logray

I know you're in a world of hurt here with your car, but your mention of a lot of rain (if that is normal there) and hard starting might be an issue with wet or cracked coil packs... might be worth mentioning to the dealer.

seanmcr6

Ok, so I got my car back from the dealer. New water pump and serpentine belt installed.

Car still does the same stumble on cold starts.

So I need to hook up the durametric to check some figures....my questions is which channels/sensors do you guys recommend I record?

sean

Topaz330ci

Did you get a new MAF?

Did you change those last 1 or 2 coils you said you didn't get to?

Full TB cleaning...

O2 Sensors.....

If those don't do the trick, then look deeper. But the car is CPO'd, tell them to fix it! You should have nothing to worry about!

logray

Yeah although it's fun to troubleshoot, I would agree to take it back under CPO.

Also as Macster pointed out O2 sensors don't play a role in cold start, the DME uses pre-programmed maps until the heaters and exhuast warm up the sensors.

seanmcr6

Which is curious, because it's precisely when the DME is NOT using the data from the O2 sensors that it seems to have problems :\

My dealer is 500 Kms away and I need to take a day off work every time I bring it to them. I've taken so many days off so far, that my 911 is becoming quite the joke around the office >

I unplugged the MAF and it did the same thing.

I've cleaned the TB, it really wasn't that dirty. Same thing.

I haven't enquired about the coils, but it it was the coils, it would do the same thing when warm.

I guess I'll have to bite the bullet and send it to the dealer again.

sean

Topaz330ci

I would look into the 02 sensors!

Also, mention to the dealer that distance and work is an issue for you. Force them to accommodate. Show a little strength and they will help you out!

CPO = No worries man!

Let them bicker at work... in the end, they know what you drive.

Ahsai

Have you checked if you have a small crack on the oil filler tube? Is your oil filler cap sealing well? Does sound like the fuel/air mixture is messed up a little at start up.

Definitely log the intake temp sensor and coolant temp sensor reading at start up. Also, check the long term fuel trims on both banks (to see if there's a consistent lean condition, which may indicate you have a small leak somewhere). Also agree with Macster most likely not an O2 sensor issue since the sensors are not even at their operating temperature and the engine is still running at open loop for the first 1.5 minute or so if you just let the car idle at rest.

Macster

If the symptoms appear before the sensors are hot enough to work, if the symptoms appear while the DME is still in open loop mode as it would be upon a cold start and some time afterwards, this strongly suggests then one or more sensors the DME relies upon for inputs to determine fueling wtihout the benefit of O2 sensor feedback are bad.

At cold start and until the O2 sensors are hot enough to work the DME only has a few other sensor data to go on along with its tables of fuel/timing/valve timing/lift maps to properly fuel the engine.

If a sensor is bad, or delivering bad/wrong data...

Sincerely,

Macster.

seanmcr6

Right...I'm asking which sensors those are??

logray

I agree.

Also, could this be an impulse sender on it's way out?

Macster

Sorry. Two critical sensors are intake air temp and coolant temp.

Now the air temp sensor is in the MAF so if you have disconnected the MAF and the symptoms still appear this points away from the MAF and towards the coolant sensor.

What you want to do is with everything cold check the intake and coolant temp sensors (reading them via an OBD code reader/data viewer connected to the OBD connector of the car) and comparing their readings to each other and ambient air temperature.

Also, keep looking at the sensor readings after you start the engine and while the engine is idling and ideally while it is exhibiting the symptoms you spoke of.

The crankshaft position sensor is a critical sensor but what I've gathered is its failure is usually present at engine start and the symptom is the engine cranks just fine but does not fire. The DME is getting an implausible signal/reading from the crankshaft position sensor and because of this the DME can't know where the pistons are and can know when to trigger a fuel injector pulse or sprark. So it triggers neither.

Since the engine is not violating emissions no Pxxxx error code is logged. A Porsche error code may be logged but over the counter generic OBD2 code readers/data viewers can't access these.

Durametric might. I have no experience using this tool.

But I'd eliminate the engine air intake and coolant temp sensors first as I covered above.

Sincerely,

Macster.

logray

I've read a few posts on renntech about dodgy impulse senders and strange things like stalling.

Also something to bear in mind. His car does appear to be operated in the cooooold cooold harsh winters of great white north 450k from Edmonton (props to driving a C4 there Sean, I bet it is fun in the snow!!!!!). A lot of wear and tear on a car in those conditions.

Macster

You make good points.

If the air temp and coolant sensors pass muster then the cause of the engine's symptoms lies elsewhere and a marginally functioning crankshaft position sensor or from your comments about the driving conditions one that might be loose or damaged from exposure to salt and the elements...well, that possibility needs to be eliminated.

Sincerely,

Macster.

seanmcr6

Man this is frustrating. So hooking the reader up requires the key to be turned on....turning the key on, it takes about 30 seconds for the reader to initialize...than 10 seconds to load...then I try to turn the key to ignition, and the key is locked. It won't turn at all.

I have to turn the key off and back on again for it to let me turn it all the way to ignition >|

Doing so resets the reader. @#$#@!#@%

Anyway, once I get things going, the intake temp and coolant temp are the exact same...which is 2 degrees C warmer than ambient. The coolant temp starts to rise, but that's expected.

My MAF and MAP both hover around 7-9 until the system is closed. then they drop to 4-5.

None of the reading I could get "look" off or disjointed.

I'm going to come back to the secondary air injector for a second...I am pretty sure it does NOT come on right away. In fact, I don't think it comes on until the DME revs the engine up to 1200.

Secondly, the MAP and MAF numbers drop when the circuit closes and the SAI shuts down. However, I thought the SAI injects air right into the TB...past the MAF. So why would the MAF numbers change based on the SAI being on or off?

With the SAI off, even if I manually rev the engine up to 1200, the MAF numbers do not come up to 9.

Anyway, I'm just about done with this whole thing. I go on vacation for 2 weeks in a couple weeks. I think I'll drop the car off at the dealer and tell them to find and fix it.

Unless, there are any other suggestions out there?

Oh, and Logray....I'm not driving it in winter ever again. It is fun in the snow, but the winters are so harsh here that the bodywork takes a beating. I have two stone chips in the windshield and several paint chips. I'd rather buy a $2000 clunker for the winter than have to repair the paint/windscreen every spring.

sean

logray

This doesn't sound right to me for some reason.

The DME setpoints for an idling engine Mas air flow and Spec air mass are:

17 kg/h +/- 2.5

Voltage for MAF

1.3 v +/- .2

What are your TRA/FRA numbers? At idle, TRA should be 0 +/- .1 and FRA should be 1.02 +/- .04.

Do you have an intake leak, perhaps a cracked oil filler tube, plenum boot damaged/loose/etc?

This might be a job left to the pros since they'll have access to a smoke machine.