Erratic Idle and Worsening Gas Mileage
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2004-2007 Cadillac CTS-V General Discussion Discussion, Erratic Idle and Worsening Gas Mileage in Cadillac CTS-V Series Forum - 2004 - 2007; I'm trying to troubleshoot some strange behavior with my '05 V and I figured some of you here might have ...
  1. #1
    RamblinV is offline Cadillac Owners Member
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    Erratic Idle and Worsening Gas Mileage

    I'm trying to troubleshoot some strange behavior with my '05 V and I figured some of you here might have some ideas....

    About two months ago I started noticing that the car felt a bit "different" at startup and when idling at a red light, etc. I usually have the music turned up pretty high, so at first I noticed it just by the vibrations I felt in the gas pedal. It really only seemed to occur when the car was stationary, or when it was rolling with the clutch disengaged. I started paying more attention and noticed that the RPMs seemed to be fluctuating. For example, when I approach a stop light and press in the clutch as the car slows, the RPMs hover at about 1150 or so. Then after the car is completely stopped for about 1 to 2 seconds, it will drop back down to 800-ish. This happens consistently, and seems to contribute to the different "feel" in the gas pedal vibrations. Basically it just makes the car feel "rougher" when it is stopped or moving at slow speeds. Any ideas?

    In addition to the above symptoms, I have also noticed a definite decline in my average gas mileage. I do mostly city driving, and for the first two years my mileage was always 13mpg give or take 0.5mpg. It has steadily declined over the past two months, and now I'm getting 10.5mpg. My driving habits and driving environment have not changed during this time. The decreased gas mileage seems to coincide with the erratic RPM behavior, but I don't know for sure if they are related.

    I had the car at the dealer last week for service. Replace the front and rear brakes and they also replaced the fuel filter and flushed the fuel injection system. While it was there I asked them to look into the problem, but they claimed that my car's idling is "consistent with other similar models." After driving the car for over two years, I know for sure that something is different. I'm just not sure what may be causing it.

    Any ideas you all might have would be greatly appreciated!

    Thanks,
    Jeremy

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    rand49er's Avatar
    rand49er is offline Cadillac Owners 10000+ Posts
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    Re: Erratic Idle and Worsening Gas Mileage

    Quote Originally Posted by RamblinV View Post
    ... when I approach a stop light and press in the clutch as the car slows, the RPMs hover at about 1150 or so. Then after the car is completely stopped for about 1 to 2 seconds, it will drop back down to 800-ish. ...
    Mine does the same thing, but it's a work in progress.

    Compared to when you first got the car, does the idle feel different? The reason I ask is the idle on these cars HAS a bit of a rough quality to it in stock form. It's the way the cam is designed, and it's part of the tradeoff for high performance at higher RPMs.
    '05 CTS-V, Maggie, Kooks, Hotchkis, Ground Control, Corsa, B&M, DSS/Hendrix, Full 3M Clear Bra, Autovation, V Headrests, CTS Console, STS-V 55w Fogs, Black Vette FRCs, Specter Werkes, Katech LS9 Clutch, SS Brake Lines, Heavymetals U-Turn Fuel Line Eliminator, Momo Combat Evo Shift Knob, Wldwhl Clear Sidemarkers, Linea Corse LC855 Wheels or OEM wheels w/'06-'07 Center Caps, plus a couple more. 472 RWHP/411 RWTQ (Mustang Dyno).

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    bleeee is offline Cadillac Owners Fanatic
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    Re: Erratic Idle and Worsening Gas Mileage

    i have the same idle issue......

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    RamblinV is offline Cadillac Owners Member
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    Re: Erratic Idle and Worsening Gas Mileage

    Yes, the idle has definitely changed in the past two months. Sure, it was rough from the beginning when compared to a typical Cadillac, but that is a quality that I have always enjoyed. Even now the difference is subtle, and somebody who is new to the car would probably not notice it... but I can definitely tell that it has changed.

    Again, I think it's mainly due to the idle "sticking" to a slightly higher RPM upon startup and when the car is rolling with the clutch disengaged. From what I can remember the normal idle for the car was always at about 800 RPMs in nuetral, regardless of whether the car is moving or not. But my idle RPM changes depending on whether the car is rolling or standing still. Also, when I start the car it idles above 1000 for about two the three seconds, then drops down to 800. I don't think it used to do that.

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    bleeee is offline Cadillac Owners Fanatic
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    Re: Erratic Idle and Worsening Gas Mileage

    Mine didnt use to do that either. IMO, I would check your tb. Build up can cause the blade to stick a little. Taking that apart and fully cleaning it may relieve your problem and it only takes 10 minutes.

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    heavymetals's Avatar
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    Re: Erratic Idle and Worsening Gas Mileage

    The computer needs to see 0 mph to drop to idle.

    You may have a broken motor mount.

    The gas mileage may be because of the weather (believe it or not).

    During the summer the refineries change the blend and the temperature changes the density of the fuel.

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    rand49er's Avatar
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    Re: Erratic Idle and Worsening Gas Mileage

    Quote Originally Posted by heavymetals View Post
    The computer needs to see 0 mph to drop to idle. ...
    I thought mine used to drop to about 800 RPM even when moving like Bleeeeeeeeeeee said. I'm probably wrong.
    '05 CTS-V, Maggie, Kooks, Hotchkis, Ground Control, Corsa, B&M, DSS/Hendrix, Full 3M Clear Bra, Autovation, V Headrests, CTS Console, STS-V 55w Fogs, Black Vette FRCs, Specter Werkes, Katech LS9 Clutch, SS Brake Lines, Heavymetals U-Turn Fuel Line Eliminator, Momo Combat Evo Shift Knob, Wldwhl Clear Sidemarkers, Linea Corse LC855 Wheels or OEM wheels w/'06-'07 Center Caps, plus a couple more. 472 RWHP/411 RWTQ (Mustang Dyno).

