GXP-MPG ?

[Arrowhead]

My"new" 2005 with less than 22,000 miles wont even get above 16.4 mpg in mixed driving on hilly country roads and 55-65 mph highway driving according to the "DIC" readout. The residue in the tailpipes is pretty black but, no check engine light is showing. What the heck, this is from driving about as mildly as somebody possibly can, any likely culprits for this lousy mileage situation ? Any specific tests that can determine what's what ? What's everyone else achieving in MPGs ? I'm tempted to drop a 3800 in it if I can't get at least 17.5-18 mpg from this North-hog !

[yonkerse]

Have you checked the DIC MPG with what your hand calculation to verify its correct?

From what I have heard, most GXP's are right around that area, maybe slightly better.

I really liked the GXP styling and additional features over the SLE, but with the miles I drive (averaging about 20k/year) I couldn't justify the difference in MPG, not to mention the difference in insurance rates either. With my most recent commute between work and my parents house, I can count on my car getting minimally 32 MPG...which consists of 80 miles of highway and 80 miles of country roads. If weather is good and I'm not trying to cut 15 minutes off of the drive time, I consistently see 35 MPG. (after removing 1 MPG from the DIC average since mine is generally reads 1 MPG high)

Luckily all the GXP parts are all directly interchangeable into the SLE so im sporting most of the GXP goodies!! :D

[dpnewfie]

I'm averaging 21mpg with a mix of Highway and City. I wouldn't know what to look for outside of doing a basic tune-up. That thing definetly needs a few WOT runs to clean the cob webs out. 22k on a 8 year old car that demands high revs can't be good.

[LeSabre in Buffalo]

I'd give it a good old-fashioned "Italian tuneup" for a tank, and see if matters don't improve. With 22k miles on it, it clearly wasn't run that much, and was likely run easy when it did run. The N* is a high-revving screamer. It's not meant to put around town constantly.

[nos4blood70]

I remember someone around here saying that it is recommended to go WOT once a day with a N* to clean out the carbon build up.

[repinS]

Yes, do verify that your hand calculation for mpg is somewhat close to the DIC. Not sure how hilly your driving is, but city and hills can obliterate gas mileage, but maybe not quite 16.4mpg bad if you're mixing with highway driving. You're coming in a bit under where you should be. I'm getting 19.7 mpg mostly highway, but mixing in with serious bumper to bumper urban traffic brings the number down considerably. On the open road 23-25mpg should be easily attainable without any effort, at speeds ranging from 60-75mph. With fresh plugs, a freer flowing Corsa exhaust, and a fender well intake, I've seen as good as 28mpg in flat, 60-65mph driving. You don't necessarily have to go slow in the GXP to get good mileage, you have to go smart. Always keep an eye on the instantaneous fuel economy on the DIC.

With the 95 SSEi I was getting a hair under 21 mpg average, marginally better but it wound up costing me MORE to fuel because the cost difference for 91 octane is higher in Canada. 19.7 mpg is ~6% worse but 91 octane costs 11% more!

[PaulyGXP]

I never get under 20mpg with my GXP... I've seen as good as 27.2 mpg with hwy only miles at 55-70 mph. Rough up the old girl a bit, maybe she'll like it... lol

[Arrowhead]

I've done a couple "WOTS" but I really suspect an electronic component etc. is causing a very rich condition in the fuel/air mix. Now I need to find a Northstar tech that's local AND knows what he's doing.

[sethjamesrimrodt]

My last tank I averaged 21.05 mpg (dic said 22.2). 300 miles of that tank were doing 80 mph on the interstate, and 75 miles in town.

[00Beast]

When ours was around home more (now with my sister ~2 hrs away) it got 20.5 pretty much like clockwork.

I agree with above, I'd throw a can of fuel system cleaner in the tank (Gumout Regane, Amsoil Performance Improver (P.I.) or Seafoam) and drive it like a teenager for a tank.

Also, have you reset the Average MPG parameter on the DIC? It averages from the last time it was reset, which could have been 20k ago in your instance...

[Arrowhead]

Yes, did the reset two fillups ago. We've only used regular gas and don't plan to switch to premium for a possible mpg gain and ending up paying even more $$ per mile. I need to check the air filter's condition first thing, it's a K & N according to a sticker on the plenum/housing. I suppose it's possible that it's restricted but I doubt it from what I've learned about the original owner who was a freak for details. Heck, he may have installed non-stock, performance injectors that have thrown things out of whack.......... Any leads on a top notch technician for these cars in the Mid Atlantic region ?? I definitely need to find one, ASAP !

[repinS]

91 octane in the GXP won't really net you extra mileage - if it does, it definitely won't offset the cost difference. You will notice some extra power, though.

[PaulyGXP]

Does is seem to misfire at all? The COPs can fail prematurely which would definately effect your mileage. Also, it wouldn't hurt to run premium for a tank or two. Check the normal stuff, fuel filter, air filter, spark plugs, throttle body, oil, etc. Even tires can effect your mpg. Really can be something that simple!

[Arrowhead]

No misses, I can't imagine that plugs could be a problem at less than 22,000 miles. What are "COP's" ? Tires are inflated properly. I want to do some reading before I mess with removing plugs from an aluminum cyl. head! I am beating the bushes looking for a Northstar maestro in my area, hope to find one soon. I get the feeling that I shoulda' gotten another 3800 car !

[01bonneSC]

Your plugs could be carboned up since they like to be high rev'rs. And with that low of miles I'm betting you have some fuel system parts a lil gummed up. I'd run a bottle of Lucas fuel treatment through and then change your fuel filter.

Nothing to removing plugs from AL, just get the engine warmed up a bit to expand the metal some so they release the plugs easier.

[00Beast]

I believe he's referring to the Coil On Plug design with "COP". They have been known to fail, but you would know.

Injectors wouldn't do anything to your mileage, and wouldn't do anything for performance either...

[Arrowhead]

Improperly metered injectors can/would/will dump excessive fuel that would not be fully burnt and make rich black exhaust tip deposits. Seen it happen with my own eyes.

[01bonneSC]

I highly doubt that he would have added injectors if all he did was drop in a K&N. Running numbers on those should be easy enough.

Does anyone even make bigger injectors for a N*? I know they JUST got around to be able to really tune them.

[Arrowhead]

Heck! I just learned about the new fangled "COP's" !

[01bonneSC]

Been runnin on LS engines since 98 lol