New 996 Owner - "Cold" Start Issues

After a year and a half of no Porsche in the garage, I just brought home an '02 C2 last week. Made the move from a 997.2, and so far I'm pretty happy with it. 115k miles, and 15 years with the last owner. A few small issues here and there, but nothing too bad. So far only thing I can't figure out is an issue with "cold" starting, which I say because I'm in Phoenix and my garage hasn't been below 100 degrees in weeks. After sitting for a day or so, it cranks normally and fires in a normal amount of time. The problem is that it doesn't do the normal rev up and high idle - instead it just kinda "begins" and idles around 500-600 somewhat poorly, which I'd expect given the cold engine. If I just barely tickle the throttle after a few seconds, it revs up and starts the normal high idle perfectly. Similarly, if I just key off immediately and start it again everything works as it should. No smoke, CEL, etc. I've tried the TB adaptation a few times with no change. Only thing I'm seeing in the Durametric is the IAT is about 10 degrees (F) higher than the engine temperature. For something that's sat for a day or two, I'd expect them to be within a degree or two. Is this enough of a variance to cause a problem? Or should I be looking elsewhere? Being an '02, I know there's no dedicated IACV - does the throttle plate itself control idle or is there some sort of valve built into the TB assy that could be sticking?

I experienced this phenomenon once, a few weeks ago, when I left my car sitting for 10 days without starting it. Like you, I pressed on the accelerator pedal lightly and the car idled normally. The problem never showed up again, as the car resumed daily driving duties. How often does the problem show up for you? Have you filled up the gas tank since you�ve purchased this car? Try cold-starting the car with the MAF unplugged to see if the situation improves. (as a side-note, I replaced my 24 year old MAF with a Bosch unit and it seemed to have restored some power) You�re right that the throttle body regulates the idle. You could also inspect it for any buildup of grime and clean it. Then, perform a throttle body reset. https://rennlist.com/forums/996-turb...ation-fyi.html edit: There isn�t a valve in the throttle body. It�s the position of the plate that regulates idle.

For me it's happened on every single cold start so far. This morning was a mixed result since it had only been sitting 8 or so hours overnight and technically the engine was still a little warm. PO filled the tank the day I picked it up, so I've taken it from full down to about half tank. I'd like to be able to try a cold start when the engine is around 80 degrees or so, but I probably won't have that opportunity until late August. I've treated MAFs as expendable items on other vehicles with decent success, but I also want to avoid throwing parts at it to no avail. I'll try an unplugged start and see what it does.

The "Cold Start" issues that are limited to a just a few seconds after the car has sat overnight or for a long peroid of time is 9 times out of 10 due to air bubbles in the fuel lines and injectors .. On the 02 and up 911 the fuel system is a "non-return" system meaning the fuel pump, regulator, and check valve is in the fuel tank and no fuel is returned or circulated in the lines . The check valve is supposed to hold pressure after then engine is turned OFF . The allowed ideal pressure drop is 2 psi per hour...This is to keep the fuel that is in the lines and injectors from "boiling" when hot after the engine is turned OFF ...IF the check valve is leaking ( not holding pressure well) or the injectors are dripping and the pressure drops too fast while hot and OFF the fuel will boil and there will be a lot of bubbles in the lines and fuel injectors. This will cause poor cold starting and rough running, low idle speeds for a few seconds when re-started after sitting over-night.. This issue can be intermittent and only happen once in a while, or it can be persistent and happen every time and too different degrees.. To diagnose and verify the issue if it is persistent and severe, you have to install a manual pressure guage on the fuel system and pressure it up, then turn it OFF and wait for hours observing the pressure lose vs time ...

Quotes source post: 19523642

Appreciate the thorough reply. Definitely an interesting thought. I hadn't actually considered that since my experience with pressure loss on return less systems has been quite different in the past. I can see how this may be the cause, though, if the leak on either end of the system is slow enough. My only observations are that in my experience so far, it doesn't seem like it even tries to do the initial throttle blip on start, which seems unconditional on whether air is present in the lines as well as the fact that it will continue to run like that indefinitely until I either cycle they key or manage to tip in throttle slow enough that it causes the Rev up. Does the DME command a priming pulse when the key is turned to ON? If so, I imagine this could also be tested by cycling the key a handful of times. Regardless, the best option is probably the manual pressure test you suggested.

