Help Rennlist! New to me 1999 996 11K mile Carrera MAS MAF Sensor will not "take"

Hello all, My first post after purchasing my first 996 two weeks ago. I was lucky to buy a garage queen, which I plan to actually drive and enjoy. It's a 1999 911 996 6-speed early build (4/98) with 11K miles on it. The original owner was retired, had too many toys, didn't drive or enjoy them. Everything is original, but the owner at least changed fluids and fired her up periodically. It did sit the past two years. All fluids and battery replaced 6 months ago prior to purchase. When I picked it up 2 weeks ago, I replaced the original tires and then drove it from Portland to Seattle. Perfect. It's been great, looks new, feels new. The CEL (O2 Sensing) CEL codes came on a week into purchase; the local shop cleaned the MAS / MAF with the caveat that it might "stick" or I might need to replace the MAS. A few days later it came on again. Along with the CEL's, it's had a rough idle that I wanted to address. I bought the replacement Bosch part (BOS-0280217007 99660612300), plugged it in, the car starts, chokes, then dies. I put the original MAF back in and it starts up fine (with rough idle and eventual CEL warnings). I've searched all these forums and haven't found the silver bullet. Here are some things I (or the shop have done). This is a Top Porsche specialist in the Seattle area, so they know what they're doing, and have the right tools and electronics...whatever that $20k scanner is.... Replaced with OEM Bosch part for the 996 with cable throttle (i.e. not e-gas). The new and old part #'s match perfectly. I've disconnected batteries, cleared the ECU, removed all history of the prior MAF, etc. We removed the MAF and it fires up just fine. The shop tried updating the "software" (DME?), but that didn't work. Oxygen Sensors are "fine". Checked the spark plugs this weekend, looks good. We tried using another brand new Bosch BOS-0280217007 99660612300 to eliminate a "received a bad MAF", but same result. Starts up, then chokes within a few seconds. I've seen other threads (just a few) that mention this same problem, but then the threads end, no resolution found. There's only one MAF Sensor for the 1999's with the cable throttle (BOS-0280217007 99660612300); i.e. no v2's or 124's or 125's etc. This *should* be pretty plug and play. Help! Any thoughts or recommendations? Is there another Bosch part that we should be using? Sincere thanks in advance for any help that can be provided. Bummed at having an awesome new car that can't have it's MAS MAF sensor replaced! Thank you!

Disconnect the mas wire and see how runs it could be just a crappy replacement second because it sat so long it could gummi try some injectors cleaner and give it a good run then change oil again

Thank you for the reply. We tried disconnecting the MAF and it fires up just fine. It's just weird that it fires up fine with the "old" MAF (the one triggering the CEL's and giving a rough idle), but then stalls out when trying the two new Bosch MAF's. It must be the sensor?! I'm gonna go purchase 2 new MAS sensors today and see what happens. Hard to believe that it would be two crappy replacements, but these things have been sitting around for over 20 years?! Thank you!

You need to get to a shop that has either PIWIS (Porsche software) or Durametric so you can read the signals from the MAF (mass air flow) sensor. Given the cost of the MAF sensor, this is one issue you cannot hit and miss on. I see the OE Porsche MAF sensor is $695. When my MAF sensor finally crapped out (after several cleanings), I just bit the bullet and got the OE sensor. I figured better quality control. I was proven right. That was about 7-8 years ago and have been trouble free all this time. The Bosch is probably made in China and they don't have any quality control.

Have you try cleaning or replace the fuel filter and injectors?

I had pretty much the same exact experience with a very low mileage 996.1 that moved very little for about seven years prior to my purchase (so far as I can tell) as it was part of a large collection. 1. You need to purchase the Porsche OEM MAF sensor. No way around it, in my experience. 2. I would take a hard look at the oxygen sensors. I needed one "upstream" sensor replaced. 3. The AOS is likely to **** the bed very soon- I would take a hard look at the unit itself along with the associated plumbing. 4. If the AOS will be replaced, I would take care of the IMS and RMS at the same time. 5. I would suggest replacing all spark plugs and tubes. With the above completed, it will feel like a new car. Please keep us posted!

