ca993twin

Now that I've gotten myself all excited about installing cat bypass pipes that can quickly be changed back to a stock cat system, I've learned that I will surely induce the dreaded "check-engine" light. This is a problem for me because:
1). I barely have my wife's permission to do this (she didn't exactly say no), and she will hate the extra noise anyway,
2). A constant check-engine light (or disabling the light) will mean I will never see a new problem that may arise, and
3). I must be an idiot to think I need more HP anyway.

But it sure sounds like there are some real nice gains to be had with the cats removed. Is the check-engine light issue real? Is there a way around it? Help me, Rhonda.

ZCAT3

Steve - what type of pipes are you installing? Will they still have the ports form threading the O2 sensors into the air stream?

ca993twin

I've been considering both the Fabspeed kit or just having a muffler shop make up a set of pipes with the necessary flanges so that the cats can be swapped in and out. I never noticed the sensor ports. Are they close enough to the cats that they'd have to be duplicated with the bypass pipes? This would complicate the muffer shop method (assuming that the Fabspeed pipe has the sensor ports). Argh. This is getting complicated.

krish

Steve,

I think if you retained all the o2 sensors you shouldn't have the check engine light coming on. I'm planning to get mine removed over the weekend (plus a hip tuck and a nose job... sorry different forum). I'm just going to get a muffler shop to do a set of 'test' pipes for me and go with that. Will be glad to report my procedure and results once done if you need me to. <img border="0" alt="[jumper]" title="" src="graemlins/jumper.gif" />

cheers,
Kris

'97 993 TT
'89 964 C4
'89 BMW 535I
'89 Nissan 300 ZXTT

ca993twin

Kris,

Yes, please by all means report back to this thread or send me email (DOHC4v@aol.com) with your experience. I believe there are 4 sensors, 2 for each bank. On each bank there is an "upstream" sensor, before the cat, and a "downstream" sensor after the cat. I haven't actually located these on my car... going by what I've been told. Its also my understanding that there must be a difference in the O2 levels before and after the cats, or you get the dreaded light. So, what if the "downstream" sensors were just left dangling in the open air? Would that fool the computer?

PorschePhD

Steve,
Simply pull the secondary O2 sensor up out of the cat. In most cases this will take care of the light. In the Gamballa unit they may also disable the light. The FVD units do that.

ca993twin

Stephen,

So this is beginning to sound like a non-issue. If my Gemballa ECU does not disable the CEL from the aftercat sensor, then I can leave that sensor dangling in the breeze, and that will take care of it? That would make life pretty simple. I'm sure I can find a safe place to tie those sensors. Yes?

viperbob

Steve,

I have the Fabspeed cat bypass kit ready to go on. Spoke to Joe at Fabspeed this morning about the kit and the O2 sensors. ODB2 cars have an upstream (of the cat) and a downstream to compare the readings. If they are the same, the system assumes no cat and will throw a Check Engine light. Fortunately, the 993TT actually learns after a while that this is OK. So while you may need to reset the light a few times, eventually it will no longer come on. Again according to Joe, the 996TT will throw the light forever (Sorry Stephen but Porsche removed this learning process), or you can replace the downstream O2 sensor with the dummy ones (ala Corvette).

Won't have the kit on for a few weeks until I get the car ready for the Unichip installation. See, yet another reason to get that ODB2 software.....

Got for it

PorschePhD

Bob,
Unfortunately I know all to well. I previously owned a 540HP 993TT. We do in-house computers on all Porsches. The 996TT setup as well as the 993 can benefit with bypasses, however 100 cell versions work well and keep the computer happy and emissions legal. The other option is to use an 02 sensor sim. They make them for many of the common cars such as Vette, Beemers etc. I know of a gent that is playing with the 996TT trying to get one to work. The 993TT should be easier to trick.
Bob....Why uni-Chip? <img border="0" alt="[jumper]" title="" src="graemlins/jumper.gif" /> Not to try to change your mind, but within 48 hours we can set it up to fit the bill. Just wondered. <img border="0" alt="[burnout]" title="" src="graemlins/burnout.gif" />

krish

Hi Steve,

Sorry for the long turnaround. My muffler guy could only schedule me in last Saturday to get the cats out!

Firstly, she runs great and I mean fantastic without the cats. The difference is subtle yet VERY noticeable. Response is so much better it's unbelievable! Both off boost and on boost! Boost comes in much earlier and on lighter throttle applications. I did sense some lag before with the cats with the power coming on strongly when the turbos spooled up. Thats gone now with a just a seamless progression of power! Exhaust note has gone deeper (but not louder) and sounds much better (more purposeful!)

Best news, NO CEL! Drove her hard over the weekend on both freeways (Took her up to 290 km/hr at one empty stretch!) and twisties. CEL never came on. I have retained all four of my O2 sensors (both pre and post cats).

You have to do this... One word though, my car's a Euro spec model. Was just wondering if that might make a difference with the CEL. Any advice from the Gurus?

My PSS-9s will be going in on Wednesday, can't wait!

cheers,
Kris
'97 993 TT Cats out!
'89 964 C4 Autothority, MAF, H&R suspension.
'89 BMW 535I Split Second MAF, Racing Dynamics springs, roll bars
'89 Nissan 300ZXTT. TO-3 Turbos, Upgraded brakes 400 Hp.

cobra06

These O2 sims may work according to the manufacurer.
<a href="http://www.casperselectronics.com/cgi-local/store/commerce.cgi?search_request_button=yes&keywords=104051" target="_blank">Casper Electronics</a>

John D II

Steve,

You can also simply remove the bulb to prevent the CEL from visually appearing. As others have mentioned, the car will run much better with the cat bypass pipes.

ca993twin

Thanks everyone for the input. I'm sold. I'll either have my muffler shop make up the cat bypass with flanges, so I can pop the cats back if need be (~$400), or I'll get the Fabspeed kit ($450 + ~$200 labor) which has the nifty quick disconnects. The cat bypass should be worth ~30HP, plus faster spool-up (as you've mentioned). Sounds too good to pass up. If I get the CEL, I can try to reset it a few times, and if that doesn't work, I can try those O2 sim gizmos. I really don't want bto disable the CEL entirely... real problems may go unnoticed.

Thanks again for all the help.

Anir

Steve,

Please update us after you do this. I have been considering nixing the cats for some time, but also do not want to deal with a constant CEL.

Joe S

My understanding from my mechanic and others is that the CEL on our cars warns of emissions system malfunctions and little else and that the real indicators of big trouble are the other warning lights (e.g. oil pressure), the traditional guages like temp and oil pressure and the good old seat-of-the-pants meter. My CEL bulb has been out for about a year now. I know there are various horror stories to the effect that an activated CEL indicated some catastrophic engine ailment or another (I recall a post where it was diagnosed as bad compression, etc.). If one of the gurus on this board has an alternative view to mine, it would be very helpful to hear it. If not, I'll continue to believe that the CEL is far more trouble than it is worth. Thanks.