3.1L engine (GM): replacing intake manifold gaskets

      The original lower intake manifold gaskets will leak and then the gaskets must be replaced. Here's how.
      This is a difficult job, so I took detailed notes and digital pictures to help.
      I successfully completed the job on 2001 Buick Century. I recently completed a more detailed post on the similar 3.4L engine.

This is the before picture of the 3.1L engine

Preliminary Removal

1) Disconnect the negative cable at battery.
2) Unplug and mark 8 or 9 electrical connectors from their devices at the top firewall side of the engine.

Here is a close up view before the air intake assembly is removed

3) Now you can remove the air cleaner duct.
4) Remove the accelerator and cruise control cables from the mounting bracket. Set off to the side.
5) Remove the bracket. Three fasteners (10mm) hold it.
6) Unbolt the evaporative canister purge solenoid valve. It bolts with a 10mm to the end of the upper manifold cover. There is a hose going to the PCV valve on the front valve cover and another to the manifold itself. Move it off to the side.
7) Remove the EGR valve. This mounts on the right rear end of the upper intake manifold by two bolts. There is a gasket. The valve will just hang there by the rigid pipe that goes to the exhaust.

The air intake hose is now off, providing a better view of the remaining components

8) Remove the MAP sensor (one 13mm bolt) .
9) Unplug the spark plug wires from the ignition coil and mark both  the wires and the coil terminals where they plug in. Remove the coil assembly. Three 10mm bolts.

10) Remove the two little heater bypass hoses from the throttle body.
11)  Remove the alternator. There are two bolts in the back that hold it along with the pinion one in the front. The bracket for the alternator will have to come off after the alternator is removed. There are four bolts holding the alternator bracket and an engine eyelet for pulling  the engine out. These are of three different lengths so keep track of them.
11) Unbolt the remaining six bolts from the upper intake assembly. It pulled right off. The bolts are different so keep track of where they belong.

View with upper intake plenum removed

                                       
Removal of  lower intake manifold

1) Relieve fuel pressure. There is a cap on the left rear end of the back fuel rail. Take off the cap and push in the spring-loaded nipple with a small allen wrench. The pressure will relieve. I put a hose over it and activated the release with a small Allen wrench. There was just a drop of fuel and that’s all.
2) Uncouple the gas lines. The gas lines are uncoupled upstream from the fuel rail. A little plastic tool is inserted into the coupling and the lines will pull apart. There is some loss of gas. I put a zipper bag on them with a twist tie to hold it on. It isn’t necessary to unplug the injectors.

3) Unplug two connectors, one large one behind the engine and one small one right in front, atop the front valve cover. Now just pull up on the whole rail assembly and wiggle till it comes off as an entire assembly.
3) Remove hose clamps from the two ends of the bypass hose and separate them from the metal tubing. These pieces of tubing are pressed into the intake manifold. An 8mm bolt threads  into a housing by the water pump.
4) Remove the coolant overflow bottle. Two nuts.
5) Remove the serpentine belt. It takes a large allen wrench on the tensioner.
6) Unbolt the power steering pump and move it out of the way.
7) Remove both valve covers. These are 8mm retentive bolts, 4 on each cover.
8) Remove a 13mm bolt holding a bracket for a heater hose that is press fit into the thermostat housing.
This requires a ¼” deep well socket. A wiring harness mounts in a hole on this bracket and must be removed to fit the socket in there.
9) Remove the pipe from the thermostat to the upper radiator hose. Two 13mm bolts hold it on. The lower one is tough to get a wrench on and impossible to get a socket on for clearance.
10) Remove the lower intake manifold. Remove the four middle bolts first and then the four outer. Put them in a cardboard box, punching holes and marking their locations. I had to use a crowbar on an exposed corner (very carefully) to get it out.
11) Remove the pushrods and similarly put them in a box. A 10 mm socket will loosen the lifter assemblies to get clearance to remove them.
12) Now the gaskets are re-moveable. Take the crappy things out and clean all mating surfaces thoroughly.

