What's wrong with my motor?

[DaveFL76]

I've always thought my new-to-me '99 996 had some weirdness going on around 5500 RPMs while at full throttle. It sputtered a bit when I tried to open it up in the 5k area, and then felt better again at 6k or so. I'd heard of the 'flat spot' in these engine's power curves, so I thought it was just the nature of the beast.

The local Porsche club had a dyno day this morning and I got to see what's going on. Everyone confirms it's not normal. She's running lean once the revs pass 5500 (check out the lambda curve). Most peoples' guesses today were that it's the Oxygen Sensor.

So far I have not run a scan on it. It feels completely normal driving around town, and seems to pull through first and second gear smoothly. Starting in 3rd gear when there's a lot of load, you can really notice the sputtering.

Anyways, I'm going to try to attach the scan of the dyno chart.

This is a bone stock (as far as I know) 1999 996 C2 Cabrio.

Any ideas what's going on here?

Thanks!

Dave

Edited December 5, 2010 by DaveFL76

[DaveFL76]

Well, a 5-minute stop at Autozone gave me this info off their scanner:

P1691

P1125

P1123

P0446

P1121

Looks like at least 3 of those errors are O2 sensor errors.

Shouldn't I have a CEL? I wonder if the previous owner disabled it for the sucker that bought it from him? (me)

I'm gonna go look and listen for intake leaks as a start.

Judging from the internet searches I've done so far, the MAF is looking to be the culprit.

I've always thought my new-to-me '99 996 had some weirdness going on around 5500 RPMs while at full throttle. It sputtered a bit when I tried to open it up in the 5k area, and then felt better again at 6k or so. I'd heard of the 'flat spot' in these engine's power curves, so I thought it was just the nature of the beast.

The local Porsche club had a dyno day this morning and I got to see what's going on. Everyone confirms it's not normal. She's running lean once the revs pass 5500 (check out the lambda curve). Most peoples' guesses today were that it's the Oxygen Sensor.

So far I have not run a scan on it. It feels completely normal driving around town, and seems to pull through first and second gear smoothly. Starting in 3rd gear when there's a lot of load, you can really notice the sputtering.

Anyways, I'm going to try to attach the scan of the dyno chart.

This is a bone stock (as far as I know) 1999 996 C2 Cabrio.

Any ideas what's going on here?

Thanks!

Dave

Edited December 5, 2010 by DaveFL76

[Loren]

Well, a 5-minute stop at Autozone gave me this info off their scanner:

P1691

P1125

P1123

P0446

P1121

Looks like at least 3 of those errors are O2 sensor errors.

Shouldn't I have a CEL? I wonder if the previous owner disabled it for the sucker that bought it from him? (me)

I'm gonna go look and listen for intake leaks as a start.

Judging from the internet searches I've done so far, the MAF is looking to be the culprit.

P1123 Oxygen Sensing Adaptation Area 1 (Cylinders 1 - 3) - Lean Threshold

P1125 Oxygen Sensing Adaptation Area 1 (Cylinders 4 - 6) - Lean Threshold

The DME thinks your mixture is too rich (and can not make it lean enough).

Possible causes:

- Fuel pressure too high.

- Fuel injector leaking.

- EVAP canister purge valve open

P0446 EVAP Canister Shutoff Valve (Function) - Below Lower Limit

Possible causes:

- Purge air line blocked.

- Flow resistance of EVAP canister too high.

These point to a charcoal canister purge valve/line problem. I would fix these first (along with the CEL bulb) and then clear the codes and see what comes back.

P1121 Oxygen sensor heating 2 behind catalytic converter – below lower threshold

Possible causes:

–Contact resistance

– Heating current too high

– Interruption in heating circuits

– Interruption in sensor signal wiring

– Short to B+ (End-stage switched off)

P1691 Check Engine Malfunction Indication light (MIL) - Open Circuit

Possible causes:

- bulb burnt out

- break in wiring

- or... someone removed it

[DaveFL76]

Great info, thanks.

I'm going to start with the easy stuff -- checking to make sure the purge line isn't blocked. (the car sat idle for a while (years?) when it was young. Maybe something crawled up there!)

I read some related topics and it sounds like a bad fuel cap can cause problems, too. The o-ring on mine is fairly cracked. I'll replace it, clear the purge line, and see if that gets me anywhere. Then move onto the next step.

