1998 Olds Intregue, 3.8L about 80K miles, started making loud engine noise. Tore down and found crack in lower intake manifold. What would cause this? Is it worth replacing? Should I be checking for anything else?

The manifold gasket failure was likely where the water went astray. Did any get down into the crankcase? You need to flush it all out. At some point, you will want to crank the engine over with all spark plugs removed to see if any more water spits out. I don't understand what manifold cover you mean had the cracks. If it is the manifold itself, you probably need another manifold, especially if it is a sealing surface. The lower intake manifold gasket is a common failure on GM V6's of these years. Good thing you caught it in time before it ruined the engine.

go to the wreckers that specialise in your make of car and get a second hand one. The intake manifold can be cracked (V8) by the incorrect tightening of the manifold bolts (tightening up all on one side first then tightening down the other side will crack the manifold.)

lous engine noise can be casued by several things let me clarify sources to you and you can try to fidn the real reason

1- improperly adjusted timing belt/chain cause your engine run on wrong ignition timing. so this cause of pinging. this cause pinging noise and this is a cause of noise in engines.
2- manifold gasket is leaking
3- exhaust manifold or exhaust is cracked.
4- intake rasonator is not attached to air filter housing and cant neutralize the rasonance casued by air vacuumed inside engine while engine is "breathing"
5-valves are out of adjustment if they are manually adjustable
6-related to valves- be sure that engine oil is in right viscosity (like 10w40 5w30 check the users manual) and engine oil is filled at correct level.

You may want to check the intake manifold coolant port tube, which is plastic and tends to get eaten away over time on this particular engine. This condition causes coolant to mix with fuel in the intake manifold, making the engine run rough, and ultimately not start.

The replacement tube is metal, and will solve the problem. A good mechanic will insure the intake is clean and dry during re-assembly.

get a tune up,compression check etc

3.8L Engine

NOTE The 3.8L (VIN K) non-supercharged engine uses a two-piece intake manifold consisting of an upper air plenum which mounts the throttle body and a lower intake manifold assembly which houses the fuel injectors. The 3.8L (VIN 1) supercharged engine uses a similar lower intake manifold. The supercharger serves as the upper manifold.
Lower Manifold

NOTE Two bolts which fasten the lower intake manifold to the cylinder head are accessible only after the upper intake manifold is removed. These bolts are located in the right front and left rear corners of the lower intake manifold. Remove the upper intake manifold to service the lower intake.
The 3.8L (VIN 1) supercharged engine uses a manifold similar to the 3.8L (VIN K), but the supercharger serves as the upper manifold.

1. Disconnect the negative battery cable.
2. On 3.8L (VIN 1) engines, remove the supercharger assembly. Please see the procedure in this section.
3. Remove the upper intake manifold using the procedure above.
4. Remove the EGR outlet pipe from the intake manifold.
5. Detach the engine coolant temperature sensor wiring harness from the sensor.
6. Remove the lower intake manifold bolts and carefully lift off the manifold.

To install:


Fig. Lower intake manifold bolt torque sequence-3.8L (VIN K) engine

1. Clean all parts well with degreaser, especially the gasket seal surfaces on the upper and lower manifold pieces and between the lower manifold and the cylinder heads..
2. Install new gaskets to the cylinder heads and new seals to the engine block. Carefully lower the manifold into place. Apply thread-locking compound to the bolt threads. Make sure to install the two hidden bolts in the lower intake manifold. Hand start all bolts, then torque evenly to 11 ft. lbs. (15 Nm). Follow the toque sequence, starting in the center, then working outwards in a circle.
3. Install the remaining components in the reverse order of the removal process.

Upper Intake Manifold (Plenum)

1. Disconnect the negative battery cable.
2. On 3.8L (VIN 1) engines, remove the supercharger assembly. Please see the procedure in this section.
3. Tag for identification, then remove the right side spark plug wires.
4. Disconnect the ignition wires from the fuel rail.
5. Remove the fuel rail. Please see Section 5.
6. Remove the cable bracket from the intake manifold.
7. Remove the throttle body.

NOTE Two bolts which fasten the lower intake manifold to the cylinder head are accessible are accessible only after the upper intake manifold is removed. These bolts are located in the right front and left rear corners of the lower intake manifold.

1. Remove the upper intake manifold bolts and carefully lift the upper intake manifold from the engine.

Fig. Upper intake manifold and bolt torque sequence-3.8L (VIN K) engine
To install:

1. Clean all parts well with degreaser, especially the gasket seal surfaces on the upper and lower manifold pieces.
2. Install new gaskets to the cylinder heads and new seals to the engine block. Carefully lower the manifold into place. Make sure to install the two hidden bolts in the lower intake manifold. Hand start all bolts, then torque evenly to 89 inch lbs. (10 Nm). Follow the torque sequence, starting in the center, then working outwards in a circle.
3. Install the remaining components in the reverse order of the removal process.

did it make a loud whistling noise went it started 2 boost up

How long have you ran the engine ? I know that they will run rough and sometimes make strange noises for a few secounds or a little bit untill everything get oiled up again. I would make sure all the push rods are back in the right place and that all the lifters are tight.

Check and or replace your speed sensor if that does not work replace the instrument cluster - you can find one at almost any salvage yard.