Running poorly @ low RPM after O2 sensor replacement

Background: I pulled off my cats to run delete pipes while I figured out whether to rebuild my rattling cats or replace them. Rattle is still there with cats out, so I guess the rattle's in one of my mufflers (mallet test indicates it's somewhere in the driver-side exhaust). So, I reinstalled the cats since they're apparently not part of the problem. For various reasons, I had to replace 3 of 4 oxygen sensors during this process. Issue: On 1999 Carrera 4 (e-gas), when the car is at idle at a stop light, it idles a little rough. It hunts around for a moment, then settles down. When I ease the clutch out in first gear, it runs rough, almost like it's missing on one cylinder, but then immediately perks back up and runs fine once it's above 1500 RPM. Power does not seem down, and there are no other symptoms other than rough running when accelerating from a stop in first gear, and even then, only for the first 1.5Kish RPM. After that, it's smooth. At low RPM in other gears, it's smooth. In just that one RPM band, it hiccups - sounds kind of like a motorcycle or lawn mower missing when it's cold. I disconnected the upstream cats and ran it for a bit - problem is still present. I checked for vacuum leaks and found nothing. I verified that the MAF is connected - it is. I checked all of the other connectors in the area of the O2 sensors and where the cables are routed to make sure I didn't disconnect/break anything. After reconnecting the 02 sensors, I disconnected the battery for a while to clear trims and trouble codes. Problem is still there. I will scan for codes this evening. ANY ideas on why the car is running rough @ idle and at low RPM? Could one of the new Bosch sensors (OEM, with the appropriate Porsche ends installed by Bosch) be bad and causing the issue? No other systems were touched while removing and replacing the cats. Thanks for any tips. I'm on the fence now whether to try different 02 sensors or take it to my indy, since I'm reluctant to keep throwing new parts at this. *EDIT* Turned out to be the coil pack on cylinder 4. Should have gone with my gut.

Hey Schnell, do you have any scanner that can show the O2 voltage reading? Sounds like an air leak to me. Have you checked the oil filler tube for leak?

Also, have you tried egas re-calibration?

It seemed like a vac leak to me, too - so I've gone over everything. But I'll go over it all again. Re: e-gas calibration: No, good thought.

If nothing helps, you can drive it more and wait till the CEL comes on (if it's going to come on) then you should have more clues. Even the CEL doesn't come on, at least you can check the fuel trims.

Also check your timing and cam chain tensioner readings. Bad o2 and bad cam timing chain tensioners give very similar symptoms

FYI I replaced the upstream O2 sensors on my 1999 C4 because I kept getting P1276 code. It lowered my idle from 900 to 800 RPMs for about 25 miles but has settled back to 900 RPMs and runs smooth and no CEL. I used the OEM sensors from Pelican parts.

How long have you been running it with your stock cats again? Have you given the ECU time to adjust adaptations back?

My guess is one of your ignition coils is slowlg failing. Had the same issue and after replacing the questionable coil with a good used one, the hunting idle and stumbling at take off went away.

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Reads like a falling VarioCam solenoid/actuator. The cam timing is changed at around 1500 RPMs from retarded to advanced. Check for pending codes. If you have the test equipment observe the short term fuel trims. When my Boxster suffered from VarioCam solenoid/actuator failure the failing bank's short term fuel trims were going crazy. Later at the dealer talking this over with the tech and he said that's to be expected. The DME isn't seeing the sensor readings it expects so will adjust fueling until it gets them or until it runs out of adjustment. Tech sometimes the engine will begin to misfire before the DME runs out of adjustment.

Another bit of info: the rough running results in uneven flow from the driver-side exhaust tip. From the smell (rich) and the interruption in exhaust, it seems to me that one cylinder isn't firing consistently at low rpm. This makes me think that Imo000 might be on to something. Coincidentally, the car was driven in heavy rain right around the time this all started, so I wonder if that plus the extra heat from the delete pipes pushed a coil pack on the passenger side of the motor over the edge. The car is due for plugs and packs anyway, so that might be the next step. Macster, would variocam interfere with a single cylinder firing? Ignition seem like a good place to look?

The passenger-side exhaust is nice and consistent, like you'd expect. So at least I'm down to maybe only having half a motor running poorly, which really doesn't feel appreciably better.

If it happened after a rain, def sounds like a coil. Do all 6 and replace plugs, spark plug tubes and o-rings while you're in there. If the plastic tubes look ok, you can get by just replacing the o-rings

