996 3.4 running very lean on high revs

[johnw7884]

My 996 3.4 is running extremely lean on higher revs. Tested on dyno, got only 120 to 140bhp. Engine starts to run a bit lean from about 3000rpm, and only revs to about 4300rpm under load, when it is running so lean it will not take any more revs.

When driving in traffic, engine feels very responsive. But when I floor it, it runs strong until 3000rpm, and gets worse the higher the revs. At 4300rpm runs completely out of power. On idle, revs very well until rev limiter.

So far I have figured out;
- No fault codes (at least does not show on Bosch tester)
- Fuel pressure stays constant, on full throttle stays at about 3.8 bar

I tried to run the MAF disconnected; it idles a bit erratic, but on full throttle same 140 bhp at 4300rpm.

Any ideas? I have tried to google for typical DME input/output values, but have not found any. I have thought maybe the variocam is not working properly, but how would I go about testing them?

I have also considered resonance flapper, MAF, O2 sensors as possible causes, but I do not know how to test them. Also I am puzzled that there are no fault codes.

Any help appreciated. Many thanks!

[bell]

sounds like its pulling in unmetered air....you need to look at the adaption numbers.......
maybe a quick smoke test to check for intake leaks......could be a number of things....
any misfires would be stored........my money is on the maf or an intake leak.....

[johnw7884]

Originally Posted by bell

sounds like its pulling in unmetered air....you need to look at the adaption numbers.......
maybe a quick smoke test to check for intake leaks......could be a number of things....
any misfires would be stored........my money is on the maf or an intake leak.....

Sounds good and plausible. I am going to try new MAF and see if that cures the problem. I will also check for intake leaks. Thanks for the advise! I will keep you updated .

[bell]

before h you just drop in the maf i would look at the adaption numbers first, if they're way out of wack then its going to take a few miles to re calibrate after you instal the new maf, if you clear the codes (even if there aren't any) it'll also reset the adaption for the system, youll need to put about 15 miles on it for it to fully re adapt to set if that fixed it.......
if you have access to a smoke tester our a shop that can check it i would check for leaks anyway.....
oh....and welcome to the board..

[bell]

you can purchase a cable from durametric then download the software for your laptop at their sore, it'll do just about everything the factory pwis can do, except program keys and such.......its a great tool for the enthusiast who can't pay 10k for the pwis........
it'll let you rest codes, diag the car properly with its live data and activations etc........been using out for years at the shop.....
shot me a pm if you need anything.......I've become very familiar with the 996 3.4-3.6.....have diaged and fixed at least a 100 over the past 2 years......
the first big project i did at our shop was swapping a 5.2 dme 3.4 to a 7.8 dme 3.6 x51 into a 99 996, with only a rewire....that was 2 years ago and the thing is still tearing up the track.....lol

[johnw7884]

Thank your for all the advise so far! Current events update;

Originally Posted by bell

you can purchase a cable from durametric

I just bought the Durametric cable, thanks for the tip! Although Bosch tester did not show any error codes, here are the error codes Durametric showed:

========
P0605
Control unit faulty
--------------
P1125 Oxygen sensing range 1 Cylinder (4-6)
--------------
P1128 Oxygen sensing range 2 Cylinder (1-3)
--------------
P1123 Oxygen sensing range 1 Cylinder (1-3)
========

Bosch tester - Durametric 0-1 .

Originally Posted by bell

sounds like its pulling in unmetered air....you need to look at the adaption numbers.......
maybe a quick smoke test to check for intake leaks......could be a number of things....
any misfires would be stored........my money is on the maf or an intake leak.....

I cleaned the MAF, but it has got no effect, so I ordered a new MAF. If I omit the dreadful sounding P0605, MAF or intake leak sounds still plausible. Before the new MAF arrives, I am going to try to look for intake leaks with starting spray (as I have not got a smoke tester).

I will keep you updated when the new MAF arrives.

