Random misfire/stumble

Gents, 2001 boxster S 72000 miles. Had for 2 1/2 years. Has a random HARD stumble at slightlly open throttle. Feels similar to light switch being turned on and off. Not a bog or soft fuel starve but like engine shut off then flicked back on. Cannot be duplicated readily, but only happens when throttle is open a bit. Will do it at 80 with cruise on or at 30. Anyone seen this before? Thanks! P.s. no CEL or codes.

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mkd628
Gents, 2001 boxster S 72000 miles. Had for 2 1/2 years. Has a random HARD stumble at slightlly open throttle. Feels similar to light switch being turned on and off. Not a bog or soft fuel starve but like engine shut off then flicked back on. Cannot be duplicated readily, but only happens when throttle is open a bit. Will do it at 80 with cruise on or at 30. Anyone seen this before? Thanks! P.s. no CEL or codes.

Air filter? Has the AOS ever been replaced?

How is the car used? Lots of short around trips? Does it ever see any sustained high speed (legal highway/freeway high speed) use?

What brand of gasoline you use? Premium octane I hope. Top tier? Have you tried switching brands?

Normally for some mild running issues with no error codes I'd advise a Techron treatment, one or possibly two treatments followed by an oil/filter service. The Techron really cleans up the fuel system and removes engine deposits. But a hard stumble like you describe it a bit past or more severe than a mild running issue.

If you want to go to the trouble get a scan tool one that can log sensor data and derived values and see if you can recreate the behavior when logging then do the same thing again logging without the symptom occurring. Compare the data. To make it as easy as you can try to duplicate the speed, gear, rpms, engine temp, etc as closely as you can.

With no error codes and a hard to duplicate symptom that's about all I have.

Grant

gee-lenahan-at-gee-mail-dot-com

but it does not mean misfires are happening.

IOWs, where there is no misfire error code there is no misfire.

The OP said in his initial post the behavior felt like a stumble or hesitation and these are not the same thing as a misfire that triggers misfire error codes.

I recall from my Boxster's first AOS failure before the diagnosis was made what had me looking for something amiss was the hesitation the engine had developed, quite uncharacteristic hesitation I might add which made it all the more obvious and frankly worrisome. But not one misfire error code. In fact no error codes at all.

So we have a stumble, or a hesitation, or a flat spot....

- tranny?
- air metering (MAF)?
- variocam oddity?

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

supply more details, more specifics as to when the behavior appeared.

Absent any error codes, absent any other symptoms, right now it could be any number of things... though I think VarioCam is probably not at the top of the list. (But given more info I reserve the right to change my mind.) I kind of sort of favor an AOS but this is not with a lot of confidence. It is just the best suspect for the moment given the info we have to go on.

...i never had any symptoms except the occasional smoke bomb at the track, followed by.......nothing.

Your theory must be something around much more oil than mine was leaking, and fouling occasionally...?

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

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grant
...i never had any symptoms except the occasional smoke bomb at the track, followed by.......nothing.

Your theory must be something around much more oil than mine was leaking, and fouling occasionally...?

Grant

momentarily upsetting the air/fuel mixture with a load of unmetered air. I do not believe oil is involved. I recall from both of my AOS failures smoking was an symptom that appeared last.

For both AOS failures the smoking appeared but it was a late appearing symptom. Prior to the AOS trouble the engine didn't seem to smoke any more or less upon startup. In fact, smoking on startup was then and is even today a rather rare event for my Boxster as early on with the car I considered what could lead to this and sought to do, within reason, to keep it from happening.

In the case of the first AOS failure it was a mild (but obvious) hesitation/stumble off idle that had me at first thinking the engine was just getting tired at around 80K miles which given my lack of Porsche engine experience seemed plausible though disappointing. Thankfully it proved to be the AOS and not a tired engine.

In the case of the second AOS failure the CEL with some error codes, which I do not now recall what they were but which had me thinking MAF and not AOS, was the first sign something was up. I recall the CEL would come on at the end of a long day's driving. One time in particular when leaving I-40 in Albuquerque. I had taken the off ramp and was stopped at a stop light when the engine gave a bit of a stumble -- nothing really severe but so out of character that I really noticed it -- concurrent with the CEL coming on. This CEL with some stumble happened several times on my drive 'cross country and each time it was about the same as the other times.

