[z06bigbird]

2004 CTS with 3.6, 37,000 miles. Used to get 27 mpg on highway. Recently noticed that my actual gas mileage is far off from what is displayed on the DIC.

DIC shows that I have used about 3 gallons and that I am getting about 18 mpg mixed driving. In all actuality, I burned about 10 gallons, and I calculate about 10 mpg.

No errors, no codes. No hesitation, no missing, no nothing. Only thing is that I failed to change the air filter for the past few years. Changed filter this week end, but I have not driven car since.

I talked to a Cadillac mechanic, and he was surprised that the DIC did not coordinate with actuality. He will look up some TSBs for me. Anyone else notice similar situations with their 3.6. Any ideas?

 

[Kingoftypos]

How did you come up with 10miles per gallon?

The way that I used to calculate my mpg in my previous cars. Was to fill them up, reset the trip meter. Drive a couple hundred miles, over a week spans. The fill them up again, took the amount of miles driven divided by how much gas I filled up with to come up with the mpg. Now I just use the dic to calculate my mpg.

KOT

 

[Spyke]

I'm sure your DIC resetted itself just like mine is used to do. Sometimes, the trip A and Trip B + fuel used resets themselves. There is a tsb about it but I dont care, I check how many miles I did versus how many gallons of fuel I paid and calculate. I usually gets approx 28 mpg.

 

[z06bigbird]

I started with a full tank, drove 100 miles, watched guage go down to 1/4 tank. Then I put 10 gallons in. Even if the driveway at the gas stations were not-leveled, that would not have allowed me to put in more than about 1.5 gallons.

 

[Kingoftypos]

Here is a real stupid question. Are you on miles or kilometers? As far as your car is reading?

KOT

 

[z06bigbird]

Here is a real stupid question. Are you on miles or kilometers? As far as your car is reading?

KOT

Miles. Austin, Tx to San Antonio. I watched the dang gas gauge move from full down to 1/4 tank left.

No smell of gasoline under car, either.

 

[Kingoftypos]

My only two thoughts are, one to change the fuel filter. Second would be have the computer relearn your car. I know of two ways to do that, disconnect the battery for at least a hour. Another way would be to connect an obd ii device and press clear. Even though there is no error codes.

An other thought that I have, is to take out the fuse for the ecm and pcm. So that you wouldn't have to relearn everything, radio presets, seat position and such. Just make sure you don't take out the fuse for the bcm, which is what I think contains the memory for the seats and what not.

KOT

 

[Black4drCTS]

There is no way even at WOT that this car would burn through 10 gallons of gas going 100 miles. I can cruse at 85+ and still get 350 miles per tank. I would assume you have a gas leak. Trace the fuel lines. If its leaking somewhere while on the road (could be in the engine compartment too) then you might not even ever smell it.

 

[ILLMATIK]

I usually get about 10-12 mpg driving around the city, which sucks for me because that constitutes about 90% of my driving. I used to really notice it when gas was like $4 a gallon, doesn't bother me too much now. That's just the way this car is. It swallows gas like a dirty whore.

I've taken it to several dealers and have been advised by all that nothing is wrong.

 

[z06bigbird]

Thanks for all the help. I hope to start climbing under/around this car within a few days. Also, one of the Caddy techs has promised to look thru the TSBs for me.

 

[JimmyH]

Second would be have the computer relearn your car. I know of two ways to do that, disconnect the battery for at least a hour.

^urban legend

 

[JimmyH]

Bad gas mileage=heavy right foot (most of the time)

I get 26-27 on my CTS. I get 22 on my CTS-V. I only loose around 3-4 mpg when it is all stop and go driving. Try to coast more when you are in traffic, and you will be amazed how much your fuel economy improves.

 

[Kenno540]

Have had mine ('06 w/23,000 miles) for a little over a week and have already filled it up twice. But then again, I'm surprised how nice a GM car is these days and driving it is always a joy (for comparison my last BMW was an 2000 540 and the CTS is as nice and as fast). But around town the CTS gets about 15 mpg, and when you jump on the loud pedal it seems the gas gauge is racing the speedo to get to the far end of the gauge.

 

[Kingoftypos]

^urban legend

What's that? To have him force his car to relearn itself? Or disconnecting the battery to relearn?

KOT

 

[z06bigbird]

Bad gas mileage=heavy right foot (most of the time)

I get 26-27 on my CTS. I get 22 on my CTS-V. I only loose around 3-4 mpg when it is all stop and go driving. Try to coast more when you are in traffic, and you will be amazed how much your fuel economy improves.

All of my gray hair and my heavy foot do not get along well. I am too old to hot rod this car. Normally, I drive a LeSabre. LOL

I have not yet talked with my mechanic friends at the dealership. Probably early next week. The more I think about the situation, the more I agree that I likely have a gas leak from the firewall back. That would especially be true if the DIC computer took its calculations from data compiled near the engine. If the DIC takes its info from the fuel pump, then I am stumped even more.

I will keep you guys posted. BTW, I used to get 27 mpg on the highway. I almost never check the mileage around town. I remember the 27 specifically because my 97 Sedan de Ville with Northstar got 30 mpg at 80 mph on the way to Austin (about 100 miles). I miss that tank.

 

[Kingoftypos]

If the DIC takes its info from the fuel pump, then I am stumped even more.

Well actually I think you ment if the computer calculated from the fuel pump as well. Because if you really think about it, if you did have a fuel leak between the tank and the engine. lets say around the firewall. Then the pump would be working twice as much/harder to keep the engine fed. Know what I mean?

Here's another though, maybe you don't have a leak what so ever. But there is a sensor that is sending wrong information to the computer. And the computer is agreeing with it, hence no error codes. So since the computer agrees with it, it's going to tell the fuel pump to work harder.

KOT

 

[Matt55330]

the millage is calculated by multiplying injector duration by flow. the computer doesn't monitor the fuel pump flow since the fuel regulator controls the pressure in the rail and dumps the remainder back to tank. you could have leaking injectors that are dumping more fuel then it should. It may not be easy to detect since you can run very rich with out noticing performance issues under normal use and the cats will just have to work a bit harder to clean it up.
I've ran into this working with the tier3 emission standards, as the manufactures bump up the injector pressure to get better atomization for emissions and performance any dirt in the fuel causes issues. We were not able to find any fuel that passed without two stage filtering.

 

[z06bigbird]

Thanks for additional advice. No news from my end yet as I have been up to my ears with other things going on.

I will keep you posted.

 

[04CTSCaddy]

Did this ever get resolved? I have the exact same issue and its eating a hole in my pocket.