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    Re: Erratic Idle and Worsening Gas Mileage

    Everything heavymetals says is correct. The car has to be at 0 MPH to drop the Idle. It is called Throttle follow or something like that. I believe it is based off IAC counts. I had to adjust mine when I put the cam in my WS6 I think it keeps the Rs up for Vaccum.
    I am not sure why it does this but all LSx M6 cars do it, and I would assume most if not all fuel injected IAC M cars do it.
    High RPM at startup is fine, that is in your program too. IAC counts again. Fuel injected cars have a cold idle at startup for a few seconds, not as severe as their old carbed out counterparts but it does do it. I have HP tunners so I can see all of this.
    You may clean up your MAF and TB if you are unhappy with it. However I would f- with it. Just take it out for some "Spirited" Driving, In other words beat the living shit out of it for a few miles.
    Do you have a catch can?
    You can do that and it helps keep the intake path clean and oil free.

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    Re: Erratic Idle and Worsening Gas Mileage

    Quote Originally Posted by LS1Mike View Post
    ...Just take it out for some "Spirited" Driving, In other words beat the living shit out of it for a few miles...
    Hmmm... This might explain why MY idle is OK...

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    Albertan is offline Cadillac Owners Enthusiast
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    Re: Erratic Idle and Worsening Gas Mileage

    The gas mileage should be better in the summer. In winter, refineries will add more light ends to the gasoline so it vaporizes better. The light stuff has less heat value thus poorer mileage. (I used to work at a refinery and made the gas) Of course, in summer if the car has the AC on, it will contribute to poorer gas mileage.

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    StealthCTSVJJL is offline Cadillac Owners Fanatic
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    Re: Erratic Idle and Worsening Gas Mileage

    You may need what was once called an "Italian Tuneup". The procedure is simple, apply maximum acceleration through redline on as many gears as possible, as long as possible.
    The benefits are removal of any carbon deposits on the valves and also adds a big smile to your face!

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    RamblinV is offline Cadillac Owners Member
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    Re: Erratic Idle and Worsening Gas Mileage

    Thanks for the input everyone. Sounds like I may be over-analyzing the idle fluctuations since it appears to be the expected behavior based on your responses.

    Somebody mentioned that I should check for a broken motor mount... if this is the problem then what is the typical symptom? The vibration that I'm feeling is pretty subtle, and it probably wouldn't be obvious to someone who is unfamiliar with the car. I'd like to check the motor mounts though. Is this something that can be easily done myself? Or should I take it to the dealer?

    As for the suggestions of "spirited driving" and an "Italian tune-up", I do it on a daily basis so I think I've got that covered already!

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    Re: Erratic Idle and Worsening Gas Mileage

    I'd add that beyond just checking the MAF, I'd also try the throttle re-learn, if you see the RPM's going above 1100 ish.

    Finally you might want to double check your intake/elbow/TB/manifold, to make sure there is no unmeetered air getting into the system (i.e. any leaks AFTER the MAF). I had an iddue with the brake booster connection at the back of the manifold, that made idle somewhat erratic.

    -Chris
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    RamblinV is offline Cadillac Owners Member
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    Update: Erratic Idle and Worsening Gas Mileage

    Just wanted to share the latest info about this situation....

    I took my car to the dealer this week to have them look into the vibrations again, since I had concluded that the idle fluctuations I was seeing were probably normal behavior. I specifically asked that they check the motor mounts. They called me today to report that they did indeed find two bad motor mounts that they will be replacing. I won't know for sure until tomorrow when I get the car back, but hopefully that will fix the problem. As for the decreased gas mileage, perhaps I can just blame the super-hot Atlanta summer and the extra workload inflicted on my A/C.

    A side note, the loaner car provided by the dealer is a 2008 CTS. Aside from the color (Bronze) I am really impressed with the looks of the car - especially the interior! Obviously the ride is a completely different experience than the V, but it's still pretty quick and responsive. I toyed around with the sport mode a bit, but in the end I prefer to just keep it in automatic mode. The six speed manual transmission is very smooth. Since it's a rental car it does not have all of the bells and whistles such as navigation or the mp3 hard drive, but overall the aesthetics of the car (inside and out) are a big step up from the previous generation. It is definitely a head-turner, as I have seen many people do double-takes in the 24 hours that I've had the car. Can't wait to see the V-Series next year!!

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    RamblinV is offline Cadillac Owners Member
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    Vibration Fixed!!

    One last update for the benefit of anyone with a similar problem who may be searching the forums in the future for answers....

    Got my car back today with two new front motor mounts. The difference is unbelievable! I had grown accustomed to the vibration and over time I guess I sort of lost track of how pervasive it was. When I picked up my car today at the dealer and started it I could barely tell if it was running, it was so quiet and vibration-less! Same thing when I turned it off... nice and smooth with a complete absence of the shuddering/shaking effect that had been occurring for so long. Feels like a new car again.

    On another note, my clutch seems to have a completely different feel to it now. It seems to be shorter and softer. Before Monday there was what felt like an inch of "play" as I release the clutch pedal from it's fully depressed position before I could feel it engage. Now it happens almost immediately with little to no "play." Also, the clutch used to be pretty stiff when pressing it in, but now it feels rather soft. Any ideas what they might have done while replacing the motor mounts that might have caused these changes in my clutch? Some other oddities that I noticed are that the g-meter was reset and my clock is also reset. Any valid reason this may have happened during the motor mount replacements?

    Thanks,
    Jeremy

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