My 996 is getting 12 miles per gallon

I purchased my 996 a few months ago and I have put about 600 miles on it. When I bought it, I had to replace the alternator. During replacement, it was tough to get the old one out and new one in. There is a lot of shifting and rotating to fit it under the plenum. I remember that one of the small hard plastic vacuum lines that passed to the right of the alternator got looped over the pivot bolt for the alternator. I had to wiggle it over the bolt to get it out of the way. I also pushed and pulled on the air pump hose that passes to the left of the alternator during the replacement process. Otherwise, the replacement seemed to go smoothly. I drove the car an it ran great, but shortly after I began driving it, I got the check engine light with two codes, P0410 and P1411. I cleared them and they came back a short time later. When I researched the codes, I thought that maybe I had damaged one of the air pump hoses during the alternator replacement. Those hoses are hard to get to behind the engine so I figured I would check them out when I drop the engine in the near future. When I bought the car, it had a relatively full tank of gas and it was obvious the trip odometer was not reset when it was last filled. When that tank got to about one quarter full I filled it up and reset the trip odometer. I drove that tank to about one quarter and filled it up. Based on the trip odometer, I go somewhere around 10 mpg, which made no sense. I am getting some grey hair so I figured I accidentally reset the trip odometer when I set the time on the clock. I filled it up and reset the trip odometer again. I filled it up today and I got slightly under 12 mpg. I know I didn't have a senior moment twice in a row, so I started looking. I gently pushed and pulled on all the vacuum lines and found one of the hard plastic vacuum lines not attached. Looking at a picture of the vacuum lines on-line, I determined it was the line that runs from the changeover valve #1 (above the alternator) to the intake resonance flap actuator (on the back crossover pipe on top of the plenum, behind the throttle body pipe). I took off the throttle body assembly, which obviously had never been removed on my car, and sure enough, I saw the tube running from the intake resonance flap actuator laying back there. Picture is a little overexposed due to the bright flashlight. Third picture shows lines reattached. I cleared the codes and drove the car briefly. Codes did not come back. My first thought was damn, the engine has been running really rich and I have gas in the oil. I could not smell gas on the dip stick, but the oil fill tube definitely had a strong gas smell. Fortunately I am running 15w50, but I will not drive the car any more until I change the oil. Hope my experience helps someone in the future. Eric

Once repaired, perform a system reset of the DME to clear the excess fuel trims.

I've had my car a year and a half. Bought it with 43k miles, now has 52k. Since day one, driving around the city a lot, using manual mode a lot of the time, my 2000 C2 cab gets about 12 mpg. On the highway its much better but I never measure. The car feels right, the exhaust pipe tips show a nice even light gray, no start up issues, no running issues. Not burning any oil, change it every 5k or 5 months. I've never had a car give me the mileage it claims on the sticker. For the moment I'll chalk it up to my right foot.

I have had my alternator out of the car 7 times so now I can do it with my eyes closed. However it is one hell of a job first time. You can in fact spin that large plastic intake tube around to the front for re-attachment of the small plastic tube you feature in your posting. Loosen up the two large jubilee clips and the whole thing will twist around. I have mine attached now so I can see it (at the front). When you remove your alternator for any reason again, remove the two very brittle evap lines first. You can also re-route the secondary tube to the outside of the space available for easier removal of the alternator.

FWIW my 2003 C2 gets 16-18mpg pretty consistently. And I don't drive it easy. At least that's what it shows on the instrument cluster, assuming that is accurate I have not done the math.

Those tubes you broke are for the secondary air injection system... they preheat the cat in cold temps to reduce emissions. I also broke these while doing an oil filler tube replacement.. Yes they are a pain to replace and after years of heat cycles are quite brittle and probably all needing replacement in all cars of this vintage. I plan to address them this winter. They should not be affecting your gas mileage though long story short.

Stop driving like I do and your mileage will improve.

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I plan on driving it when I feel like it, but don't want to put a ton of miles on it in a short time period. I plan on keeping it for a while. I also have an F250, my wife's, and both of my boys cars around, so there is always something else to drive if I want to. I realize there may be something else going on. I bought an oil filter today and will change the oil this evening and drive it around tomorrow. I will see if the codes come back and see what kind of gas mileage I am getting. I will reset the DME as Jake suggested. I will update the thread when I have more info.

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None of the tubes were broken, but the one going to the intake resonance flap actuator was disconnected, so I have a vacuum leak and the resonance flap wasn't functioning. I made an earlier assumption that the check engine light was associated with the air injection system. I also did a little reading on resonance and the intake plenum. There is definitely an impact on efficiency with the air/fuel mixture if it is not working correctly.

