Re: You promised not to tell ...

Hi..

I usually average about 220-240 miles on about 12-13 gallons of gas (18-19) which is kind of low to begin with. That's a mix of highway and city. Recently I had the dealer do an oil change, new spark plugs and an air filter. Since then I have noticed I am getting about 4-5 mpg less. I took it back to the dealer and they didn't find any fault codes. Even if I drove only city at 18mpg x 13 gallons = 234 miles..

The car does have a history of a faulty oil separator. The dealer replaced that a while back under warranty. At that time they didn't replace anything else other than the separator.

Any ideas? I thought it could be something like the MAF sensor...but surely the car would throw some codes if it was bad?

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SoundMaster
Hi..

I usually average about 220-240 miles on about 12-13 gallons of gas (18-19) which is kind of low to begin with. That's a mix of highway and city. Recently I had the dealer do an oil change, new spark plugs and an air filter. Since then I have noticed I am getting about 4-5 mpg less. I took it back to the dealer and they didn't find any fault codes. Even if I drove only city at 18mpg x 13 gallons = 234 miles..

The car does have a history of a faulty oil separator. The dealer replaced that a while back under warranty. At that time they didn't replace anything else other than the separator.

Any ideas? I thought it could be something like the MAF sensor...but surely the car would throw some codes if it was bad?

Most often a large drop in gas mileage is due to the (mis) measuring of the gas mileage. It is really important that you fill the tank to the same level each time. Unless one is very careful the nozzle can shut off prematurely. I have found that for really consistent fill ups turning the nozzle almost 180 (turning it almost upside down) has the auto shut off the most consistent. But even so once in a while I catch the nozzle shut off shutting off fuel flow prematurely. I actually walk around to the other side of the car and turn the key on to check the level is "full". If not then I resume adding fuel.

Be careful to not overfill. The car mags to check fuel consumption fill the tank right to the brim. While this helps deliver consistent numbers this is hard on the active charcoal canister. IOWs, Avoid filling the tank too full. Let the auto shut off work -- properly -- and do not be tempted to really top off the tank to the brim.

Plain old properly measured gas mileage varies quite a bit. My 996 is quite sensitive to city driving. It doesn't take much city driving to have the gas mileage really drop. Filled up today and the OBC reported 18.0mpg over 149 miles. This includes a daily (M-F) 30 mile each way work commute (that unfortunately while mostly freeway miles has the car traveling at less than freeway speeds...) and some city driving only errands as my other car is still in the shop awaiting parts.

Pure highway driving can have the gas mileage up to 26mpg. But it takes attention and restraint to get it there and keep it there. Just a few indiscretions with the accelerator can have the fuel mileage drop and it takes a long high miler mindset drive to restore the fuel mileage and even then one doesn't get it all back.

Couple fo things: Check tire pressures. Be sure these are inflated properly. Also in some regions of the country gas is being switched to the summer blend. This blend can have gas mileage drop some. But at most this will be just a few (barely) percent. Not the 4-5 gallons per mile you report.

If you have the right equipment -- OBD2 code read/data viewer -- you can view the fuel trims and O2 sensor readings. But really with no CEL, no engine misbehavior, it is hard to believe the actual fuel mileage is as low as you report/believe.

Thank you for the reply.

I realize the amount filled up can vary, so I go by gallons filled based on the fuel gauge level. If the light comes on, then assuming about 3 gallons left...I figure I should be filling up at least 13 gallons. I don't really go by the auto shut off since it turns off prematurely most of the time. If I fill up between 1/4 tank left and the before the light coming on, I do about 12-13 gallons. I try not to overfill.

Tire pressure and alignment is good.

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SoundMaster
Thank you for the reply.

I realize the amount filled up can vary, so I go by gallons filled based on the fuel gauge level. If the light comes on, then assuming about 3 gallons left...I figure I should be filling up at least 13 gallons. I don't really go by the auto shut off since it turns off prematurely most of the time. If I fill up between 1/4 tank left and the before the light coming on, I do about 12-13 gallons. I try not to overfill.

Tire pressure and alignment is good.

Well it is my opinion that you are not filling the tank in a consistent manner and this accounts for the change in gas mileage.

You have to rely upon the auto shut off but you must be careful that you use the gas nozzle in a manner in which this shut off is itself consistent. There seems to be some considerable difference in shut off sensitivity.

