Ford 4.6L Intake Manifold Replacement
The 4.6 liter engines found in the Ford Mustang, Crown Victoria, and other full size sedans originally came equipped with an intake manifold made entirely of plastic. At the front of the manifold there is a coolant passage which, after thousands of heat cycles, begins to crack and leak coolant. This crack is usually in the center of the cross-over tube just behind the alternator. Depending on the severity of the leak, it may or may not be necessary to pressure test the cooling system to locate the fault. When diagnosing the source of the leak, keep in mind that there is a coolant pipe that runs just under the intake in what would be called the lifter valley on a conventional push rod engine. Ensure that the coolant leak is not coming from this tube before condemning the intake.
After this coolant leak was recognized as a pattern failure, manufacturers began to produce an updated manifold that utilized an aluminum coolant passage to correct the issue of the plastic cracking. While it is an improvement over the original design, the new manifold is not without its faults. The coolant fitting at the rear of the manifold (which is still made of plastic) has a tendency to crack. Also, the aluminum coolant passage at the front of the manifold is sealed to the plastic portion of the intake with O-rings. These seals can degrade and shrink over time, causing leaks in this area as well.
In this photo, the crack in the original-style intake is visible at the end of the red arrow (the alternator has been removed for clarity). It was spraying coolant into the 'lifter valley' and leaking out of the rear of the engine as well as being visible from the front. Another common source of a coolant leak is indicated by the green circle.
This guide is intended to assist the technician in replacing or re-sealing either intake manifold. The procedures depicted below are by no means the only way to complete the task.
Disassembly
Begin the disassembly process by disconnecting the battery and draining the cooling system. Remove the air intake hose and the throttle linkages from the throttle body.
Take out the bolts that retain the throttle cable bracket. There is no need to disassemble the cables from the bracket, simply locate the throttle cable assembly out of the way of the other components.
There is a bracket behind the EGR valve visible in this photo. Remove it and set it aside. Loosen the large nut on the EGR tube, using penetrating oil if necessary. It is not necessary to remove the EGR valve from the throttle body. Remove all vacuum hoses and lines at the throttle body in addition to any electrical connectors.
Position all of the cables, hoses and brackets out of the way up near the cowl. Be very careful with the colored plastic vacuum lines, as age and under hood temperatures make them very brittle and prone to cracking. If they begin to fall apart, it is possible to repair them with short sections of vacuum hose. Once the throttle body is clear of all connections, remove the five bolts that retain it to the intake manifold. Four bolts are around the base of the throttle body, one is located towards the side of the engine on a short bracing leg of the throttle body.
Disconnect all electrical connectors at the intake manifold including the coolant temperature sensors, fuel injectors and coil packs (if applicable). Remove the serpentine belt and the alternator. The factory alternator bracket is typically discarded, as many updated intake manifolds require a redesigned bracket to retain the alternator. Disconnect the fuel lines at the driver's side of the engine and gently move them aside. Removal of the spark plug boots is not necessary, but will make removal and installation slightly easier. If the vehicle is equipped with coil-on-plug ignition, it will be necessary to remove the coil packs.
Remove the heater hose from the fitting at the rear of the intake and place it out of the way. Remove the upper radiator hose at the thermostat housing and place it aside also. Take out the two bolts retaining the thermostat housing and remove and clean it in preparation for re-installation. There is a large wiring harness that runs across the back of the manifold and is held to the manifold with plastic clips that must also be removed. The clips will likely break when this harness is disturbed. Ensure that, if the clips do break, the harness is secured with tie straps upon reassembly to prevent possible shorting of the wires.
The top of the engine should look something like this just before the intake manifold comes off. There are four bolts on the driver's side and five on the passenger's side of the engine that retain the intake to the cylinder heads. The EGR tube may be positioned too low to get the intake out of the vehicle. If this is the case, VERY GENTLY pull upwards on the EGR tube, until enough clearance has been gained to allow removal of the intake manifold. If the EGR tube refuses to flex, or if it feels as if it may break, it must be taken off of the exhaust and removed from the vehicle.
Preparing the new Manifold
With the intake out of the car, it is now time to transfer parts to the new manifold. Remove the two coolant temperature sensors, clean the threads and install them in the new manifold. Some technicians feel the need to coat the threads with Teflon or other pipe thread sealant. While this is not wrong, it is also not typically necessary as long as the threads are in good condition. The brass threads are soft and will self-seal when properly tightened. If sealant is used on the threads, make sure to apply it carefully so that none of it can enter the cooling system. Remove the four studs that retain the fuel rail, and gently remove the injectors.
To safely remove the fuel injectors, grasp each one with a pair of medium hose pliers and carefully rotate them side to side until they spin freely. Next, pry upwards with the pliers until the o-ring seal is free. It may be necessary to spray the injector O-ring with penetrating oil in order to facilitate removal. Repeat this process with the other seven injectors, then remove the fuel rail from the old manifold. With a clean rag, wipe down the sealing O-ring and inspect its condition; replace as needed. Coat each O-ring with a lubricant before installing them in the new manifold to ease insertion and reduce the chance of the seal tearing.
Here, the updated coolant passage can be seen at the front of the new manifold.
Before the new intake can be installed, the sealing surface must be cleaned. Here is an example of before and after cleaning. Be very careful to keep debris out of the ports in the heads. Once the loose grit has been removed, go back with a razor blade or other flat scraper and remove the carbon and coolant residue. Then wipe down the sealing surface with solvent and dry with a clean rag.
Re-installation
When the heads are clean, the new manifold may be installed. Ensure that none of the sealing O-ring gaskets are knocked loose when setting the manifold down. Start all of the manifold bolts and thread them in a few turns. Now install the new thermostat and gasket, and the thermostat housing and bolts. This photo also shows the coolant tube that runs under the intake. Make sure it is inspected for leaks before the intake is installed.
Once all of the intake manifold bolts are in place, run each one down until it contacts the surface of the manifold. Then, starting in the center and moving in a criss-cross pattern, torque each one to 18 ft. lbs. This step is very crucial to the integrity of the job. Over- or under-torquing will cause coolant or vacuum leaks. Once the manifold has been torqued down, the rest of the job is simply installing parts in the reverse order of removal. One difference is the updated alternator bracket that comes with many new intakes. The original bracket will not mate up to the new manifold, so the new bracket must be used.
Once the engine is re-assembled, fill the cooling system, start the engine and check for leaks. Remember, as with any other repair, work at a pace you feel comfortable with, take your time and do it once.
Adam Riggs
ASE CMAT L1