Much appreciated guys! The local shop has the PIWIS + Durametric, so they're seeing all good readings. Based on your feedback, I will skip the Bosch hail mary and go straight for the OEM. Looks like I can get it for a bargain price of $450! I'll also add the following to the list: clean / replace fuel filter + injectors; replace the passenger side Oxygen sensors (I replaced the drivers side this weekend which was a huge pain in the **** since I didn't remove muffler); keep an eye on the AOS; and then swap out the spark plugs. I should get the Porsche OEM MAF by tomorrow, so I'll provide an update. Thanks again all!

I think you got a lot of good advice above, but I would offer up the following suggestions as well. If your car has the original injectors they are now nearly 25 years old and are probably dirty, leaky, or both, and I would recommend replacing them. The last thing you want is to be fuel washing your bores. Also, cable throttle cars have an Idle Control Valve (ICV) and it is very common for this valve to get dirty and cause rough/surging idle. When I first bought my '99 idle would surge a little at start up, but after cleaning the valve it idle is perfectly steady. Also, if your current shop continues to be unsuccessful with helping you fix this, feel free to PM me and I can recommend a new shop a couple shops in the Seattle area.

If the car idles with the "old" MAF ( faulty one), but doesn't with a new correct one, and also runs with the MAF disconnected, that is the key When the MAF is dissconnected, the DME runs on an Alfa 1 default program that ignores the actual air flow in the engine and only uses RPM and Throttle position to calculate injector flow. When there is a MAF sensor connected, the DME will use the reading of the air flow through the MAF sensor,, IF there is air coming in on the engine side of the MAF Sensor ( leak) it will not be measured and the engine will stall. CHECK FOR AIR/VACUUM LEAK First thing to look for is the last thing that was monkeyed with, or something a Rodent could have nibbled on....or since it was a Garage Queen, could be just old rotted rubber/plastic parts. ( AOS ect.)

Thank you Porschetech3! So I'm only 2-3 weeks into my Porsche 996 ownership and 2-3 days as a member of Rennlist... This place and people and knowledge are the best! I'll let the shop know about the Air / Vacuum Leak and bump that to the head of the list. My car is on the standby list since I was a drop-in, so they're working on it as time allows. They / I haven't ordered the new OEM Porsche MAS...which just might work out if the vacuum leak is the culprit. My initial thoughts about Porsche maintenance... Porsche maintenance is like the game "Whack-A-Mole"! Hit (Fix) one thing, then on to the next think that pops up! It's fun though and thank gawd for the expertise here. Thank you! Again, updates to follow as they sort things out.

where do you find a Porsche MAF for 450?

Quotes source post: 18088894

https://www.getporschesilverspringpa...or-99660612300 In addition, please enjoy extra savings up to $75 through April 30th with promo code: NEWNAME $441.23 with discount right now.

Quotes source post: 18088894

Sunset https://www.sunsetporscheparts.com/o...or-99660612300

Thanks guys!

Hello all, just heard back from the shop. They did a smoke test...everything checked out fine. Just out of random luck and being cautious, they ordered a THIRD MAS Bosch sensor. This 3rd sensor works fine; problem solved! Ugh. I've heard one other example of this happening via the posts (3rd MAS works), but just assumed they bought from Crazy Eddie or Craigslist or who-knows-where, but it just happened again, from reputable dealers. I bought the first one (Bosch 0280217007 99660612300) from a good shop (came out unopened, hologram intact), the shop bought the second one locally from their go to local supplier (0280217007 99660612300), the shop ordered the third one from another respectable dealer (0280217007 99660612300). All the same part, but the third one worked. Frustrating for all! I think I've seen commentary about the MAS sensor being finicky and temperamental....probably goes back to "Only order a Porsche OEM replacement MAS". Surprised to see such hit and miss results with the Bosch "OEM" sensor. Sorry guys for the weird scenario and hopefully not too much of a waste of time, but we got there in the end. Crossing off my list (for now): Vaccuum Leak / Hoses. Keeping on my list: clean / replace fuel filter + injectors; replace the Oxygen sensors; keep an eye on the AOS; swap out the spark plugs, and clean / check the ICV valve. Thank you all! Until the next adventure...the next whack-a-mole problem!