While my Boxster was pretty immune to this the 996 proved to not be immune. I finally hit upon the technique of when filing the gas tank to turn the nozzle upside down, to have the handle pointing nearly vertical. This resulted in the auto shut off being much less sensitive and when the thing did shut off fuel the level was based on my monitoring gas mileage every fill up very consistent.

As I touched upon in my first post, even with this technique once in a while the nozzle shuts off prematurely. When I suspect this has happened -- to avoid possibly over filling the tank -- I check the fuel gage and if it reads a bit less than full I add more fuel. Sometimes I can add a couple of gallons more fuel. You can imagine how a shortage of a couple of gallons can really affect gas mileage.

By filling the gas tank this way the fill up levels are very consistent and any change in fuel mileage clearly arises from the difference in how I choose to drive.

It is in fact easy to under-fill. One of my cars fills consistently to the top, the other has a mind of its own.

But its very clear from a quick look at he the fuel gauge - i will be visibly low, as much as 1/5 tank. I'll also note that if you reset the computer each fill, this gives a check that is independent of the fill. Any my mileage never varies except predictably (lower with oxygenated fuel, higher for true, pure highway trips, lower with trailer, etc.)

By any chance did your gas just change to the summer mixture? That's good for 1-3 mpg for me.

How many times did you get this lower reading? How consistent was it?

Have you checked for oxygen sensor codes, or checked the exhaust gas mixture? Rich mixtures are easy to measure (high HCs). Heck, i can often smell it if its that bad.

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

that after an unusually low MPG tank the next tank registers unusually high. This is a consistent pattern over 70 tanks. I attribute this to inconsistent fuel shut off at the pump. I totally ignore any readout from the OBC. And I don't react at all when I get inconsistent readings since I know over time and many tanks, things will even out.

I use an app called fuelly.com available online and from my phone.

Sounds like your intake was cleaned when the seals were replaced along with the AOS. And the MAF is upstream of the AOS hose that dumps into the intake so the MAF should not usually be suspected of oil contamination unless there is an oiled air filter being used.

I know you filled up at the "best gas" pump setting but that doesn't mean the fuel truck stuck his nozzle in the right hole.

What plugs were used? Heat range? Gap?

What air filter?

Not a good thing to happen when gas prices are rising again.

Does the tail pipe look a different color than it did?

Spark plugs are Bosch FGR5NQE04 (999 170 130 90)
Air filter - 987 110 133 00

Tail pipe doesn't look any different.

I agree the first instinct should be look at the auto shut off and gallons filled..but I am trying to see what else it could be. From the needle being at 4/4 to the needle coming down to 1/2...that's about 8 or so gallons.
Currently I am at 110 miles on 1/2 tank... 110 miles / 8 gallons = 13.75mpg. That's pathetic because I am driving it like a grandma to see what's the best I can get.

Prior to this maintenance, I was getting 150-160mpg on half a tank. I know that's not great but I was "ok" with it.

If the spark plugs weren't a pain to get out, I could have someone look at it...but 2.5 hours of labor ain't cheap.

Just to clarify, it sounds like you are only getting about 14 mpg - about168 mi. on 12 gal. of gas. Is that correct? That is pretty low. Over how many tankfulls have you averaged that?

If you get into more traffic, have to make more frequent stops than usual, that can have a significant effect on mileage. Or, as Marc suggested if you don't completely fill the tank it will appear that you are getting poorer mileage on one tank of gas.

Have you used the OBC to measure mileage? I found that on both my 06 and 14 Boxters that it measures quite accurately. I always found it to be within 0.1 mpg of what I calculate. I normally reset it at each fillup.

That's correct. I have done 2 tanks so far but I noticed it right way since I keep a keen eye on the fuel gauge and miles driven. The OBC is showing 15mpg since the last fill up. It used to be in the 20s.

The only thing I can think of is maybe the MAF sensor is dirty especially since it was not replaced after having a failed air/oil separator. Even when I had a failed AOS, i didn't get a CEL.
According to the service note..."intake manifold seals were leaking a substantial amount of oil caused by a faulty oil separator. replaced oil separator and intake seals".

It just seems like such a huge difference in mileage.

Is it a manual or PDK?

Do you smell any gas around the car?

I don't know what type of driving you do. If I was driving in a place like New York City, I'd probably be happy to get 15 mpg.