I'm getting 11.5-12mpg (all city) over the past 6 fill ups, using physical gallons divided by the trip odometer. And this is driving incredibly mellow, and skipping shifts (like 1 to 3). I've done everything I can think of including; replacing air filter, cleaning MAF, proper tire pressure, not using A/C, etc. The car doesn't throw any codes, and runs incredibly well. My mechanic believes its the wheels I have on the car, and the extra width of the tires that are fitted to them. (19's w/ 305s and 235s). Though I must add that I live in the middle of Los Angeles. My entire commute is stop and go traffic, mixed with some hills. Regardless, I never figured the car would be getting worse gas mileage than a 5,490 lbs Rolls Royce Phantom w/ a V12, but I digress...

2002 996 purchased with 76,676 miles, suffered IMS failure at 100,280 Best average was on a 5500 mile road trip (brown line was average)—ave over the 5500 miles as 69+ mph with a max speed 124 mph. Typical non-roadtrip driving was mixed between freeway and street driving. Then the average was ~23 mpg. Granted, I am a hypermiler, but this shows what a 996 is capable of. P.S., my computer read about 0.5 mph higher than the actual measured gas consumption

and just to be obnoxious

I get about 16 mpg around town and up to 24 mpg on a long highway drive. YMMV - literally.

I stopped calculating after the first couple of tanks resulted in 15mpg...better not to know

I am about 60/40 highway to stop n' go downtown traffic. Just filled up this morning and I am at 19 mpg for my 98 model

You could always purchase a Prius'. Who cares about MPG when driving this or any other Fun car, your right foot is the culprit. DUA.

^ I agree, some vehicles just need to be enjoyed for what they are. If you want better gas mileage get a fuel efficient car.

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I always thought once you got messing with tire/wheel diameters you will affect the accuracy of the speedo which could affect the gas mileage calculations...No? Does the OP have stock size wheels and tires. Rolling resistance could also be a factor. If you don't think so just try to push your 911 then turn the steering wheel just a little! Seems like a "heavy" 3000# car to me!

In my '02 C2 MT, I get ~22 MPG in mixed use, and 26+ on the highway. The PO put larger sized tires (235/35-19 and 295/30-19) on the car, so I have that going against me. It's an "occasional" car, so a a tank lasts a month or so in mixed use. I get ~325 miles out of a tank. Fuel is the least of my expenses.

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Don't mean to highjack the thread but thanks for the input. Appears by the calculations my front is almost .5" larger in diameter than stock which decreases the Revs/Miles by -13. The rears are almost spot on with regard to diameter. Im wondering if the fronts being out by that amount can disrupt the actual speedo, even though the rears are correct.

I have the original stock 17 inch wheels and tire size (Z rated street tires) on my 99 C2. I talked to the previous owner on Saturday and he stated he got as much as 26 mpg on the highway and typically around 17 or 18 in town. He was surprised I only got 11.9 mpg on the last tank. Based on the smell or gas in the oil fill tube, it was definitely running too rich. I attempted to change the oil yesterday, but I used an incorrect oil filter wrench and I cracked the oil filter housing when I was reinstalling it. Good news, I think, was that there were almost no metal particles in the filter or drain pain (the few I saw were not ferrous, appeared to be aluminum), but there were several dozen small plastic particles, from the chain guides I assume. I will definitely be changing them out when I drop the engine. I put in an order for new housing, an oil filter wrench, and an LN Engineering magnetic drain plug. I searched for the LN Engineering oil filter adapter, but the first few places I looked were sold out. I should have it up and running by the weekend.

Around 18 in town and 23.5 on the highway at 75 or so.

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I didn't know you needed a wrench to reinstall a filter cartridge/housing! I have always just hand tightened.

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I gave it a little "extra" with a set of oil filter pliers to ensure it was snug in there. I squeezed too hard and cracked the plastic. I was being stupid, I know.

Some controversy here, but next oil change buy the LN spin-on adapter and get rid of the plastic "can". Use recommended spin-on filters. No oil by-pass. Kinda pain to cut open if you do that but easy maintenance. My European car Indy was surprised when I asked his opinion..."Why do that many European cars have the same setup..." So I did it myself...easy conversion.

100 mile round trip to work. Purchased an 01 C4 with 95K last September. I've put approximately 13K as of today. 20 mpg in winter (North Dakota) and 23 summer. I like summer a lot better than winter. I wonder why?

Winter blend fuel sucks. Last winter's fuel was the absolute worst I have seen yet, it makes engines use more oil, gets crappy mileage, and makes less power. I can tell the exact week when they change the mixtures, we see power elevate, and things run better on the dyno.