I had a dual exhaust system installed on a 82 351W. The driver side leaks, I did not have the oem flanges for the exhaust pipe to manifold. How can I stop this from leaking?

Check this for 1996-1999 FORD 3.0L (182) V6 DOHC...
- Install the exhaust manifold with a new exhaust manifold gasket.
- Install the exhaust manifold retaining studs and tighten to 13-16 ft. lbs. (18-22 Nm) in the sequence illustrated.
Raise and safely support the vehicle.
Position the Y-pipe assembly using a new flange gasket and install all the retaining nuts and bolts loosely.
Starting at the front of the system tighten the Y-pipe to exhaust manifold nuts to 26-34 ft. lbs. (34-46 Nm).

- Tighten the converter to transaxle nut and bolt to 30 ft. lbs. (40.3 Nm). Tighten the converter outlet bolts to 26-34 ft. lbs. (34-46 Nm).



Fig. Right side exhaust manifold mounting bolt tightening sequence-3.0L DOHC engine (click image for zoom)




Fig. Left side exhaust manifold mounting bolt tightening sequence-3.0L DOHC engine (click over image for zoom)

Hope this helps (remember to rating this answer).

Removal & Installation 2.2L Engine To Remove:

1. Remove or disconnect the following:
   • Negative battery cable.
   • Heat shield.
   • Oxygen sensor.
2. Raise and safely support the vehicle.
3. Remove exhaust pipe from the exhaust manifold.
4. Lower the vehicle.
5. Remove the exhaust manifold mounting nuts from the cylinder head.
   
   
6. Remove the exhaust manifold.
7. Clean the mating surfaces.
8. Check for cracks, broken flanges and gasket surface damage.
9. Use a straight edge and a feeler gauge. If the flanges or the mating surfaces do not align, the manifold is warped and should be replaced.

To Install:
NOTE: The manufacturer recommends using new exhaust system nuts.

1. Install the manifold gasket.
2. Position the exhaust manifold to the cylinder head. Exhaust manifold 2.2L
   
   
3. Install the new exhaust manifold nuts. Tighten the nuts starting from the center and working toward the ends to 110 in lbs (12.5 Nm).
4. Raise and safely support the vehicle.
5. Install the exhaust pipe to the manifold using a new flex coupler gasket.
6. Install new exhaust flange mounting nuts.
   • Tighten to 33 ft lbs (45 Nm).
7. Lower the vehicle.
8. Install the oxygen sensor.
9. Install the heat shield.
10. Connect the negative battery cable.

2.4L Engine To Remove:

1. Disconnect the negative battery cable
2. Disconnect the Oxygen (O2) sensor electrical connector.
3. Remove the (O2) sensor.
4. Raise and safely support the vehicle. Exhaust brace 2.4L
   
   
5. Remove the exhaust manifold brace-to-manifold bolt and the oil pan nuts, if necessary. Exhaust heat shield 2.4L
   
   
6. Remove the upper heat shield. NOTE: Do NOT bend the exhaust flex decoupler more than necessary to remove it. Excessive movement will damage the flex decoupler
   Exhaust flex decoupler
   
   
7. Remove the manifold-to-exhaust flex decoupler fasteners.
8. Remove the exhaust pipe.
9. Carefully lower the vehicle.
10. Remove the exhaust manifold-to-cylinder head mounting nuts/bolts. Remove the exhaust manifold.
11. Discard the gaskets and/or seals. Clean the mating surfaces.

To Install:
Exhaust manifold 2.4L

1. Install new exhaust manifold gaskets. Position the exhaust manifold on the cylinder head. Tighten the mounting bolts in sequence to 110 in lbs (12.5 Nm).
2. Raise and safely support the vehicle.
3. Install the exhaust manifold heat shield. Tighten the mounting bolts to 10 ft. lbs (14 Nm).
4. Install the exhaust manifold brace-to-manifold bolt and the oil pan nuts. Tighten the mounting bolts to 41 ft lbs (56 Nm) and the nuts to 18 ft lbs (24 Nm).
5. Position the exhaust pipe to the manifold. Install the manifold-to-flex decoupler mounting bolts. Tighten to 26 ft lbs (35 Nm).
6. Lower the vehicle.
7. Coat the threads with a suitable anti-seize compound and install the O2 sensor.
8. Connect the O2 sensor electrical connector.
9. Connect the battery ground cable and check for leaks.

Right-Hand
Removal

1. Remove spark plug heat shield (12A406).
   

1. Remove distributor to spark plug wires (12286) using Spark Plug Wire Remover T74P-6666-A.
   
1. Disconnect muffler inlet pipe.
   
1. If equipped with secondary air injection, disconnect the secondary air injection check valve tube (9B480).
   
1. Remove attaching bolts and right exhaust manifold (9430).
   
1. Inspect joint faces for signs fo exhaust leaks, cracks or any defect that would make exhaust manifold unfit for use
   

Left-Hand Removal






1. Remove oil level indicator tube (6754).

1. Remove spark plug heat shield.
   

1. Remove distributor to spark plug wires using Spark Plug Wire Remover T74P-6666-A.
   

1. Disconnect EGR valve to exhaust manifold tube (9D477). If equipped with air injection, disconnect the secondary air injection check valve tube.
   

