up4speed

I searched all over the Internet for my problem, but the solutions seem to be all over the place. I also didn't find the exact symptoms as my car. Hopefully someone knows what the problem is, or someone can help me with step by step troubleshooting.
I noticed that when my car is started cold for the first time of the day, it idles rough. It won't stall, it just feels lumpy. This will last for a few seconds, or until I rev the engine a few times. Once it smoothens out, it stays smooth and runs strong. The check engine light did not come on and the car will start right up without hesitation also.
The spark plugs and coils were changed within a few thousand miles and I recently changed the air filter and fuel filter. I also cleaned the MAF/MAP sensors.
I have an obd2 reader....where do I start?
I was thinking of starting with hooking up the obd2 on a cold engine to see if the readings on all the temp sensors make sense.
Please help!

Kevin

You need to datalog the lambda circuit. Wideband O2's and a NEW MAF, and pressure testing your intake system is a good start.

rmc1148

Good advice from Keven and agree new MAF not cleaning.

Macster

Could be normal. My 2003 Turbo engine is a bit rough running at cold start. More so than for instance my Boxster's engine.

With no background with these engines one could very well believe there is something amiss.

I hear the Turbo engine backfiring through the exhaust. The engine is over fueled to get raw fuel to the converters where it is burned to help them heat up quicker. The engine runs rough because of this over fueling. Not helped by the low lift being in effect.

Do the engine RPMs rise/fall any? Of is the engine just a bit rough until it gets a bit of heat and the idle speed drops? That is what my Turbo engine does. I just let it idle and after not too long the engine smooths right out and the idle speed drops to near hot idle and I then just drive off.

With no CEL, there is not much to go on. You can check for pending codes I guess. You can datalog various engine operating parameters if you want. Coolant temp and intake air temp to name two. If either of these differ too much from ambient that could be a sign a sensor is not reporting the correct temperature and the engine could be too lean at cold start. If you tell me you hear the engine spitting back through the intake rather than backfiring through the exhaust this makes a temp sensor more likely.

mcbit

TBH mine does the same and I really have no issues with it.

"02996ttx50

mine is 25k miles+ and there is nothing "rough" at startup. just 1k rpm until it settles and richer mixture.

up4speed

Thanks for the reply. I'll take a look into the OBD2 reader I have and see how much info I can pull from it. How do I pressure check the intake? Do I need to hook up a smoke machine to look for leaks?
Also, is there a reliable way to check the MAF sensor? I don't want to just throw parts at the car that may not be bad in the first place.

I'm pretty sure that the RPM's don't vary that much and it seems to have an appropriate idle speed (I will pay more attention next time I start it up), the best way I can explain it is that it feels like a lumpy idle. Almost like an old American car with a big cam. There is no back firing or awkward noises, just a shaky car for a few seconds. I kind of feels like half of the cylinders are not firing at full potential.

James88

Mine does it as well, I mentioned this to my indy and he cold started 2 other 996 turbo's and they both did the same thing.

up4speed

A little additional info. I just hooked up my OBD2 reader with key on, engine off on a cold engine (approx. 50 deg). I attached the photos of the reader showing all the pages of readings for the live data (there were no pending codes).

I then started the car and it was rough again (it's 100% consistent when cold). It idled at 1200 rpm, and when it was rough, it varied by about 100 RPM. I also was able to smell a rich mixture from the exhaust.

I hope this info helps a little.

Macster

It is impossible for any of us to know for sure if the idle is just the normal rough idle -- rough here is a relative term -- or if there is something more going on.

If you are unable to make this call I advise you to take the car to someone who can then experience the car and its rough engine at cold start behavior and advise you.

My Turbo engine has always been a bit rougher at cold start -- all other things being equal -- compared to my Boxster. Not scary rough just rougher. Kind of like my 2006 GTO at idle. Kind of like the Turbo had a big old high lift/long duration cam installed.

But the Turbo engine smooths out concurrent with the idle speed dropping to its near hot idle speed. Then the engine is pretty smooth. Not quite as smooth as the Boxster but close enough.

An odor of gasoline at cold start can be normal. Everything is cold and the engine is fueled richer than it would be otherwise. Most of my engine starts occur with the windows up. And I seldom get out of the car right after starting the engine but once in a while if the wind is very light and blowing just so, I get a whiff of the exhaust gases in the cabin as they are pulled in by the cabin vent fan. There is an odor of fuel. It is not an overpowering odor.

As I offered above you need someone who has experience with these engines and who then can tell you if the cold start/idle behavior is normal or needs looking into to.

I would not delay or put this off. If there is a problem, it can have excess fuel in one or more cylinders and this fuel will wash away the oil from the cylinder/piston/rings and that ain't good.

mcbit

100% agree with Macster. I don't do real cold starts over here, my cars are never at fast idle for more than about 30 seconds. During that time it's not quite a sweet flat six rhapsody.

rmc1148

Its cold here and my car is as smooth at start up as in warm weather.

Dock

Can you post up a video clip that will allow us to hear what you are talking about?

up4speed

Thanks for the great description. Based on your description, the car may actually be normal because I can also explain it exactly as if the car was an old American car with a high performance cam. It feels lumpy. I also agree fully with your comparison to the Boxster because I had a Boxster S and I never had any idling issues.

I cleaned the throttle body today (what a pain in the butt! I hate working on this car!). It actually looked spotless, so it's probably not the issue. After I started it, it stumbled a little as expected since I cleaned it, but I won't know if it improved unless I start it cold tomorrow morning.

I'll report back tomorrow. If that doesn't fix it, I have to agree that it may be normal, or I may have to seek a professional that has experience with the 911TT.
One more quick question. When I put the turbo pressure hoses back on, do I have to treat the rubber o-ring with anything? A coating of oil, or maybe Molykote o-ring grease?


I tried doing a video, but it didn't really show much. The engine sounds smooth in the video and you can't see how the car has a shake. The only thing is that the RPM's vary 100rpm at most. It doesn't even seem too abnormal. It's just the shake of the car that feels abnormal to me.
I do have the Ultimate Motorwerks tune, but I don't think that it has anything to do with it because I don't remember the car doing this for the first 5 or 6 months that I've owned it (bought June of last year). I wonder if it's related to the cold weather?

Dock

My car has EVOMS Stage II and a Europipe Stage II Loud, and on cold start it feels like the engine is plugged into my spine; plenty of high frequency vibration transmitted into the seat and passenger compartment. It smooths out as the engine warms up.