Just pulled my exhaust pipes off and there is no gasket Between the manifold and the pipe, do all manifold to pipes require gaskets?

Look for a bad gasket where the pipe connects to the manifold. Could just be loose bolts or a bad gasket.

2.3L Engine

1. Disconnect the negative battery cable
2. Detach the oxygen sensor connector.
3. Properly drain the engine coolant into a suitable container. Remove the heater hose from the thermostat housing for more complete coolant drain.
4. Remove upper and lower exhaust manifold heat shields.
5. Remove the bolt that attaches the exhaust manifold brace to the manifold.
6. Break loose the manifold to exhaust pipe spring loaded bolts using a 13mm box wrench.
7. Raise and safely support the vehicle. NOTE: It is necessary to relieve the spring pressure from 1 bolt prior to removing the second bolt. If the spring pressure is not relieved, it will cause the exhaust pipe to twist and bind up the bolt as it is removed.
   
8. Remove the manifold to exhaust pipe bolts from the exhaust pipe flange as follows:
   1. Unscrew either bolt clockwise 4 turns.
   2. Remove the other bolt.
   3. Remove the first bolt.
9. Pull down and back on the exhaust pipe to disengage it from the exhaust manifold bolts.
10. Remove the radiator outlet pipe from the oil pan and transaxle. If equipped with a manual transaxle, remove the exhaust manifold brace. Leave the lower radiator hose attached and pull down on the outlet pipe to remove it from the water pump.
11. Carefully lower the vehicle.
12. Remove the exhaust manifold, seals and gaskets.
13. Loosen and reposition the rear engine mount and bracket for clearance, as required.
14. Remove the water pump mounting bolts and nuts. Remove the water pump and cover assembly, then separate the two pieces.

I’m happy to assist further over the phone at https://www.6ya.com/expert/jeremy_d728a59f986299fa

Check this procedure, according with the repair guide for...

2.3L and 2.4L Engines
(see Figure 9)

1. Disconnect the negative battery cable
   
2. Detach the oxygen sensor connector.
   
3. Properly drain the engine coolant into a suitable container. Remove the heater hose from the thermostat housing for more complete coolant drain.
   
4. Remove upper exhaust manifold heat shield.
   
5. Remove the bolt that attaches the exhaust manifold brace to the manifold.
   
6. Remove the lower exhaust manifold heat shield.
   
7. Break loose the manifold to exhaust pipe spring loaded bolts using a 13mm box wrench.
   
8. Raise and safely support the vehicle.
   

It is necessary to relieve the spring pressure from 1 bolt prior to removing the second bolt. If the spring pressure is not relieved, it will cause the exhaust pipe to twist and bind up the bolt as it is removed.

1. Unfasten the two radiator outlet pipe-to-water pump cover bolts.
   
2. Remove the manifold to exhaust pipe bolts from the exhaust pipe flange as follows:
   1. Unscrew either bolt clockwise 4 turns.
      
   2. Remove the other bolt.
      
   3. Remove the first bolt.
      

On the 2.4L engines, DO NOT rotate the flex coupling more than 4° or damage may occur.Pull down and back on the exhaust pipe to disengage it from the exhaust manifold bolts.

1. Remove the radiator outlet pipe from the oil pan and transaxle. If equipped with a manual transaxle, remove the exhaust manifold brace. Leave the lower radiator hose attached and pull down on the outlet pipe to remove it from the water pump. Leave the radiator outlet pipe hang.
   
2. Carefully lower the vehicle.
   
3. Unfasten the exhaust manifold-to-cylinder head retaining nuts, then remove the exhaust manifold, seals and gaskets.
   
4. For the 2.4L engine, remove the front timing chain cover and the chain tensioner. For details, please refer to the procedure located later in this section.
   
5. Unfasten the water pump-to-cylinder block bolts. Remove the water pump-to-timing chain housing nuts. Remove the water pump and cover mounting bolts and nuts. Remove the water pump and cover as an assembly, then separate the two pieces.

