6.5 Diesel Upper & Lower Intake Manifold

6.5 Diesel Intake Manifold

The 6.5 GM turbodiesel utilizes a two piece intake manifold. The lower intake manifold resembles a largely traditional casting for a V-8 engine with a centrally positioned common entry point dividing into individual intake runners attaching atop the cylinder head intake ports. The upper intake manifold, sometimes also referred to as the intake plenum or upper plenum, connects the turbocharger compressor outlet to the lower intake manifold. The manifold absolute pressure sensor and intake air temperature sensor are mounted through the top of the upper intake manifold. The fuel filter housing (fuel filter manager or FFM) secures to mounting tabs at the rear of the lower intake manifold.

Both pieces of the intake manifold are cast from aluminum, yielding a lightweight component but with brittle properties. The lower intake manifold is particularly susceptible to cracking if it is not torqued in the proper sequence, if it is over-torqued, or if it is not seated flat against the both cylinder heads before being secured into place. Cracking of the intake runner mounting pads is a common occurrence when one of or a combination of the aforementioned conditions are met.

The lower intake manifold conceals the rear half of the injection pump, which is where the outlet connections are located. It is impossible to inspect or access this portion of the injection pump or remove any injector hard line without first removing the upper and lower intake manifolds. Furthermore, the lower intake manifold hides a section of the wiring harness and several electrical connectors integral to the proper operation of the DS-4 injection pump. The fuel supply and bleed/drain hoses are also hidden below the lower intake manifold, but can typically be fished out and replaced without necessitating removal of the intake manifold.

Cleaning Gasket Mating Surfaces

For the intake manifold to seal properly, the gasket mating surfaces on the engine block and intake manifold runners must be reasonably clean. Unless the gaskets were recently replaced, this is likely to require mechanical removal of gasket remnants from both surfaces. The intake ports on the cylinder heads should be blocked off with lint free clothes or rags prior to removing any piece of the intake manifold gasket that has stuck in this region. Any debris that should fall into an intake port is the potential for a severely damaging event when the engine is started. After cleaning, the intake ports should be vacuumed to remove potential debris.

Neither mounting surface should be cleaned with sandpaper or harsh abrasives. The industry standard for cleaning such surfaces are called conditioning discs or conditioning brushes. The 3M Roloc product line is dedicated to these needs, and their products are often specified by OEMs for such tasks. Unlike sandpaper or similar solutions, conditioning discs do not remove the base material when used properly, reducing the risk of over-treating an area and creating peaks, valleys, or craters in the mating surface. The aluminum intake runners are especially prone to excessive material removal from sanding and grinding tools.

Do not rush the cleaning and conditioning process. These products are designed to be used with light pressure at medium to high RPM, gentle removing unwanted material a little at a time. Excessive force may result in undesirable results. After all gasket material has been removed, any oil or residue should be removed by wiping the surfaces with a solvent (brake cleaner) applied with a clean cloth/rag. Part Numbers for our recommended surface conditioning products is outlined in the table below. We prefer the bristle disc type because they are more versatile in tight spaces.

6.5 Diesel Intake Manifold Parts List

[1] - Replacement of this part is required if removed; do not reuse
[2] - Replacement of this part is not required; recommend inspection and replacement as necessary
[3] - Highly recommend replacement of this part if removed
[4] - Additional information on thread sealant and threadlocker requirements can be found below in the subsection below

6.5 Diesel Lower Intake Manifold Torque Sequence

The torque specification for the lower intake manifold bolts/studs is specified at 25 to 37 lb-ft. Some GM service material lists the torque specification at 31 lb-ft, which represents the median value of the specified torque range; we recommend torquing all lower intake manifold bolts and studs to 31 lb-ft. The upper intake manifold bolts are of a much smaller diameter and are required to be torqued to 17 lb-ft (204 lb-in). The torque sequence for the lower intake manifold is outlined in figure 2 below. Each number represents the sequence in which that bolt/stud should be torqued to 31 lb-ft, starting numerically with "1" and ending with "16".

The lower intake manifold is susceptible to cracking and thus this torque sequence should be strictly followed. If the fasteners are tightened to less than the specified torque, the intake gasket(s) may not seal properly. If the fasteners are over-torqued or torqued out of sequence, an intake runner or mounting tab may crack. At time of publishing we were unable to find new replacement intake manifold castings, thus if one is damaged you'll be limited to finding a second hand unit.

