draining all the fluid and replacing it with fresh. It can't hurt, and it's a lot cheaper than a rebuild. Unless you've got a drain plug in your pan, you'll have to loosen the pan bolts to drain it so while you're there might as well replace the filter too.

I agree there's likely a rebuild in your future, but this might buy you some time.

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post #5 of 12 (permalink) Old 12-16-2016, 05:44 PM Thread Starter
 
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Ya I'm toying with the idea of just fluid and filter, might as well I gone this far with toying with it. I was quoted prices from 800 to 1200 low end with me pulling the trans myself. Side note I pulled the speedo cable to see why it doesn't register on my gauge, the clip that holds the driven? gear in popped off is there a way to install the clip without dropping the tail shaft? and still so far so good with the trans doing its thing. It surprised me also but I noticed results after a few minutes of driving it was doing what it said it would do.
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post #6 of 12 (permalink) Old 12-21-2016, 12:36 PM
 
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I find it interesting that your shift pattern is related to warm up. If I recall correctly, in 1969 and later models vacuum is routed through a temperature controlled valve or "tree" mounted on a water passage on the manifold. As your engine warms up, vacuum is routed differently. The upshift timing and harshness are at least partially controlled by your modulator, which is controlled by vacuum. Your shift symptoms are very similar to a low vacuum signal to your modulator. The fact that it changes upon warm up indicates a vacuum leak somewhere in the system AFTER the temp controlled vacuum valve. I wouldn't assume a rebuild is necessary just yet. Check for vacuum leaks first.

When the only tool in your toolbox is a hammer, you tend to see every problem as a nail.
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post #7 of 12 (permalink) Old 12-22-2016, 01:15 AM Thread Starter
 
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Where else could I check for leaks? Your post gives me hope, the vacuum line is hooked up to the carb, I don't know where to put it on the manifold
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post #8 of 12 (permalink) Old 12-23-2016, 09:41 AM
 
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Quote:
Originally Posted by 1970lemonhead View Post
Where else could I check for leaks? Your post gives me hope, the vacuum line is hooked up to the carb, I don't know where to put it on the manifold
The vacuum for the modulator has to be full manifold vacuum. Where is it hooked up right now? Is it above the base plate? If it is, then it is more than likely ported vacuum and not enough. Now, in addition to it being hooked up correctly, you'll want to make sure you have enough vacuum. With a stock cam you should have somewhere between 14 and 20 Hg (inches mercury). If you have less than this, you may have a leak somewhere, or you may need a good old fashioned tune up. Late ignition timing will cause low vacuum, so make sure your timing is set correctly. I like to use a vacuum gauge along with a timing light when I set timing.

If all that is right and it still shifts crappy, you may have a bad line at the modulator, or a bad modulator, or an adjustable modulator which is not adjusted correctly.

Lots to check before you pop for a rebuild.

Hope this helps.

Chuck

When the only tool in your toolbox is a hammer, you tend to see every problem as a nail.
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post #9 of 12 (permalink) Old 12-23-2016, 01:02 PM Thread Starter
 
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Thanks for the info I'll definitely make a list and take it from the top, the modulator is adjustable, with new hoses. I had a friend tune it's timing by ear so that can be another issue. The car has manual brakes so I'll just use the port on the back of the carb to rule that out, right now it's just plugged anyway. I'll report back what I find after I run threw the list. Then engine has had a tune up, new msd ignition and dist, plugs and wires but I'll pull some and inspect them
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post #10 of 12 (permalink) Old 01-26-2017, 09:36 PM Thread Starter
 
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I'm back, sorry been busy with work and school. So I finally got around to dinking with the car again and decided to check for vacuum at the modulator it held steady I just used my mighty vac and put 20lbs to it. So next I hooked it up to the other side and capped off the end that goes to the modulator, the steal line would not hold a vacuum and had a fast leak of pressure. So my limited experience says pull the steal line and inspect for holes, does it just pull out from the top? or is there a trick to yanking it, and if it is bad can i source a used part from a yard or is this something I want brand new? thanks for all your guys help as I clearly have no business working beyond my skill level but that is how we learn.
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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, Lemans & GTO Undercarriage, Frame, Transmission and Differential Discussions.

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