I don't see spark plugs making a huge difference unless old ones are really worn. I had the plugs changed in my 2006 after 60 K miles and found no noticeable difference in mileage and performance.

PDK. I am in Houston, TX.

It definitely is a noticeable difference in mileage. I am guessing it's the new spark plugs that are causing the issue...considering it started after the dealer changed.

What's funny is when I took it back there, I probably waited about 30 mins at the most..guy comes out and goes "nope everything looks good, spark plugs look fine..". Huh? You looked at the spark plugs in 30 mins..but you charged me for 2.5 hours of labor 2 weeks prior when I had you put new ones in?

Luckily I found a good indy mechanic. I guess I will take it there but I was hoping to have something specific they can look for...other than redoing the spark plug job.

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SoundMaster
PDK. I am in Houston, TX.

It definitely is a noticeable difference in mileage. I am guessing it's the new spark plugs that are causing the issue...considering it started after the dealer changed.

What's funny is when I took it back there, I probably waited about 30 mins at the most..guy comes out and goes "nope everything looks good, spark plugs look fine..". Huh? You looked at the spark plugs in 30 mins..but you charged me for 2.5 hours of labor 2 weeks prior when I had you put new ones in?

Luckily I found a good indy mechanic. I guess I will take it there but I was hoping to have something specific they can look for...other than redoing the spark plug job.

Not sure what could have happened to the car/engine during a service to account for the change (drop) in gas mileage. Generally problems after a service are misfires -- coils not installed correctly or "bad" plugs (more on this below) -- or a vacuum hose got damaged or knocked off and there's an air leak.

About "bad" plugs... Were the plugs installed the right plugs or something you wanted to try or plugs that someone recommended to you?

On another forum I came upon a IIRC a Cayman (but could have been a Boxster) owner who was having problems and the short version is he installed plugs that while the cross reference chart indicated were the "same" as the plugs specified by Porsche -- and I double checked the cross reference -- the plugs proved to be just lousy. The were of some kind of a recessed tip style and were just not working very good at all. He went through some other things first though and it was only after I viewed the new plug vs. the old ones and noticed the type style and suggested this could be a problem did he replace the plugs. He reported the problems were gone after.

The plugs used in these engines have an exposed/extended tip. The word I get is the tip sticks out and at low speed operation runs hot to keep the tip clean and generate a good spark for efficient low speed running. At high speed the tip gets cooled by the incoming air charge.

I have to believe the tech didn't check the plugs in just 30 minutes. When I have a question about my cars the tech calls me back and shows me the part or assembly or whatever it is I had a question about and discusses this with me until I'm satisfied. I'm aware of the time so I'm pretty careful to keep on subject and try to keep my questions to a minimum, but I am entitled to have this discussion with the tech and leave with confidence the right action is being considered, since I will be the one paying for it.

Would I advise you to change plugs at this time? Well, let this new tech advise you. However, I note the plug you installed is a single ground electrode and the correct plugs are the 4 ground electrode type. These multi electrode ground plugs are superior in at least this application. The incoming charge swirls about and with just one electrode this can have the spark blown out.out.

I'm leaning towards the plugs being the cause of the sudden drop off in gas mileage. But let the tech at the new shop you have elected to use advise you.

Thanks again for the detailed reply.

Are you saying the 999 170 130 90 ones are not right? It's also the same one listed in the parts catalog. I didn't pick it out..it's what's listed on my service paper.

Can the indy shop verify if the O2 sensors are working properly?

using "Bosch FGR5NQE04". The picture I looked at the last time, was of a single electrode plug. However, I noticed something I didn't notice before on the picture -- the largest picture of the plug -- and that is the words: "Representative photo".

I did some more searching and the rest of the pictures were of a 4-electrode style plug. So my mistake. Based on the one picture I found I believed the plug was a single electrode style when in fact the actual plug -- not a preventative one -- is the 4 electrode style.

Thus the plugs in the engine are (should be) the right ones.

Sorry for the goof. And again thanks to Grant and his taking the time to post which prompted me to revisit this.

Then as to what is going on with your car's engine and its drop in gas mileage, I think you'll have to wait and see what the tech tells you. I'm having a hard time thinking of ways an engine could suddenly increase its fuel consumption and yet not have any CEL.