It sure seems too simplistic that a bad gas cap or a blocked hose in the evap system could be robbing me of 50 horsepower, though! I'll keep my fingers crossed.

Well, a 5-minute stop at Autozone gave me this info off their scanner:

P1691

P1125

P1123

P0446

P1121

Looks like at least 3 of those errors are O2 sensor errors.

Shouldn't I have a CEL? I wonder if the previous owner disabled it for the sucker that bought it from him? (me)

I'm gonna go look and listen for intake leaks as a start.

Judging from the internet searches I've done so far, the MAF is looking to be the culprit.

P1123 Oxygen Sensing Adaptation Area 1 (Cylinders 1 - 3) - Lean Threshold

P1125 Oxygen Sensing Adaptation Area 1 (Cylinders 4 - 6) - Lean Threshold

The DME thinks your mixture is too rich (and can not make it lean enough).

Possible causes:

- Fuel pressure too high.

- Fuel injector leaking.

- EVAP canister purge valve open

P0446 EVAP Canister Shutoff Valve (Function) - Below Lower Limit

Possible causes:

- Purge air line blocked.

- Flow resistance of EVAP canister too high.

These point to a charcoal canister purge valve/line problem. I would fix these first (along with the CEL bulb) and then clear the codes and see what comes back.

P1121 Oxygen sensor heating 2 behind catalytic converter – below lower threshold

Possible causes:

–Contact resistance

– Heating current too high

– Interruption in heating circuits

– Interruption in sensor signal wiring

– Short to B+ (End-stage switched off)

P1691 Check Engine Malfunction Indication light (MIL) - Open Circuit

Possible causes:

- bulb burnt out

- break in wiring

- or... someone removed it

[DaveFL76]

I went in to the dealership to get a new gas cap (hoping that'll get rid of my P0446 error) and talked with the tech about my other error codes and performance issues.

He's skeptical that anything in the EVAP system is going to cause a loss of 50HP at high RPMs. He says even if the purge valve were stuck open, it's still only a 1/4'' tube and isn't going to make a difference to the huge amounts of air that the engine is gulping down at 5000 RPMs.

He noted that the P1123 and P1125 errors are low range errors, meaning that that data is only good at <800 RPMs and at idle. So they're telling me that I'm idling rich, basically.

Best guesses he had were (1) that the fuel pressure regulator may not be boosting fuel pressure at high RPMs, (2) that it's possible that there's bad coils and if a cylinder were missing spark at high RPMs, then it wouldn't burn the oxygen and that'll come across as running lean, or (3) the MAF is sending bad data. He couldn't think of why my problem would show up so dramatically on the dyno chart right at 5500 RPMs.

So I guess my question now is, is there a way for me to test my fuel pressure regulator at home?

And, if it comes to a MAF, he suggested to go OEM and not aftermarket, that the aftermarket MAFs can be just as bad as the one I'll be replacing. Any info about that?

many thanks!

Ahh.. and the CEL was burned out. I got that fixed, at least!

[mp911]

Regarding the MAF Sensor, you can get an original equipment (Bosch) one from someone like Pelican parts at a fraction of the cost of the Bosch sensor in a Porsche box.

[DaveFL76]

Regarding the MAF Sensor, you can get an original equipment (Bosch) one from someone like Pelican parts at a fraction of the cost of the Bosch sensor in a Porsche box.

You read my mind. I was actually just looking at Pelican Parts. They're $215 there vs. $600+ for a 'Genuine Porsche' one. The more I research, the more I think my problems tie back to the MAF. Think I'm gonna go ahead and order one.

[DaveFL76]

Just installed my new MAF purchased on Ebay for $129.

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

What I thought was a reasonably quick car just turned into a legitimately fast one!

So far no check engine lights at all.

Regarding the MAF Sensor, you can get an original equipment (Bosch) one from someone like Pelican parts at a fraction of the cost of the Bosch sensor in a Porsche box.

You read my mind. I was actually just looking at Pelican Parts. They're $215 there vs. $600+ for a 'Genuine Porsche' one. The more I research, the more I think my problems tie back to the MAF. Think I'm gonna go ahead and order one.

[crazyed71]

glad you were able to fix your problem...