Mass Air Flow / gas mileage

My '01 996 is running fine but seems to be getting lousy mileage. Is the mass air flow sensor a likely suspect? If it is, is a careful cleaning likely to solve the problem, or should I replace it? 2nd, the Porsche MAF on Pelican is listed at $753, while Ebay offers a range of manufacturers with prices from $250 all the way down to the low $20's - and even $16. Opinions? Last question; where the hell is it, on the engine... the videos... (continued at bottom of photo) on Youtube must be Boxters or 997's or the earlier model 996, because I don't see it where they show it. (I suspect it's in, or near the throttle body - still can't find it.) Thanks. PS. Dumb question of the day: I think I have the original radio, but can't find the **** letting me adjust the balance between front and rear speakers. The **** on the right side doesn't seem to affect it.

Middle right edge of your picture.

You can buy a Genuine Bosch MAF on Amazon for $172. https://amzn.to/2InhHbr

If you just gonna throw parts on 19 year old car to get better gas mileage, throw 2 new Bosch upstream 02 sensors on it. It will probably do more for fuel mileage than anything else.

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I consistently get 14mpg no matter how I drive and am looking for ways to diagnose. I have a durametric. Is there a value I can watch that would indicate new upstream 02 sensors are old/bad and replacing them would help? I monitored the data and saw the typical sine wave. Other data: FRA ~ 1.07 on both banks RKAT is ~1.3 on Bank 1 and 0.15 on Bank 2 MAF is ~20kg/h at idle Idle is ~800 rpm Camshaft deviations are about -3 degrees on each bank (working on bringing this down to 0) I have a KN CAI

Hmm, that seems really low. For reference, I get around 23 mpg with a fair amount of acceleration mixed in with every tank. 2002 C2 3.6 MT not sure what to look for but am interested to hear what others have to say.

Just as a point of reference, my engine runs great. I typically get 14-15 mpg around town, stop and go traffic, and 23-24 steady on the highway. I have gotten as much as 26-27 on the open road. I do tend to accelerate rapidly from traffic signals in first gear up to around 4000 rpm (after I'm warmed up). Just leaving the traffic behind. I really don't care too much about fuel mileage....it just is what it is.

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Start there. The KN Hot Air Intake™ is allowing the heated air from in the engine bay to be used. Go back to the stock air intake that pulls cool air from above the wing into the isolating box will result in a denser air charge.

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Makes sense. I've been keeping an eye out for one since I got the car. I guess I should be able to verify this is the cause though by monitoring my air intake temperature. I'll see if I can get some data and compare with whatever references I can find on the forums.

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Read your first sentence again slowly. Try it with me... say it, "My '01 996 is running fine" Good good... again... "My '01 996 is running fine" Getting better... AGAIN, "My '01 996 is running fine" ONE MORE TIME LOUDLY! "My '01 996 is running fine"!!!!! Now, how does that feel? Get off the computer and go drive and enjoy the car. You're sippin' the hypersensitivity kool aid. You don't buy a Porsche concerned about the fuel efficiency.

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I don't disagree. But if, for example, my 02 sensors are causing my car to run rich and therefore impact my MPG, fixing that will pay for itself in fuel savings and potentially save me from possible follow on issues of running rich. With all the sensors / data we have I'd hope I can diagnose a specific component clearly, so I don't have to waste money on just replacing things and seeing if there's improvement. If I am being hypersensitive, then the data should confirm, and I'll go on my merry way filling up every 200 miles.

Ha ha - okay, I get it... I guess I'm with FrozenApples, though, thinking poor gas mileage could be an indication of some kind of issue that should be addressed.

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Using Durametric, select bank 1 02sensor voltage only, select graph display, should display a sawtooth up and down with 5 switches per second (new). Old, worn out, aging sensors will have a slow response as slow as once per second, and can even be curved pattern or delayed in switching. Do same test for Bank 2...

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If you want to trade that K&N, I got an OEM air box I'll trade.

Tech - thanks! This is exactly what I was looking for. I'll get the results early next week when I'm back from vacation. And I'll PM you about that swap as well.

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I may be interested if the OP didnt take it.

I have a lightly used K&N set up I might sell if anyone is interested. PM me.

