High maf voltage and HFM, Please Help!

[RedRotors]

I picked up a 996 a few months back, not running.  It will start and runs for 10-15 seconds and sounds perfect, after that the idle drops and it stumbles around 500-600 RPM until it eventually stalls.  It runs pig rich.  With the Durametic, in a keyed on state, not running, I am seeing high maf voltage and high mass air flow HFM numbers.  Even with the MAF unplugged the 11 kg/h never drops or goes away.  I've tried another MAF with the same results.  Any clue why the MAF voltage is so high?  I've included a Durametric log.  Other data seems to be in line, cam deviation is 1-2 deg.  Any insight would be very helpful!

 

2017_07_23_16_26_30.xls

[RedRotors]

I also should've mentioned that the car will not rev.  The throttle body is opening and adaptation was done.  It runs severely retarded timing -9 deg or so while it's trying to idle.  I figure the retarded timing is why it won't rev, and the timing is retarded because of the high maf voltage at low rpm.  

The maf voltage and high kg/h  is the root of the problem, would you agree?

Edited July 26, 2017 by RedRotors

[JFP in PA]

Not necessarily.  At idle, a good MAF would read in the 12-24 Kg/H range.  What kind of readings did you see while it was still running?

 

I would also remove connector of MAF sensor, connect ohmmeter to MAF sensor pins 1 and 3.  Display should read : 2.3 - 2.7 k ohms at 20 °C

Also check for a short to ground.

 

When it was running, did you capture fuel trim values?

[RedRotors]

Thanks for the quick reply.  I was thinking the 11 kg/h maybe a default for open loop cold start.

 

I attached an excel spreadsheet of the log while it was running.  IIRC it's around 30 kg/h while trying to idle.

 

Ive replaced the maf and I get the same readings.

 

I can certainly log fuel trims and post back.

[DBJoe996]

Here is what I would do.  Undo the battery negative lead for an hour to reset the DME.  Disconnect the MAF sensor and leave it disconnected.  The engine doesn't really need the MAF for idle up to about 4,000 RPM.  Connect up the battery and fire it up and see what happens.  At least that way you can take something off the list.  Did you replace the MAF with a brand new Bosch unit specific for your car?  Is it installed the correct way?  Makes a huge difference.

[Ahsai]

With key ON engine OFF, the MAF voltage should be ~1.0v. At idle ~680rpm, it should be ~1.3v.

The MAF reading from Durametric could be a substituted value (when the DME has detected issues with the MAF).

 

You need to remove the MAF and check the resistance like John said. Then with key ON engine OFF, check that you get 12v on pin 2 and 5v reference voltage on pin 4 (on the harness connector side with the MAF removed). Also, only use MAFs from Bosch that is specific for your model and not the cheap Chinese knockoffs.

[RedRotors]

Gentlemen, thank you.

 

Yes, the maf was replaced with the same Bosch 986 part number that I removed.  Yes, it's install in the correct orientation.

I will check the 12V supply and the 5V reference voltage and reset the dme.

 

The tuner has my dme currently.  I had I flashed back to stock to see if anything would change.  Right now he is reflashing the proper tune back.  

I didn't want to over complicate my original post, but the car has a VF charger kit on it.  Most recent diagnostic work was done with the charger disconnected, stock flash, and the stock maf housing.  The dme was verified good and ran another car.

 

Now it's being flash back to the custom tune that matches the injectors and the larger maf housing. 

 

Any idea why its pulling so much timing?

[DBJoe996]

Well, that explains a lot.  So you had the custom tune for the VF super charger kit, and then reverted to the stock tune w/stock MAF housing and with modified injectors and larger MAF housing....and you are wondering why the timing is retarded?  This is not a simple MAF sensor problem.  Too many variables.  Thought we were talking about a stock engine and so on.  I hope your tuner knows what he is doing.

[RedRotors]

I understand your comments above, and almost expected it.  This has been an on going issue and I am doing my best to eliminate possibilities.  

 

The car was dyno tuned by Frank Smith and ran perfectly for years, I know because it was previously a good friends car.   I have nothing but respect for his tuning abilities.

 

Since then, they car was sold to another local that was not knowledgeable.  The supercharger needed the bearings replaced, the car sat, and I purchased it.  Put it all back together and it has this issue.

 

This is my first Porsche, but far from my first tuned, modified, or supercharged car.

 

I posted here after a couple months of pulling my hair out, with the car on the lift, in hopes someone has come across this in their past.

 

Thank you for the helpful advice, 12V, 5V reference, and ohming the maf are the next on my list.  2 bad mafs are unlikely, but not unheard of I suppose.

[Schnell Gelb]

So to eliminate a lot of variables,why not do as suggested and idle the engine with the MAF cable disconnected at the MAF ?

Not a brilliant suggestion but quick,simple,does no harm and costs nothing.

MAF problems sometimes set no codes.

You must have  codes associated with the rich stall condition?