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After re-reading your first post I do not see that you read the codes. You disconnected the battery to clear the codes (and fuel trims) though. You did not say the CEL was on so I believed the CEL was in fact not on. Thus I recalled the time my Boxster's engine suffered from a bad solenoid/actuator. The engine acted about the same and yet there was no CEL. There was a pending error code though and it pointed to the VarioCam hardware. You should have read the codes and even if the CEL was off checked for pending codes. I have to stress if you are going to diagnose behavior like your car is manifesting you should have an OBD2 code reader/data viewer in your tool box. We'll never know but if the codes are, make that were, misfire codes then sure one or more coils are suspect, obviously the ones indicated by the misfire codes. If you want to throw plugs and coils at the behavior then do it. Or not. (More on this below.) Since the plugs are due to be changed anyhow this is not really throwing money away. While you are there if the tech or you if you do your own work believe the coils need replacing then replace the coils at the same time. However, I'm a bit reluctant to recommend you go in and do plugs/coils. While they may be due they were not acting up before you were at the engine. Generally when some untoward behavior appears after someone having been at the engine I prefer to focus more on what was done. Could it have been done wrong? What possible collateral damage could have been done during this work? IOWs, last thing touched first thing suspected. Going off and doing the plugs/coils now is in some ways a distraction, a diversion from where the real source of the problem resides. If you go ahead with the plugs and coils, and you probably will, you must be particularly careful in doing this work. The last thing you want is to say make a sloppy connection of a coil to a plug or a coil to its wiring harness connector. Also, while you are at the engine look things over very carefully looking for something you might have done -- inadvertently -- the time before you were at the engine.

Thanks, Macster. I have been over and over the engine bay looking for anything I might have disturbed, a vac line I might have broken, a sensor I might have unplugged. I've been under and over the motor a dozen times looking for anything that looks wrong, or even looks like it might have been in the wrong place at the wrong time. The sum total of what I did was to remove the cats and replace them with delete pipes, and subsequently remove the delete pipes and replace them with cats. The downstream and upstream sensors are in the downstream and upstream holes; I tagged the connectors before pulling parts to ensure things went back in just the way they came out. I did the work, I was sober while I did the work, and I remember everything I did - and I just can't think of anything I did in the process that should have resulted in rough idle/poor acceleration from a stop. I have finished a tank of gas and replaced it with a full tank while this has been going on, so bad gas, though I don't think anyone threw it out there, is a possibility. I will be hanging out with the car and my Durametric this evening to see if any stored or pending codes exist, or if any appear in the meantime.

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While racking my brains last night for anything pertinent, I recalled that during my smog check about a month ago, when the guy was testing the car at low idle, the car was also hiccupping VERY slightly during idle...nowhere near the extent to which it seems to be missing now, but in retrospect, the pattern of missing seems very similar. So, at this point I'm trying to figure out if there's a pattern, or if I'm trying to shoehorn something I noticed into being a pattern.

Easy test is to spray down your car with garden hose and also spray the underside of the car and see if it is more pronounced.

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If the engine is truly suffering misfires then the CEL should be on and one or more misfire error codes should be present. If the engine is not running right -- though this may be attributed to or described as misfiring -- the root cause is probably something other than plugs or coils. And not all misfires are caused by bad plugs or coils. If the engine is not making scary noises, if the oil filter housing oil and filter element are free of metal bits and you feel comfortable running the engine then run it and check for pending codes. Or if the check engine light comes on read the associated codes and post for comment. If the engine runs poorly but there is no CEL and active or pending error codes then you will have to rely upon monitoring various engine telemetry in the hope you spot something that gives you a clue. The engine has two cylinder banks. If the problem is confined to one bank then you have a good bank and its sensor data and telemetry to refer to in the hope you will spot something significantly different on the other bank. For instance, fuel trim differences, O2 sensor readings/behavior, camshaft timing deviations if you get to it, and so on. The tendency is to try to rush in and do a fix right away. Sometimes, if the spur of the moment or WAG diagnosis was correct this delivers results, good results. Other times it can be as I touched upon before a distraction, a diversion. Analysis of the data you have available is at least to me the better way to approach this. That you have assured yourself that your previous excursions into the engine bay have not resulted in something that could account for this behavior then strongly points to the problem being from something else. What this could be has a number of possibilities. Since you replaced the O2 sensors they are suspect. (Why did you replace just 3, why not all 4?) Hopefully you did not touch the tips, you did not use any thread lube, the sensors are the correct sensors, and so on. Since you were at the exhaust hopefully there are no exhaust leaks. This may have been covered but I missed it, but how about the AOS? While the problem/symptoms at any rate, appear confined to one bank it may still be a common failure to both banks, it is just one bank is more sensitive to the problem than the other bank. (I know of one case where a bad MAF was causing misfires on just one bank.) So, further away from the engine proper what about fuel supply and pressure? Has the fuel filter been changed recently? Does the air intake show any signs of water incursion? Does the MAF look clean? While a properly functioning stock filter system should keep this free of any dirt I'm thinking maybe the air filter setup has been compromised? Or if you have added an aftermarket air filter system.

Update: Durametric says misfire on cylinder 4. Having a conversation with my indy - I'll either replace the coil pack and plug on #4 or see what he comes back with. Stay tuned...

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Personally I would replace all 6 instead of one. It's not going to be too long until others go bad.(plugs, coils, spark plug tubes)

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+1

Update: I should have stuck with my gut - it was just the coil pack on cylinder 4. All six coil packs and plugs are in, and the car is running perfectly.

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I hope you replaced your spark plug tube o-rings at least...

Glad to hear it's something simple!

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That's good news. Glad you solved it. Oh, and to heck with your gut. The error code was the key. Always, read the codes first. Would have saved yourself some time/trouble if you had.

By "gut" - I mean that sometimes, a misfire is just a bad coil pack or plug. That's what I proposed earlier in this thread, and it was met with extensive supposition about possible alternative causes. That's awesome, and helpful, but sometimes, if it quacks like a duck and it looks like a duck...