The DME DOES prime the system for a couple of seconds when the key is turned ON. But if there is AIR/Bubbles in the metal lines and/or injectors , it is just "pressurized AIR /FUEL" ...If no AIR was introduced during the "cool down" phase, then it will be pressurized FUEL.. The "re-start cold" strictly depends on what happens during the "cool down phase"...

The bubbles (=gasoline in its gaseous instead of liquid state) dissappear when the gas cools down again. There is no air introduced to the fuel line at any time (as long as the fuel pump intake is submerged in gasoline). No air when the gasoline is boiling nor when it cools down again. After sitting overnight all gasoline will have cooled down to its liquid state and the lines and hoses will be filled with liquid fuel only. The hard starting the next morning is due to the injectors already firing whilst the fuel pressure has not reached the operating spec of ~45 psi yet. As such less than required amount fuel to start the engine has been injected and once the proper amount is injected when FP is at 45psi the engine is flooded. A defective check valve would not cause any issues if one could run the fuel pump for 10sec or so and only start the engine once 45psi is achieved.

Quotes source post: 19525004

I have to strongly disagree...Gasoline vapors can and do exist at room temperature ... Once gasoline has boiled/separated it will not simply go back together when cooled to room temperature.. Gasoline is a complex mixture of 100's of elements , some will start to vaporize/boil at room temperature , some at 95F, others can take up to 400F to vaporize/boil .... Once these elements have vaporized/boiled/separated some will condense back to a liquid at room temperature, some will not, some can turn back to liquid "IF" cooled down below 30F ... Yes there is no "Air" in the fuel, I used the less scientic term aimed at Novice readers when I should have just said "gaseous vapor" , but somehow I thought air would be better to comprehend the symptoms better.. Also FWIW :: winter blend gasoline has a much lower boiling point than summer blend gasoline...

Alright, quick update a week later. I took the first suggestion and tried cold starts with the MAF unplugged three separate times. Each time, the start was perfect with a solid high idle. Lots of warning lights and didn't run great after a bit, but I assume that's just from having no MAF. Finally replaced the MAF last night thanks to FCP's (unusually) long delivery time, and we're back to square one. Started today with a low idle again. Assuming the fueling issue is the next most likely, how is it that the car starts perfectly with the MAF unplugged? It's insanely hot here this week, so I won't be able to check the "cold" engine temps until morning, but I also want to confirm IAT is actually reading correct now with the new MAF.

Doesn�t the DME also use ECTS to determine cold start rev blip? Do you have an ODBII module and app you can plug in to read some of these values while the car if off and has been sitting? Or Durametric?

My guess is with it being 100+ degrees air temp, the MAFS is sensing the temperature and not allowing the car to rev. as it doesn't need to build temp. I live in Houston and my 991.1 does the same thing....but when the air temp was lower (60's) the car would rev high on start up and level after about 30 seconds.....I wouldn't worry about it.

Quotes source post: 19540526

Yep - I've been watching values via Durametic. That was my other factor in replacing the MAF. After sitting in the garage a day or two, engine bay temp and engine temp (I assume this is coolant temp) were reading around 100 degrees but the IAT was 115-120 degrees. I really need it to cool down here a bit so I can see if colder temps help at all. No end in sight for the heat, unfortunately.

Quotes source post: 19540979

I'm pretty much in agreement with that, and I'd be fine with that if it didn't stall on throttle after the first start. Post MAF-replacement, first start was no good. Yesterday morning it recovered and brought the idle up. Next week lows are finally mid-80s so maybe I can get a lower temp start for comparison.

Remember, the DME/ECU has a learning curve. When replacing the MAF sensor, always a good idea to disconnect the battery to erase all of the learned values in the DME. Then start driving again. Just a suggestion...

Quotes source post: 19540994

Good luck and let us know what you find.

If you do decide to disconnect the battery, you could touch the two lead cables together to drain any residual capacitance in the vehicles electronics, or wait 5-10 mins with the battery disconnected. It�s best practice to disconnect the negative cable first when handling the battery. On electronics, I�ll sometimes try to turn them on while unplugged to try to drain any charges.

Quotes source post: 19542020

I can't see how the result achieved by 'touching positive to negative' is any different from that by the 'vice versa'. Is it to do with the battery being a large 'sink' and thus less or no spark when touching?

Quotes source post: 19542492

No difference. I edited my post because I must have had dyslexia that day and was backwards. The reason to disconnect the negative cable first is because a ground short could occur if the positive side touches the body (which is grounded) with a metal wrench.

