911cedric

Hello,

i found out working on 993s, that the 96' and 97' with varioram use to have shaky idle. I mean that idle is quiet fine, but the engine shakes on a pattern of few seconds every 15sec; exhaust coughs pretty badly at the same time.
Performances and mpg are unaffected.

i found out that this problem is pretty common on late 993s.
Without being a real threat, it's kind of annoying to feel the engine coughing and shaking when you are at idle.

I scanned many cars with this problem without pulling relevant error codes.
What drew my attention, was the varioram air injection system, and those 4 O2 sensors in the exhaust system. As the 95 never have this problem, i assumed that it was related to the additional systems they fitted on the late models.

It seems to me that without being sure about it, O2 sensors appear to be the problem.
The 95 as only one central o2 sensor when the 96 97 98 have 4, which make them more vulnerable and sensitive to emission and sensors related problems.
The problem is that when checked with a PST2, the O2 sensors values seem normal and do not trigger error codes. Replacing the 4 O2 sensors is not a cheap fix: almost $600 the 4 OEM sensors; so i'd better be sure about it.
It might also be caused by tiny vacuum leaks.

I might be interested by your experiences, if you had the same kind of problem

cheers

911cedric

i forgot to add that it seems to happen only when engine is hot

Mike J

My understanding is that Porsche designed the system to be quite lean during idle, which leads to a slight marginal misfire...or at least that is something I read long ago.

Are you saying that you have done research, and have evidence its systemic ?

I do know that on my car, before I rebuilt it, it had a slight shake on idle. After the rebuild, it seemed quite a bit smoother, and now its got a slight bit of shake, but the exhaust is steady.

Its very subtle, not sure if what you are describing is that subtle.

Cheers,

Mike

911cedric

i have met this problem on many 993s;
however, i never met this problem on 993 turbos and on 94' 95'.
Of course i had many late 993s running perfectly at idle.

most of the time, concerned owners leave it like this; looking for the cause might not always be worth the time and money spent.

Basically, it's always the same problem more or less pronounced; engine shakes intermittently and exhaust coughs like to rich or too lean.
while sitting in the car at idle you feel it very well, even through the gearshift.
It's not just 'a little shake'; most of the time, it shakes so hard that you can hear the clutch and the flywheel moving in the transmission bell.

I have read many posts from guys having hard time with this problem, but nothing relevant came out of the discussions.

I never tried to fix this problem exactly with new O2 sensors, as i never got related CEL.
But i suspect them to make a too sensitive match with the DME settings.

Otherwise, the varioram air injection system an also be the problem;
the things is that due to the complexity of the intake system on late 993s, it's pretty difficult to efficiently spot air leaks. There are many tiny hoses, T's, valves, etc...

do you know if some parts of the air injection are known to be vulnerable?

JM993

IMHO most of these issues are caused by vacuum leaks. I don't believe this typical behavior for healthy 993s.

The vacuum system on the VRAM is not so complex that it is difficult to spot air leaks. A leak can be identified by isolating individual "circuits" at the "TT" connector (driver's side - directly connects to the vacuum tank) and pressurizing with a mitivac.

FWIW, I installed VRAM in my 95 (still OBD1) and it has a rock stable idle.

Cheers,
Joe

Mike J

This sounds quite a bit more than what I am describing. What I can feel is, sitting in the car idling, is a tiny bit of random vibration that almost feels like it disturbs the smoothness of the engine. However, it does not "shake", it's more of a vibration.

If you are getting something that you describe as a shake where you can hear the clutch and flywheel, then that is different than what I am describing.

Cheers,

Mike

JimPA

My '98 has a small flutter at idle, perhaps every 10-15 seconds. Barely noticeable and then only if I'm paying close attention. There's nothing dramatic or significant enough to chase and attempt resolution.

It's been there since I can remember (almost 9 years of ownership), both before and after the top end rebuild.

I asked my indie shop about it and they said it's common. The OP seems to be describing a more severe condition.

J.B.H.

+1 on the possible vaccum leak or more likely cap and rotor issue

axl911

+1 on the vacuum leak. One quick test is to pull a vacuum line while the engine is running. If it doesn't run worse, then it's for sure a vacuum leak.

On a vram motor, one of the capped port in the front (near the fan) is a good place to pull.

BlackCar

My 96 with 44K miles has had the "cough" and lumpy idle since I got it 12 years ago. I've run SeaFoam through the car, use Techron, had new plugs, rotor, and dist cap, and it's the same. It's there regardless of the ambient temperature or brand of gas. The only time it's absent is the first 5 minutes after starting.
My mechanic (former chief mechanic at a local PCar dealer) said that's the way the car is. I'm sure if it was symptomatic of a problem such as vacuum leak or other fixable item, that he would have offered to track it down.

But yes, if there's a way to smooth it out, I'd want to get it adjusted.

mcipseric

loose dual mass flywheel?

chris walrod

thats part of it, absolutely. Also, i've attributed a rough idle to the notion of camshaft timing deviations from one another.

4X4SCHE

On my 97 (with lightweight flywheel) I traced it to faulty coils. I could see blue sparks on the coils with the engine lid open in the dark. Replaced the coils, now smooth.

Cowhorn

I wanted to resurrect this thread because my car has this same set of problems, namely a regular intermittent shaking of the engine when warm and at idle. So far it sounds like the potential problems are:

-faulty O2 sensors
-vacuum leak
-faulty plugs/wires
-loose dm flywheel

Does anyone have any other suggestions or experiences with this problem? I am going to try a new DME relay to see if that makes any difference before I pursue the other potential causes. This is very annoying problem, and I'd love to get it remedied ASAP.

mstolo

If the 02 sensors aren't throwing a fault, just replace the pre-cat sensors. As I understand it, its the pre-cat's that impact the fuel mixture and interfaces with the engine controls. Post-cat is merely for emissions. At least this cuts your cost down. I'm about to replace mine and I'm only going to do the pre-cats, and wait till I throw a CEL before I spend the money on post-cats.