Smoking constantly or on startup is a sign that the AOS is pretty wasted.

But very, very early - and sometimes on a good AoS even, you can get a rop or two through under high G forces (only on track) and a total bomb comes out the tailpipe. When it first happened to me the folks behind me were sure my motor had let go. Ran just fine, once my heart restarted.

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

Car service history is always so important when buying a new car

It sounds like something I had last year. 2003 Boxster S, about 60,000 miles at the time. Would be running fine, cruising highway at 50/60/70/80, and then I'd get a slight hestiation/buck and then back to normal. The CEL would flash briefly but nothing else. Was very intermittent. Felt like a misfire. Had Maurice check the codes at BRBS but nothing came up.

Tried replacing the spark plugs (they were due) but that didn't help. In the end, pretty sure it was the ignition coils. Was kind of ticked that neither the Indie nor dealer had ever heard/seen this and didn't look into it, especially when they were in there doing the plugs. I think someone on this board said to try. Car has been running smooth ever since.

You had a CEL/MIL flash. The OP did not have any codes or MIL/CEL

So this is a very different circumstance....

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

Wow thanks for all the feedback.. I live in MN so the car is used about 5 months. Bulk of driving is short 15 minute trips. We take it on a few 300-1500 trips. Service history is spotty. It runs lime a champ aside from the issue. Hoppy44 gave an excellent description. Hesitation/buck. Happens 10-100 MPH as long as throttle is open slight and fuel demand is low. Mine foes not blink the CEL. The only code i get from time to time is P0507 idle air control system RPM higher than expected. I only use premium gas, which in this crappy state means 91 and always buy BP non-oxygenated when possible. Never smokes at all, uses no oil. I use a K&N filter, desnorkled and has Pedros replacement air intake tube. None of these have had any effect on the issue. I will probably take Hoppy44 advice and do ignition coils.

Hope it works out for you mkd. Like Grant said, our situations are somewhat different in that you don't get a CEL, but it sounded close enough that I would post it. Same driving pattern for me... I live in NYC and use the car April-November, but other than BRBS only get to use it a few times a month at best. Garaged in pretty cold temps in Boston Nov-March. Premium gas. Ran great other than that issue.

This board and the Boxster community has helped me enjoy my car so much, that I'm always looking to give back. It was so frustrating to ask two dealers and an indie and this board and extensive google searching and be like, REALLY?!? NO ONE has ever heard of this? Not a soul? Dealer sees hundreds of cars a year (a month?) and the indie claims 47 years working on Porsches, and somehow I'm the ONLY one to ever get these symptoms? Every time that CEL would flash I'd get a mini IMS heart attack.

I'm no mechanic and definitely no guarantee that the coils will correct your problem, but if you've tried everything else, figured it's worth a shot.

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mkd628
Wow thanks for all the feedback.. I live in MN so the car is used about 5 months. Bulk of driving is short 15 minute trips. We take it on a few 300-1500 trips. Service history is spotty. It runs lime a champ aside from the issue. Hoppy44 gave an excellent description. Hesitation/buck. Happens 10-100 MPH as long as throttle is open slight and fuel demand is low. Mine foes not blink the CEL. The only code i get from time to time is P0507 idle air control system RPM higher than expected. I only use premium gas, which in this crappy state means 91 and always buy BP non-oxygenated when possible. Never smokes at all, uses no oil. I use a K&N filter, desnorkled and has Pedros replacement air intake tube. None of these have had any effect on the issue. I will probably take Hoppy44 advice and do ignition coils.

As much as I detest the blame that so often gets heaped upon dirty MAFs, which is unjustified, in this case, it might be justified. It might be time to think about cleaning the MAF...

Also, not sure what you mean by de-snorkled?

And while I have the greatest respect for Pedro I'm always leery of any changes to the shape of the air box and intake.

What can happen is this upsets the laminar air flow that the MAF needs and this non-laminar air flow can cause these kind of inexplicable problems.

Also, that P0507 error code suggests to me that at idle the MAF is not getting laminar flow and this results in DME adjusting idle speed up and encountering an error threshold.

Given the car gets used only 5 months out of the year it probably doesn't see any snow/ice and road salt -- unless it is your winter beater -- so the coils are probably ok. But you can do the ignition coils if you want, of course.

Honestly, my first choice would be to revert to the stock air intake and carefully treat the MAF to a thorough cleaning.

