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NOTE: The information in this document is correct to the best of the author's knowledge. However, due to a change in employers the author will not be able to update/correct this document in the future. The author accepts no responsibility for any damage, injury, or other "loss" you encounter while working on your car.
This portion of the FAQ was written by Andre Molyneux. It's labeled as Part 4A, as my intention was to make part 4 of the FAQ contain common failures and fixes. This obviously would get too long to handle in a single section, so I planned to do Part 4A, 4B, 4C, etc. Anyone adding on to the FAQ in the future may want to keep the same naming convention.
Converted to HTML by William Keller (wdklists@jps.net).
The cooling system is a known trouble spot on Mustangs. Water pumps and heater cores go out with distressing regularity. Some of the more common problem symptoms/causes are:
The water pump write up only covers 5.0L cars at this time.
The water pump is a consumable on 5.0L Mustangs. Expect it to go out around every 50k miles or so, although some certainly go sooner and others last much longer. Fortunately the water pump isn't too difficult to replace. The replacement procedure in the factory manual is pretty decent, so only a few additional hints will be added here.
There are two main failure modes for the water pump, which can be diagnosed by where the coolant is leaking from. They are:
Both failures will result in coolant leaking from the underside of the pump. If the gasket has failed, you'll see coolant spraying out from the pump/engine block junction. Input shaft seal failures typically don't spray as high a volume of coolant out, and it will be coming from further forward on the pump body, where it tapers down around the shaft.
Input shaft failure requires replacement of the pump. For gasket failures you can just replace the gasket, but based on the age of the pump you may want to go ahead and replace the whole thing anyway. If the pump's already got 50k miles or more on it, it doesn't make much sense to replace just the gasket when an input shaft seal failure probably isn't too much further down the road.
Unfortunately, getting the correct pump isn't always easy. Since the 5.0/302 has been around for a long time, there have been a variety of sublty different pump designs used over the years. This is complicated by the fact that later cars with a single serpentine-belt spin the pump in the opposite direction compared to earlier cars. It's not at all unusual for a store to give you the wrong pump for your application.
Although it's a pain, you should pull the old pump off the car before heading for the auto parts store. Things to check when comparing the old pump and the new pump:
Several people have commented about problems with rebuilt pumps. Brand new pumps aren't overly expensive, and are probably worth the money.
Replacing the pump doesn't take anything special (aside from a 5/8" deep socket, which is an absolute necessity). A well-stocked socket set, a tube of sealant, and a scraper are about all that's required.
Since you're draining most of the coolant and disconnecting one of the hoses anyway, you might want to go ahead and pull the radiator when doing this job. It's very easy to do, and getting it and the fan shroud out of the way gives you a lot more elbow room.
The pump-to-thermostat hose is a pain to get on and off. Remove all of the other necessary hoses before unbolting the pump, but leave this hose until you actually have the pump unbolted from the block. The hose comes off easily when you can move the pump around. Likewise, install this hose onto the new pump before bolting it up.
The heater core is a well known weak-spot on Fox Mustangs. In fact, a local radiator shop claims that replacing the heater cores in Mustangs (as well as Taurus's and T-birds) is what keeps them in business. The information in this section applies to '79 to '93 Mustangs - I don't know if Ford changed the design of the heater core and/or evap case for the '94 and later cars.
Failures are typically pinhole leaks along the seams where the core was soldered. However, some people have damaged their cores by flexing the input and output tubes while replacing the heater hoses. Take your time and be careful when replacing these hoses - creating a stress-fracture on the core is a very expensive mistake.
The core itself is relatively inexpensive, but replacing it can be quite difficult. In most cases, the dash has to be pulled back, and if you have A/C the manual says you have to discharge it before pulling the core. However, a number of people have managed to replace the core in cars with A/C without discharging - it can be done, but it's not easy.
On earlier cars without A/C, heater core replacement can be very easy. Simply remove the glove-box door, and look for a removable panel on the heater case. If you've got one of these, all you have to do is remove the panel and you've got access to the core. Unfortunately, this only applies to cars without A/C, and only up to maybe the mid-eighties. Later non A/C equipped cars (and apparently all A/C equipped cars) require that the dashboard be pulled back, and that the heater/evap case be pulled back as well, to get at the core.
You really need the factory manual for this job, as it clearly calls out the location of all the fasteners that must be removed in order to pull the dash back. Once you've managed to get the dash pulled back about 8 inches or so, you'll have a good look at the evaporator case. The problem with accessing the core is that the access panel has to be pulled off from the top, but the evap case is tucked under structural metal. Doing the job "right" would involve pulling the evap case back about 6 inches, but the A/C lines prevent this - thus the instructions tell you to discharge the A/C system and disconnect the lines.
The case can be pulled back a little bit before the A/C blocks further progress. This is just enough that with some prying (and perhaps a little bit of judicious cutting of the plastic case) that you can pull the old core out of the case. You need to be very careful when inserting the new core - you don't want to rub it hard against the case, or get it jammed and have to force it in, as you could damage it and end up with a brand new leaking core. Cutting the case (and using some sealant to patch it up afterwards) is much preferable to risking any damage to the core.
When you install a new core, make sure that is has a plastic restrictor on the inlet (the core may come with this, or your old core may have one that you can re-use). If you don't have one, get one from a radiator shop or other source. You need one because the Mustang's heater core's inlet is larger than its outlet. This means that a pressure drop is created across the core, and this is probably one of the reasons why they fail so often. Adding an inlect restrictor causes the pressure drop to occur before the core, and will hopefully lead to a longer life. Ford started putting the inlet restrictors on sometime in the mid-late eighties, but it's a good idea for any year of car.
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