Cat Bypass Questions
 

Hi All,

I recently replaced my cat with a Dansk bypass (test pipe). I saw an immediate gain in horsepower, however I'm billowing exhaust at stop lights. From a cold start, no noticeable exhaust. But when at a stop light, it's a steady stream of exhaust.

Questions:
Is this normal?
Is this a sign my rings are worn?
Should I replace my O2 sensor to counteract the issue?
Would a Steve Wong chip correct the behavior?
If I put my cat back in, is it gonna just be clogged up given the amount of exhaust? Or did the test pipe cause the increase and it should cease if I put the cat back in?

Thanks for any help.

1987 Porsche 911 with 190K.

what's your oil consumption like (if a quart or more per 1k miles, you might just be seeing the oil consumption)? Has your engine's top end/valve guides been rebuilt? A 911 Carrera engine's more likely to smoke due to (prematurely) worn valve guides than bad rings. A healthy engine shouldn't smoke/produce excessive exhaust, European market 911s of your vintage didn't come with cats. That said, the catalytic converter does a great job of cleaning up an engine's exhaust and, if you didn't see smoke/dirty exhaust before swapping out the cat for the test pipe, suspect things would return to "normal" once you swapped the cat back in. Doubtful that switching chips will have any real effect on this issue, similarly, doubtful that replacing the O2 sensor would make any real difference, especially if you're not having drivability issues that would point to a failing O2 sensor.

How long have you run it after the change? Maybe some build up in the muffler getting burnt off.
BTW How does it sound now with the Dansk and any drone

Did you remove or unplug the O2 sensor? If so, then the ECU is expecting a signal it's not getting, and I will guess that it's running too rich.

What color is the exhaust smoke? Dark would be running too rich, bluish white would be oil.

I tend to agree with Pete, straight pipe runs way cooler than the CAT and you may be really rich. At least a decent starting point to isolate the issue.

@Tinkerer You may be on the right track. My mechanic buddy asked me to drive it on the highway for 45 min to burn off anything that may have collected in the exhaust system.

It sounds a bit more guttural. There's a definite increase in horsepower.

@PeteKz - smoke is white. I screwed the existing O2 sensor into the provided inlet on the Dansk.

@darrin - I think you're correct in that the heads haven't been rebuilt. I'm leaning toward the guide seals being worn. I'm gonna do a wet and dry compression test to see if the rings/cylinders are the culprit but guides are the most likely culprit. Will let you know.

Drive it around some more. Give it an "Italian tuneup." If it still smokes white, then start planning for a top end overhaul. You could try a new O2 sensor, and if it's the original, it's overdue for replacement, so I would do that in any case, but would not expect that to have a lot of effect.

Also, get a leak down test or at least a compression test to check the health of the rings.

190Kmi is about time for valve guides, and you got service out of it so far. Whether you want to split the case and do a full rebuild is up to you. From what I've seen of 3.0 and 3.2 cases, the lower end doesn't wear much. The pro's here will counsel doing a full rebuild, but it's always easy to spend other people's money! ;)
In fairness to them, they are dealing with customers and they want happy customers, no oil leaks, and no come-backs, so it's in their interest to recommend a full rebuild, rather than a top end only.

Will you do it yourself, or have a shop do it? If DIY, you can inspect the pistons and cylinders to see if they are within specs. If you like wrenching on your own car, then your time is free, whereas shop time is expensive.

@frogsickle what’s the verdict?

@Tinkerer - I've discovered it's eating oil. I was 1.5 quarts low. I'm putting the cat back in then pursuing next steps. Has to be rings or valve seals... Will check back in once I find the culprit.

@frogsickle assuming that after putting the cat back the smoke stopped. I have a Dansk test pipe that I’ve been planning to add but can’t get my baked-on factory cat off without destroying it which I don’t want to deal with so it’s going to the Indy. I’ll check oil consumption before making any changes now.

My mech won't install a premuffler/cat bypass. Cannot blame him as he does not want to get fined. Instead I have him replacing all of the exhaust HW and the oil return tubes.

@190K, it's well over the mileage where things need attention. Not surprised it smokes a bit.

you have to have CAT in your state? strange, i thought only CA has the law like that? Then again, you have so many 911 why don't you put it in yourself?
Ivan

Ivan, that is the plan. Since they had the engine out and car on the lift had them do the tubes and hardware.