Fig. 9: Water pump and cover mounting - 2.3L and 2.4L engines (click over image for zoom)

To install:

1. Thoroughly clean and dry all mounting surfaces, bolts and bolt holes. Using a new gasket, install the water pump to the cover and tighten the bolts finger-tight.
   
2. Lubricate the splines of the water pump with clean grease and install the assembly to the engine using new gaskets. Install the mounting bolts and nuts finger-tight.
   
3. Lubricate the radiator outlet pipe O-ring with antifreeze and slid the pipe onto the water pump cover. Install the bolts finger-tight.
   
4. With all gaps closed, tighten the bolts, in the following sequence, to the proper values:
   1. Pump assembly-to-chain housing nuts-19 ft. lbs. (26 Nm).
      
   2. Pump cover-to-pump assembly-106 inch lbs. (12 Nm).
      
   3. Cover-to-block, bottom bolt first-19 ft. lbs. (26 Nm).
      
   4. Radiator outlet pipe assembly-to-pump cover-125 inch lbs. (14 Nm).
5. Using new gaskets, install the exhaust manifold. Make sure to following the tightening sequence and torque specifications given in the exhaust manifold procedure located in this section.
   
6. Raise and safely support the vehicle.
   
7. Index the exhaust manifold bolts into the exhaust pipe flange.
   
8. Connect the exhaust pipe to the manifold. Install the exhaust pipe flange bolts evenly and gradually to avoid binding. Turn the bolts in until fully seated.
   
9. Connect the radiator outlet pipe to the transaxle and oil pan. Install the exhaust manifold brace, if removed.
   
10. On the 2.4L engine, install the timing chain tensioner and front cover.
    
11. Install the lower heat shield.
    
12. Carefully lower the vehicle.
    
13. Fasten the bolt that attaches the exhaust manifold brace to the manifold.
    
14. Tighten the manifold-to-exhaust pipe nuts to specification.
    
15. Install the upper heat shield.
    
16. Attach the oxygen sensor connector.
    
17. Fill the radiator with coolant until it comes out the heater hose outlet at the thermostat housing. Then connect the heater hose. Leave the radiator cap off.
    
18. Connect the negative battery cable, then start the engine. Run the vehicle until the thermostat opens, fill the radiator and recovery tank to their proper levels, then turn the engine off.
    
19. Once the vehicle has cooled, recheck the coolant level.

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Front Pipe REMOVAL & INSTALLATION Pipe Without Catalytic Converter

1. Before removing any component of the exhaust system, always squirt a liquid rust dissolving agent onto the fasteners for ease of removal..
2. Raise and support the front of the vehicle on jackstands.
3. Remove the exhaust pipe-to-manifold nuts.
4. Support the catalytic converter, then disconnect the pipe from the converter. Remove the pipe.
5. Remove the exhaust gaskets. To install:
   
6. Be sure to use all new gaskets when required. Assemble the system, check the clearance and tighten the bolts to about 15-30 ft. lbs. (20-41 Nm). Fig. 1: Front exhaust pipe connection common on vehicles with the catalytic converter not included
   
   
7. All clamps should be tightened to about 18-25 ft. lbs. (24-34 Nm). Do overtighten the clamps. If pipes become dented they can leak.

Pipe With Catalytic Converter

1. Before removing any component of the exhaust system, always squirt a liquid rust dissolving agent onto the fasteners for ease of removal.
2. Raise and support the vehicle.
3. On some vehicles you will need to remove the brace from the transaxle.
4. Remove the exhaust pipe nuts from the manifold.
5. Support the catalytic converter pipe bolts from the rear of the converter.
6. Lower the catalytic converter/front pipe assembly from the vehicle. To install:
   
7. Be sure to use all new gaskets when required.
8. Attach the new catalytic converter/front pipe assembly to the vehicle. Tighten the pipe to manifold nuts to 20-40 ft. lbs. (27-55 Nm).
9. Attach the bolts at the rear of the catalytic converter/front pipe. Tighten them to 30-35 ft. lbs. (41-47 Nm).
10. Connect the brace at the transaxle if removed.
11. Lower the vehicle.

