Street News

An Unfortunate Series Of Fluid Dynamics

I’d rather deal with a spun rod bearing than a fluid leak.

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Over the past couple of years, the garage concrete underneath my 1966 Mustang has been freckled with little red dots. I was able to deal with it because, as any vintage Mustang owner knows, leaks are just part of the C4 automatic transmission experience. But over time, the little drops became a puddle, which eventually expanded into a small pond to the point where I have to watch where I step when getting into the car. Now, taking the Mustang out for a Sunday drive means having to add a quart of transmission fluid, only to watch it all leak onto the floor within a few hours of returning home.

The good thing about a transmission leak is that it’s easy to diagnose – the Type F tranny fluid used in older Ford automatics is red and has a distinctively stinky odor. The bad thing about a transmission leak is that it’s difficult to determine where it’s coming from. Gravity causes the fluid to collect around the lowest point on the transmission, so it could be coming from a number of places above. And by the time air flowing underneath the car pushes the fluid around, nearly the entire transmission is covered in red liquid.

So I did what any 21st century Mustang owner would do – I searched the Internet Mustang forums. Typing in “C4 leak” revealed hundreds of threads from other early Mustang owners who are just as baffled and frustrated as me. So, since I’ve already researched this topic and as a free service from this column, here are the most common areas for a C4 leak:

Dip stick tube: A simple o-ring failure can make it look as if the transmission has suffered a gunshot wound. The tranny fluid will seep out and run down the side of the transmission, then collect in the gap where the pan attaches to the case, looking for all the world like a leaking pan gasket. Replacing the gasket requires pan removal, thus draining the transmission and refilling once the otherwise simple R&R procedure is complete. Then you discover that the leak was coming from the dip stick tube and you’ve wasted your time, tranny fluid, and new gasket.

Modulator valve: An obvious spot because it screws into the transmission. If it’s loose or you don’t apply enough thread sealer to stop Niagra Falls, it will leak. Also check the crimp that secures the two metal halves. If someone tightened the modulator valve using vise grips on the body of the valve, it could damage the crimp. Ask me how I know. But that was another leak from another time.

Shift linkage: Another cheap o-ring, another potential leak source.

Pan gasket: Yes, the actual pan gasket is, more often than not, the actual culprit, sometimes caused by the pan itself, which could be warped after years of bouncing off traffic humps and overzealous tightening by backyard mechanics. There are also debatable theories about the best gasket material – cork or rubber – and whether or not to avoid silicone sealant or use enough to win a Dolly Parton look-alike contest.

Unable to determine the starting point, or points, for my C4 leak, I decided to call in a professional. Merv Rego at Classic Creations of Central Florida who restored my Mustang a couple of years ago and has years of experience with vintage Fords. I was confident that he would be able to find and fix the leak. Over a frustrating two-week period, Merv replaced the pan gasket, removed the pan again to reshape the lip, installed it again with yet another new gasket, ordered a new reproduction pan, installed a heli-coil to repair the stripped-out threads for one of the pan bolts, and finally installed an aftermarket pan with a thicker, stronger lip – with yet another new gasket. None of this even slowed down the leak. At one point, Merv suspected a hair-line crack in the case. Not a best-case scenario.

At least I’m not the only one who has trouble tracking down a C4 leak.

Finally, after days on the lift and many quarts of Type F fluid, Merv finally confirmed the source of the leak – the front pump seal, which leaks fluid inside the bellhousing where it gets tossed around and eventually seeps out all along the circumference where the bellhousing joins the case. Seal replacement is simple but, unfortunately, requires transmission removal and partial disassembly.

If it had been a spun rod bearing, it would have been diagnosed and fixed weeks ago.

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