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Old 07-13-2011, 05:39 PM   #1
DDubbs
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Default cat pipes same on '96 thru '98?

I have a '96 throwing a check engine light with code for underperforming cat. Car runs great, gets regular highway mileage but reduced city mileage. Mileage was reduced for about 6,000 miles before the check engine light came on. My mechanic asked $3000 grand for a new cat and replacing the 02 sensors. That's not going to happen.

So I got a lead from another member here on a used '98 cat. I didn't realize it's an entire section of pipe and not just the cat until I looked it up on realoem. Here's the link for the part, and now for the question:

The diagram shows two different part numbers for the cat. Can I use a 98 pipe on a 96 car or are the different in some fundamental way? I'm also up for anyone with some experience or information on why the car has run great for the last 4,000 miles with the CEL lit and a supposedly bad cat.

Thanks!
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Old 07-13-2011, 06:52 PM   #2
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It will bolt right up. I'm not sure what is different between 1996 and 1998 cats, but it must be internal to the cat and most likely the later one works better. If it were me, I would go for it.
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Old 07-15-2011, 05:54 AM   #3
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Thanks for the answer. That's what I thought too. Glad to hear it. The part is a good many miles away with Bill Proud in TN. I'm considering a drive to get it. Hate to find it the wrong part after going all that way.

Anybody care to comment on why the car runs fine with a 'bad' cat? I'm curious what long term negatives it could have on the rest of the car.

Thanks!
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Old 07-15-2011, 12:06 PM   #4
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I have had issues with the cat CEL (P0420). Someone correct me if i'm wrong, but the code is thrown when the 02 sensors don't read the value they are expecting. So the culprit must be the cat?

Well, I just changed out my coolant temperature sensor (the one thing I forgot to do when I rebuilt the cooling system last year ) and that turned off my P0420 code. I guess the temperature the cats were expecting was getting skewed by the bad temp sensor.

Anyway, P0420 is a pita CEL and it can be the cat, 02 sensors, or any number of things apparently.
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Old 07-15-2011, 05:00 PM   #5
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No kidding? The temp sensor? Wow, I would have never guessed that. My coolant pipes ruptured while my wife was driving home a few years back, before the CEL came on. THe shop replaced all the rubber but I don't think the changed out the sensor. I wonder if it got cooked and went bad later? My gut says no since the CEL came on about a nine months later after many thousands of miles.

Man, so I could replace the cat and still have a bad cat? My mind has been officially blown.

How did you know your sensor was bad? Car overheat but the needle didn't move on the gauge?
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Old 07-18-2011, 03:35 PM   #6
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I didn't know it was bad, I was reading a thread and someone mentioned that the sensor can get cooked and that they should be replaced when you do a cooling system rebuild.

Yeah, P0420 code sucks.
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Old 07-18-2011, 04:13 PM   #7
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You can check the coolant sensor faily easily.
If you have the Bentley manual there is a chart which shows the resistance value vs temperature. First you check it cold, then at operating temp. This sensor is located under the front intake runner. You can reach this without disassembling anything.
The section in Bentley that deals with this is 130-15.
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Old 07-19-2011, 06:55 PM   #8
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The coolant sensor would throw off the A/F mixture calcs, and then would mess up the O2 sensor readings.

The OBD1 cats will bolt up also, you would have to add a bung after the cat for the rear O2 sensor though
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Old 07-21-2011, 08:14 PM   #9
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Dave, thanks for the tip. I think it's time invest in a Bentley manual so I can start doing something about these issues.

A new wrinkle:

I've been getting an exhaust groan at 2500 rpms which is new. I was at Southampton Imports getting a 2nd hand a/c condenser and asked the mechanic there about it. He says the cat dampens a lot more sound than people give it credit for. If mine's making noise and is throwing the code maybe the cat itself is going bad after all.

Anybody on the board replace their cat and notice a difference in acceleration or mileage? Or is this just a PITA maintenance item with no noticeable benefit?
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Old 07-21-2011, 10:33 PM   #10
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I have a nice vibration at 2500rpm, lasts till ~2800rpm. I also have an aftermarket cat, no noticeable gains, just more noise. I would like to get a stock cat again.
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Old 07-21-2011, 11:37 PM   #11
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You should see how horrible they are with no guts in the cat.....Lots of sound cancelling going on in there.


If it's bad or clogged, there will be a huge gain in power and drivability
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Old 07-23-2011, 09:23 PM   #12
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I put a magnaflow 94305 in to replace my cat and it throws no CEL.

http://www.amazon.com/Magnaflow-9430.../dp/B000COTISY
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Old 07-24-2011, 12:11 AM   #13
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Any weird vibrations from it?

I was looking at those magnaflow spun metallic cats the 59955, http://www.amazon.com/Magnaflow-5995...1462616&sr=1-1. It seemed like an interesting design without the heat shields that could possibly vibrate. I just wonder how loud it would be.
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Old 07-24-2011, 05:09 AM   #14
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No weird vibrations. Check the exhaust clips thread: http://318ti.org/forum/showthread.php?t=9665&page=7

Those were all recorded with the Magnaflow cat on.
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