PLEASE DON'T FORGET TO RATE! THANKS! 2.3L & 2.4L Engines

1. Disconnect the negative battery cable
   
2. Detach the oxygen sensor connector.
   
3. Properly drain the engine coolant into a suitable container. Remove the heater hose from the thermostat housing for more complete coolant drain.
   
4. Remove upper exhaust manifold heat shield.
   
5. Remove the bolt that attaches the exhaust manifold brace to the manifold.
   
6. Remove the lower exhaust manifold heat shield.
   
7. Break loose the manifold to exhaust pipe spring loaded bolts using a 13mm box wrench.
   
8. Raise and safely support the vehicle.
   

It is necessary to relieve the spring pressure from 1 bolt prior to removing the second bolt. If the spring pressure is not relieved, it will cause the exhaust pipe to twist and bind up the bolt as it is removed.

1. Unfasten the two radiator outlet pipe-to-water pump cover bolts.
   
2. Remove the manifold to exhaust pipe bolts from the exhaust pipe flange as follows:
   
   1. Unscrew either bolt clockwise 4 turns.
      
   2. Remove the other bolt.
      
   3. Remove the first bolt.
      

On the 2.4L engines, DO NOT rotate the flex coupling more than 4° or damage may occur.

1. Pull down and back on the exhaust pipe to disengage it from the exhaust manifold bolts.
   
2. Remove the radiator outlet pipe from the oil pan and transaxle. If equipped with a manual transaxle, remove the exhaust manifold brace. Leave the lower radiator hose attached and pull down on the outlet pipe to remove it from the water pump. Leave the radiator outlet pipe hang.
   
3. Carefully lower the vehicle.
   
4. Unfasten the exhaust manifold-to-cylinder head retaining nuts, then remove the exhaust manifold, seals and gaskets.
   
5. For the 2.4L engine, remove the front timing chain cover and the chain tensioner. For details, please refer to the procedure located later in this section.
   
6. Unfasten the water pump-to-cylinder block bolts. Remove the water pump-to-timing chain housing nuts. Remove the water pump and cover mounting bolts and nuts. Remove the water pump and cover as an assembly, then separate the two pieces.
   

Water pump and cover mounting-2.3L and 2.4L engines

To install:

1. Thoroughly clean and dry all mounting surfaces, bolts and bolt holes. Using a new gasket, install the water pump to the cover and tighten the bolts finger-tight.
   
2. Lubricate the splines of the water pump with clean grease and install the assembly to the engine using new gaskets. Install the mounting bolts and nuts finger-tight.
   
3. Lubricate the radiator outlet pipe O-ring with antifreeze and slid the pipe onto the water pump cover. Install the bolts finger-tight.
   
4. With all gaps closed, tighten the bolts, in the following sequence, to the proper values:
   
   1. Pump assembly-to-chain housing nuts-19 ft. lbs. (26 Nm).
      
   2. Pump cover-to-pump assembly-106 inch lbs. (12 Nm).
      
   3. Cover-to-block, bottom bolt first-19 ft. lbs. (26 Nm).
      
   4. Radiator outlet pipe assembly-to-pump cover-125 inch lbs. (14 Nm).
      
5. Using new gaskets, install the exhaust manifold. Make sure to following the tightening sequence and torque specifications given in the exhaust manifold procedure located in this section.
   
6. Raise and safely support the vehicle.
   
7. Index the exhaust manifold bolts into the exhaust pipe flange.
   
8. Connect the exhaust pipe to the manifold. Install the exhaust pipe flange bolts evenly and gradually to avoid binding. Turn the bolts in until fully seated.
   
9. Connect the radiator outlet pipe to the transaxle and oil pan. Install the exhaust manifold brace, if removed.
   
10. On the 2.4L engine, install the timing chain tensioner and front cover.
    
11. Install the lower heat shield.
    
12. Carefully lower the vehicle.
    
13. Fasten the bolt that attaches the exhaust manifold brace to the manifold.
    
14. Tighten the manifold-to-exhaust pipe nuts to specification.
    
15. Install the upper heat shield.
    
16. Attach the oxygen sensor connector.
    
17. Fill the radiator with coolant until it comes out the heater hose outlet at the thermostat housing. Then connect the heater hose. Leave the radiator cap off.
    