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ok, I'll let you know

Got some snaps. Really curious about the timescale in the durametric interface. Seems to be labeled in seconds but the plotting seemed to be happening faster than that would indicate. Bank 1 looked like would hang (or be lazy) sometimes. This would be consistent with my tailpipe observations of bank1 having slightly more carbon. Definitely not seeing switching at 5x a second or even 1x a second. My question is, shouldn't this O2 sensor deficiency show up in my long term fuel trims? Measurements were taken at idle (still 800 rpm) and engine hot (~100C) Bank 1 precat O2 Bank 1 precat O2 2nd shot - getting a bit squirrely Bank 2 precat O2 was consistently looking like this

You know what, I should save the datalog and then plot in a another tool. I'll do that this evening.

Looks like the time scale is in seconds, read the comma (radix character) like you would a period.

The fastest switching will occur at 2500rpm, don't expect 5x per second at idle, test at 2500rpm, but idle switching should be consistent, not hanging or delaying.

Definitely collecting the logs and plotting in another tool is the way to go. My plotting skills leave much to be desired tho. Retook the data at 2500 RPM. Kinda tough to hold it there, but the fastest switching area on these plots should be about right. Looks like the fastest switching period is ~1.5 seconds on bank 1 and ~1 second on bank 2. That's a far cry from 5x a second. The sampling rate of the durametric on these is a touch slower than 10x per second. So it's not aliasing. I think at that sampling it would be tough to confirm a 5x/s switching rate. Precat O2 2500rpm

What tire pressure you are running? What engine oil? Your durametric readings (fuel trims, cam devaition, etc) look good. Try this. Get on the highway then reset the mpg, then cruise for a few miles. You should get 25-30mpg. You said your idle was at 800. Was that with the a/c ON? If not, that's not normal. Fully warmed up normal idle rpm should be closer to 680-700. Not 800.

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If you look at the time span from 80-85s, bank 2 switches 11 times in 5 seconds, bank 1 switches 7 times in 5 seconds.

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34 psi F and R 5W-40 Motul X-cess 8100 I don't believe the 996.1 has a MPG reading. Is there a way to get it on Durametric? 800rpm is without AC on. AFAIK this is not adjustable for e-gas. I do have the KN which looks like I'll be swapping for stock this week. Not sure if that will bring it back down. There's a Durametric value called Spec RPM, it also shows 800. Is the spec RPM what the engine is targeting? Ah, I was looking at full periods, you're talking about each switch from high to low. But this is still far short of 5x/sec if that's the baseline for new. Not sure if we've reached a conclusion here. This article indicates a minimum switch time of 100ms: https://www.callasrennsport.com/Blog...ll-performance Which they verified with an o-scope. Looking at mine, even the faster switching bank2 is at best hitting a ~200ms transition time. What's interesting is the article said it's on the verge of setting a check engine light, but that doesn't seem to be the case from the reading I've done. There's not much the DME can do to know how fast the O2 sensor is switching. It's kind of forced to trust whatever value it gets within reason.

2001 996 idle speed should be 700rpm +/- 40 (a/c off) so 800rpm seems a bit high. Your idle fuel trims TRA look good so I don't suspect intake air leak. 34 PSI is a little on the low side for the rear. Have you tried 38 PSI for the rear? I think Durametric can read fuel consumption rate but I'm not sure. Never used it.

Also can you confirm your MAF reading at idle fully warmed up with a/c off? You mentioned 20kg/hr and that sounds very high. Is the MAF voltage ~1.3v?

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Yeah I've seen the spec and always wondered what I could try to fix the idle speed. Didn't find many useful things on searches. It's not off by much, and I've seen many others comment their idle is close to 800 on the 3.4L. Hopefully, I'll get some answers and be able to post here for future reference. I'll put tire pressure on the list. I'm going to try to only change one thing per tank of gas to confirm the impact. I'll start with the airbox this week and go from there. I'll look through the Durametric values to see if there's anything that would indicate fuel consumption. I've been just reviewing past data logs for getting some values. The MAF was collected with the AC on, but my intake air temps were pretty low (30C), so it was a cool evening drive. I sat at a light during the drive for 1 min and the idle ranged from 780-830 rpm while the MAF ranged from 19.5 - 23 kg/hr. Looked through a couple other data collections over the past few weeks and they are consistent as well. I do have a log with the MAF sensor voltage as well. With the engine hot at idle (likely AC on, but again ~30C intake temps) the voltage is 1.38-1.44.

