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Well, like the title reads ... I replaced the heads on a Dodge Ram V8 5.9L gas engine. It's now running like the cat is plugged. Can blowing the white plume of coolant exhaust out the tailpipe cause the cat to plug?

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According to this website, the cat can surely be plugged by coolant entering the catalytic converter. Here are the reasons given on the website:

  • Engine Tune-Up Required.

A number of problems could occur to the catalytic converter as the result of an engine that is out of tune. Any time an engine is operating outside proper specifications, unnecessary wear and damage may be caused to the the catalytic converter as well as the engine itself. The damage is often the result of an incorrect air/fuel mixture, incorrect timing, or misfiring spark plugs. Any of these conditions could lead to a catalytic converter failure or worse.

  • Excess Fuel Entering Exhaust

The fuel that powers your vehicle is meant to burn in the combustion chamber only. Any fuel that leaves the combustion chamber unburned will enter the exhaust system and light-off when it reaches the catalytic converter. This can super-heat the converter far above normal operating conditions and cause a Melt Down. Possible causes are an incorrect fuel mixture, incorrect timing, corroded spark plugs, a faulty oxygen sensor, sticking float, faulty fuel injector or a malfunctioning check valve.

  • Oil or Antifreeze Entering Exhaust.

Oil or Antifreeze entering the exhaust system can block the air passages by creating a heavy carbon soot that coats the ceramic catalyst. These heavy Carbon Deposits create two problems. First, the carbon deposits prevent the catalytic converter from reducing harmful emission in the exhaust flow. And second, the carbon deposits clog the pores in the ceramic catalyst and block exhaust flow, increasing backpressure and causing heat and exhaust to back up into the engine compartment. Your engine may actually draw burnt exhaust gasses back into the combustion chamber and dilute the efficiency of the next burn cycle. The result is a loss of power and overheated engine components. Possible causes are worn piston rings, faulty valve seals, failed gaskets or warped engine components.

  • Deteriorated Spark Plug or Spark Plug Wires.

Spark plugs that don't fire or misfire cause unburned fuel to enter the exhaust system. The unburned fuel ignites inside the converter and could result in a partial or complete melt down of the ceramic catalyst. Spark plugs and spark plug wires should be checked regularly and replaced if damaged or if wires are worn or cracked.

  • Oxygen Sensor Not Functioning Properly.

An oxygen sensor failure can lead to incorrect readings of exhaust gasses. The faulty sensor can cause a too rich or too lean condition. Too rich and the catalyst can melt down. Too lean and the converter is unable to convert the hydrocarbons into safe elements and may not pass a state inspection.

  • Road Damage or Broken Hangers.

The ceramic catalyst inside a catalytic converter is made from a lightweight, thin-walled, fragile material. It is protected by a dense, insulating mat. This mat holds the catalyst in place and provides moderate protection against damage. However, rock or road debris s triking the converter or improper or broken exhaust system support can cause a Catalyst Fracture. Once the ceramic catalyst is fractured, the broken pieces become loose and rattle around and break up into smaller pieces. Flow is interrupted and backpressure in the exhaust system increases. This leads to heat build up and loss of power. Possible causes of a catalyst fracture are road debris striking the converter, loose or broken hangers, potholes or off-road driving.

In the case of the original question, the actual problem was that the plugged cat had caused the problem of the blown head gasket, not the other way around.

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    "plugged cat had caused the problem of the blown head gasket" - wow. I didn't realize that that was even possible! – Bob Cross May 30 '14 at 16:44
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    @BobCross ... I found out after the fact, from the driver's description, that the cat had been plugged for a while, but he continued to drive the truck harder and harder until the head cracked due to over pressure. This happened over time. I mean, he just kept pushing the throttle trying to make the truck go faster not even worrying about the why it was happening or if something needed to be fixed. – Pᴀᴜʟsᴛᴇʀ2 May 30 '14 at 16:50

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