If you disconnect the battery ,remember to follow the Initialization Procedure

Edited July 25, 2017 by Schnell Gelb

[DBJoe996]

I do hope you get it sorted.  Most of the knowledgeable people on here can help with stock setups, but very few have experience with superchargers and custom tuning.  There are so many variables with a stock engine, much less one that has what you have.  I would suggest keeping it simple and sticking with the basics.  Checking the reference voltage is a very good first start.  I was wondering, if the tuner got the car to run right perfectly for years, is there any way to get the car back to him?  Maybe he could get it sorted out.

[RedRotors]

Schnell, I've ran the car with the MAF unplugged and it runs in a similar fashion, maybe slightly worse.  With the MAF unplugged the DME falls back on known good MAF values.  I do not think those values are manipulated with the custom tune, so with larger injectors and larger MAF housing, the car no longer has a known good value to fall back on.  This is why I had it flashed back to stock, and used the stock housing, granted it still had larger in injectors, but still behaved the same way with the MAF unplugged.  Crude I know, but the injectors are a PIA to replace. 

 

DBJoe996,  I stopped in and saw the tuner Tuesday to drop off the DME.  He has more work that he can handle at the moment.  He offered to take it, but pointed the the other cars that have been sitting...

 

Testing the MAF IAT, I have 3 of them now, the all test 1.8K ohms at 26 deg C.  Waiting on the DME to test voltages.  

I've also smoke tested the engine for vacuum leaks, only 1 small one from the brake booster O ring, it was replaced.

I'll keep you posted.  

Thank you for the advice.

[DBJoe996]

I'm going to ask some stupid questions....in your original post you said " I picked up a 996 a few months back, not running.  It will start and runs for 10-15 seconds and sounds perfect, after that the idle drops and it stumbles around 500-600 RPM until it eventually stalls."  You suspect that the MAF is responsible and we have been running down that rabbit hole.  These things make an engine run - fuel, air and spark.  You also said it is running "pig rich".  I am wondering if it is something more fuel related than air related.  Does the car have fresh gas in it?  Has the fuel filter been replaced? What about the fuel pressure regulator?  Have you cleaned the Idle Control Valve?  When was the last time the plugs were pulled and checked?  How are the coil packs looking?  You also said it does not rev but the throttle plate is moving. Have you checked the throttle position sensor and if it is plugged in correctly?  It sure seems a strange set of conditions.  I would expand your search to some other things.  Just throwing some other things to check into the mix, and just trying to help.

 

Once you get your DME back, use the Durametic and go to Actual Valves and see if the Throttle Position sensor is returning values.  Is it throwing any codes at all?  A bad TPS can wreak havoc on idle and running.

 

I sure wish Ahsai  would weigh in again...he is far more knowledgeable than I am.

Edited July 26, 2017 by DBJoe996

[RedRotors]

All good questions.  

Fuel is a couple months old, but good.  I did all the basics as preventive maintenance.  New plugs, updated coil packs, and checked fuel pressure.  The car is a 2002, so it runs a returnless fuel system and E-gas.  The throttle is opening and the proper adaptation is done every time I disconnect the battery.  I've verified the TPS with Durametric.  I've also replaced the pre cat O2s.  I also replaced the chain tensioners, properly locking the cams and crank, nothing moved or slipped.  Verified deviation on durametric.  I even replaced the AOS for good measure.  All maintenance work was done before attempting to start it for the first time.  I've retraced my steps many times to check my work.  I'm at a loss.

When it runs from cold start for those first 15 sec it does sound great.  From cold, it starts in open loop, its seem that when it goes to closed loop it falls apart.  Another interesting fact, if I spray MAF clean at the intake filter, just one tiny spray, the car smooths out and runs fine for another 10-15 sec.  It definitly runs smooth way longer that it would take to burn off the MAF cleaner.  I've done some stand-alone tuning in my past, mostly on speed density systems, but it seems that the MAF needs to be scaled.

Just weird...

[Ahsai]

Fuel enrichment is there during open loop so everything is nice. Once in close loop, it sounds like to me the engine is starving of fuel. Have you checked the fuel pressure and flow rate? You could check the pre-cat and post-cat sensor voltage to see if the engine is running lean or rich and if close loop control is effective.

 

With non stock injectors and intake, I don't even know how you begin to diagnose with a stock tune. Note that the MAF is coupled to the stock intake so if you change the intake and use the same MAF, it may not be accurate anymore. 

Edited July 26, 2017 by Ahsai

[RedRotors]

Ahsai,  it is definitely running rich.  If you check the excel log I posted (link below the durametric screen shot pic) youll see the maf kg/h and voltage.  Both are substantially higher then they should be reading.  This was taken after a dme reset with stock maf housing. 

[JFP in PA]

Again, I would like to see fuel trim values.......

[RedRotors]

Jeff,  indeed that's my next step.  Thanks!

[JFP in PA]

I'm with Duncan on this one, with oversized injectors and a different intake, I don't think the stock DME program is up to the task.