I got my new-to-me '99 996 earlier in the year and it's been a really fun car. The only thing I thought was a little weird about it was that if I was on the highway and tried to pull through 3rd gear at wide open throttle, I'd get a sputtering around the 5k RPM area.

I'd heard that there's a flat spot in that area and assumed that was what it was. The car seems completely normal driving around town and there is no check engine light (sidenote: there's also no CEL when I turn the key to the Accessory position. Should there be?)

Anyways, the local PCA had a Dyno Day yesterday and we put my car on. It was a sort of scary-odd chart. She's going lean high up in the rev bands (see the green Lambda curve), which is obviously killing the power up there. About 50 horseys are missing!

The hunch around the shop was that it's a bad Oxygen sensor. Does that sound plausable?

It's a stock 3.4L 996 cabrio with about 70k miles. Spark plugs and air filter just changed at 60k.

The thing I find oddest is that it idles fine, and drives around town completly fine and peppy. It's just when it's under load high in the revs that something is obviously wrong.

Any ideas? (and, yeah, I'm going to take it to the dealer next week to get the codes read).

Here's the dyno:


thanks!
Dave

 

Check the resonance flap operation. Years ago my 99 996 dyno was 252 rwhp

Switching points of the resonance flap:
Resonance flap not actuated between 700 rpm and 3,120
rpm and from 5,120 rpm.
Resonance flap actuated between 3,120 and 5,120 rpm, if
the throttle is open by more than 30%.

 

Gotcha. So the theory there would be that the flap is stuck open then, right? If more air goes in by the flap actuating, then that could explain the lean issue.

Is this something that would show up on a computer scan? Or is there anything I can do in my driveway to check it? Check the connector, at least? I don't know where the resonator flap is, or if I can get to it. I'll do some more research on it unless anyone has any advice on how to check it.

thanks!

 

Check the changeover valve right above the alternator (circled in red). Also, check for flap operation by applying a vacuum with a mityvac engine off (vacuum line with the 90 deg elbow). You should hear the flap moving

 

I just stopped by Autozone, who ran a quick scan and gave me this info:

P1691
P1125
P1123
P0446
P1121

Looks like at least 3 of those errors are oxygen sensor errors.
Shouldn't I have a Check engine light? I wonder if the previous owner disabled it for me?!?

 

it appears that your CEL isnt working, as it should light up when you turn the key to the on position.

 

okay

 

I misread your 1st post and thought you had no CEL. Jwhipple is correct and P1691 indicates your CEL is either burnt out or was removed. I would check your purge valve and the purge air line. It is located behind the right front wheel liner. Pull the vent line off and use a mityvac to pull a vacuum and the valve should close IIRC.

 

Just installed a new MAF and found my missing 50 horses!

I just took her out for a test drive and the motor pulls strong and smooth all the way up to redline. I got the car earlier in the year and didn't really realize there was a problem due to the car's CEL being 'burned out' so I didn't know what I was missing!

So THAT's how a 996 is supposed to accelerate!

 

I'm facing aparently a similar issue, but not sure it's the MAF.

I was to reflash my 996 C2 (MY2000) and first found max power was at 277 hp instead of the expected over 300 (bypass, gt3 throttle body, ipd plenum, bmc air filter...).

Then the guy checking the numbers found too lean mix at higher rpm which I did not expect as car was running perfect IMO. Anyhow we postponed the reflash till the lean issue was solved.

See the dyno bellow:

[IMG][/IMG]

SInce this test last week I've clean the MAF, checked for leakages and read a LOT of posts about it but still could not get a clear idea. Since I have a durametric cable I did a couple tests on 3rd gear WOT a recorded the following parameters:

1-RPM
2-Mass air flow (MAF)
3-Adaption, range 2 (FRA), bank 1
4-Adaption, range 1 (RKAT), bank 1
5-O2 sensor voltage ahead of cat. conv., bank 1
6-Adaption, range 2 (FRA), bank 2
7-Adaption, range 1 (RKAT), bank 2
8-O2 sensor voltage ahead of cat. conv., bank 2


1 2 3 4 5 6 7 8
1.681 160 1,02 -0,19 0,84 1 -0,52 0,87
1.928 188 1,02 -0,19 0,88 1 -0,52 0,88
2.176 221 1,02 -0,19 0,86 1 -0,52 0,87
2.418 237 1,02 -0,19 0,88 1 -0,52 0,88
2.673 244 1 -0,19 0,88 1 -0,52 0,89
2.921 295 1 -0,19 0,88 1 -0,52 0,87
3.156 323 1 -0,19 0,87 1 -0,52 0,86
3.400 342 1 -0,19 0,88 1 -0,52 0,88
3.679 390 1 -0,19 0,87 1 -0,52 0,87
3.985 437 1 -0,19 0,87 1 -0,52 0,88
4.253 468 1 -0,19 0,87 1 -0,52 0,87
4.539 516 1 -0,19 0,87 1 -0,52 0,86
4.816 561 1 -0,19 0,86 1 -0,52 0,86
5.104 589 1 -0,19 0,85 1 -0,52 0,85
5.424 624 1 -0,19 0,84 1 -0,52 0,84
5.697 667 1 -0,19 0,84 1 -0,52 0,83
5.988 695 1 -0,19 0,83 1 -0,52 0,83
6.286 680 1 -0,19 0,81 1 -0,52 0,81
6.565 704 1 -0,19 0,81 1 -0,52 0,76
6.857 764 1 -0,19 0,83 1 -0,52 0,82
7.056 784 1 -0,19 0,85 1 -0,52 0,83
7.306 643 1 -0,19 0,84 1 -0,52 0,81


Any clues?

Thanks in advance

 

Just as a FYI, you will NEVER reach 300whp on a 3.4 with just those mods. 277whp is a very healthy and realistic figure for a 3.4L.

You got a chart for your af graph?

 

Thanks Alpine for your answer.


I also thought 277 hp were OK, but note they're corrected figures at the crank and not on the wheels, so I guess a typical 15% is added to real numbers.


About the MAF I did use almost half of the cleaning spray but have to say the liquid was falling of very clean, and didn't feel any improvement.


On the other hand I've been doing some work with the previously showed data and the MAF vs RPM graph under 3rg gear aceleration looks like this:







How about the MAF gap at 6000 rpm? Could it be the variocam effect or something is looking bad with MAF or airflow?


Thanks all for your help

 

Sometimes it doesn't matter if you clean the MAF. An old MAF may be distorted from contamination and and worn out. I had cleaned the MAF on my car and felt I had gotten back a lot of hp and then for kicks I ordered a new MAF and voila - 15 extra hp appeared!!!

 

X2. This is a good possibility too. I would either try to borrow another local members or just plunk down the cash for a new one. IIRC, you can get a bosch one for around $100.

6k is too late for Variocam switchover and signs do point towards a MAF issue.

Also make sure you had a chance for car to adapt to new mods with new adaptation values. When I first put on those mods, I got beat by a Audi A5 and my car didn't feel like it wanted to rev all the way up.

As a FYI, ~265whp seems to be the avg on a dynojet for a 3.4L

 

New MAF on its way. Closer to 200€ than 100€ but still less than half compared to OD. Hope for the best.

I'll keep you updated and thanks for the good tips!