seanmcr6

ok, this has been happening for about a month now...when cold starting, the engine fires immediately...tries to set idle at about 700...then stumbles a couple times and then revs to 1100-1200. After about a minute...everything is ok, revs settle back down and everything is smooth. If I try to drive off right away, it will keep stumbling unless I keep the revs way up. I can also smell gas. So I'm pretty sure it's way too rich on cold start.

Dirty MAF?

I just changed my cats yesterday to Fabspeed 200 cel cats. On the next 3 cold starts, the issue was worst.

I'm under the impression the secondary air injection system would help with a cold rich situation no? Or is it only for emissions?

4 of the coil packs are new from last summer, as are the spark plugs. The air filter is 1 month old (oem)

The only thing I can think of is the MAF....so I'm going to try cleaning it.

Can I use any MAF cleaner or is there a Porsche specific one? (shouldn't be, but you never know with the Germans )

Here is a vid of the stumble on cold start. Note that my foot isn't touching the gas pedal at all until about 30 seconds in....

http://vimeo.com/25356604

Any suggestions would be great.

sean

Topaz330ci

Try cleaning the MAF and throttle body, but I do not think that's the issue. The other 2 coil packs were good I'm guessing because you didn't change them?

Any MAF cleaner will work.

SSST

Cleaning the MAF isn't going to hurt, standard MAF cleaner is fine. However, if the MAF is going bad, it may not help either.

If you have the original MAF, it might be time to replace it.

Macster

You can clean the MAF but I don't think that's the cause of the symptom.

If the MAF is dirty then you have an engine air filter/intake leak that lets dirty (unfiltered) air into the engine. (Have to mention I've driven my 02 Boxster over 160K miles without even touching the MAF. MAFs just don't get dirty if the intake system is stock and working properly.)

From the description of the engine's behavior I suspect the coolant or possibly air intake temp sensor is acting up and supplying an optimistic coolant or air temp value to the DME and the DME is not adding enough fuel for a cold engine.

(The intake air temp is provided by the MAF so the MAF may be bad. Which reminds me: Unplug the MAF from the wiring harness. Under the same conditions as before start the engine. If the symptoms are still present probably not the MAF. There is a slight possibility the DME has learned fuel maps based on the input from a going bad MAF and the way to reset this learned mapping is to use an OBD2 code reader/data viewer to erase DTCs. Even if there aren't any this resets the DME's learned maps back to their defaults.)

The secondary air injection system is there only to add air after the chambers to help exhaust gases with unburned fuel combust to help heat up the converters.

The gas smell is the result of a cold engine, cold converters and an engine that is not running right. The exhaust gases are 'rich' but the air fuel mixture being fed to the engine (so to speak) is lean.

If you want to 'clean' something you can remove the throttle body and clean it. If it is dirty (along with some signs of oil wetness) this points to an AOS problem.

Sincerely,

Macster.

seanmcr6

I've wiped out the throttle body before...right at the butterfly. There was carbon buildup, but no wetness. Maybe I'll take it right off and give it a thorough cleaning...

I'll also try unplugging the MAF when I go home tonight and see if it happens again.

I can erase DTCs with my Durametric tool right?

I hope it's not the AOS....I was looking at it when the airbox was out (see my other thread about installing new cats) and I don't see how I could possibly reach it without dropping the tranny.

I was going to get that done with the IMS/etc next spring.

Thanks for the advice

sean

Macster

There should be no carbon build up at the TB. Combustion chamber gases should not get that far upstream from the combustion chambers.

My guess is what you are mistaking for carbon build up is a very light coating of oil. The oil is dark because oil as it accumulates miles picks up soot which darkens the oil.

The oil is carried in vapor form by fumes from the crankcase directed to the intake manifold at a point (at least in the case of my Boxster not sure about the 996 engines) immediately behind the TB. There will be some flowing of these fumes upstream and the TB and its butterfly valve are very close to where these fumes are fed into the intake manifold.

If you remove the TB check for oil behind the TB at a hose connection of a hose that runs from the AOS to the intake manifold.

Also, after you have the TB removed carefully -- and with watches/rings removed -- put your hand into the intake manifold as far as you can and touch the walls of the intake manifold. If your hand comes out oily... Either you've been running the engine overfilled with oil, or the AOS is suspect.

Sincerely,

Macster.

seanmcr6

ok, thanks.

So the inlet to the throttle body...where the plastic hose form the airbox attaches...should be sparkling silver clean after 120,000Kms and 8 years?

Macster

Can't say for sure. The only time I had my car's intake hose off and the TB out was when the AOS (as it turned out) was going bad.

I can't recall the condition of the TB at its inlet side, but at the outlet side next to where the hose from the AOS connected it was quite dirty, oily. There was a drop of dirty oil hanging from the TB butterfly valve. At the time I didn't know what this meant and I never stuck my hand down in the intake manifold.

What I can say for sure is after 160K miles of driving my Boxster (since the 1st AOS failed) I've never had the TB out and cleaned it.

In fact the replacement AOS failed -- I can't recall the mileage -- and I never even bothered to remove the TB and clean it before or after the AOS was replaced. I just had the car towed to the nearest Porsche dealer and had the AOS replaced. I'm sure the dealer didn't clean the TB for the labor never showed up on the work invoice.