A bluetooth or wifi ODB-II reader with an app will show MPG. I normally get around 16 mpg around town and around 25 mpg at highway speeds. It's really interesting to see how your diving affects the fuel efficiency in real-time.

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The 02sensor does not switch on its own, it just reads oxygen, that's all it does. In the example from Callas, they forced a switch from rich to lean by artificially inducing propane or air (vacuum leak) to force switching and measure reaction time. More than 100ms reaction time is too long and is signs of a lazy sensor. If the sensor takes longer than 100ms to react, then by the time it has reacted, then the DME reads and reacts in the opposite value, then the sensor reacts to that change, much time has lapsed and cannot get the desired frequency. It is easier for the sensor to switch from rich-to-lean (high voltage to low voltage) than vice versa. Therefore a lazy sensor will spend more time at low voltage (causing the DME to increase injector pulse). 02 sensors usually fail or are lazy to the lean side and is why it effects fuel mileage. If for example a sensor spends 100ms at .9v and 150ms at .1v, then back to 100ms at .9v , the DME does not measure the time spent at .1v, it just knows it got the switch and thinks the average is .5v ( actually .450mv stoich 14.7 AFR), and is happy with no STFT or LTFT adjustments, and thinks everything is fine..But the 50ms spent at .1v was wasted fuel !!

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Now we're having fun! I guess I don't know how fast the mixture is being changed, so I can't do the same test without the artificial rich to lean transition. It's not clear to me why the STFT or LTFT would not be impacted as the O2 sensor gets lazy. I mean it's using the reading from the O2 sensor to confirm the mixture. So in the example above, if I swap in an O2 sensor that's not lazy and has exactly 100ms transition from high to low and low to high, then the average voltage would move up to (maybe 0.5V) and the DME would compensate (lean out) the fuel trims to bring it back down to the target 0.45V. I'm failing to understand how the laziness doesn't show up in fuel trims.

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I have one of these for my Leaf. I'll swap it over and give it at try.

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The reason STFT and LTFT is not effected (sometimes)by lazy 02sensors is the "amplitude" is still correct (ie still .9v and .1v). The DME watches "amplitude" and not "switch times"(although is it doesn't switch eventually, it will know there is a problem). If it is a "true" lean condition (vacuum leak) the high reading would be like .75v or .8v , then the DME would trim richer to get the amplitude to .9v.. The DME "assumes" that the 02sensor that switches is reading correctly and is spending the same amount of time high and low, and believes that .3v is richer than .1v and that .6v is leaner than .9v.. All that said, switch times per second is a good indication that an 02sensor is slowing down and aging..

A little more info on 02 aging from the Porsche workshop manual. You won't see this anywhere else, and I'm not entirely sure what a "period" is in Porsche Engineering Speek. But it says the 996 should be less than 3.0 seconds, and a new sensor value will be 1.0, and that the DME needs to see 16 of the "periods".?.?. to be called OK..

I think the period refers to the period of the O2 signal at 1,000 rpm. It says the period of a new sensor should be 1s. That means it takes the sensor 1s to go from low to high and back to low again (one period). The 16 seems to refer to how many of these periods observed (for more confidence). It probably means you take 16 consecutive measurements and each will be at most x seconds, where x=3 for a 996 and 2.3 for a Boxster.

This was my first thoughts as well, but then I was puzzeled by the "16" value, why not 18, or 15,20,25, it seams to be an unrestrained value, or is it restrained to 16 by something?

My guess is they did a computation indicating 16 samples are good enough for the confidence interval they were after.

