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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
Last edited by Splitting Atoms; 06-02-2017 at 10:45 PM.
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.
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.
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.
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.
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
06-02-2017, 09:02 PM
My 996 is getting 12 miles per gallon