18. Connect the negative battery cable, then start the engine. Run the vehicle until the thermostat opens, fill the radiator and recovery tank to their proper levels, then turn the engine off.
    
19. Once the vehicle has cooled, recheck the coolant level.
    

There are several areas to check for possible exhaust leaks. i will list them below.

1. Exhaust manifold gaskets - This will be the most common area. There is a gasket that separates the block from the exhaust manifold. these gaskets can fail, thus, causing a exhaust leak at the manifold.

2. Lower manifold gasket(donut gasket) - This will be a possible leak spot as well. This small circular gasket is placed in between the upper exhaust pie and the lower side of the manifold. It basically separates the pipe from the exhaust manifold.

3. Exhaust pipe - The entire pipe can rust overtime, thus, creating small holes along the pipe structure. These holes will cause the exhaust to leak ,thus, filling the vehicle will moderate fumes and causing louder than normal engine sounds during revving.

4. Muffler - This component can also deteriorate to the point that it cannot filter out the noise. It will also cause leaks as well.

You will find the leak in one or all of the areas stated above in the list.

start by removing nuts from exhaust pipe on the underside, then remove the 8 bolts that hold manifold to engine and pull it up. make sure when reinstalling you inspect the gasket and replace if needed. there is also a circular gasket between the exhaust manifold and the exhaust pipe, inspect and replace if needed. reverse for install.

2.4L Engine
See Figure 3

1. Disconnect the negative battery cable.
2. Drain and recycle the engine coolant.
3. Disconnect the oxygen sensor wiring.
4. Remove the upper exhaust manifold heat shield.
5. Raise and safely support the front end of the vehicle securely on jackstands.
6. Remove the exhaust manifold brace bolt.
7. Unbolt and remove the lower exhaust manifold heat shield.
8. Break loose the manifold-to-exhaust pipe spring loaded nuts.
9. Unbolt the radiator outlet pipe assembly from the water pump cover.
10. Disconnect the exhaust pipe from the manifold.
11. Pull down and back on the exhaust pipe to disengage it from the exhaust manifold bolts.

Do not rotate the flex coupling more than 4°. this could damage the component.

1. Separate the radiator outlet pipe from the oil pan and transaxle.
   1. On manual transaxle, remove the exhaust manifold brace.
   
   
2. Next, leave the lower radiator hose attached and pull down on the outlet pipe to detach it from the water pump. Allow the outlet pipe to hang.
3. Lower the vehicle.
4. Remove the exhaust manifold-to-cylinder head nuts.
5. Extract the manifold, seals and gaskets.
6. Remove the timing chain cover. Refer to the procedure later on in this section.
7. Remove the timing chain tensioner. Refer to Timing Chain removal later in this section.
8. Unbolt the water pump from the cylinder block.
9. Extract the pump assembly-to-timing chain housing nuts.
10. Remove the water pump and cover assembly.
11. Separate the pump from the cover.
12. Clean all mating surfaces of gaskets and sealant materials.

To install:
Prior to installing the water pump, read the entire procedure. Pay special attention to the tightening sequence to avoid component damage and ensure a proper seal.

1. Attach the water pump to the cover and hand-tighten the components.
2. Lubricate the splines of the water pump drive with chassis grease or equivalent, then install the pump and cover assembly. Hand tighten the cover-to-block bolts.
3. Install the pump-to-chain housing nuts and hand-tighten.
4. Lubricate the O-ring on the radiator outlet pipe with an anti-freeze solution and slide the pipe into the water pump cover, install and hand-tighten the bolts.
5. With the specified gaps, tighten in sequence as follows:
   1. Pump assembly-to-chain housing nuts
   2. Pump cover-to-pump assembly
   3. Cover-to-block, bottom bolt first
   4. Radiator outlet pipe assembly-to-cover
   
   
6. Install the exhaust manifold using new gaskets.
7. Install the manifold-to-cylinder head nuts, follow the tightening sequence and tighten to specifications.
8. Raise and support the front of the vehicle.
9. Index the exhaust manifold-to-exhaust pipe flange bolts.
   1. Turn bolt bolts in evenly to avoid cocking the exhaust pipe and binding the bolts. Turn the bolts in until they are fully seated.
   
