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Old 10-18-2012, 06:21 AM   #1
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Rotten Cat on a 2002 S

So as some of you know, 2002 S (52K miles) has been down for a while getting a clutch, IMSB retro, tranny, bypass pipes etc..

The genesis of this project was a reoccurring CEL on bank one for the catalyst. When we pulled the headers, the Cat was rotted and there was debris scattered down stream through out the exhaust system.

So my question has to do with the ECU and how it performs in my scenario.

With a bad catalyst and the associated CEL How does the ECU alter it's management of the engine? The car ran fairly well, but was a bit of a gas hog (avg just under 16mpg with my heavy foot). While the car felt reasonably strong, acceleration was not in keeping with it's power to weight ratio.

I compare it to my 98 GS400 with 210K miles on the clock. The GS has a slight power/weight disadvantage, but it still pulls much stronger and turns better mileage.

Can I expect the car to run better when I get it back with the primary cats performing properly?

Last edited by shadrach74; 10-18-2012 at 11:51 AM.
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Old 10-23-2012, 02:59 PM   #2
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The boxster S's output is unusual imo. I too have a Boxster S 2002 and find the power delivery progressive and smooth. It's certainly no slouch but i find you can feel where Porsche have backed off power to keep the Boxster S treading on the toes of the carrera. Power from 1,000 - 4,000rpm is reasonably flat, increasing in power mildly and a hike in power is felt at around 4,500rpm. You can feel the power starting to flatten out at around 6,500rpm and it feels pretty pointless reving beyond say 6,900rpm as you'll get moving quicker by shifting up again.

The onboard computer is reading an average of around 27mpg so there is certainly a vast improvement to be had from your car gas mileage wise. Unsure if it will be slightly different given I'm from the UK and my Boxster S has just 2 cat's as oppose to your 4 cats (fitted to the exhaust header too if I recall correctly?).

At 52K, you seem to be having serious work carried out... Gearbox rebuild? Mines covered 80K now and the gearbox is still unopened from factory. What were the troubles you had with it?

Hope you get it sorted.

Last edited by gazrawly; 10-23-2012 at 03:05 PM.
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Old 10-23-2012, 03:20 PM   #3
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Gearbox issue was an external issue (cracked mount boss).
My car will only have 2 cats when I get it back!

The power delivery you describe is the same in my car with the exception that I feel little to no "mild increase" @ 4000 or "hike" at 4500rpm. It's pretty much flat across the rev range. I agree that little is gained by reving past 6500.

I will be purchasing a durametric soon to see what I can find on my own.

Thanks for the well wishes.

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Old 10-23-2012, 03:26 PM   #4
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When you get the durametric, just log the MAF sensor readings at wide open throttle. I did have an issue with my car a while back with a faulty MAF sensor which led to the MAF effectively seeing 75% or so of true air flow and so was down on power. It actually performed better with the MAF sensor disconnected. I did notice a decent increase in power once I changed the sensor
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Old 10-23-2012, 06:32 PM   #5
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How the ECU reacts depends on which CEL (error) that it is detecting. My best guess is that a bad cat would not clean up any unburned hydrocarbons in the exhaust gas and thus, the downstream O2 sensor would then detect a slight rich condition. If this was the case, then the ECU would lean out the mixture. As the rich condition is continued to be reported, the ECU would continue to lean the mixture to try and correct it. At some point, the engine would likely start to knock due to running lean (you might not ever notice this low level of knocking) and then the ECU would retard the spark timing in order to avoid damaging the engine from knocking. Retarding the timing will typically reduce the power of the engine.

When you repair the cat, then the O2 sensor will report correctly and the ECU will correct the timing and mixture setttings as you drive, thereby restoring normal engine operation and power.

Alternatively, you can disconnect the battery which will erase the learned settings in the ECU and then the ECU will start up using the default settings and make adjustments based on valid sensor feedback as you drive.
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Old 10-24-2012, 05:34 PM   #6
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Picked up car...llittle to no difference in performance after replacing cats and cleaning the blockage from exhaust. Time to get a durametric.
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Old 10-24-2012, 05:50 PM   #7
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Def get the Durametric, great tool! Also, it will take a few mikes and start/stop cycles for the ECU to "relearn".
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