Thread Sealant & Threadlocker Requirements

Fasteners 9, 11, 13, and 15 from figure 2 above are exposed to the crankcase. This means the fastener threads into a through-hole in the cylinder head where crankcase fumes can potentially vent through. These studs thus require thread sealant to prevent crankcase gases and fumes from leaking. ACDelco 10-2024 thread sealant is indicated as the correct product to be applied at the bottom tip of these threads. It is a non-hardening paste with a 300° F maximum exposure temperature rating suitable for use on aluminum, steel, brass, and plated metals. Teflon based thread sealants (pipe joint compounds, "pipe dope") that meet this minimum criteria may be suitable replacements if they are compatible with oil exposure. Do not substitute with RTV silicon or threadlocking compounds, which may infiltrate and contaminate the crankcase.

GM calls for high strength threadlocker to be applied on all the remaining lower intake manifold fasteners (all except 9, 11, 13, and 15 from figure 2 above). This is a preventative measure put in place to improve vibration resistance and prevent the intake manifold gasket(s) from losing their seal. High strength threadlocker makes fasteners very difficult to remove and increases the risk of shearing during removal. For these reasons, we opt for medium strength threadlock. The choice is entirely yours, but be aware that high strength threadlocker is required per GM specifications.

How to Remove the Intake Manifold on a 6.5 Diesel

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• Disconnect both negative battery cables.
• Remove the electrical connectors from the manifold pressure sensor (boost sensor) and intake air temperature sensor. The sensors do not need to be removed, but can be if desired. If removed, the manifold pressure sensor will require a new seal.
• Loosen the turbocharger compressor outlet boot (8 mm socket) and remove the retaining bracket (13 mm socket).

• Remove the (6) upper intake manifold bolts with a 10 mm socket.
• Separate the upper intake manifold from the lower intake manifold, maneuvering the inlet out of the turbocharger boot.

• Remove the fuel filter housing retaining ring, then pull the filter assembly upwards and out of the housing.
• Completely drain the fuel bowl using a suction tool.

• Remove the (2) bolts securing the fuel filter housing to the lower intake manifold with a 15 mm socket.
• Pivot the top of the fuel filter housing towards the firewall to increase access to the outlet and drain hoses, then loosen the clamps on both hoses.
• Disconnect the outlet and drain hoses from the fuel filter housing (simple barb-type fittings).
• Pivot the top of the fuel filter housing forwards towards the intake manifold to increase access to the inlet hose, then loosen the clamp and disconnect the hose at the rear of the housing.

• Maneuver the fuel filter housing upwards and out of its compartment, then trace and disconnect the water-in-fuel sensor and fuel heater connectors.
• Remove the fuel filter housing from the engine compartment and set aside.

• Disconnect the wastegate solenoid air lines and electrical connector.
• Remove the (3) wastegate solenoid and glow plug controller bracket retaining nuts using a 15 mm socket (the glow plug controller and wastegate solenoid brackets share a common stud).
• Let the glow plug controller float at the rear of the engine, but remove the wastegate solenoid bracket from the engine and set aside.
• Relieve the serpentine belt tension by applying torque to the belt tensioner pulley with an 18 mm socket, then slip the belt off of the alternator.

• Remove the power lug and electrical connector from the alternator (note; alternator style and orientation may vary with application).
• Remove the small bolt securing the alternator to the bracket at the 10 o'clock position (viewed from the front of the alternator) with a 13 mm socket.
• Remove the nut off the long bolt securing the alternator to the bracket at 5-6 o'clock position; leave the bolt in place.
• While supporting the alternator's weight, remove the small bolt securing the alternator to the bracket at the 3 o'clock with a 13 mm socket.

• With the alternator now pivoting on the lower bolt, get a good grip on it and remove the lower bolt to free it. Remove the alternator from the engine compartment and set aside.
• Remove the (2) alternator bracket retaining nuts with a 15 mm socket. Note that one of the bolts is shared with the driver side engine ground, which should be repositioned away from the intake manifold.
• Remove the alternator bracket from the engine compartment; it is mounted to (2) intake manifold studs.

• Remove the (2) heater hose bracket retaining nuts from the passenger side intake manifold studs. Position the bracket upwards and off of the studs so they are accessible.

• Using a 15 mm socket, remove the turbocharger support bracket retaining nut, passenger side engine ground, and small ground terminal near the rear of the intake manifold.
• Verify that any clamps, grounding lugs, or other accessories that could interfere with the removal of any intake manifold bolt/stud have been removed.