Quotes source post: 19540979

I'm in Phoenix as well - I recently bought a fairly low mileage 996 C2, just had it fully serviced (new plugs, 997 coils, oil change, etc, etc) and mine does the exact same thing when starting it for the first time each day. The temperature in my garage is 108 currently. I'm inclined to agree with Viper after reading through this thread - my car runs perfectly in every other way so I have no issue waiting for temperatures to drop to see if my car decides it's actually now cool enough to bother keeping the rpms up when first starting.

Quotes source post: 19543176

Interesting! Sounds like that�s the reason. Little tidbit of info: the Porsche M96 996 engines start with an initial higher rev blip is largely due to the oil pump system design. That blip is meant to ensure the pump can lift all oil quickly on startup to critical components. At 100F+, the oil is probably thin enough that this is not required. If it bothers you, you could blip the throttle yourself. I actually hold my revs at 1500-2000 RPM on cold start myself, for 5-10 seconds. because otherwise, I get a lot of rattling at idle when the engine is cold. I�d rather have more oil circulating faster in any cold start situation. Similar to: �start the car and drive off immediately,� which is good practice for these engine anyway.

I�m just now seeing this thread�I made a thread about a similar issue my car is having�I did a search and older threads showed up, but not this one�then I just scroll down and here�s the thread�wtf

An update - I did a cold start yesterday at 6:00am and it started normally - higher revs, typical start that I'd expect from a warm start as well. I immediately checked the ambient temperature and my garage was at 95 degrees. Thirteen hours later - 7:00pm - I did another (relatively) cold start and the engine did not do the higher revs; it held the lower idle as described by the op and myself in earlier posts. Again I checked the ambient temperature - my garage was at 105 degrees. I continue to agree with Viper's post. Looks like a temperature thing. I wonder how the Porsche Engineers decided what temperature would indicate the engine was already warmed up - looks to be somewhere between 38-40C* - I envision them saying, "there's no way any sane person would be anywhere where the LOW temperature is 40C! Let's use that!" *Celsius used in my imaginings because Europe. Yes, they're all wearing white lab coats too.

Yep, 40C in Europe is a bit warmed up... You have to realize that most of Europe is about the same latitude/climate as our NY/Mi/Canada....lol

There�s are still plenty of places in Europe that don�t have AC - richer regions too. It�s been unseasonably hot there already this summer and last year was worse. 105F is presumably a car that was started just recently and driven a short distance, so they assumed oil lubrication is pretty much already present, which it�s not. So that�s why I would manually hold 1200-1800 RPM on your 105F cold start car for a few seconds after start. In the end, probably doesn�t even matter, but we Porsche types are a bit ****.

Subscribing because my '99 just did this to me yesterday after sitting in the airport parking lot for 48 hours. Ambient temp was only in the high 70's, with high humidity though. Very odd.

Jumping back into this thread, and not surprisingly, @Porschetech3 seems to be correct. The car went back to the PO's shop for a bit to fix one lingering issue, so I only just got it back. While it was up there (N. Arizona) I had them check on this issue, and naturally they couldn't reproduce it, since it's in the mid-80s up there with overnight lows in the 60s. Works totally fine in that weather. Next question for the group here is which component actually contains the check valve? Is it the fuel pump itself or the upper sender/regulator piece? I already bought the new pump since I found it really cheap at my dealer, but now that I have it apart, I'm starting to question if that's the right part to replace. Another side note - what's the best way to take care of debris in the fuel tank? Seems to be a lot sitting down there.

To hold fuel pressure on the injector rails it needs 2 check valves.....one at the outlet of the fuel pump and one on the outlet of the pressure regulator .. On the 996.1 cars the pressure regulator is on the fuel rail on the engine, on the returnless 996.2 cars the pressure regulator is in the fuel tank made into the fuel sending unit/housing...

Got it. Makes me feel a little better that there's one in both pieces inside the tank (996.2 over here). I only fully realized the scope of my misunderstanding once I pulled it out this morning. Basically shooting in the dark here on which it is, I guess. We'll know in the AM if it was the pump itself. Very much hoping it's not an injector leaking down. That seems detrimental to other parts of the engine.

I must have "dodged a bullet" or something with hot weather. I bought my C4S around the time of this thread and temps near Palm Springs CA were in the 112*F-116*F range (and all over the west). AV was hotter than anything... No starting problems at all. Car ran fine and did a great job to get to the "cooler" mountains. (posted to find this if needed) YMMV