The upper intake manifold is a very common failure point on the venerable GM 3800 Series II engine. Often when it begins to fail, the car will show the symptoms of a head gasket failure, however actual head gaskets failing on this engine is VERY rare. One of the first signs of the intake manifold failure is coolant loss, these intakes can leak internally into the engine, or externally near the thermostat housing. In extreme cases, the leak can get so bad internally that it can actually hydro-lock (when an engine fills with so much liquid that the pistons cannot move up in their bore). This was the case in the example car I will show you today. In the case shown below, the owner of the car thought the head gaskets were bad, and it would need major repair or a new engine. After a quick inspection, I knew it was only the intake manifold that was bad. The car had stalled on him, and would not turn over at all, acting like the engine was seized.
Before I start, here are the exact vehicles affected by this leak:
1995-2005 Pontiac Bonneville 3.8L
1998-2005 Chevy Monte Carlo 3.8L
2000-2005 Chevy Impala 3.8L
1996-2005 Buick Lesabre 3.8L
1996-2004 Buick Regal 3.8L
1995-2005 Buick Park Avenue 3.8L
1995-1997 Buick Riviera 3.8L
1995-1998 Olds Regency 3.8L
1995-1999 Oldsmobile Delta 88 3.8L
1996-1999 Oldsmobile LSS 3.8L
All 3.8L (3800) engines with VIN code K (8th digit of VIN) Non-supercharged
Before you tear into it, there are two important steps,
1) DISCONNECT the negative battery terminal
2) Drain any engine coolant that may still be in the car (there is a small drain near the bottom of the radiator). After these two steps, you must gain access to the intake manifold by removing some things that are in your way.
3) Remove the engine cover by turning the oil cap to loosen, then without pulling it off, continue to twist it counterclockwise, when it stops completely, pull the oil cap AND oil cap tube away from the engine. You can then lift the engine cover off of the engine. Remove the air filter housing. Loosen the two Philips screws on the housing itself, then pull the throttle body hose end off. (Peel back the top of the hose, then work around until the whole thing comes loose.) Disconnect the plug going into the intake hose, and remove the whole assembly from the car.
4) If the spark plug wires aren’t marked, mark which one goes where on the coil pack, and pull the three plug wires that run to the rear off of the coil, and let them fall to the rear of the engine bay.
5) Remove the serpentine belt.
6) Loosen the three alternator bolts and remove wiring from the back of the alternator. Remove alternator and set aside. NOTE: To remove one of the bolts, you must remove a bracket that runs from the alternator to just below the ignition coils. TIP: there is a bolt that is hard to see just below the belt idler pulley.
7) Remove the six fuel injector electrical plugs. (Squeeze in on the metal locks and pull up.) Remove the three electrical connectors going to the throttle body. Remove the electrical connector going to the MAP sensor (sits on top of the intake manifold near the alternator).
8) Remove all vacuum lines going to the throttle body and intake manifold.
9) Remove the fuel lines from the fuel rail. You will see two plastic lines going to the fuel rail, they are both held in by small plastic clips. To remove them, squeeze in on the bottom of the clips, then pull up on the fuel line. **CAUTION** There may be some residual fuel pressure in one of the lines, so remove the lines very slowly and carefully.
10) Remove the fuel rail with injectors. There are four nuts that keep the fuel rail and injectors in place. Once you remove those four 10mm nuts, carefully wiggle and pull upwards on the fuel rail, and the injectors will unseat themselves from the lower intake manifold, and the whole fuel rail will come out. Set aside.
11) Remove the throttle body. There is a bracket that connects the throttle body to the cylinder head, this little bracket blocks access to one of the throttle body nuts. My trick is to remove the bracket-to-throttle body bolt, then carefully pry the bracket back until you have enough access to reach the nut with a deep 10mm socket and extension. You can leave the throttle cables attached to the throttle body and just position the entire assembly aside. At this point, you should have clear access to the entire upper intake manifold.
12) Remove all of the upper intake manifold bolts, and remove the intake from the car. If the intake does not want to separate from the lower, then you most likely missed a bolt. it should NOT require any prying to get it loose.
13) Now if the intake was leaking internally badly, you will likely see an alarmingly large puddle of coolant sitting inside the engine. It is CRUCIAL that you remove all of this coolant, use paper towels or old rags to soak it all up. Clean the gasket mating surface on the lower intake manifold. TIP: It is very important to have a clean mating surface on the top of the lower intake manifold, or you may encounter leaks.
14) Snap the upper intake gasket into place onto the bottom of the new upper intake manifold and place the upper intake back onto the car, make sure all the bolt holes are aligned.
15) Install and tighten all the bolts in the following sequence. NOTE: the specified torque is ONLY 89 INCH pounds, which is less than 10 ft. pounds...be very careful not to over-torque and risk cracking the new intake.
16) Reinstall all accessories and wiring, and fuel rail in the reverse order of removal. Refill the coolant system.
17) IMPORTANT STEP: If you saw ANY coolant at all inside the intake manifold upon removal, you MUST replace the spark plugs. Also, it is very important that you remove most of the coolant that may have entered the combustion chambers, which is simple to do.
17a) Remove all six spark plugs, and turn the engine over (pretend you’re starting it) for at least 30 seconds. Install six new spark plugs and you are good to go.
18) Start car and let idle for a while, check for any coolant leaks, carefully watch your temperature gauge to make sure no overheating takes place. Once everything looks good, you are done! Congratulations, you just saved yourself hundreds of dollars by replacing your own intake manifold on a 3.8 Liter 3800 Series II car!