Exhaust noises

[2barhaven2]

I'm afraid I need some advice again! My '07 V (37000 miles) has a very deep "droning" noise at around 30mph in 4th gear, when using a light throttle under slight load. The noise is almost a pulsation but not quite. It is so deep and loud you can almost feel it as well as hear it. It's not transmission or tire noise, but exhaust-related I'm sure. It goes away if the transmission is shifted to 3rd manually or automatically. It sort of "slides" in as the trans shifts into 4th, getting louder and lower until it reaches a steady state after 2-3 secs. I wonder if it could be a malfunction in the exhaust flaps, perhaps opening up well below 3500rpm. These flaps create a loud "tinny" sounding rattle when the car is first started in the morning; that noise fades away after 10-15 seconds or so. I read in a bulletin somewhere that this is normal (but really embarrassing!), caused by the flap control system building up vacuum. But sometimes when accelerating from a stop or changing speed around town I hear a very brief rattle again for a second or so. As well, the exhaust sound has always been very "wimpy", including at idle when warmed up; sort of like a "purr" with no V-8 "rumble" at all. I wonder if all these symptoms might be related? Or could it be some other cause entirely? (The dealer here says that they can't find any obvious holes/leaks in the exhaust, and there are no fault codes or lights showing up). Unfortunately as far as we know mine is the only V in the entire province where I live, so we have nothing to compare the problem with. I would be grateful if anyone has the time to offer an opinion/suggestion.

 

[Phrede]

IMHO Exhaust sounds will be based on RPM, load and throttle position rather than what gear the transmission is in. Maybe looking more at the tachometer could help. Of course, normal road sounds will depend on the cars speed.

I have a "mild to wild" control installed on my '08 V that lets me select the two inner tips independent of RPM and throttle position. I really like it and prefer the sound of the more open exhaust most of the time. Running on the "open side" can cause some annoying droning at highway speed with roof up.

There may be a way to temporarily keep the system from switching over to the quiet side and see how that effects what you hear. I am thinking the vacuum line could maybe be clamped closed. The electrical line could be cut, test driven and then spliced back together. Some have installed a switch in the line or installed a relay (like my mild to wild). The wire is very easy to get to near the hydraulic pump in the trunk. I can give you a picture if you like.

Mine takes nowhere near 15 seconds to quiet down on starting. 2-3 seconds max. for mine.


HTH

 

[2barhaven2]

Thanks for your thoughts and advice. I'm going to try and pin down the noise more precisely to RPM as you suggest. Right now I am hoping that the Cadillac dealer will ultimately find the cause, although they tell me there are no service bulletins covering the issue.

As for some modification to the flap system operation itself, at the moment I don't want to play with it, but I will keep your suggestion for a temporary modification in mind. So I won't need to trouble you for a pic just yet. Thanks for the offer though!

I'll let you know as things progress further.

Regards

 

[Cadillac Tech]

Rattle @ start up

LC3 leak noise
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<TABLE width="95%"><TBODY><TR><TD width="7%">Subject:</TD><TD>Exhaust Rattle Or Buzz At Engine Start</TD></TR></TBODY></TABLE>​

<TABLE width="95%"><TBODY><TR><TD width="7%">Models:</TD><TD>2006-2007 Cadillac XLR-V Series Only</TD></TR></TBODY></TABLE>​

<HR>
Condition/Concern:


Exhaust rattle or buzz at engine start. This condition may last up to 20 seconds after start up, usually after the vehicle has set at least over night or longer.
This condition is driven by the amount of available stored vacuum which is used to close the exhaust flow control valves located in the inner two tail pipes. When the amount of stored vacuum is low, there is insufficient force hold the valves tightly closed against the exhaust pulses. Due to the length of the vacuum supply pipes and cold start engine vacuum characteristics, it may take a few seconds to build enough vacuum to hold the valves closed firmly enough to prevent the noise.
Note: After the engine is shut down and the BCM goes to the sleep mode, the system releases the vacuum to the vacuum motors which opens the valves to allow ventilation of the exhaust system. The system commands the valves closed at BCM wake up to minimize exhaust noise at engine start. Actions like opening the vehicle's doors and or deck lid will cause the system to close the valves. If this happens a couple times without engine start, this can also cause the available vacuum to be low, leading to the brief rattle noise at engine start.
Recommendation/Instructions:


This is considered to be a normal condition and no repairs should be attempted if the condition is only experienced under the above conditions.
Please follow this diagnostic or repair process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.