1. Disconnect muffler inlet pipe.
   

1. Remove attaching bolts and exhaust manifold (9431).
   

1. Inspect joint faces for signs of exhaust leaks, cracks or any defect that would make the exhaust manifold unfit for use.
   

Installation

1. Clean mating surfaces of right exhaust manifold or left exhaust manifold and cylinder head (6049). Clean mounting flange of right exhaust manifold or left exhaust manifold and inlet pipe(s). Clean mounting surface for air injection tube or right exhaust manifold. Apply light film of Premium Long-Life Grease XG-1-C or -K or equivalent meeting Ford specification ESA-M1C75-B to right exhaust manifold or left exhaust manifold.
   

1. Position right exhaust manifold or left exhaust manifold on cylinder head. Install attaching bolts and washers, starting at fourth bolt hole from front of each right exhaust manifold or left exhaust manifold. Tighten bolts to 34-47 Nm (25-35 lb-ft), working from center right to both ends.
   
1. Position inlet pipes to right exhaust manifold or left exhaust manifold. Install attaching nuts and tighten to 34-47 Nm (25-35 lb-ft).
   
1. Position the secondary air injection check valve tube and the secondary air injection tube gasket (9C435) at rear of right-hand exhaust manifold. Install attaching bolts and tighten to 19-25 Nm (14-18 lb-ft).
   
1. Attach the secondary air injection manifold tube (9B449) to left-hand exhaust manifold fitting (located under EGR tube). Tighten nut to 30-40 Nm (22-30 lb-ft).
   
1. Attach the secondary air injection check valve tube to rear of right-hand cylinder head. Tighten bolts to 75-85 Nm (56-62 lb-ft).
   
1. Connect EGR valve to exhaust manifold tube to left exhaust manifold, if removed. Tighten nut to 53-72 Nm (39-53 lb-ft).
   
1. Install spark plug heat shield and distributor to spark plug wires.
   
1. Install oil level indicator tube to left exhaust manifold, if removed. Tighten nut to 13-24 Nm (10-18 lb-ft).
   
2. Start engine (6007) and check for exhaust leaks.
   

Hope this helps you...WF †

There is one on the exhaust manifold ( if it's a six cylinder there is one on each manifold). Then there is one on or right after the catylitic converter.

Possible carbon build up need to clean system depends on your mileage and condition of your engine watch also for possible bit of overheating during testing look for vacuun 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.
   

Have you checked the flex pipe section under the engine, and the gasket between the manifold and cat/flex pipe section? Those are two places they usually start to leak from either a blown gasket, or the flex section rusts out under the protective covering.

Removal Procedure
1. Disconnect the negative battery cable.
2. Drain the engine oil from the engine crankcase.
3. Remove the exhaust flex pipe retaining nuts from
the exhaust manifold.
4. Remove the exhaust flex pipe retaining nuts from
the catalytic converter or the connecting pipe.
5. Remove the exhaust flex pipe.
6. Remove the oil pan flange–to–transaxle retaining
bolts.
7. Remove the oil pan retaining bolts.
8. Remove the oil pan from the engine block.
Cleaning Procedure
1. Clean the oil pan sealing surface.
2. Clean the engine block sealing surface.
3. Clean the oil pan retaining bolts.
4. Clean the oil pan attaching bolt holes in the engine
block.
5. Clean the oil pan scraper.
Installation Procedure
1. Coat the new oil pan gasket with sealant.
Important : Install the oil pan within 5 minutes after applying
the liquid gasket to the oil pan.
2. Install the oil pan to the engine block.
3. Install the oil pan retaining bolts.
Tighten
Tighten the oil pan retaining bolts to 10 NSm (89 lb–in).
4. Install the oil pan flange–to–transaxle retaining
bolts.
Tighten
Tighten the oil pan flange–to–transaxle bolts to 40
NSm (30 lb–ft).
5. Install the exhaust flex pipe.
6. Install the exhaust flex pipe retaining nuts to the
exhaust manifold.
Tighten
Tighten the exhaust flex pipe–to–exhaust manifold
retaining nuts to 35 NSm (26 lb–ft).
7. Install the exhaust flex pipe retaining nuts to the
catalytic converter or the connecting pipe.
Tighten
Tighten the exhaust flex pipe–to–catalytic converter
or connecting pipe retaining nuts to 35 NSm (26 lb–ft).
8. Connect the negative battery cable.
9. Install the oil pan drain plug.
Tighten
Tighten the oil pan drain plug to 35 NSm (26 lb–ft).
10. Refill the engine crankcase with engine oil.