Cat Converter Over Temp Protection

[Biggsy]

I got a question for some of you knowledgable guys out there.

I mate of mine recently had an issue with a brand new Cat Converter that had all its 'guts' crumble and block the exhaust. When he took it back to the exhaust place to get it changed, they said Cats are very sensitive to the gasses run through them and could fail due to some of the following reasons:

1- running too lean
2- running too rich
3- Lead contaminates

I have heard of this before.

Now, in our LS1 systems, we have this over temp protection, it will richen the A/F to help cool the cats.

Does this not create a problem to try to solve another?

If you richen the mixture, there will be an incomplete burn of fuel causing the leftover fuel to go through the cats which are not good for them???

[joecar]

Overly rich causes the cats to burn the excess fuel causing the cats to glow bright orange/red...
they will get internally damaged if this condition is prolonged (say more than a few days worth of driving).

[Redline Motorsports]

You'd think with GM's overly rich commanded AFR's in PE mode, more converters don't burn up!

[joecar]

You'd think with GM's overly rich commanded AFR's in PE mode, more converters don't burn up!GM's PE is fairly rich and does heat up the cats pretty hot, but it's nowhere near what happens if you have 1 or 2 cylinders not firing.

GM's idea is to make the cats last just a little longer than their warranty... :|

[stigmundfreud]

how do high flow cats cope? Are they able to handle the abuse a little more?

[joecar]

The hiflow cats (metallic substrate) are more robust than the stock cats (ceramic substrate)...
so I would imagine they could handle more abuse.

[stigmundfreud]

here's hoping!

[dfe1]

Actually, "robustness" doesn't have much to do with converter failures. What kills converters are high levels of hydrocarbons in the exhaust stream. Under normal circumstances, converter core temperatures run between 1000 and 1200 degrees (F). When an engine runs extremely rich, a fair amount of unburned fuel enters the exhaust system and when it hits the alread hot converter core, internal temperatures can exceed 2200 degrees, at which point you have converter melt-down.

Converter over-temp protection utilizes a relatively rich mixture to lower EGTs, and consequently cool the converter down a bit. As long as combustion remains efficient the strategy works. However, in cases of modified engines, if actual air/fuel ratios are very far off of commanded, the strategy can cause more problems than it cures (if the mixture is so rich that it causes unburned fuel to exit the combustion chambers).

The best way to put converter operation into perspective is to understand that a catalytic converter doesn't "do" anything-- it causes a reaction to occur. The intensity of those reactions is strictly a result of pollutant levels in the exhaust stream. If combustion inside the enigne was 100% efficient and there were no pollutants remaining after combustion, converter core temperature would not be any higher that exhaust gas temperature.

One of the reasons that engines with long duration cams frequently have converter problems is that the excessive overlap allows a lot of unburned fuel to enter the exhaust stream.

Hope this helps.

[hrodgmc]

FWIW, cats are covered in the US for 80,000 by govt. regulation.

[Redline Motorsports]

DFE,

Nice explaination. Seems to me that after a "good" performancec tune, COT is not really going to be an issue. The way GM sets up there WOT fueling it sounds like they cause more of the issue then us guys that modify the tunes!

[joecar]

Interesting TSB regarding cats...


Info - Misfire DTCs P0300, P1380, P1381 and Catalytic Converter Damage Due to Installation of Alarm Systems #02-06-05-004A - (Jan 9, 2004)

Misfire DTCs P0300, P1380, P1381 and Catalytic Converter Damage Due to Installation of Alarm Systems

2004 and Prior GM Passenger Cars and Light Duty Trucks
2003-2004 HUMMER H2
2004 and Prior Isuzu Light Duty Trucks

This bulletin is being revised to add the 2004 model year and include specific DTCs in the title line. Please discard Corporate Bulletin Number 02-06-05-004 (Section 06 - Engine/Propulsion System).
General Motors Engineering, in an effort to determine the root cause of catalytic converter damage, has determined that aftermarket alarm systems incorrectly installed in vehicles have the potential to cause misfire codes and damage to the converter. These alarm systems use a circuit interrupt which utilizes the ignition circuit on the vehicles.
These alarm systems utilize mechanical relays and normal vehicle movement can trigger these relays to engage and disengage the ignition circuit while the vehicle is in motion. These disruptions of the ignition circuit, which occur in milliseconds, may cause more fuel to be commanded. Over time, this dumping of fuel on and off again can cause misfire codes and ultimately damage the converter assembly.

Important

Engineering could not identify any alarms that utilize solid state circuitry that would eliminate this concern. Because of this, it has been determined that all alarm systems must be routed through the starter circuit in order to avoid this condition.

Dealers must be aware of this issue and take note of the wiring on vehicles with alarm systems that come in for repair, particularly for catalytic converter damage that seem to have no known root cause.


© Copyright General Motors Corporation. All Rights Reserved.

[Biggsy]

Interesting TSB regarding cats...
I find it interesting that they narrow it down to only a few models!