Bisan

I got dragged by a buddy to go see a 996 Carrera even though I have a wicked C4s for sale, but he doesn't want an AWD car. I digress.

He is almost certain to purchase it. But I noticed something that concerns me, it doesn't seem to concern him so I figured the quickest way to get an answer is the forums.

2001 996 Carrera 2
78k miles
Non PSE exhaust
No CEL
No noises or rattles
Power is good (stock) car
Showed us paperwork for new plugs, maf, and coils within 6 months, almost makes me wonder if he was chasing an issue.

For some reason the driver exhaust pipe (should be bank 2) is just low on exhaust flow.

I had my trust duramtetic cable and the owner allowed me to hook it up to the car but it showed no issues. I thought maybe cam timing was off but its within spec from what i can see.

Bank 1: -.22
Bank 2: 8.01

I am pretty sure this is measured off the cam so at the crank its 4 degrees off, that still within porsche spec of 10 degrees off.

Any ideas from the masses?

808Bill

Does he know that the C4 can be converted to 2 wheel drive?

Bisan

Lol. He does. I even have some GT3RS uprights I told him I'd sell him, but he doesn't want to mess with it. A C2S widebody with some GT components would be awesome.

AWDGuy

how do you know it has low exhaust flo?

Soul Performance

It's possible that one of the catalytic converters could be broken off and clogging that bank while NOT throwing a CEL code.

Really that's just an educated guess, the only way to be sure would be to perform a compression and leak down test.

Bisan

Car has had two new Cats within the past year as well. He had a heat gun we checked inlet and outlet temps by the Cats. And they were dead nuts close to even and about 125-150 degrees hotter on the outlet than the inlet.

Tells me the cats are working properly. So assume its not the cats, any other ideas? Because I immed thought it was the cats to be honest.

If you put your had on the outlet of the exhaust pipe when the car is running the amount of pressure on bank 2 pipe is almost non existent. You'd think that entire bank has no compression.

Car is going in for a ppi. And after doing some googling I can't find anything about the subject.

1) clogged cat
2) low compression across the entire bank (far fetched)
3) ?

Could it be an issue with the exhaust box itself on that bank?

Soul Performance

First, it just sounds like it would be wayyyy easier for your friend to buy your car!

Second, out of curiosity, what brand cats did he put on, are they OEM? The outlet temps being the same definitely seems to indicate they are working but they could theoretically "be working" and still block the flow.

Has it been verified that there are NO exhaust leaks anywhere? The leak down test will at least verify the condition of the head gaskets, valve seats, rings etc. and tell you if you are dealing with a serious mechanical issue.

Bisan

Soul Performance. #1 I love the name, #2 I agree he should buy my car but I think he doesn't want it to be always known as "Andy's old car"

I'll let ya know about compression and leak down. I have a weird feeling about this issue. Specially since I've googled my lil heart out before I posted this and no one has mentioned a similar issue anywhere that wasn't the cats.

From the bill it seems like they are OEM, but i'll have him ask to be sure. Stay tuned.

Soul Performance

Well thanks for the kind words!! Also, feel free to PM/EM/Call me if you run into any dead ends. I might not be the end all be all of M96 knowledge, but I know people that are and can get the answer!

Evan

RRTEC

I looked at one that had this issue. Or a similar one.. it would clear up after a quick drive and only did it on initial start up. I passed on the car, although later he called me back to let me know he fixed the issue, it was leaky injectors causing lower pulses out of the exhaust due to washing of the cylinders. I had already moved on to another 996. But could be that...

Bisan

Slightly low across the board but relatively but uniform.

These number were all done HOT. Would have that they should be higher but they are all close within each other. They also smoke tested the car which came back negative.

I also doubted the injectors could be causing the issue, they showed me a flow chart of an injector cleaning from less than a year ago about 2000 miles ago.

Cyl Compr Leak
1 165 6.5%
2 160 9.5%
3 157 5.0%
4 155 8.5%
5 165 6.5%
6 160 7.0%

My buddy purchased the car. Even though the exhaust issue has really been bothering me, he thinks I am just trying to force my car on him, which I am not.

He already has cat bypass pipes which were are going to install ASAP, I'd like to see if its a fixes the exhaust issue. Hopefully in the next few hours since him and I are both off work.

With all that being said... other than a clogged cat what are the other ideas that can be causing this???

Possible reasons: Reasons list
1) clogged cat
2) injectors (fairly recent service)
3) Spark plugs (new plugs as of today; he paid the ppi for new plugs)
4) Maybe coils? I might switch coils from one side to the other....
5) Vacuum leak/airleak (passed smoke test)
6) No CEL/Codes

If its not the cats.... couldn't it be the actual muffler? Something dislodged and causing a blockade? Wouldn't this cause other issues? Doesn't this cause knock? Since its too much exhaust pressure?

Help lol. My head is spinning.

OKB

cat or muffler can be bad, but if its had cats replaced the answer is one of them is not normal, either the old one is good or bad, or the new one is good or bad, but they are mismatched

Bisan

Well both cats are out and the bypass pipes are in. Its safe to say it was not the cats.

Same issue is still apparent. Exhaust output flow/pressure from bank 2 side is weak to almost nonexistent.

Maybe I should have had him start her up without the muffler boxes on to see if the issue was in them. I guess the saga will continue. Since we are done and the car is already on the ground.

So is that all we are left with fellas? The issue is in the exhaust box itself?

Bisan

So update: We straight piped the car pressure felt much better but still not even. So its not the actual exhaust/muffler. And I did make a mistake when I checked camshaft deviation.

Bank 1: .22
Bank 2: -8.01 This number has fluctuated slightly -8.21, -8.28

So bank 2 is retarded. Is there a way to figure out if the motor is out of time or maybe it has a bad hydraulic tensioner?

Cam Timing under load seems to be more uniform.

Macster

IIRC cam timing at idle and to around 1400 RPMs should be retarded. If my memory is correct bank 2 is ok but bank 1 is not.

Any pending codes? With Durametric I believe you can manually trigger cam shaft timing advance and retard and then note the cam timing readings which should go a long way to helping you know what's working and what may not be working.