exhaust temp

joe_mnNovember 5, 2005

I got a check engine lite last week. the code refers to cat conv. efficency. I did a search online for info about cats and it seems the exhaust gas should rise at least 100F as it passes thru the cat. I used an infrared thermometer and measured the inlet/outlet of my cat. it is 380F in and 280F out. that does not seem right. I talked to a muffler/tuneup shop and he said cat temps have nothing to do with how the cat is functioning. or put another way, he said i was over analyzing the situation. my friends car measured 380F in and 500F out. but the mechanic said that was a different brand car and all cars run different. I still think I need a cat. but i also think the temps are valid and an indication of a cat problem.

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What yr make model car and how many miles might help someone answer your question... but i agree with your advisors on 20 degrees.

    Bookmark   November 5, 2005 at 1:32PM
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First if your setting a catalyst efficiency code, it's quite likely that the convertor is bad with noteable exceptions. It's also quite likely that the convertor is fine, and you actually need a software update to change the way the convertor is tested.

A dead cat will definately not cause a rise in temperature as the exhaust goes through it. Unfortunately there are other reasons that this could occur as well.

One of which is how do you know the cat needed to be working under the conditions that you measured the temperatures? How do you know if the pipe you measured the outlet temperature is not in fact double walled and insulated?

Years ago we were taught to take convertor temperatures as a diagnostic tool. It only took a little more heavy use of this technique to find out it didn't always give us the right answer. In otherwords, we would replace convertors and not fix the car. (Thats a bad thing....)

Some of the best minds got together and started devising ways to allow us to test more accurately to see whether a convertor is functional or not. One of the things they figured out is a convertor has to be able to store oxygen in order for it to work. Using a four gas analyzer, a technician can litterally watch the convertor as it lights. The O2 content of the exhaust will drop from 1.5 to 2% down to .4 or lower depending on the convertor and how hard it's working. The other indicator is that while the O2 reading drops, the CO, and HC also drop and CO2 will rise and exceed 15% in most cases. In the past we also would do a check where with a known hot catalyst, we would disable ignition, and while supplying extra fuel if necessary, crank the engine and read the CO2 output. Since there was no combustion in the engine with the ignition system disabeled, the only place combustion (burning of the fuel) was taking place was in the convertor. Readings exceeding 10% CO2 were judged as a good convertor, and basically below that suspect.

But here lies the hard part today. Todays cars, especially PZEVS, ULEVS, SULEVS all have different thresholds for convertor efficiency and what they are allowed to deteriorate to before a code seets and the light comes on. PZEVS and SULEVS are set to light the MIL at 90% efficiency. (Approximately anyway, the exact spec might be a little higher or lower, but just suffice it to say it's pretty high) We have no way to test a convertor that is working at 90% efficiency other than to rely on the car itself. From there we have to rely on the code, check carefully for outside interfeerences, such as a sluggish or shifted O2 sensor, or a leaking exhaust (which will allow extra air in and have an impact on the actual air to fuel ratio entering the engine). Then it's onto TSB's and checking for software updates (which are not all referenced in the TSB's)

    Bookmark   November 5, 2005 at 4:25PM
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its a 96 dodge intrepid with the 3.5L motor. it has 170k miles and the O2 sensors are original. it also has the original plug wires. the car seems to run fine. i know it has a pre-cat on each manifold and than a dual inlet/single outlet cat. the check engine lite went off after several days. i do notice a rattle/buzz under the car when i accelerate. it has been doing this for several yrs. its just that lately the car seems a little sluggish and the code 72 sorta suprised me.

    Bookmark   November 5, 2005 at 5:05PM
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Thanks for the additional information. Now we are ready to take the next step in the diagnostics/repair. First code 72, otherwise known as P0432 in OBDII is the RH catalyst efficiency. You say you have a rattle? Is it inside the RH catalyst shell? If it is, then of course that will need to be dealt with. If not then your going to have to go through all of the steps to confirm what the trouble is.

If you follow the trouble tree, you'll notice the first thing it has you do is clear the codes and road test to run the vehicles monitor. If the code does not reset, your done at that point. From your description, that the lamp has gone back out, it is apparent that the convertor has been tested by the on board computer, and it has passed the catalyst monitor two times! The trouble tree has you check for exhaust leaks. It has you checking for oil consumption concerns, (you would know this by how much you have to add between oil changes, provided you don't have any leaks)

Following the chart further you will see that it asks if the down stream sensor has been replaced, and if the upstream has not. This is the little piece of information I hinted at last time when it comes to the O2 sensors and the computers ability to test the convertors. A sluggish or slightly shifted sensor might not be detected by the computer as faulty, and that can affect the results of the catalyst monitor. The trouble tree chart finishes with the statement, "IF" the upstream O2 sensor has been replaced in the last 5000 miles, replace the convertor! That of course means every step prior to that one has not revealed a problem. But I see a problem with your last response, you said " it also has the original plug wires. the car seems to run fine." And " its just that lately the car seems a little sluggish".

Don't worry people do that all of the time, give a shop/tech contradicting information. How can it seem to run fine, and seem a little sluggish? :) I've talked a number of years on this board about things like this, it does impact a shops ability to solve a customers vehicle problem efficiently, but does not prevent it in most circumstances.

Lastly the repair. If this still has the original wires, then I don't care how old the plugs are, this needs plugs and wires before you replace the convertor, if it's actually bad. A missfire can destroy a convertor in a relatively short amount of time. So if you replace the convertor, you need to protect it by making sure the engine is operating correctly all of the time. If you do reach the point of replacing the convertor, have you realized what else you should do? Replace all four O2 sensors! This will ensure that the fuel ratio's are controlled correctly and allow the convertor to operate exactly as designed. Lastly, and this is the really hard part. All of these parts should be replaced with O.E. parts from the dealer. IMO the aftermarket stuff simply does not work good enough.

    Bookmark   November 6, 2005 at 12:44PM
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