   
10. Install and secure the radiator outlet pipe to the transaxle and oil pan.

On manual transaxles, install the exhaust manifold brace.

1. Install the timing chain tensioner and chain cover.
2. Attach the lower heat shield.
3. Lower the vehicle.
4. Attach the exhaust manifold brace. Tighten the manifold-to-exhaust pipe nuts to specification.
5. Attach and secure the upper heat shield.
6. Connect the oxygen sensor wiring.
7. Fill the radiator with coolant water mixture until it reaches the heater hose coolant outlet, then install the hose and continue to fill. This will assist with the complete cylinder block fill.
8. Connect the negative battery cable.
9. Start the engine and top off the cooling system and check for leaks.

Changing the oil pan gasket on a GMC Yukon XL is realtively simple, but does require removal of the exhaust pipe. You will need a new oil pan gasket and a new exhaust pipe flange gasket.
Put the car on ramps or jacks to access the undercarriage.
Remove the exhaust pipe by removing the 3 nuts holding the exhaust pipe flange to the exhaust manifold. These nuts often rust and prior application of penetrating oil may be required to remove them without damage. Once the pipe is unbolted, work the exhaust system loose from the hanging brackets. In most cases the entire system does not have to be removed since there is enough room to remove the pan with only the front hangers removed.
Drain the oil from the engine
Remove the 10 mm bolts holding the pan to the block.
Remove the old gasket material and scrape any remaining gasket from the block and pan.
Install the new gasket. Forma-Gasket or a similar product may be used in small quantities to "glue" the new gasket in place and prevent shifting while the pan is reattached to the block.
Reinsall the 10 mm bolts in a cross pattern to ensure the gasket is compressed evenly. Do not overtighten the bolts.
Replace the pipe to flange gasket between the exhaust pipe and the exhaust manifold with a new part.
Reattach the exhuast pipe to the hangers.
Install the 3 bolts connecting the pipe flange to the exhaust manifold.
Fill the engine with oil.

2.4L Engine

1. Disconnect the negative battery cable
   
2. Detach the Oxygen (O2) sensor electrical connector.
   
3. Properly drain the engine coolant into a suitable container. Remove the heater hose from the thermostat housing for a more complete coolant drain.
   
4. Remove upper exhaust manifold heat shield.
   
5. Remove the bolt that attaches the exhaust manifold brace to the manifold.
   
6. Remove the lower exhaust manifold heat shield.
   
7. Break the manifold-to-exhaust pipe spring loaded bolts loose using a 13mm box wrench.
   
8. Raise and safely support the vehicle.
   

NOTE It is necessary to relieve the spring pressure from 1 bolt prior to removing the second bolt. If the spring pressure is not relieved, it will cause the exhaust pipe to twist and bind up the bolt as it is removed.

1. Unfasten the two radiator outlet pipe-to-water pump cover bolts.
   
2. Remove the manifold to exhaust pipe bolts from the exhaust pipe flange as follows:
   
   1. Unscrew either bolt clockwise 4 turns.
      
   2. Remove the other bolt.
      
   3. Remove the first bolt.
      

NOTE DO NOT rotate the flex coupling more than 4° or damage may occur.

1. Pull down and back on the exhaust pipe to disengage it from the exhaust manifold bolts.
   
2. Remove the radiator outlet pipe from the oil pan and transaxle. Leave the lower radiator hose attached and pull down on the outlet pipe to remove it from the water pump. Leave the radiator outlet pipe hang.
   