I wonder what the short term and long term fuel trims look like? And what the intake air temperature and coolant temperature readings look like? All I can think of is the DME thinks the engine is colder than it actually is and is thus more richly fueling the engine but I would still have believe the DME would note something amiss from the O2 sensor readings and set one or more error codes and turn on the CEL.

Went by the shop today..and they said if there's no CEL, there's not much they can do. The tech doesn't think the new plugs would cause a drastic change in mileage without causing an CEL.

Bummer.

Would anything show up in durametric without a CEL? Maybe I can buy one and scan it myself? I am tempted to take the MAF out and clean it..but I was hoping the shop would do it.

... but several years ago I had a customer with very low mpg symptoms and lack of power, especially at higher RPMs.
He had just had it serviced by another independent shop before the symptoms showed up.
That shop washed their hands. He then took it to a local dealer who kept it for a few days, didn't find anything either.
He then brought it to me. I did all the battery of tests with the PST-II computer, and couldn't find anything obvious.
Finally, on a whim I decided to check the engine air filter. Nothing.
Disconnected the throttle body. Nothing.
I stuck my fingers into the air intake tube and felt an obstruction.
Turned out to be an oily rag. Apparently the first shop had stuffed it there and forgot to remove it when they reassembled.
You never know.
Good luck,
Happy Boxstering,
Pedro

Pedro Bonilla
1998 Boxster 986 - 311,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)

---

Racecar spelled backwards is Racecar

"Racing is life. Anything before or after is just waiting" ... Steve McQueen as Michael Delaney in "LeMans"

"If you wait, all that happens is that you get older"... Mario Andretti

"Being second is to be the first of the ones who lose" ... Ayrton Senna

You promised not to tell, Pedro.

Great story!

"A mile of highway will take you one mile. A mile of runway will take you anywhere."

Gary your story involved an abandoned Snap-On screwdriver which I still use to this day!
Happy Boxstering,
Pedro

Pedro Bonilla
1998 Boxster 986 - 311,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)

---

Racecar spelled backwards is Racecar

"Racing is life. Anything before or after is just waiting" ... Steve McQueen as Michael Delaney in "LeMans"

"If you wait, all that happens is that you get older"... Mario Andretti

"Being second is to be the first of the ones who lose" ... Ayrton Senna

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Pedro (Weston, FL)
Gary your story involved an abandoned Snap-On screwdriver which I still use to this day!

BUT.....it didn't reduce my mileage at all, not that I keep records on that.

"A mile of highway will take you one mile. A mile of runway will take you anywhere."

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Gary in SoFL

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Pedro (Weston, FL)
Gary your story involved an abandoned Snap-On screwdriver which I still use to this day!

BUT.....it didn't reduce my mileage at all, not that I keep records on that.

Gary, did you find that your acceleration after the removal of that tool was not quite as snappy?

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Guenter in Ontario

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Gary in SoFL

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Pedro (Weston, FL)
Gary your story involved an abandoned Snap-On screwdriver which I still use to this day!

BUT.....it didn't reduce my mileage at all, not that I keep records on that.

Gary, did you find that your acceleration after the removal of that tool was not quite as snappy?

Glad Pedro didn't mention where the tool was extracted from.....

"A mile of highway will take you one mile. A mile of runway will take you anywhere."

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MarcW
However, I note the plug you installed is a single ground electrode and the correct plugs are the 4 ground electrode type.

I just looked the Bosch number up. It is a 4-ground electrode and crosses to 987s/997s.

Grant

Grant

gee-lenahan-at-gee-mail-dot-com

Steady long distance driving at about 75 mph through the Great Plains gives me ≈ 34-35 mpg.* Driving in my area, definitely not "City" and it drops to 15-16. About a year ago I tried following the upshift indicator, but the thing will have you in 6th at 40 mph, which is ridiculous. I don't think the Sport throttle setting makes much difference mpg-wise. As I mentioned some time ago, because it's like gaining about a quarter of a gear down, I found myself upshifting sooner because the initial response is somewhat comparable, so the aggressive mapping is offset by my earlier upshifting.
* What amazes me is that's about, or even an mpg or two more than what my base 01 could get, and the 981 Boxster S has a nominal 98 more horsepower.