Now regarding the Boxster, I had the MAF out once and cleaned it -- over some white paper towels and didn't get enough dirt off the MAF to even show up on the paper towels -- and ended up replacing the MAF though my diagnosis of a bad MAF proved to be wrong.

The problem proved to be a leaking oil filler tube cap not the MAF. However, the replacement MAF eventually (after approx. 70K miles) started acting up and I replaced this MAF with the original MAF which worked just fine and continues to work just fine to this day.

At this point, I have put over 160K miles on my Boxster since I cleaned the TB and over 56K miles on my Turbo, and have never cleaned nor had cleaned the TB or MAF.

Both engines run just fine. Whether the inlet side of the TB is sparkling silver clean I can't say but if it ain't it ain't dirty enough to cause any symptoms.

My WAG would be there might be a very light layer of very fine (very fine) dust, enough to put some color on one's finger but not enough to affect air flow into the engine.

Sincerely,

Macster.

RPMulli

Updates?

seanmcr6

I unplugged my MAF and the stumble was still there. I got a PSM and ABS failure notice....but the engine still did the exact same thing, so I don't think it's MAF related.

If anything, since changing the cats, it's worst. So not sure why less back pressure would make the situation worst...but the first 30 seconds is really rough, almost stalling a couple times (but it's NEVER stalled)

So I guess this weekend I'll be taking the TB off for an inspection and cleaning.

Also, I'm thinking of hooking up my Durametric and recording some numbers while cold starting. Specifically temps, AF ratio, timing advance and look for anything that seems out of whack.

update to follow on Monday...

sean

logray

MAF shouldn't affect idle at all.

First and foremost, check for intake leaks. This can really affect idle, but not as apparent on higher revs. For example if the T/B is not completely secure or if a boot in the intake plenum has a leak or is not tightened down properly. Or the oil filler tube has a crack, oil cap ring, T/B clamp or hose broken, brake booster, etc. On older cars the idle air control valve needs to be cleaned every so often.

A dirty T/B shouldn't affect the idle that much, the clean side of the T/B should be oil free but could have a very small bit of carbon on it if it's never been cleaned. The plenum (back) side can have a very light coating of oil, normal.

Also vacuum leaks can attribute. A more free flowing exhaust might make intake leaks become more evident/apparent since the entire system can "breathe" more freely, however the DME should adjust accordingly, as it sounds like is happening. Although it sounds like you were having the issue prior to the cats...

Have you reset the DME by unplugging battery for a period of time to have it relearn?

AOS can affect idle as well if it's failing, usually other symptoms follow like difficulty removing oil cap and white smoke.

O2 sensors can affect fuel mixture, usually one bank will run richer than the other if a sensor is on it's way out - they can be monitored using Durametric, PST2, etc, also bench tested using a torch. A rich fuel smell is normal for these cars on cold startup, especially if they idle alot or are not driven hard enough, for example city driving makes them use more gas.

Also does this happen on a warm/hot start?

seanmcr6

Yeah, I was having this isue before the cats.

It doesn't happen on warm starts. If I let the car sit for more than 3 or 4 hours, it will happen.

I couldn't find anything obvious that was leaking vacuum pressure.

I reset the DME when I installed the cats (as you suggested)

I will run my durametric and see the values for the O2 sensors at startup and see what they are compared to when I'm warm starting the car.

It only lasts about 30 seconds ..then the idle seems correct.

My worry is that it's the AOS. I have a new one on a UPS truck somewhere in Canada (by now)

seanmcr6

When it first starts...it seems to set idle correctly...for the first 2-3 seconds...then there is a "condition" that causes it to start to stumble. The DME raises the revs to compensate. After about 30 seconds or so, the idle is normal.

I'm thinking this might well be an O2 sensor issue as they would not be providing data for starting, just operating. It's only once they (all of them, maybe just one of them) get some heat into them that the idle is correct.

hmmm, going to investigate this further...

sean

Macster

It is almost certainly not an O2 sensor problem since at engine cold start and for some time after a cold start the sensors are not hot enough to work properly and the DME does not rely upon them for mixture monitoring.

If you have an OBD2 code reader/data viewer connected to the car you can view the sensor voltages and see the voltages are not 'right'.

My experience (observation) is when the secondary air injection pump shuts off or shortly thereafter that is when the sensors are working. The DME will switch from open loop mode to closed loop mode (you can see when this occurs too with the data viewer).

I have not gone back and refreshed my memory on this whole thread but from the info you posted in your latest posting I suspect the coolant or air temp sensor is delivering too high of a temp reading and the DME is based on this leaning the fuel mixture when it should be leaving it rich. The bad sensor starts out ok but within a few moments after engine start I believe then goes bad and supplies an overly optimistic temp value.

With an OBD2 code reader/data viewer you can view both the coolant temp and intake air temp sensor values at engine start and over the span of time when the engine starts to misbehave. A real good code reader/data viewer would let you log these and view the data either at the reader's display or download the data at your PC and view it on your PC.

Have you tried disconnecting the MAF at the wiring harness and seeing if the symptom still happens? The MAF supplies the DME with the air temp value the DME uses (along with other inputs) to properly fuel the engine.

Sincerely,

Macster.

seanmcr6

yeah, based on your recommendation, I tried unplugging the MAF....the car did the exact same thing.

I'm going to plug the Durametric in this afternoon before work and log all the sensors that you've mentioned and look for spikes/changes.