Another thing that got me slightly concerned is that Durametric shows:
- Camshaft position 1 deviation -3.0 crk
- Camshaft position 2 deviation 0.0 crk ***euro engine, so no worries here, though

Camshaft position 1 deviation stays dead straight at -3.0 crk, no matter if I blip the gas till 5000rpm. Does this sound like problem with variocam?

[bell]

don't worry about the cam deviation too much, won't cause any drivability issues until it gets over 6-7 degrees, at which point will throw a code. this happens as the chain stretches and components wear (including the ims bearing)...the fix is to re-time the cams.....
now that you have the cable you can monitor the deviation....

you have an intake leak somewhere after the maf sensor.....
idle range 1 o2 codes on both banks is a leak 99% of the time....

what are the adaption numbers for idle range 1?

the maf sensors are very sensitive, a single drop of oil can kill one fyi.....so be care full how you clean it (i never even touch them)

[johnw7884]

I finally managed to do DIY smoke test for vacuum leaks. I concluded that the engine is airtight. When I opened the oil filler cap, lots of smoke exited with good pressure. I also double checked for airleaks with starter spray, I sprayed it literally everywhere. Only reacted when I sprayed directly to air filter.

I thought what to do next and looked at the trouble codes again. I have got one more (P1130) since I last checked the codes;

P0605: Control unit faulty
P1123: Oxygen sensing range 1 Cylinder (1-3) - Lean Threshold
P1125: Oxygen sensing range 1 Cylinder (4-6) - Lean Threshold
P1128: Oxygen sensing range 2 Cylinder (1-3) - Rich Threshold
P1130: Oxygen sensing range 2 Cylinder (4-6) - Rich Threshold

These trouble codes seem to me that there is no vacuum leak, because vacuum leak would most likely show on idle. Initially I though completely the opposite, I misunderstood "lean threshold" to mean "running lean", although it should mean "running rich". Would someone please confirm have I got it right?

Originally Posted by bell

what are the adaption numbers for idle range 1?

Here they are;
================
800rpm
air mass: 18.50kg/h
oxygen sensing cylinders 1-3: 0.98
oxygen sensing cylinders 4-6: 0.98
range 1 cylinder 1-3 (TRA): 0.10
range 1 cylinder 4-6 (TRA2): 0.00
================

What do these mean?

I then decided to try new maf, I bought this "OEM quality" part. The results were that the lambda goes to 0.75. I also got error P0103 (Mass Air Flow Sensor - Above Upper Limit). On idle, engine warm Durametric showed 63kg/h air mass, 760rpm. This is obviously way out, I thought should be under 20kg/h, more like 17 or 18. I then tried the new maf in my mates 996, and it gave readings 30.25kg/h with the new maf. Next I tried my old maf in my mates car, and the readings were normal. So it seems I have got a faulty new maf. Also I cannot understand why two similar cars give different readings for the new (faulty?) maf. Could this be because my ecu has been "chipped" or tuned while with previous owner?

Before I buy a new ecu, which is more expensive than many other parts, would someone know a way to test (a) the resonance flapper, and (b) variocam. I would like to know if I can test it by multimeter and/or by supplying battery voltage to the solenoids. I tried to google this, but got to say I did not understand an awful lot.

[pjalexandre]

Early 996 and 986 fuel bumps are known to go bad and lean out the car at WOT at max rpm.

[johnw7884]

I checked fuel pressure while dynoed; fuel pressure increases from 51 to about 55 PSI, where it stays till 4300rpm. Could not test any further as it is so lean will not take more revs under load.

- checked MAF, it is not faulty
- how do you test for resonance flapper?

[johnw7884]

Problem is now resolved. Root-cause was a faulty MAF -sensor. Verified with a new sensor, on a rolling road, engine now gives full power. Advise given was spot on! Thanks for all help!

[Xabi_one]

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

[Xabi_one]

Some further info.

RPM vs MAF signal graph:



Doesn't it look weird at the 6,000 rpm mark? VAriocam, MAF or something else?

Thanks for your help