View of the 3.1 with the lower intake removed

                                                                            Re-assembly
Lower manifold
1) Put new gaskets in place. I used a Fel-Pro kit (see below post for a link to the kit) that includes metal gaskets that are much more stout than the original plastic ones. See the link below these instructions to this excellent gasket kit.
2) Install pushrods in original positions. Tighten arms to 14 ft/lbs plus a 30 degree turn. This is as far as they will go.
3) Install lower manifold. Lay a 3/8” bead of black RTV to the section in the end of the valley between the two banks. Tighten the four inner bolts first to 5 and then to 10ft lbs. It is impossible to get a torque wrench at the inside end bolts (one on each end). I had to guesstimate the torque.
4) Install the valve covers. Should be about 7 ft lbs,  I purchased a 3/8”-1/4” socket adapter to allow use of the torque wrench with a 8mm ¼” socket.
5) Install the heater hose in the thermostat housing. Replace the o-ring in the end of the pipe first. Use a 13mm deep well here to attach the bracket welded to the pipe. Re-attach the wiring harness in the hole on the bracket.
6) Install the thermostat. Install the thermostat radiator side housing. It’s easier if you start the troublesome lower bolt first and then work the hose housing over the thermostat. Use a short 13mm wrench to fit around the exhaust pipe cover.
7) Install the heater bypass pipe. It goes under the upper radiator hose. Reattach two clamps and an 8mm bolt to the housing that attaches the assembly to the water pump.
8) Mount the fuel rail assembly at the same time as the power steering pump. Work them down into position together. If the pump is put in first the rail assembly will not fit and vice versa. I found that thin coat of motor oil on the injector gaskets was very beneficial. Mount the two 10mm fuel rail bolts.
9) Reattach the fuel line to the fuel rail. They just snap into place. Put the retainer clips on. Put the fuel lines 10mm mounting bolt back in.
10) Mount the power steering pump with the three 13mm bolts.

Upper Manifold

1) Place the new gaskets for the upper manifold in place. They have nice plastic guides for proper placement.
2) Set upper manifold in place and loosely tighten the six mounting bolts. The middle rear bolt holds the MAP sensor, so it goes with it when mounting. The two front left bolts hold a wiring bracket and the front right holds a spark plug wire holder. Torque to 18 ft lbs. There is guesswork on the MAP sensor bolt, since no torque wrench would fit there.
3) Plug in the Map sensor and the wiring in front of the upper manifold. Attach vacuum hose to fuel regulator upper right of manifold.
4) Mount the alternator bracket with the three 15mm bolts on the front and one in the back. Make sure the power steering lines are properly routed  between the bracket and the pump. There is holder down below.
5) Mount the alternator with the long through bolt on the front, a 13mm nut to a small bracket on the left rear stud of the upper manifold and two 15mm bolts to the bracket in the rear. Attach wiring connector to lower alternator.
6) Plug in oxygen sensor and small vacuum line to left rear of upper manifold. Plug in large injector wiring harness.
7) Mount the ignition coil assembly. Note the spark plug holders that go on the right side. Plug in the two electrical connectors on either side of the coil assy. Plug in the spark plugs. Snap them into the routing channels.
8) Mount the EGR valve. I reused the old gasket. Two 10mm bolts.
9) Attach the coolant lines to the little hoses that run though the throttle body.
10) Install the evap canister solenoid valve with a 10mm bolt to the upper manifold, right rear
11) Install the throttle assembly bracket.  A tie-wrap is helpful here.
12) Install the air intake duct.
13) Plug in all remaining electrical connectors. Nice work! Pop the top off your favorite
beverage and celebrate…after you start it up.

Conclusion

     The Buick has been leak free for several years now. This is a challenging repair for sure. I hope these instruction helped you "get it done." They, too, can be a lot of work and a challenge to create. If you found them useful I hope you would consider making a donation to this blog (see the "Pay Pal" link on the left side bar).  Have a good one and come back again.


                

 About the "Original Mechanic"