Edited 1 time(s). Last edit at 04/18/2015 09:34PM by Laz. (view changes)

... filling up with high octane?
Dropping the octane will dramatically lower mileage.
Happy Boxstering
Pedro

Pedro Bonilla
1998 Boxster 986 - 311,000+ miles: [www.PedrosGarage.com]
PCA National Club Racing Scrutineer - PCA National HPDE Instructor - PCA Technical Committee (Boxster/Cayman)

---

Racecar spelled backwards is Racecar

"Racing is life. Anything before or after is just waiting" ... Steve McQueen as Michael Delaney in "LeMans"

"If you wait, all that happens is that you get older"... Mario Andretti

"Being second is to be the first of the ones who lose" ... Ayrton Senna

Nope. Have always done the highest grade at either Chevron or Exxon.

Both reputable shops. Neither of them can think of any reasons off top of their head and said there's really not anything they can do. Rennsport in Sealy, TX and Swiss Garage in Houston.

I don't have the experience to take stuff apart for diagnosis purposes and I am afraid I will do more harm than good. I can always buy the durametric software and get any readings off that..but not sure if it will show anything...

I am mainly concerned with the performance and longevity of components than cost savings. The money I can potentially save on gas will just get negated by the amount I will have to spend to diagnosis this.

I'm sure you didn't knowingly do this, lol, but no one tossed a few duffel bags full of bricks or cinder blocks (or the equivalent) in your front trunk, right? Hey, you never know..

All kidding aside, I hope you get it resolved.

Regards, -rick.

You'd think that you'd notice a difference in performance.

Are you watching what gear you're in? Maybe the PDK is staying in too low a gear?

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SoundMaster
Both reputable shops. Neither of them can think of any reasons off top of their head and said there's really not anything they can do. Rennsport in Sealy, TX and Swiss Garage in Houston.

I don't have the experience to take stuff apart for diagnosis purposes and I am afraid I will do more harm than good. I can always buy the durametric software and get any readings off that..but not sure if it will show anything...

I am mainly concerned with the performance and longevity of components than cost savings. The money I can potentially save on gas will just get negated by the amount I will have to spend to diagnosis this.

to 14mpg, that's a big drop. By my calculation a 28% decrease in fuel mileage. And with no change in driving style and no untoward engine behavior and no CEL, frankly it is hard to believe.

If I give you the benefit of the doubt and accept you are measuring the fuel consumption correctly, the only thing I can think of is there is a fuel leak or perhaps somehow the odometer is off, not reporting miles traveled accurately.

My 1996 Mustang GT odometer did this. While the speedo was "accurate" I could drive miles and miles -- 15 to 20 -- and the odometer would only advance a few miles. I reported this and the dealer tested the speedo and because it reported the right speed claimed there was no problem. I found out later there was a TSB on the odometer problem. Seems the grease was too thick or something. Because the car was under warranty I said what the heck. Not my problem I cover maybe 100K miles with the odometer only registering maybe half that or less. Stupid Ford. Stupid Ford dealer. Now you know why while I've bought 6 cars since then not one was a Ford.

Perhaps during the service the battery was disconnected and some odometer compensation setting was lost or got garbaged upon the restoration of power?

It might be worth it to give the odometer a check, drive over a known distance and see if the odometer -- both the main one and the trip odometer -- register the same distance. I recall when I first noticed my Boxster's speedo was reading approx. 5mph less than actual I was concerned about the odometer and with a GPS device in the car drove 200 miles and found the GPS and the car's odometers differed by just 0.2 of a mile over 200 miles.

It might be worth the effort, though I'm never much in favor of messing with the battery unless I have to, to disconnect the battery for a while then reconnect it. if you do this be sure you follow all the proper battery disconnect and reconnect precautions.

Afterwards you should reset the window limits and calibrate the E-Gas. Which reminds me: I don't think it will help, but what the heck. Try an E-Gas calibration. It is easy to do and costs nothing. See if that helps gas mileage.

Really, though, I can't believe a modern engine could increase its fuel consumption by over 25% without the engine misbehaving, without the CEL coming on. The air/fuel mixture is very tightly controlled to ensure the engine gets fed a mixture right close to 14.7:1. The exhaust gases produced then are most efficiently processed by the converters. Both the exhaust gases just before they enter the converters and the exhaust gases after they exit the converters are constantly monitored for correctness. 25%+ more fuel into the engine and it is hard to understand how the DME could not fail to see this, so to speak. While the DME could dial back fuel, it would exceed its the limits by which it can adjust fueling and this should have the CEL on.