INTMD8

If it runs good on either distributor it's likely fine. Mine was backfiring on one and not the other. (no check engine light)

Cogito_Ergo_Zoom

Came home from dinner the night before with the car warm and decided to go ahead and start looking into how the fuel system might be doing, so plugged in my OBD monitor and checked the fuel trims with the car warm and they did not look good. They improved when I raised and held the RPM to around 4,500, which reinforced my theory because of the low overall manifold pressure I found previously, that there was still a significant vacuum leak I hadn't found:


So I decided yesterday to start removing the MAF in order to smoke test the manifold. What I found made me really angry with the shop in Houston that did the almost $20K of work:

The airbox snorkel was left disconnected and unclamped:


A filthy air filter was left in:


The lower hose was left unclamped, with the clamp dangling on the hose below:



They cracked the flange to the airbox that hose attaches to:


They overfilled the oil and it's all over the intake and manifold:



Gritted my teeth, connected the smoke test machine to the intake and the root problem made itself apparent quickly:

So now I'm left with a decision about how to deal with the shop and whether or not I attempt to just take the entire intake system out with the engine in the car and do a full rebuild of it..

pp000830

Hi Cogito,

Dont get too upset yet, see my notes below:


The airbox snorkel was left disconnected and unclamped:
Easily reconnected.

The missing clutch vent hose is on the outside of the air filter media and its presence or lack thereof is of no consequence. Some feel attaching the hose just directs clutch debris into the engine intake. Other put a mesh over the steel down pipe and nix the hose.

A filthy air filter was left in:
Easy to replace


They cracked the flange to the airbox that the hose attaches to:
Easily repaired with epoxy or potentially plumber's ABS pipe cement that acts to melt the existing crack allowing it to be fused back together.


They overfilled the oil and it's all over the intake and manifold:
This may be the case, quite common. even if not overfilled a lot of oil vapor from the oil tank condenses on the inside of the intake manifold causing an occasional puff of blue smoke at startup.


connected the smoke test machine to the intake and the root problem made itself apparent quickly:
The smoke test tests the manifold under slight pressure where when operating it runs under a slight vacuum making the test not valid in all cases. I would tighten the clamps and leave it alone. The ECU can adjust for some minor level of intake leaks and if not it will throw an OB Diagnostic Code.

Don't look for love too deeply. If it holds idle, runs well, and does not list any engine-related On Board Diagnostic Code faults it may be best to just enjoy driving the car. A deep dive into stuff may not be warranted.
Andy

Cogito_Ergo_Zoom

Hi Andy, appreciate your perspective and all the help you've provided with your extensive documentation.

To clarify, the car has lit the CE light twice and was misfiring which is what resulted in me stopping and dropping the car in Houston in the first place. I'm just not comfortable with any car producing 8"HG of manifold pressure (and I haven't seen any documentation here or anywhere else on the web this is normal behavior for a VarioRam 3.6). The lackluster performance, the continuing idle issues, the need to change the leaking checkvalve on the tree, the oil leak on the resonance flap valve, and the compromised runner rubber sleeve collectively tell me the VarioRam should come out.

Regarding the performance of the shop, that's a personal issue. If I spend $20K at a reputable specialist Porsche workshop, I expect a certain level of attention of detail and certainly that parts on the car will not be damaged in the process (they cracked the rubber upper fan shroud cover, too, and put it back on the car.) I guess I'm just a hardhead. : )

jcochran1

Thanks for posting all of your findings, I need to follow this path with mine at some point soon.

For what its worth and regarding your most recent findings, I removed my intake with the engine in place and it wasn't awful. I also replaced the sagging engine sound pad with oem while it was out and it also wasn't bad and drastically improved the cosmetics back there.

Cheers and good luck

Cogito_Ergo_Zoom

Hopefully it's of some use to you in the future. I've been in communication with the shop that did the original work and they have asked me to bring the car back and have vowed to make things right.