"Problem Solved" I know, I know, I shouldn't declare victory since sometimes we don't know symptoms vs root cause!

When you can't trust "New Bosch Parts" >>>>>

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I would be shocked if they aren't currently made in China...

I would ohm out the sensors to see what you get ?

I've used Bosch MAF sensors in a couple of 996s and never had an issue with them. I wonder if new Bosch parts are not as good as they once were.

Let's blame it on Covid...chip shortage! Comparing the two boxes, it does look like the older looking box WITHOUT a QR code on the label, was the one that worked. The "newer" looking box WITH the QR code on the sticker, was the one that did not work. BOSCH!!!

Oh who knows, they're probably both counterfeit!!! The one on the right worked....Left, nope. Who knows?!

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Glad I have 2 of the old ones in stock.

Right, we should start collecting the old ones like they're fine aged wine....Bosch!!! You can tell the right box is much older since it has 3-4 "Location stickers on it"; meaning it's been moved around at that particular suppliers location / bins over the years / decade(s).

I think I would be writing an E-mail to Bosch saying "Tell me whats going on !!! WTF !!! "

I bought a Bosch unit in March of '21 for my '99 C2 and it worked fine. Having said that, when it comes to ignition components, I prefer Bremi and Beru if available.

Looks like you might have gotten an older one (no QR code above the "wrench guy")! Just guessing here, but aligns with my experience...hypothesis... Again, who knows, except for Bosch and manufacture dates. I should email Bosch with this experience plus tack on the $900 bill from the shop! Fair bill mind you since they ordered and put in a new MAS (after my install failed); re-programmed DME to new MAS (Porsche dealer involved); troubleshooted via PIWIS; smoke-tested for vacuum/air leaks; bought new MAS; success). At least 4-5 hours of troubleshooting... BOSCH!!! Ok ok, you're right, I'm gonna create an email. Not sure who to send it to, but it's always fun sending a blast to the board of directors on the web page. Surprisingly good response rate (i.e. forward forward forward, "Jan Ulrich will look into this for you").

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It's there, just in a different spot.

Sorry for being vague! Both of my boxes have that QR code at the box seal, I just noticed one had it above the wrench guy, the other did not. Look for my faint blue circle in my photo on the left box. In other words, the one without the 2nd QR worked, the one with the second QR code above the wrench guy did not work. Similar to yours that worked, without the QR code above the wrench guy. On a side note: I don't think there have ever been two instances of the words "Wrench guy" stated in back-to-back sentences in the history of the English language! Again, just spit balling, but something is off with the Bosch Quality Control!

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My bad, I didn't catch that.

I've heard plenty of people talk about bad parts new out of the box. I've never actually seen a documented case before though. Darn, Greg. Welcome to the madness buddy. ​​​​​​

Despite the seals and holograms they may still be counterfeit.

Now that the OP seems to have it solved, I would like to chime in. In my case the car starts and idles fine. But it just falls down when trying to come off idle. I have 3 Bosch MAF sensors. 2 work the same and 1 does not work at all. I've taken them to my friends Boxster track car and plugged them in. Its also a 5.2.2. ECU. 2 of them work fine, and the bad one is still bad, this is a new part BTW. I have done all of the same checks as above, and I also sent out the ECU, Immobilizer and key to the ECU Doctors, and they found no problems. Durimetric reads the MAF voltage at 1.0 at idle, shows air temp, air flow, throttle position If I open the oil cap the engine starts to stumble, and returns to idle when closed. One interesting note is that I can crack the throttle a bit, then give a 1 second spray of starting fluid into the airbox, and the RPM's pick up. I had an adjustable FPR and changed back to stock, no change in the problem. If anyone has any ideas, or things to try, i am all ears! TIA Lon