Grant

gee-lenahan-at-gee-mail-dot-com

I guarantee you that if the intake tube was modified per my instructions that is not the issue.
There are hundreds, if not thousands of cars out there including mine that have that mod and have been de-snorked without any hesitation issues.
My guess would be an air leak in the intake which happens intermittently.
Check all of the intake thoroughly including clamps, rubber sleeves and make sure that the rubber gasket between the throttle body and the intake tee is installed.
Modification to the shape of the intake tube between the MAF and the TB will NOT alter the air flow at the sensor.
Happy Boxstering
Pedro

Pedro Bonilla
1998 Boxster 986 - 311,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)

---

Racecar spelled backwards is Racecar

"Racing is life. Anything before or after is just waiting" ... Steve McQueen as Michael Delaney in "LeMans"

"If you wait, all that happens is that you get older"... Mario Andretti

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Pedro (Weston, FL)
My guess would be an air leak in the intake which happens intermittently.
Check all of the intake thoroughly including clamps, rubber sleeves and make sure that the rubber gasket between the throttle body and the intake tee is installed.
Modification to the shape of the intake tube between the MAF and the TB will NOT alter the air flow at the sensor.
Happy Boxstering
Pedro

That's exactly what I was thinking, that it's one of the rubber sleeves maybe with a crack, based on my experience. See my Vacuum Leak post below.

I see that Stefan feels differently, but i'm not truly convinced.

I also dont see hwo the airbox changes would cause this, assuming it was done well, doesn't leak, etc.

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

If they are not oiled perfectly, they can deliver oil to the MAF and cause metering issues.

I wont use such a filter - any risk is too much since they do so little.

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

Remember K&N did that experiment where they submerged a MAF in K&N filter oil and installed it in a car and within seconds the reading was correct. And they guarantee it. It's not impossible, just very unlikely.

Grant

gee-lenahan-at-gee-mail-dot-com

Within seconds, yes, but what about after days, weeks, etc?

in oil? My recollection is the active surface of the MAF is located in the middle of an upwardly curved passageway that in the event the MAF was submerged vertically in a container of oil this would result in an air pocket forming and thus preventing oil from contaminating the hot film surface.

Regardless there are other possible explanations to the behavior of the OP's car not just the possibility of a dirty MAF.

Update: I am a bad poster. I have the k & n at the house, not installed yet. Has factory filter right now. I cleaned the MAF, still bucks. I believe it has a vacuum leak, as it has an idle fluctuation between 800-1000 rpm that is constant. Taking it this week to have vacuum checked. Not sure if/why small vacuum leak would cause bucking.

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mkd628
Update: I am a bad poster. I have the k & n at the house, not installed yet. Has factory filter right now. I cleaned the MAF, still bucks. I believe it has a vacuum leak, as it has an idle fluctuation between 800-1000 rpm that is constant. Taking it this week to have vacuum checked. Not sure if/why small vacuum leak would cause bucking.

an intake air leak to the top of the list.

As for the bucking the air leak could be intermittent with a short cycle time. IOWs the leak is not constant but varies rather quickly. And this variability accounts for the bucking. Maybe. First eliminate the air leak.

it could also be from a malfunctioning throttle body. Maybe. The TB is a vital system and well protected by redundancy. So I'm reluctant to envision a scenario where it could act up without any error codes. Well, not entirely without error codes: That P0507 (IIRC) you mentioned in an earlier post might be a clue.

However, I recall one owner who encountered some odd behavior -- though unfortunately I do not recall the behavior now -- took apart the TB and cleaned its gears and greased and then reassembled the thing and apparently the behavior was gone. I recall this because I posted a reply that urged caution as the TB is not a field serviceable item and disassembly could ruin it. But he was convinced and went ahead and reported success. The success could have arisen from fixing (inadvertently) a leak by removing an then installing the TB. Or it could have been fixed by the disturbing of the electrical connections removing some surface corrosion. One never knows since these were not attempted first.

Now I'm in no way suggesting you do anything with the TB. Start with the assumption there is an intake air leak. However, intake air leaks just don't happen out of the blue. Well, not often. So I have to ask what was done to the engine recently that could possibly account for an air leak?

