Cold start / sputter / dies /then fires up (yes, I've searched!!)

Hello, I have a 2004 C4S convertible w/ 94k. I've only owned it for a few months, but purchased it knowing of a few things it needed. I've replaced the clutch, IMS, RMS, oil filler tube (cracked), and a couple other misc things. MY PROBLEM: Every morning after it's sat overnight (or even at work after it's cooled down after a few hours) it will almost ALWAYS sputter and will sometimes die right after initial startup. I did an induction and injection cleaning service which truly seemed to fix the problem for about a week, but it's back!!! I've seen similar posts about the Air Oil Separator on other ones, but my symptoms seem to be different than those posts. I asked my Porsche dealer if I needed new plugs/injectors/coils and they said they'd actually start by testing the MAF. Any ideas? Thanks!

Reads like a temp sensor input is off and the fueling starts out too lean. 'course the symptom description is a bit vague. The mixture could be too rich. Does the engine start ok when hot? A too rich mixture generally causes problems when the engine is hot while a too rich mixture when the engine is cold is less likely to cause problems. You need a scan tool to view the coolant and the intake air temperatures at cold start and right after. These should be close to the same and should be close to ambient. They should also continue to agree as the engine starts and begins to run though shortly after the engine begins to run the coolant temp will climb approx 2 degs every second while the intake temperature should remain close to ambient. If you want to get your hands dirty disconnect the MAF at the wiring harness and use the scan tool to erase DTCs. Even though there are none this resets the fuel trims among other things to their defaults. Then under similar conditions start the engine and see if the engine behaves the same. If it does then the MAF is probably not the problem. If the engine doesn't behave the same then reconnect the MAF, clear the codes again, and do the test again. If the engine then behaves as before the MAF was disconnected the MAF is probably the problem. Treat that MAF with care. Static discharge into its connector or the connector it plugs into and you could be buying a new MAF even though it was ok before you zapped it. Or you could be on the hook for a new DME. Or take the car in. The tech can hook up the diagnostics computer and check the MAF signals for any signs of problems and eliminate other possible causes of the symptoms. Sometimes an hour of diagnostics saves a lot more time and money than it costs.

Sounds like you may have a vacuum leak. Makes it too lean when cold. Did you deploy the roll bars? If so could you post more pics?

If you join your search would include all areas of Rennlist instead of only visitor areas.

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Starts perfectly when the car is warm

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Thanks and good to know. Any input on my challenge?

Does it continue to idle rough or does it settle down after warmup? Do you have any codes? Any other information would be helpful. Sputtering on startup can have an AWFUL lot of causes and on a higher mileage vehicle it's usually a combination of things.

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Thanks for the reply. It settles down after only about 3-5 seconds once it's actually fired up and started. If I give it ANY throttle, literally any, while it is sputtering, it will instantly die. After it stops sputtering/starts properly, it is perfectly fine. It's just annoying as hell (and somewhat embarrassing when others are around) when I get in my car to start it after its been sitting for a while. I'll try to take a video tomorrow am when I start it up. Possibly later today too

Sounds to me you have one or two leaking injectors. They drip amd flood the cylinder when it sits for a long time.

^^^ this I have a cold start stumble for the better parts of 8 months. After Porsche looked at it a half dozen times, they finally changes 2 injectors and it's fine now.

Although a video may be helpful, we're gonna need those codes (or lack thereof). If you don't have a reader the nearest auto parts store will do it for free. I would definitely do Macster's procedure for checking the MAF as well. That's a great write-up.

There are no DTCs that a generic scan tool shows. Also no check engine light. Im sure the PORSCHE specific diagnostic may pull some, but my cheapie does not

Hmm, maybe try techron injector cleaner? How is the battery voltage?

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Not surprised. We're going to need to track misfires though (assuming it's not the MAF)

A leaky fuel pump internal check-valve could cause this.

So, car cold start it stumbles and dies. Warmed up it works perfectly. No codes or CEL. Too lean a mixture at cold start. Check: 1) Is the secondary air injection pump running at cold start? 2) Pull and clean the MAF sensor. However, the MAF has more to do with higher RPM's than idle. Leave it disconnected and see if there is any difference in cold start. 3) Install clean new air filter. Make sure all intake hose clamps are tight. 4) Clean the throttle body, check the idle control valve and throttle position sensor. Make sure they aren't stuck or gummed up. Check for vacuum leaks around throttle body and the o-ring seat. Make sure the clamps on the intake plenums behind the throttle body are tight. 5) Crank position sensor? Although this usually results in a no start vs. stumble start. 6) Replace all spark plugs and check if any are wet or carbonized. 7) Check the coil packs for cracks, replace all of them if you can afford it. 8) Running some Techron through might make a difference, but agree with others, bad or leaking injectors could cause the problem. Injector seals might be bad as well. A good shop can test your injectors and you can replace the seals. 9) O2 sensors could be fouled and giving a bad reading to DME. But that usually would result in a CEL. Does the CEL light work? 10) Does your coolant temperature gauge read correctly when warmed up? This provides feedback to the DME as to whether the engine is cold or warm and it adjusts the fuel mix accordingly. Either buy the Durametric software and get some real readings of what is happening, find a friend that has it, or get a full diagnostic as Macster recommended. That's all I can think of for now.

