FlyingLow

I've already searched but couldn't find anything like I'm experiencing. I'm wondering if anyone else has experienced this. When driving at a constant rpm, like say 3000 or so, every few minutes the car jerks pretty hard like the engine is stumbling or something. It's a pretty violent jerk. I'm hoping that the dealer can reproduce it and diagnose it. Hopefully it's just a software upgrade or something. Very weird, I've never had any car do this before. Any ideas?

KPG

I get a slight bump around 3K.... I have been told that is when the cam switches over to the larger lift. Kevin

FlyingLow

Hmmm. It's more than a slight bump and it happens at any constant rpm, not just 3000. Somethings not right here.

Oak

could need a new maf or try and clean it, possible plug change. how many miles?

wross996tt

Sounds like plugs to me as well.

Tippy

My cars occasionally mis-fires but it does not constitute a jerking, more like a slight vibration from the cylinder that misses. It also makes a droaning noise in the cab until she fires on all 6. Sounds like either a intermitent complete loss of ignition (not a plug) or fuel. That is why I rule out plugs. You could have some water in the fuel, this causes jerking of the engine. Water would cause random jerking where a plug mis-firing would have a set pattern and a little bit of power loss. When you say jerking, I think of complete loss of engine power intermitently causing an abrupt slamming of the driveline.

FlyingLow

Tippy, your description is very accurate. It is a very abrupt slamming of the driveline. The car only has 7K miles on it. I'm going to see what the dealer says. I'll have to learn more before I try cleaning the MAF, this is my first turbo car. I'm very interested in what you guys have to say. Thanks for any comments. I hope it's not water in the gas, I try to use Chevron as much as possible and haven't had a problem with their fuel in the past. Sure does make it hard to go for a nice cruise.

xilvar

Not sure if this happens to the 996 TT since I've only just gotten one myself, but it sounds exactly like what was happening to my e36 bmw 325i for about a year before I sold it. When I was preparing it for sale an ex bmw formula mechanic caught a listen of it and told me that he believed the timing chain was failing and that I shouldn't drive it anymore until I had it fixed. I drove it some more anyway to bring it to a mechanic near my house and he verified that the timing chain was failing and that likely because of that the head gasket was also failing. What seemed to be happening when I experienced the sudden jerks on the highway was that the engine was falling slightly out of sync. It felt almost like the engine was trying to suddenly come to a halt at whatever RPM it was at but that the car's momentum was forcing it to continue and eventually it would fall back into sync and continue.

99firehawk

your feeling the vario cam and vario lift, the car wasnt designed to be cruised at 3000-4000 rpm it desigend to be full throttle for accleration and expects to be placed in a high gear for cruising out of the variable timing and lift range. YOu do not have a failing timing chain. Acclerate and be done with it put the car in a crusing gear or keep it over 4000 rpm if your on a track, or on the street just deal with it as a fact of life if you want to cruise in the 3000-4000 rpm range

1999Porsche911

That is a ridiculas statement. The variocam will not make the car jerk if cruising in the range of 3000-4000 rpm. You can cruise all day long in a properly running TT at 3300 rpm, 3500 rpm, etc with absolutely no engine miss, hesitation or jerkyness. Normal cam engagement is nothing like the abruptness described.

Flyinglow: there is something that is not right. I would check what is mentioned above and would not overlook the obvious, such as bad gas, MAF cable connection (remove and reconnect), check all plug wire connection, throttlebody connections, etc. Also check O2 sensor wires for bare spots as a quick touch to ground can cut the system off without blowing the fuel pump fuse. Your description makes me think electric problem more than anything else. But, obviously, we are all guessing.

If your have a laptop with scanning software that can log sensor activity, you stand a good chance of identiftying exactly what the fault is.

How long have you had the car and has it always done this?

PorscheMD

Is your check engine light comiing on at all?

99firehawk

[QUOTE=1999Porsche911]That is a ridiculas statement.QUOTE]



You would be amazed how often i get this complaint and after a drive with the customer with the tester hooked up and watching the vario cam engage and disengage when the customer complains of
customer states misfire at part thottle 3000 rpm
customer staes vechile hesitates while accelerating
customer states vechile shakes and bucks while cruising
and many other complaints
thats usually what they are feeling.
Turbos are usualy really good at telling you when theres something wrong the tolerances anything must excede is very very small to set a check engine light.
Cam timing off (streched chain, failed tensioner, out of time) a few degrees will set cam timing and misfire faults.
Maf sensor will throw maf sensor faults, or o2 adaptaion faults or misfires.
Its not impossible to not have a fault and there to still be a problem but in this situation I would bet money that on a test drive with a tester you can wacth the vario cam come on right about the time he feels his jerking.

FlyingLow

I appreciate the comments guys. I've only owned the car for eight days and it has only 7K miles on it. It wasn't doing this during the test drive or the first couple of days I owned it. I have owned 911's for the last 16 years and have never felt anything like this, on turbos, or my Carrera S which had variocam. It is at the dealer right now and hope to hear something today. Something is amiss (no pun intended). The variocam should not be sending a jolt to the driveline like I'm feeling. We'll see.

Bob M

This has happened to me on more than one occasion. Your issue could be due to something else but mine was related to the MAF being dirty. It is easy to fix: undo the one bolt holding it in place (don't drop the bolt!), carefully pull out the MAF, spray the sensor with CRC MAF cleaner and reassemble. It will take you all of 5 minutes to do, if that. Problem solved.

Wachuko

A 2003 car with 7K miles... taking it to the dealer was the smart move... leave the working it yourself after the warranty runs out.

Do let us know what ends up being the culprit.