No fluid change is going to help that. MY M-20 did that before it's rebuild.
How do you overfill a M21??
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From: Baton Rouge LA
How do you overfill a M21??
My tranny keeps popping out of 4th gear on occasion. It usually happens if you don't get it all the way in gear or if you keep acceleration and letting off it will work itself out of gear. I know it's probably an internal problem but I was going to go ahead and change the fluid thinking that it could be low. Well when I pulled the fill plug fluid came running out. How the heck can you overfill one of these? I didn't get to change the fluid since I have no drain plug and didn't have anything handy to get the fluid out. I let the fluid run out until level with the fill plug so now I know it has the right amount of fluid in it.
As far as the tranny problem goes. I'm sure it's internal unless anyone else has any ideas.
.
As far as the tranny problem goes. I'm sure it's internal unless anyone else has any ideas.
.
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From: Charles Town WV
Was your car on an incline back when you filled it or on an incline forward when you popped the plug? That could account for the fluid running out.
I agree with george, it sounds like you have bad synchro teeth on 4th, which is an expensive gear to change, because it's part of the input shaft.
I agree with george, it sounds like you have bad synchro teeth on 4th, which is an expensive gear to change, because it's part of the input shaft.
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From: Baton Rouge LA
Well I just got the car a few months back so I don't know how they filled it but it was level when I checked it. Yeah I'm thinking it's probably the shift fork or the sliders. If it were the Synchros shouldn't it be hard to shift? It shifts into 4th perfectly. It just doesn't like to stay there. 
Paul
Paul
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From: In Dreams There Is Truth Ohio
Originally Posted by Paul @ Thunder
If it were the Synchros shouldn't it be hard to shift? It shifts into 4th perfectly. It just doesn't like to stay there.
Paul
Paul
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From: Charles Town WV
If it were the Synchros shouldn't it be hard to shift? It shifts into 4th perfectly. It just doesn't like to stay there.
An old trick for curing this is to pull it apart and flip the rings around, but IMO if you've gotten that far, you might as well replace them. There are a few companies that make "grabber" rings that hold onto the gear a little better. I put them in my M21 the last time I rebuilt it and there is a noticable positive lock when you put it into gear.
In any case, I think you're due for a rebuild. An M21 is an absolute piece of cake to rebuild if you're up for a fun project. I rebuilt mine entirely for about $400 and it's nice and quiet and shifts perfectly now.
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From: Baton Rouge LA
Originally Posted by Steve Straus
An M21 is an absolute piece of cake to rebuild if you're up for a fun project.
Is there any kind of service book for doing this? I'm sure we can do it here at the shop.
Paul
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From: Little Rock AR
The gear oil will get all foamed up if you have just driven it for a while so the fluid will seem to be overfilled.
I agree with you guys. The clutch teeth on the input shaft/4th gear are worn and/or the clutch teeth on the slider are toast. That is usualy why it popps out of gear when you get off-on the gas. It might could be the input gear rollers but either way, you probably need an overhaul.
-Mark.
I agree with you guys. The clutch teeth on the input shaft/4th gear are worn and/or the clutch teeth on the slider are toast. That is usualy why it popps out of gear when you get off-on the gas. It might could be the input gear rollers but either way, you probably need an overhaul.
-Mark.
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From: Baton Rouge LA
Originally Posted by Steve Straus
There are a few companies that make "grabber" rings that hold onto the gear a little better. I put them in my M21 the last time I rebuilt it and there is a noticable positive lock when you put it into gear.
Paul
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From: Charles Town WV
Is there any kind of service book for doing this? I'm sure we can do it here at the shop.
http://www.5speeds.com/video.html
Doing this was probably the most fun I've had since I've bought the car, and I'm going to do the same with my ST-10 on my Camaro (basically the same tranny).
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From: Charles Town WV
Also any suggestion on the best place to get the rebuild parts?
Transmission Products in Tucson, AZ
(520) 884-7134