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Uh-oh that doesn't sound fun The BG injector cleaner that I used is virtually the same stuff if I had to guess this doesn't sound fun either!

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Thanks for the detailed reply. Im actually going to forward this to my Porsche service writer It sounds like the injecorts as the symptoms seem Identical to his

If your Porsche service writer needs help from an internet forum then, might want to find a better mechanic. For some reason I thouhht you were doing all the work yourself. Since you aren't last thing you need is start throwing parts at it. A lot of the above suggestion do just that so be careful.

Secondary air pump, crank sensor, O2 sensor, most ignition issues, all these things would have thrown a code pretty early on. Which is why I asked for codes. Of course if you don't mind having your mechanic run down all these issues, hey, it's only money What we have left on the table ... bad gas, idle control valve, MAF, injectors, fuel pump, vacuum leak. Injectors are pricey so if things go that way we want to get it right.

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Thanks a bunch. One of my mechanics (I work at a car dealership, but not a Porsche dealer) is going to smoke test it (for a case of beer) for a vacuum leak on Friday and we'll go from there. I know it's good gas at least for the last 2,000 miles (91 octane with NO ethanol) The Porsche service writer says hes seen vacuum leaks due to bad AOS housings....does that seem like a possibility? I had trouble finding this part on pelicanparts for some reason

Try this thing for me Percy311. When the car is cold tomorrow and you are about to start it, press the gas pedal down about 15%. Start the car with pedal still pressed about 15% down. Immediately let go of the gas once revs get past 1,500.

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Always a possibility, but I would expect a rough idle all the time not just on startup. Good suction on the oil tube (but not TOO good) implies a good AOS. This is not a complete check however. Man I hate vacuum leaks. Even a small vacuum leak along with aging/flaky emissions controls (AOS) and little problems like this can be hard to get right.

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Oddly enough it fired up this morning without any real stumble. I read your post about the 15% throttle just now but I'll try tomorrow Am I looking for anything specifically?

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I am understanding your situation as: on cold start up , car idles low, does not self correct, if you give it any throttle it dies. Car can die seconds from first start-up ,however,it will restart and idle perfectly fine on 2nd start-up. Your description suggests that you can shut off the car immediately after first cold start-up, restart it and it will idle perfectly fine. Vacuum leak, coils, spark plugs and MAF would produce constant symptoms most of the time. You problem only happens on first cold start-up, and disappears on the 2nd start-up. You also stated that there are no codes and no other symptoms. Car is not at all warmed up from 3 seconds of running after it stalls on first start up. Because from your description it starts fine and idles good on the second start-up, it leads me to eliminate the MAF, coils, plugs and vacuum leak. If they were were the problem, your symptoms would persist on the 2nd start-up and while driving. I am leaning towards the Macster's suggestion that Reads like a temp sensor input is off and the fueling starts out too lean. As far as starting it with gas depressed 15%. Opening throttle on the start up delivers more fuel and air, overriding the factory parameters for the start-up. I believe that in your situation this will solve the sputter and dying on the first start up.

2004 Porsche 911 996 sputter - YouTube http://www.youtube.com/watch?v=MA9aYNDYKeg

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Video was with 15% gas pedal Sputter was consistent with the normal sputter without throttle. Once fired in the video, it obviously jumps to a higher RPM than normal bc of the gas

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Did you have the gas pedal down when you cranked the car? If you had the gas down, revs should go up immediately after you crank the car. Your car tries to die from 4-7 seconds, then it revs. Did the car stall afterwards? Did it idle normally after the 15 seconds in the video?

Diagnosing the car like this is a little bit crazy if you ask me. With the sophisticated engine management systems on our cars, this is a hit or miss diagnoses. Somebody is going to have to get the car to a shop where they can run a diagnostic test or the OP is going to have to get the Durametric and start getting some hands dirty. In my suggestions was an idea for the coolant sensor reading, if it was working correctly and reading accurately. Also, in recommending pulling the spark plugs, I was leaning towards a common problem - loose spark plug(s). Also, we are assuming without knowing, has all other maintenance been up to par...air filter change, etc.?? Continuing to start the car and experience the same problem over and over again...well something has to physically change, be fixed, or it will just keep doing the same thing. Everybody has pointed out some very good places for the OP to start. Just has to happen. Dart board anyone?

Many people don't like to get their hands dirty and hope the car will fix itself.

So I hate to drag up an old thread but did you ever resolve this issue? Appears my 04 911 Carrera is displaying the exact same issue. I just had the Spark Plugs replaced so that isn't the issue. I haven't had a chance to read the codes yet I am going to try and get them read today.

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Hopefully the person who replaced the plugs evaluated the condition of the coils and deemed them ok? Regardless, read the codes and post them for comment. I would add that if you are going to try to diagnose this by yourself or with help from here you need an OBD2 diagnostics tool, a code reader/data viewer, so you can not only read codes, active and pending, but you can view engine telemetry like intake and coolant temperature, fuel trims, etc. in real time as you start the engine and goes through its period of misbehaving.