Description & Operation
The catalytic converter, mounted in the exhaust system, is a muffler-shaped device containing a ceramic honeycomb shaped material coated with alumina and impregnated with catalytically active precious metals such as platinum, palladium and rhodium.
The catalyst's job is to reduce air pollutants by oxidizing hydrocarbons (HC) and carbon monoxide (CO). Catalysts containing palladium and rhodium also oxidize nitrous oxides (NOx).
On some trucks, the catalyst is also fed by the secondary air system, via a small supply tube in the side of the catalyst.
No maintenance is possible on the converter, other than keeping the heat shield clear of flammable debris, such as leaves and twigs.
Other than external damage, the only significant damage possible to a converter is through the use of leaded gasoline, or by way of a too rich fuel/air mixture. Both of these problems will ruin the converter through contamination of the catalyst and will eventually plug the converter causing loss of power and engine performance.
When this occurs, the catalyst must be replaced. For catalyst replacement, see the Exhaust System procedures under "ENGINE AND ENGINE OVERHAUL."
When working on the exhaust system always keep the following in mind:
For a number of reasons, exhaust system work can be among the most dangerous type of work you can do on your truck. Always observe the following precautions:
Removal & Installation
A number of special exhaust system tools can be rented from auto supply houses or local stores that rent special equipment. A common one is a tail pipe expander, designed to enable you to join pipes of identical diameter
System components may be welded or clamped together. The system consists of a head pipe, catalytic converter, intermediate pipe, muffler and tail pipe, in that order from the engine to the back of the truck. Some extensions may be fitted between major components to compensate for variations in wheelbase, transmissions, or other chassis obstructions.
The head pipe is bolted to the exhaust manifold, on one end, and the catalytic converter on the other. Various hangers suspend the system from the floor pan. When assembling exhaust system parts, the relative clearances around all system parts are extremely critical. Observe all clearances during assembly.
In the event that the system is welded, the various parts will have to be cut apart for removal. In these cases, the cut parts may not be reused. To cut the parts, a hacksaw is the best choice. An oxy-acetylene cutting torch may be faster, but the sparks are DANGEROUS near the fuel tank, and, at the very least, accidents could happen, resulting in damage to other under-truck parts, not to mention yourself!
The following replacement steps relate to clamped parts:
- Raise and support the truck on jackstands. It's much easier on you if you can get the truck up on four jackstands. Some pipes need lots of clearance for removal and installation
- Remove the nuts from the U-bolts. Don't be surprised if the U-bolts break while removing the nuts. Age and rust account for this. Besides, you shouldn't reuse old U-bolts. When unbolting the header pipe from the exhaust manifold, make sure that the bolts are free before trying to remove them. If you snap a stud in the exhaust manifold, the stud will have to be removed with a bolt extractor, which often necessitates the removal of the manifold itself.
- After the clamps are removed from the joints, first twist the parts at the joints to break loose rust and scale, then pull the components apart with a twisting motion. If the parts twist freely but won't pull apart, check the joint. The clamp may have been installed so tightly that it has caused a slight crushing of the joint. In this event, the best thing to do is secure a chisel designed for the purpose and, using the chisel and a hammer, peel back the female pipe end until the parts are freed.
- Once the parts are freed, check the condition of the pipes that you had intended keeping. If their condition is at all in doubt, replace them too. You went to a lot of work to get one or more components out. You don't want to have to go through that again in the near future. If you are retaining a pipe, check the pipe end. If a clamp crushed it, it can be restored to its original diameter using a pipe expander.
Check the condition of the exhaust system hangers. If ANY deterioration is noted, replace them
Use only parts designed for your truck. Don't use "universal" parts or flex pipes. "Universal" parts rarely fit like originals and flex pipes don't last very long.
- When installing the new parts, coat the pipe ends with high temperature lubricant. It makes fitting the parts much easier. It's also a good idea to assemble all the parts in position before clamping them. This will ensure a good fit, detect any problems and allow you to check all clearances between the parts and surrounding frame and floor members.
- When you are satisfied with all fits and clearances, install the clamps. The header pipe-to-manifold nuts should be torqued to 20 ft. lbs. (27 Nm). If the studs were rusty, wire-brush them clean, and spray them with a lubricant. This will ensure a proper torque reading. Position the clamps on the slip points. The slits in the female pipe ends should be under the U-bolt, not under the clamp. Tighten the U-bolt nuts securely, without crushing the pipe. The pipe fit should be tight, so that you can't swivel the pipe by hand. Don't forget: always use new clamps.
- When the system is tight, recheck all clearances. Start the engine and check the joints for leaks. A leak can be felt by hand.
- If any leaks are detected, tighten the clamp until the leak stops. If the pipe starts to deform before the leak stops, reposition the clamp and tighten it. If that still doesn't stop the leak, it may be that you don't have enough overlap on the pipe fit. Shut off the engine and try pushing the pipe together further. Be careful; the pipe gets hot quickly.
- When everything is tight and secure, lower the truck and take it for a road test. Make sure there are no unusual sounds or vibration. Most new pipes are coated with a preservative, so the system will be pretty smelly for a day or two while the coating burns off.