• Use a vacuum cleaner to clean around each intake manifold runner, removing any debris that may infiltrate the cylinder head intake ports once the manifold is removed.
• Starting at the center and alternating between the left and right sides of the engine, remove each intake manifold bolt. Note that there are several different bolt/stud lengths and types; each type and its location is outlined in figure 2 above. There are (16) bolts/studs total, (8) on each bank of the engine.

• With all (16) intake manifold bolts/studs removed, slide each injector line bracket away from the intake manifold so that it can be removed.
• On the passenger side, position the fuel return line retaining brackets away from the intake manifold runners so that they are not bent or damaged during remove of the intake manifold.

• If necessary, lightly tap the intake manifold to break it free.
• Maneuver the intake manifold off of the engine and out of the engine compartment. It is helpful to work the intake manifold towards the firewall where there are less wires and hoses to get caught on the runners. Note that the passenger side front intake runner has a labyrinth of electrical wires crossing over it that tend to become tangled if the intake manifold is pulled straight upwards.
• If further work is to be performed, plug each cylinder head intake port to prevent debris from infiltrating the combustion chamber.

How to Install the Intake Manifold on a 6.5 Diesel

• Use a vacuum cleaner to thoroughly clean out the engine valley and around/inside each cylinder head intake port. Preventing debris from infiltrating the combustion chamber during reinstallation of the intake manifold should be an absolute priority.
• Thoroughly clean the gasket mating surfaces at the engine block, lower intake manifold (at base of intake runners and upper manifold mating surfaces), and upper intake manifold. All remnants of old gaskets should be completely removed and the surfaces should be free of residue.
• Install the new lower intake manifold gaskets.

• Note that the replacement intake manifold gaskets should have locating pins at the front and rear of the gasket. These pins should be secured into the appropriate hole in the cylinder head to keep the intake gasket from sliding around while the intake manifold is installed. If the gaskets are installed incorrectly and these pins are not properly set into place the chances of cracking the intake manifold casting are significant.

• Install a new filter into the fuel filter housing and reposition it at the rear of the engine.
• Connect the inlet and outlet hoses. Note that the two outlet fittings are labeled. The passenger side fitting, labeled "DRAIN", must connect to the tee fitting mounted to the thermostat housing. The driver side fitting, labeled "OUT", must be connected to the injection pump supply hose. If these two hose connections are reversed, the injection pump will be supplied with unfiltered fuel. We highly recommend replacing the 3/8 inch inlet hose and 1/4 inch outlet hoses at this time.
• Reconnect the water-in-fuel sensor and fuel heater connectors.

• Maneuver the intake manifold into place. Ensure that the intake runners are sitting flush with the gaskets at the cylinder head ports and that they are not pinching part of the wiring harness, the return fuel line brackets, or any other accessory.
• Verify that the gasket is positioned correctly and that the alignment pins remain properly installed. In the intake runners are not perfectly flush with the gasket when the fasteners are installed it is highly likely to crack.

• Reinstall all fuel injector line retaining brackets and line up the holes as close as possible.

• Install and torque each intake manifold/bolt stud to 25-37 ft-lbs per the numbered sequence (starting with 1) at left. Failure to torque the intake manifold in this sequence and per these torque specifications may result in cracking or damaging the casting.
• Note that fasteners 9, 11, 13, and 15 are exposed to the engine crankcase and a small bead of thread sealant (ACDelco 10-2024 or equivalent) must be applied to each the base of the threads to prevent oil leaks. A small amount of threadlocker should be installed on all the remaining fasteners (do not use threadlocker on bolt/stud 9, 11, 13, or 15; thread sealant only).

• Orient and reinstall the fuel filter housing; torque the (2) bolts to 31 lb-ft.
• Reinstall all brackets, retainers, clamps, grounds, and accessories that were previously removed. Anything installed on an intake manifold stud should be tightened snug, and final torque should not exceed 37 ft-lbs.
• Reinstall the alternator and serpentine belt.

• Install and orient the upper intake manifold gasket.

• Install the upper intake manifold. Torque the (6) bolts to 17 ft-lbs, starting with the (2) center bolts.
• Reconnect the manifold pressure and intake air temperature sensors.
• Secure the turbocharger outlet (upper intake inlet) boot clamps and reinstall the retaining bracket.
• Bleed to the fuel system and check for fuel leaks at the filter housing and all hoses.
• Check for fuel system leaks once more once the engine is started for the first time.