With others mentioning vacuum leak and bucking, it reminded my of my VW Jetta doing the same thing - that is "bucking". It turned out to be a deteriorated/cracked bellows going to the FI system. When accelerating or decelerating, it would expand the bellows enough to allow extra air to be drawn in. this ended up being the cause. Replaced the bellows and it ran like a charm again.

Just because a lot of people misdiagnose their problems and send it up to the internets doesn't mean it's so. The oil doesn't come off the filter and even if it did, it wouldn't cause a problem.

(Source [www.knfilters.com])

"We are aware of the “urban myth” (K&N News Story) created by a few dealerships that a vehicle's MAF sensor can be contaminated by K&N filter oil. No evidence has ever been provided to support this “myth” and years of diagnostic testing by K&N has shown that not only is this allegation not real, it is not even possible"

[...]

"The idea that oil comes off our filter throughout its life is truly ridiculous. Just like oil treated disposable air filters, once our oil is properly and evenly absorbed through the cotton, no oil will come off, even under extreme engine conditions. We have even conducted a test with an over oiled K&N air filter in which we flowed 1,000 cubic feet of air per minute for over twelve hours (few cars or truck could generate even 500 cubic feet of air flow). The use of an absolute filter confirmed that no oil came off the K&N filter tested, even in these harsh conditions."

And (from [www.knfilters.com])

"We coated both hot wire and film style sensors with K&N filter oil in both controlled (laboratory) and uncontrolled (real world) environments - none triggered check engine lights. We created extreme conditions, beyond anything an engine would ever experience such as submersing a MAF sensor in filter oil and monitored the sensor readings while spraying it with test dust. Even under these circumstances, the MAF sensor was not damaged. In addition, we were able to take this same MAF sensor, clean it, and found that the readings were identical to the ones taken prior to the extreme testing."

MAF test video:
[www.knfilters.com]
(Novel use of a Paasche airbrush, btw.)
Annnd- as with no more IMS, no more MAF with the 9A1 engine!



Edited 1 time(s). Last edit at 07/07/2013 10:00PM by Laz. (view changes)

No codes. Check the crankshaft position sensor. If its intermittent, it will not throw a code but it will shut down the car. Otherwise, check the fuel delivery, a fault there will not svhow acode but will affect drive ability.

Had smoke test done. No vacuum leaks. Mechanic thought pressure at oil filler was excessive. It is getting a new AOS and plugs and having coil packs checked. Did Techron treatment. Running premium top tier non O2 fuel 91 oct is best avail in MN. Bucking was reduced in severity some but still there. Will report after AOS. Still coding 1103 on CEL.

Well, the AOS was replaced, new plugs, oil/filter and no change. Still has bucking and throws P1130 code every now and then. Not sure what else to try. Thanks for all the input everyone.

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mkd628
Well, the AOS was replaced, new plugs, oil/filter and no change. Still has bucking and throws P1130 code every now and then. Not sure what else to try. Thanks for all the input everyone.

that takes care of that. (I'm ignoring the small chance the replacement AOS is bad out of the box or was not installed correctly.)

Oh: P1130 Oxygen Sensing Adaptation, Idle Range, Bank 2 – Above Limit

Did you check the oil filler tube cap for a leak?

How about the oil filler tube?

Did you ever disconnect the MAF, clear the error codes (to reset the fuel trims back to their defaults) and then drive the car to see if the error code comes back? You have to be careful. The CEL can come on due to the MAF being disconnected. So you have to read the codes and ignore any that relate to the MAF. Do not clear the codes though. The other thing is you have to do is drive the car more than around the block. A drive long enough to set the readiness codes to complete should suffice. Roughly 15 miles of mixed driving. Do not too far from home or out of cell phone coverage in case the car acts up and becomes undrivable.

Another possible explanation is the fuel pressure is too low, or the pump can't supply enough volume (pretty sick pump if it can't keep up with an idling engine), fouled fuel injectors, or a leaking exhaust system.

For low pressure that could be the pump, or the filter, or the fuel pressure regulator. Low volume is a nearly empty tank or a bad pump. Fouled injectors can come from real old or bad gasoline.

Leaking exhaust can just happen.

Stating the obvious you have to work through the possible causes until you get to the cause.

I had very similiar symptoms in my '01S: normal driving then WHAM, engine felt like it had totally shut off for a split second. Given the poor history these cars have had with ignition switches I preemptively replaced and it's not happened since. Hate to suggest just randomly swapping parts, but something to consider given the age of the car...

