kpd122

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?

996love

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.

kpd122

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.

Porschetech3

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 ...

kpd122

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.

Porschetech3

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"...

hardtailer

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.

Porschetech3

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...

kpd122

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.

Mike Murphy

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?

Viper1000

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.

kpd122

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.

kpd122

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.

DBJoe996

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...

Viper1000

Good luck and let us know what you find.