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Required Reading


By Mike Davidson

No, it’s not the next remake of Home Alone…but it is the stains in your driveway and smoke billowing out from under the hood of your prized Mustang.

The 5.0 Fox bodied Mustangs are showing their age. Many of the cars coming into the shop these days are leaky disasters. Previously we had gone in-depth about preparing a Mustang to go fast, with the chassis and suspension being a priority. The problem now is that customers looking for modifications only to be thwarted by dangerous and inefficient leaks. We’ll start with the back and move forward.

Rear End Leaks 

Before you consider upgrading that rear end with gears, control arms and the like, you must address that sweaty cover plate. Usually made of steel or even aluminum, these covers are often a source of loss of fluid. An eventual leakage of all axle lubricant will lead to a costly repair, as not only the differential is at risk of failure, a lockup can cause severe transmission damage…stick or automatic. Another source of leakage can come from the pinion seal, the front of the housing where the driveshaft flange mates to the rear. These should always be replaced whenever a gear change is done. The only other rearend lubricant loss takes place is at the axle shaft seals. Once again these should be checked whenever the axles are removed for any reason, gear swaps, 5-lug kits, etc. The only other leak potential at the rear is brake fluid from a wheel cylinder or caliper, and hopefully you are not reading this to figure that out.

Transmission Rear seal leaks are common on both T5 and AOD based cars and both are repairable without any transmission disassembly. Only the driveshaft is required to be removed for this service. The driveshaft should carefully be inspected at this point because the front yoke is often worn out or chafed, which is the cause of the seals failure. Many customers come in to the the shop with a blown up T5, usually due to a burn-up, or lack of any fluid at all inside the unit! These trannies are not even acceptable cores as no part of the transmission can be reused.

Engine Rear main seals on late model 5.0 motors are one piece units that should almost always be replaced whenever flywheel removal is taking place (clutch jobs). Unlike early 5.0 and 302s (1983 and prior), the one piece seal is easily changed without any main cap or oil pan removal. One of the problems of having your car serviced by a non-Mustang specific mechanic is that they don’t automatically swap this seal, even though it is a quite simple fix. In this same area you might also find a leaky oil pan seal. Not an easy fix but it should be noted that many times a leaky oil pan gasket is a mistaken diagnosis. Valve cover leaks and intake manifold pan rail leaks tend to drip down to the area of the oil pan causing the confusion. I always start at the top of the motor when taking care of these evil drippings. All these leaks must be taken care of before a plan to add aftermarket parts to your Mustang. For example, leaking valve covers drip burning oil onto the headers, ruining the gaskets and discoloring or even worse, starting a fire on your headers! Also, any oil leaking out the back of the motor can eventually penetrate the bellhousing and clutch area. This can cause glazing and premature clutch failure.


The following are sources of many coolant or anti-freeze leaks. Timing covers, water pumps, intake manifolds, radiators, overflow tanks, hoses and heater cores are all sources of disastrous drips which are highly frowned upon by your local Drag Strip operators. Besides loss of traction, they can cause loss of life. If you have ever skated over someone’s leaks or hit a patch of slick roadway, you know what I’m talking about.  A pressurized system tester will check where the leaks are coming from. 

Remember, you must start with a dry motor before you consider your hot rodding and racing.  That’s the law, or mine at least.  Safety first and you won’t be ass-first into the guardrail or head-on into another vehicle.