It is the difference the two sensors readings have compared to each other and to the actual temperature of the coolant and intake air.

If the two sensors disagree with each other that's one thing. One sensor is probably bad, going bad.

If the two sensors read very close together but do not report close enough to the actual temps of their respective coolant or air that's another thing.

To avoid being fooled by legitimate differences in temps it is important to check these temp sensor their readings after the car has sat for 12 hours or more and not in the sun.

The assumption is that after 12 hours or so the coolant and air temp have reached equilibrium with each other and the ambient air temp. Now you'd think intake air temp would be ambient but not as long as there is heat in the car/engine compartment.

After you verify both sensors produce temps that are close to each other and also reflect reality you want to continue to monitor them as the engine runs/warms up to confirm while they read ok at from the start they do not start delivering wrong or incorrect numbers as the engine gains heat.

While the engine is warming up from cold the coolant temp reading can jump by 0F to 4F degrees per approx. every second. Intake air temp can climb a bit as the engine and engine compartment heats up and this heats the intake air.

Even if the car is moving down the road the air intake temp will always (my experience) read higher than ambient. The reason is that the intake air is made up from air that in part comes from the radiators and this is heated air.

The very closest I could see the intake air temp and ambient air temp (measured with a digital thermometer stuck out side the window of the car) get is within 10F to 12F of each other.

The worst is of course when you restart a hot engine that has heat soaked the air intake system. In this case the intake air temp can be 30F or more degrees hotter than ambient. But I have never observed either of my cars' engines reacting in any way to this difference in air temperature.

Oh, the variable valve timing system may not work properly from cold but begin to work once the engine warms up. Another thing I have noticed with some failing components (O2 sensors) is they go bad, the check engine light comes on and the error code points to the sensor. But the engine gets shut off and upon hot or warm restart the component is working just fine.

Lastly, you have other possible causes to suspect before you start condemning the variable valve timing hardware.

Sincerely,

Macster.

 

Sean, that's cold start fuel enrichment, part of the ECM program during cold start so I'm not sure that is not normal.

"The ECM software controls fuel enrichment during cold-start by altering the timing of the main injectors based on engine temperature, no 5th injector is required. The idle speed is also fully controlled by the ECM, including fast-idle during warm-up (no thermo-time switch is required)." from http://en.wikipedia.org/wiki/Motronic

I smell gas too at cold start.

 

sigh....well, the dealer has it now. we'll see what they "find"....

 

I wasn't "condemning" the VVT hardware at all...just looking for possible answers.

My car is parked in an underground lot. All of my tests are after the car has been sat 12+ hours overnight. The temps are fine. The all read what they should read, as per your suggestion.

O2 sensors aren't even used when the engine is cold. So I fail to make the connection on how their status could effect cold starts.

I am not saying there was a faint gas odour...I'm saying it reeks of gas on cold starts. That's not right...for any car.

If you can smell it...it's NOT being burned.

If it's not being burned...it's doing all sorts of damage to your engine.

I've had two small backfires actually (if I try to rev it higher when it's stumbling)....tell me that's normal!

Anyway, I'm done. The dealer has it. I'll post an update when I have one.

sean

 

HEY!

back from vacation. Tip for you all...don't break up with your Cuban gf in the middle of the Cuban country...with no transportation of your own. Lesson learned xD

Anyway...get back to Porsche dealer to get my baby. The service advisor gets the tech that worked on my car and says she wants him to talk to me directly as I seem to know more technically about the vehicle than she does. Nice guy. Brings me through his 5 day diagnosis and repair of my 911.

Found a vacuum leak in a PCV crankcase hose coming off the throttle body. It was rubbing again the top of the engine and wore a hole. This was sucking dirt from outside the system into the throttle body, which explains why I found my TB to be dirty. Replaced system. Retest...issue still there.

Check fuel pressure...58psi, good

Remove fuel supply line, drive link fuel pump, fuel pump output is good.

Compression test. All cylinders are equal on initial stroke and even on final stroke. All values almost identical across all cylinders, around 260 psi.

Check cam timing. Actual versus spec. Cam timing good.

Inspect spark plugs....2 and 4 are wet from fuel.

Check roughness values. 2 and 4 are higher than other, even when engine is warm.

Using his PIWIS (or whatever it's called) he was seeing 12-18 misfires on cylinders 2 and 4 when the cold stumble would happen. They decided to replace all my plugs. Issue still there.