3. Carefully lower the vehicle.
   
4. Unfasten the exhaust manifold-to-cylinder head retaining nuts, then remove the exhaust manifold, seals and gaskets.
   
5. Remove the front timing chain cover and the chain tensioner. For details, please refer to the procedure located later in this section.
   
6. Unfasten the water pump-to-cylinder block bolts. Remove the water pump-to-timing chain housing nuts. Remove the water pump and cover mounting bolts and nuts. Remove the water pump and cover as an assembly, then separate the two pieces.
   

Water pump and cover mounting—2.4L engine
Click to Enlarge To install:

1. Thoroughly clean and dry all mounting surfaces, bolts and bolt holes. Using a new gasket, install the water pump to the cover and tighten the bolts finger-tight.
   
2. Lubricate the splines of the water pump with clean grease and install the assembly to the engine using new gaskets. Install the mounting bolts and nuts finger-tight.
   
3. Lubricate the radiator outlet pipe O-ring with antifreeze and slide the pipe onto the water pump cover. Install the bolts finger-tight.
   
4. With all gaps closed, tighten the bolts, in the following sequence, to the proper values:
   
   1. Pump assembly-to-chain housing nuts:19 ft. lbs. (26 Nm).
      
   2. Pump cover-to-pump assembly: 106 inch lbs. (12 Nm).
      
   3. Cover-to-block, bottom bolt first: 19 ft. lbs. (26 Nm).
      
   4. Radiator outlet pipe assembly-to-pump cover: 125 inch lbs. (14 Nm).
      
5. Using new gaskets, install the exhaust manifold. Make sure to following the tightening sequence and torque specifications given in the exhaust manifold procedure located in this section.
   
6. Raise and safely support the vehicle.
   
7. Index the exhaust manifold bolts into the exhaust pipe flange.
   
8. Connect the exhaust pipe to the manifold. Install the exhaust pipe flange bolts evenly and gradually to avoid binding. Turn the bolts in until fully seated.
   
9. Connect the radiator outlet pipe to the transaxle and oil pan.
   
10. Install the timing chain tensioner and front cover.
    
11. Install the lower heat shield.
    
12. Carefully lower the vehicle.
    
13. Fasten the bolt that attaches the exhaust manifold brace to the manifold.
    
14. Tighten the manifold-to-exhaust pipe nuts to specification.
    
15. Install the upper heat shield.
    
16. Attach the oxygen sensor connector.
    
17. Fill the radiator with coolant until it comes out the heater hose outlet at the thermostat housing. Then connect the heater hose. Leave the radiator cap off.
    
18. Connect the negative battery cable, then start the engine. Run the vehicle until the thermostat opens, fill the radiator and recovery tank to their proper levels, then turn the engine OFF.
    
19. Once the vehicle has cooled, recheck the coolant level, then install the radiator cap.
    

3.1L Engine

1. Disconnect the negative battery cable.
   
2. Properly drain the cooling system into a suitable container.
   
3. Loosen, but do not remove, the water pump pulley bolts.
   
4. Remove the serpentine belt.
   
5. Unfasten the water pump pulley bolts, then remove the pulley.
   
6. Remove the five water pump mounting bolts, then remove the water pump.
   

To install:
View of the water pump mounting and bolt tightening specifications—3.1L engine shown
Click to Enlarge

1. Clean all the gasket surfaces completely.
   
2. Apply a thin bead of sealer around the outside edge of the water pump along the gasket sealing area, then install the gasket onto the water pump.
   
3. Position the water pump on the engine, then tighten the water pump mounting bolts to 89 inch lbs. (10 Nm).
   
4. Install the water pump pulley and finger-tighten the pulley bolts.
   
5. Install the serpentine belt.
   
6. Tighten the water pump pulley bolts to 18 ft. lbs. (25 Nm).
   
7. Fill the cooling system.
   
8. Connect the negative battery cable, then start the engine. Let the engine run until it reaches normal operating temperature, then check for leaks and coolant level. Add coolant, if necessary, then turn the engine OFF.