So I attempted to perform the workshop manual test. I held the engine at ~1500 rpm. It's very clear at the lower RPM that bank 1 and bank 2 are performing differently. I didn't collect a full 16 periods, but I'm certain the average would still be <3 seconds even on bank 1. You can see that 'laziness' though in bank 1, where it likes to hang on the downswing. I swapped back to an OEM intake (thanks Tech!) and collected MAF readings with the A/C off. It's now 15kg/h. But the idle still roams between 760-840 and generally just seems a little lumpy. I tried some CRC QD Contact Cleaner on the MAF sensor and did a e-gas calibration after disconnecting the battery for 15 mins. This made everything seem generally worse for the next 2 drives, but now it's settled back to where I started. I got an OBDII app (Car Scanner) which logs MPG. It worked perfect with the OBD BT adapter and iPhone: https://www.amazon.com/gp/product/B00QJRYMFC/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1 https://www.amazon.com/gp/product/B00QJRYMFC/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1 It seemed pretty optimistic about my MPG, so I did 60 mile calibration. It was about 25% optimistic. But I did log ~17mpg with the new intake. Very subject to inaccuracy on the short distance tho. Next step seems to be to go ahead and do the O2 sensors. I'll swap bank 1 and post a graph with both banks again for comparison. One other thing of note, since the intake swap my FRA fuel trims have moved up to ~1.15 on both banks. They went right back to that level even after the battery disconnect reset. Maybe my lazy O2 sensors are getting bad enough to show up on the trims now.

Ended up doing them both at the same time. Easier that way. Definite difference in the buildup on bank 1 vs bank 2. Bank 1 bottom, bank 2 on top

As promised, new O2 sensor graphs at 1500RPM. New on top and old on bottom. I think it was worth it for bank1, but a waste for bank 2. I saved them though, so now I have backups. Bank1 Bank 2

I�m here to attest that an M96 will run for weeks with a disconnected MAF and not trigger a check engine light. She had a slightly rough/fluctuating idle (+/- 50 to 100 rpm) which made me dig around the engine bay. I found that the connector was placed on the air box mount but it wasn�t snapped in to fully complete the connection.

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You had better check that your CEL light works at all !! Something is not right about that, the CEL should come on with the second trip with a disconnected MAF.....

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It works. I suspect my car has a RoW flash...maybe that is related?

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Yes, if your car has had some kind of reflash then all bets are off on it functioning "as designed". Lol

Is there an easy way to verify if the DME was reflashed via Durametric?

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Yes, on the page where you find overevs, there will be a line that says "number of programming operations" .If your car still has the original programming it will say 1.

If anybody happens to read this and has a minute... What is that measuring device called that can be attached to the engine to diagnose fault lights or other problems? Is that something anyone can buy, or is it a shop tool & crazy expensive? I started this post to try and figure out why my 996 is getting relatively poor mileage; I don't understand a lot of the more technical discussions posted back and forth between various owners here, and I still have questions. One would be - someone suggested I look at the 02 sensors as a possible cause, rather than the MAF; would a faulty 02 sensor show up in the line of dashboard warning lights, or could it be bad or dirty without any light coming on? 2. I have no idea where the 02 sensors are on my car - a 2001 .1 version, and can't find any videos on Youtube showing the location, or the steps to replace it. (I'm assuming the Boxster's are different) I posted a photo of my engine in the very first entry, & somebody responded, but I still can't find it. Last thing; I caught some grief for driving a Porsche & worrying about gas mileage - I'd just say that one of the things I always liked about Porsches is that, besides being incredibly fast, they always managed to accomplish that very efficiently & with a relatively small engine. There are plenty of American muscle cars there are that are fast too, but get that way by burning boatloads of fuel - any dumb engineer can do that. Thanks, Chris

You can get a generic OBDII reader or a Durametric. The generic one will work for monitoring the O2 sensors and MAF, but missed some of the Porsche specific stuff. I recommend you get a Durametric if you want to keep your car for a while. It will save you $ in the long haul and even if you sell, they don't lose much value. Once you get one you can read a bit or I can make a quick step by step to collect the O2 sensor data. A light will come on if your O2 sensors are bad enough, but the concern seems to be they can get pretty bad before a light will come on. My two cents is that fuel trims should show an impact if it's bad enough to show up in MPG. I haven't gotten to drive enough to confirm if my mileage has improved yet, but I'll tell you that swapping my O2 sensors saw no appreciable gain. Unfortunately, that data point is a bit questionable because my car had the 996.2 MAF sensor instead of the correct 996.1 MAF sensor (I have a 2000 cab). I have only run the back half of a tank with the new corrected MAF. Mileage across the whole tank was 17.7mpg. I'm hoping that means the correct sensor is helping. It certainly improved my fuel trims which came down from 1.2 to 1.04 max. My car is stuck in a garage with no driveway for now, so I won't have anything useful to add for a while.