Swapped the injectors from 2 and 4 with 3 and 6. Misfires during stumble moved to 3 and 6. They replaced those injectors and emptied/cleaned the fuel rails. reset everything and the stumble as gone. Engine now runs smoothly.

(Awesome, so it was the injectors. Although the work order lists injector valves...not injectors themselves, so maybe he just repaired them? Although he did tell me he replaced them.)

BTW...we noticed 2 small oil leaks. One is either RMS or IMS...the other is the transmission shifter shaft assembly. Took photos, sending them to Porsche to ask permission to replace seals under warranty.

He tells me the transmission leak is an expensive job as he's not only got to drop the tranny, but he has to open it up. So I asked him...if Porsche says yes to that work....could I pay for some new syncros and have him replace them while he's in there? (if there is any significant wear). He's cautious about it and said he didn't think he could without asking them first.

Whatever, I got her back and she's fixed. I pay for my new sparkplugs (parts only) and my oil change and head out. I get in the car, which has been parked for two days, and guess what?

It does the exact same stumble on start

I took her home as I was late for a shoot already. This morning, she started better, but there was still a hesitation and an rpm dip. Also, as with before, I cannot simply drive away...if I try to drive right away, the engine bogs and I have to over-rev and run the clutch to get it moving. So I don't think it's fixed. Sadly.

She does idle more smoothly now (once warm)

So when I bring it back for them to fix the leaks, I'll let him have another kick at it I guess.

sean

 

Ouch Sean! Both on the Cubanita situation, as well as your car! Good thing you've got such a sense of humour, I'd be pretty frustrated about this. You're lucky that your "Veronica" is still under warranty! Keep us informed... Maybe you need to send the question to the guys on "Car Talk"?

 

Well, I have no one to blame but myself for the cuban princess

I just went out for groceries after letting it sit all day and the stumble was less pronounced. I'll leave it for a week and then decide if they need to see it again.

BTW...while I had the mechanic's attention, I asked him about putting in my brand new, bought 8 months ago, still in the box, Stage 2+ clutch kit with LWFW...and if he thought there was any risk to my crank/engine and he said it should be fine, he was going to asked another tech who races and works on several track machines to get some feedback, but he's never heard of a 996 that went down due to LWFW.

I also asked him what he thought about doing the LN retrofit. He said they've only ever seen 2 996s that had any kind of IMS issue and those were many years ago. He said he think the internet blows things out of proportion and said if I don't hear anything coming from it, don't worry about it. He said you can really obviously hear it when the IMS is on it's way out.

I'm not preaching or evangelizing, I'm just forwarding his thoughts to the group. YMMV.

 

Sean, this IS frustrating. Sorry to hear that. I was hoping to see a happy ending in the middle of your post. Any updates?

 

I think the fact the problem only happens in the first minute or so after a cold start really limits the time window for diagnostic. I wonder if the missfire was the cause or a consequence.

 

well, like I said...there is way too much fuel flowing somewhere. It's not being burned and on one occassion, attempting to rev the engine produced a backfire!

Since he found the plugs on 2 and 4 were fouling/wet and replaced those injectors...I'm hoping it's just another injector that decided to give up. When the stumble happens now, the engine really chakes from side to side...so now it's unbalanced, leading me to believe the issue is with only one injector.

Heck, maybe he didn't install one of the injectors right? unlikely but possible.

They are getting the car at the end of the moth to repair the RMS and transmission leak..so he'll have to look at it then.

Good news is that Porsche said he's allowed to do extra work for me while he's got the transmission out....so if any of the syncros are worn, I can save 90% of the labour charge to have him replace them. I'm happy about that!

I will update....in early Sept when I get her back.

Cheers
sean

 

Sean,

Still experiencing the stumbling problem?

Mark

 

Any update?

 

Yes. still happening....no, no update.

Was supposed to give the car back to the dealer a couple weeks ago but I couldn't be without my car. It's hard for me since the dealer is 5 hours away and won't give me a loaner. So I'm faced with renting a car for a week just so the dealer can tinker with it.

 

Are you under CPO? Bummer no loaner and 5 hour drive...yikes that's not convenient. I won't tell you how close my dealer is.

 

Are the misfires on the cylinders that still have old coil packs? I know everyone has been focused on afr issues but bad packs will cause similar behavior.

S.