</FORM>

 

[2barhaven2]

Much obliged! I did see this info before somewhere, but thaks for directing it to this thread. It seems to deal with the start-up rattling (tho' the noise sure is embarrassing when someone else is there to hear it ). But my other problem is with the loud, deep resonance on the road at 1500 RPM in 4th gear.

I will bring your new info to the dealer's attention, in the hope that it will help in resolving the whole problem.

I'll keep you posted!

 

[XLR-V Lover]

Just wondering what they did to fix this for '08 & '09.

 

[2barhaven2]

Good point! Anyone out there with an 08-09 having this problem with rattling?

 

[graytoad]

I have a 09, there were no problems except lack of attitude so I replaced with a Corsa system anyway, but mine is not a V with the electric cut out deal.

 

[Phrede]

Good point! Anyone out there with an 08-09 having this problem with rattling?

I have never noticed it on my '08.

If the lack of vacuum turns out to be the problem, you may want to consider installing a switch in the control wiring that would delay the "quiet mode" until there is enough vacuum to prevent the rattling. A simple timer circuit could take the system bake to normal. It would be louder, but not rattling.

A small can used for vacuum storage near the actuators may also provide a solution. A blocking valve that would only open if the engine was running might keep things in quiet mode with out the rattling.

Both of these ideas are "band-aids" which none of us like to do, especially on a vehicle of this class.

Does the rattling only happen after the car has been off for a while?

You may want to check the actuators and make sure things are moving smoothly and that there are no vacuum leaks. They are very easy to see. If something is binding it may require more vacuum than usual to get it to move. Although I would think that rattling would indicate something loose rather than tight.

HTH

 

[Phrede]

Much obliged! I did see this info before somewhere, but thaks for directing it to this thread. It seems to deal with the start-up rattling (tho' the noise sure is embarrassing when someone else is there to hear it ). But my other problem is with the loud, deep resonance on the road at 1500 RPM in 4th gear.

I will bring your new info to the dealer's attention, in the hope that it will help in resolving the whole problem.

I'll keep you posted!

From this and your other post I would look into a vacuum leak. I can get the droning at about that RPM with the top up. I notice it more in 6th gear, but I suspect that it could happen in any gear. Keep in mind that vacuum holds the inner two exhaust tip valves shut. I seem to remember that it should take in excess of 3000 or 3500 RPM and lots of throttle to get them to open up (no vacuum). I'm not 100% sure on the exact required conditions (Help Bruce?) but am reasonably sure that the valves should be shut (quiet mode) at 1500RPM.

A vacuum leak in this circuit would explain both the rattle and the droning. I suppose it could also be "dirty" relay contacts, but I'm betting on a vacuum leak.

 

[2barhaven2]

It's taken longer than I guessed to be able to give an update on my XLR exhaust noise issue. After assessing things, my Cadillac dealer replaced both mufflers. Unfortunately, when the work was finished the weather here meant that the car went into storage right away, out of the snow. So I only had a chance to drive it very briefly. But it looks like the problem is solved. Obviously, with new mufflers the sound is as per factory specs. I am really impressed with the willingness of GM and the local dealer to work this problem through with me. On this point I have seen praise from other forum members mentioning a couple of great Cadillac dealers in the U.S.. Well I can tell you that our local dealer here, Hickman Chevrolet Cadillac, is right up there with the best of them!

Again, thanks to all for your helpful suggestions throughout.