Update for fun. I don't expect to really solve this, but... We just returned from a 2650 mile trip to the Great Smoky Mountains. Usual bucking, which I mentioned has been part of this car since I bought it. Intermittent and not reproducible. Will buck 10-15 times over 5 minutes and then may not do it for 500 miles. The CEL is usually on with a P0507 which I reset. However, it did throw a code fest on the second day with 1275, 1276, 1130 and 0133 all appearing. All related to O2 sensors. I reset and they have not appeared over the next 4 days and 1400 miles. I did unplug the MAF as suggested. It immediately sent the car into "limp" mode. I reconnected and for a day or so, the idle stabilized at 650 or so and did not wander. However, on the trip it began to go between 600 and 800 in a cycle and then it just started idling at 1000-1100, which is where it is now. While driving, I tried to make it buck by tapping/jiggling the key, as jlegelis said he had a bad ignition switch and his description is the same as mine. I did notice that during the bucking, which is hard enough to make you move forward in your seat, the tach never changes and none of the dash lights etc. are affected by it. I would love to solve it, but in the end, the car performs like a champ and we got 30+ MPG at 80mph, not too shabby. Still, any advice is welcome. Thanks everyone.

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mkd628
Update for fun. I don't expect to really solve this, but... We just returned from a 2650 mile trip to the Great Smoky Mountains. Usual bucking, which I mentioned has been part of this car since I bought it. Intermittent and not reproducible. Will buck 10-15 times over 5 minutes and then may not do it for 500 miles. The CEL is usually on with a P0507 which I reset. However, it did throw a code fest on the second day with 1275, 1276, 1130 and 0133 all appearing. All related to O2 sensors. I reset and they have not appeared over the next 4 days and 1400 miles. I did unplug the MAF as suggested. It immediately sent the car into "limp" mode. I reconnected and for a day or so, the idle stabilized at 650 or so and did not wander. However, on the trip it began to go between 600 and 800 in a cycle and then it just started idling at 1000-1100, which is where it is now. While driving, I tried to make it buck by tapping/jiggling the key, as jlegelis said he had a bad ignition switch and his description is the same as mine. I did notice that during the bucking, which is hard enough to make you move forward in your seat, the tach never changes and none of the dash lights etc. are affected by it. I would love to solve it, but in the end, the car performs like a champ and we got 30+ MPG at 80mph, not too shabby. Still, any advice is welcome. Thanks everyone.

This error points to the IAC valve sticking (based on my sources) but IIRC the 2001 Boxster S didn't come with an IAC (and a cable operated throttle) but with E-Gas.

However, you and your mechanic are better positioned to confirm this than I. If not the IAC then my source offers up the air cleaner is blocked. Did you lose a rag in the intake air box? Or when the filter was changed perhaps a leaf or some other trash (the top of the air filter when I change the one in my car is littered with big pieces of trash that I take care in making sure I do not *drop* into the air box when I remove the filter element) fell into the air box and has possibly fouled the MAF. That is I do not seem to recall you mentioning you have removed the MAF to say clean it. If you buy into the possibility the MAF is fouled you should disconnect the air intake tube where it connects to the throttle body in case the trash that has fouled the MAF falls away as you work the MAF out of its seat. Otherwise the trash could foul the throttle body or lodge in the intake port and cause problems there. I don't want to think about the possible ramifications of any trash making it into the engine combustion chamber or beyond.

P0133 - Ageing of oxygen sensor ahead of TWC, bank 1. The sensor is taking too long to reflect the change in oxygen level in the exhaust gas. (As an aside I read where "frequent driving in the low load range like city driving can be a major could [sic] [ I think this should read "cause"] of this problem".

P1275 - Ageing of oxygen sensor ahead of TWC, bank 1
P1276 - Ageing of oxygen sensor ahead of TWC, bank 2
P1130 - Oxygen sensing adaptation, idle range, bank 2

Given the number of errors above that point to the #1 sensors (the sensors ahead of the TWC (three-way converters) I'd say if you want to toss money and parts at the behavior new #1 sensors would be a the top of my list. Given the car has 72K miles two new #1 (and possibly even two #2) sensors right about now is anticipating the sensors are close to the end of the their life, and maybe at the end of their life if one believes the error codes.