They will likely have to drop the engine and trans again to address some issues with the latter I haven't even documented in this thread. If they do that, obviously it's a lot easier to address the VarioRam related issues while the engine is out. We'll see if there's a happy ending to the story, but good to hear your feedback that it's not impossible to do engine in. I won't hesitate to take it on in my garage if I can't come to terms with the garage in Houston.

gbgastowers

I have a smoke machine and was wondering where you place the output hose from it on the 993 engine. Can you circle the area it connects to from your photos for me? Thanks. Gunar

Cogito_Ergo_Zoom

I'm not sure what year you have or type of smoke machine, but you simply remove the airbox assembly, filter, and MAF sensor (#10 in drawing below) and insert into the rubber intake connecting tube (highlighted in yellow) that leads to the throttle valve assembly. My smoke machine has an inflatable bladder attachment that allows you to easily seal off the tube completely. If yours doesn't, you might want to think about pulling the hose to the Idle Speed Valve (#11) and insert from there. There's no perfect drawing in the PET to reference, but despite year, varioram vs. non-varioram, etc., the above will apply.

Fair warning, the re-assembly of the MAF and airbox assembly the first time can be a little tricky.


This is it on my car:

Cogito_Ergo_Zoom

Have been too busy to spend much time on the car but have been monitoring the live fuel trim data while I've been driving this week. Originally, I assumed the intake runner vacuum leak on Cyl 2 was causing the AFR "hunting" I was seeing with regard to the continuing idle problems but then I realized the bank with the constant +/- 20% STFT overshoot was actually on bank 2. The LTFT on both banks 1 & 2 are staying below 5%.

If I let the car sit and idle effectively heat soaking, the stumbling/cycling seems to correspond with watching the STFT on bank 2 fluctuate. The STFTs on both banks "correct" and approach zero when I raise the RPM and/or increase load on the engine. All this is leading me to believe I have a sticky or clogged injector somewhere on bank 2 or an O2 sensor problem that's not bad enough to throw a CEL.

Opinions?

Cogito_Ergo_Zoom

At risk of talking to myself here on this thread, but nonetheless figured I would document what I finally traced down to be the root cause of the idle and a good bit of the performance problems as it could help someone in the future.

I know that the shuddering, chuffing idle problem is well documented here and can be hard to trace down. The Motronic system is trying to keep the car right at stoichiometric and as lean as possible in closed loop mood trying to walk the fine line of emissions between HC/CO (too rich) and NOx (too lean). The fact that the car wasn't doing this at cold start up was the major clue.

Having already noticed the constant +/- fluctuations in the bank 2 STFT, I decided to graph them and superimpose them over the 02 sensor data and a strange pattern became evident when visualized:


The shaking idle corresponded perfectly with the trims and voltages diving down to lean like clockwork. The ECM appeared to be chasing its own tail. My first thought was it was likely a bad pre-cat sensor but the post-cat voltages exhibited the same behavior and I don't believe in coincidences.

Studied the electrical wiring diagram and noticing all the sensors share a common ground, I started doing basic tests for ground, continuity, 12v etc. It was curious to me that the individual wire colors on the loom side did not match the respective sensors after I was sure I had verified their placement in the exhaust stream. I was sure I was testing the pre-cat sensor, yet the wires it was connecting to in the loom were telling me that that signal was going to the ECM on the wires intended for the post-cat sensor. Double and triple checked.

Obviously Porsche anticipated the potential confusion here and shape and color coded the connectors. Black for pre-cat, gray for post-cat. So it's literally impossible to cross connect them at the harness.

What it's not impossible to do is prevent a tech from swapping them in the wrong bungs in the actual exhaust and then connecting them to the "right" connector.

Swapped them back (they were a cinch to get off and clearly had been recently removed) and problem solved. Idle and performance improved dramatically. Manifold pressure at idle also gained 2"Hg.

Fuel trims now look exactly as you expect understanding there is a manifold vacuum leak on bank 1 I still need to address:

boomboomthump

Awesome sleuthing and glad you got it licked.

Sometimes we have our blinders on when we look at color-coded plugs/connectors and all seems well. Good thing you double-checked the other end and found the sensors had been swapped at the cat end. Certainly the right analogy to re-enforce your data.. "chasing it's own tail".


Great work and thanks for the conclusion. Certainly will help others in the future and even folks reading it now who will "trust nothing" when troubleshooting and "double-check everything".

Cogito_Ergo_Zoom

She's running night and day better. Next up is a VarioRam reseal and check valve replacement and then sort out this alignment.

A few pics never hurt: