Exhaust Leak/Carbon Monoxide?

[JeniorNV]

I'm posting this in "off topic" because it's a question about my '05 Pontiac Vibe (162K miles, completely stock). About a month ago, the car started making a loud "buzzing" sound at certain RPM's. It sounded exhaust-related. I visually inspected the exhaust and didn't see any obvious problems. I also tightened the bolts on the heat shields above and below the exhaust manifold, which made the noise significantly quieter, but it didn't go away completely. There are no other symptoms of a problem except for a vibration at idle. I occasionally smell exhaust briefly when I start the car (even with the windows shut) but it has never struck me as abnormal.

Since tightening up the heat shields brought the sound to a tolerable level and the car runs great otherwise, I was tempted to ignore the problem. But the possibility of a dangerous exhaust leak was in the back of my mind. I read a Q&A on CarTalk where Tom and Ray suggested that someone put a carbon monoxide detector in their car and let it idle for 10 minutes to check for an exhaust leak. So I decided to give it a shot.

My carbon monoxide detector displays the CO readings above 30 ppm. About 2 minutes after I started the car, the display showed 33 ppm and then immediately shot up to 80 ppm. For the next 2 minutes, the CO level fluctuated between 60 and 80 ppm. Then the display went blank (meaning less than 30 ppm) and stayed blank for the next 6 minutes until I shut off the car. The car was parked outside. There's no wind outside at all today, so exhaust from the tailpipe wasn't blowing forward.

So before I freak out, here is what I'm wondering. Is there any logical explanation as to why my car may have been registering a borderline dangerous level of CO for 2 minutes? Or is that a sure sign of an exhaust leak?

Thanks.

[Mylt1]

What CO level is dangerous to my health?

The health effects of CO depend on the CO concentration and length of exposure, as well as each individual's health condition. CO concentration is measured in parts per million (ppm). Most people will not experience any symptoms from prolonged exposure to CO levels of approximately 1 to 70 ppm but some heart patients might experience an increase in chest pain. As CO levels increase and remain above 70 ppm, symptoms become more noticeable and can include headache, fatigue and nausea. At sustained CO concentrations above 150 to 200 ppm, disorientation, unconsciousness, and death are possible.

i wouldnt worry to much about it. plus, how often do you sit idling for 10 min or more on any given day?

[JeniorNV]

Originally Posted by Mylt1

i wouldnt worry to much about it. plus, how often do you sit idling for 10 min or more on any given day?

Right now, I'm more worried about the exhaust system in the car than health problems in myself. I'm not overly concerned about carbon monoxide poisoning because the levels are low enough that I wouldn't get acute poisoning. I have found a couple of sources that state chronic problems can occur with long term exposure of more than 9 or 10 ppm (which I would have no way of knowing because my CO detector won't register anything below 30). I frequently get headaches after driving this car long distances, but lots of things cause headaches.

But if I have an exhaust leak, I only see it getting worse and not better. What I'm most curious about right now is whether the 60-80 ppm points to an exhaust leak, or whether it could be explained by something else. I know that exhaust fumes are worst on initial cold startup, but what I can't seem to find any information on is what the normal level of CO is inside an idling vehicle.

I'm probably going to bite the bullet and have the exhaust system checked out. I didn't see any obvious problems when I looked, but I'm not a mechanic so it's possible I overlooked something.

[Skibane]

Modern vehicles don't produce nearly as much CO as in the old pre-pollution-control days.

According to Wikpoopia:

Before 1975, motor car exhaust contained 4–10% carbon monoxide, but newer cars have catalytic converters that eliminate over 99% of the carbon monoxide produced.

Smelling exhaust during the first cold start of the day isn't abnormal. When the engine is cold, the engine computer increases the fuel-to-air mixture, resulting in a lot more exhaust emissions than after the engine warms up. Often, you can smell this exhaust even with all the windows closed and the HVAC blower turned off - some exhaust smell still enters through HAVC outside air intake.

Exhaust rattles are often traced to the catalytic converter - Chunks of the catalyst material break off, and rattle around on the inside bottom of the converter. Sometimes, you can hear 'em rattling if you whack the outside of the converter with a rubber mallet.

[JeniorNV]

Originally Posted by Skibane

Exhaust rattles are often traced to the catalytic converter - Chunks of the catalyst material break off, and rattle around on the inside bottom of the converter. Sometimes, you can hear 'em rattling if you whack the outside of the converter with a rubber mallet.

When I did the rubber mallet trick initially, I heard a rattling noise when I hit the cat so that was my first thought too. But after I tightened up the exhaust manifold heat shields (they attach right next to the cat), I couldn't duplicate the rattle. Isn't it unusual to have a bad cat without other symptoms like a CEL, decreased performance, shitty gas mileage, etc?

Originally Posted by Skibane

The noise sounds like it is coming from the front of the car. That combined with the fact that the sound changed when I played with the heat shields around the exhaust manifold makes me think that my problem is BEFORE the catalytic converter. Wouldn't a leak from the exhaust manifold contain a significantly higher level of carbon monoxide than a leak coming from behind the cat?

Originally Posted by Skibane

When the engine is cold, the engine computer increases the fuel-to-air mixture, resulting in a lot more exhaust emissions than after the engine warms up.

That explains why my CO detector was registering higher levels during the first few minutes. I still wish I could figure out whether 80 ppm inside the car is perfectly normal, or whether that's a sign that exhaust is entering when it shouldn't be.

Sorry for picking apart your post into so many pieces. Several questions came to mind when I read it.

[x_mods]

Originally Posted by Skibane

Smelling exhaust during the first cold start of the day isn't abnormal. When the engine is cold, the engine computer increases the fuel-to-air mixture, resulting in a lot more exhaust emissions than after the engine warms up. Often, you can smell this exhaust even with all the windows closed and the HVAC blower turned off - some exhaust smell still enters through HAVC outside air intake.

The smell is also worse at start up because the cats don't work properly until warmed. In addition to the engine "chocking" itself

Originally Posted by JeniorNV

When I did the rubber mallet trick initially, I heard a rattling noise when I hit the cat so that was my first thought too. But after I tightened up the exhaust manifold heat shields (they attach right next to the cat), I couldn't duplicate the rattle. Isn't it unusual to have a bad cat without other symptoms like a CEL, decreased performance, shitty gas mileage, etc?



The noise sounds like it is coming from the front of the car. That combined with the fact that the sound changed when I played with the heat shields around the exhaust manifold makes me think that my problem is BEFORE the catalytic converter. Wouldn't a leak from the exhaust manifold contain a significantly higher level of carbon monoxide than a leak coming from behind the cat?

If the secondary cat is bad, then it won't normally throw a code, only if the primary is blown. And I'm not sure but I though a busted cat resulted in better performance as far as air flow. Since the first O2 sensor is before the primary cat, and it is what controls engine mixture

And yes, if there is a leak before the cat then there will be more CO as the cats converts CO to CO2 and water for the most part.

Sorry I can't be much more help But maybe try testing the CO levels in the truck as a control?

[JeniorNV]

Originally Posted by x_mods

If the secondary cat is bad, then it won't normally throw a code, only if the primary is blown. And I'm not sure but I though a busted cat resulted in better performance as far as air flow. Since the first O2 sensor is before the primary cat, and it is what controls engine mixture

And yes, if there is a leak before the cat then there will be more CO as the cats converts CO to CO2 and water for the most part.

Sorry I can't be much more help But maybe try testing the CO levels in the truck as a control?

I read in several places that a bad cat would cause a decrease in performance.

I didn't know that about the CEL. The thing that makes me think that the problem can't be the secondary cat is that it is nowhere near the exhaust manifold heat shields.

I know that heat shields are the easiest and most obvious cause of this kind of buzzing or rattling noise, but I feel like I've ruled them out. The shield that goes under the exhaust manifold was missing three bolts (it was just hanging there by one bolt). I used an assortment of Nissan bolts to tack it back on and tightened up the fourth bolt. There was no change in the sound. The shield on top of the exhaust manifold I removed completely for a while. The sound got quieter (but didn't go away) when I took the shield off - and then didn't change much at all when I put it back on. It seems like the change in sound must be from the noise resonating off of the heat shields, rather than a problem with the shields themselves. The only other heat shield that could be an issue is the one that is welded to/wrapped around the primary cat. It feels nice and tight, but I can't see the whole thing because of the location of the primary cat.

Using the truck as a control is a great idea. I'll have to wait until tomorrow because I drove it today. I think I'll sneak into the garage first thing in the morning and check the Mazda too (probably should back it into the driveway to prevent CO build up though...lol).

[x_mods]

Originally Posted by JeniorNV

I read in several places that a bad cat would cause a decrease in performance.

I didn't know that about the CEL. The thing that makes me think that the problem can't be the secondary cat is that it is nowhere near the exhaust manifold heat shields.

I know that heat shields are the easiest and most obvious cause of this kind of buzzing or rattling noise, but I feel like I've ruled them out. The shield that goes under the exhaust manifold was missing three bolts (it was just hanging there by one bolt). I used an assortment of Nissan bolts to tack it back on and tightened up the fourth bolt. There was no change in the sound. The shield on top of the exhaust manifold I removed completely for a while. The sound got quieter (but didn't go away) when I took the shield off - and then didn't change much at all when I put it back on. It seems like the change in sound must be from the noise resonating off of the heat shields, rather than a problem with the shields themselves. The only other heat shield that could be an issue is the one that is welded to/wrapped around the primary cat. It feels nice and tight, but I can't see the whole thing because of the location of the primary cat.

Using the truck as a control is a great idea. I'll have to wait until tomorrow because I drove it today. I think I'll sneak into the garage first thing in the morning and check the Mazda too (probably should back it into the driveway to prevent CO build up though...lol).

lol, you don't want something to happen with the Mazda in the garage, because the papers will all say that you couldn't take the stress of pinpointing exhaust leaks anymore and decided to go out in an ironic way I would try and test both vehicles with the same variables as the Vibe.

I was going to say try running without heat shields but since you already did that, and your hypothesis of the sound reverberating makes sense, it probably wouldn't do anything.

Maybe the noise and CO levels are not related? Maybe you have a pinhole leak causing the CO to reach the cab, and you have something wrong inside the cat leading to the noise and stronger CO levels overall? Do you have any way to read AFR? I know the computer won't throw a CEL as soon as something goes wrong, it waits till it's a % off then it throws the code.

My old vibe used to smell a bit on the inside too when cold, but no vibes (pun intended).. I thank that good ol' Toyota engineering

[JeniorNV]

Originally Posted by x_mods

lol, you don't want something to happen with the Mazda in the garage, because the papers will all say that you couldn't take the stress of pinpointing exhaust leaks anymore and decided to go out in an ironic way

You know what else is ironic? I'll be using a vehicle that's throwing an intermittent CEL as my control vehicle.

Originally Posted by x_mods

Maybe the noise and CO levels are not related? Maybe you have a pinhole leak causing the CO to reach the cab, and you have something wrong inside the cat leading to the noise and stronger CO levels overall? Do you have any way to read AFR? I know the computer won't throw a CEL as soon as something goes wrong, it waits till it's a % off then it throws the code.

It's very possible that the noise and the CO aren't related at all. Question: If there is a problem with the secondary cat that isn't tripping a CEL, what are the chances that the car will pass smog?

I don't have any way to read AFR but I'm sure my mechanic does. I feel like this car might be headed there. I've gotten so used to working on my own cars in the past few years that I'm resisting the idea, but I don't really want to **** around if I might have something like an exhaust leak. I guess I'll make that decision tomorrow after my little carbon monoxide test.

Originally Posted by x_mods

My old vibe used to smell a bit on the inside too when cold, but no vibes (pun intended).. I thank that good ol' Toyota engineering

The "vibes" could be caused by so many different things, so I'm not putting any money on the vibration being related to the noise or CO level, although it could be. Usually it's just a vibration in the steering wheel, but occasionally it gets bad enough to make the seat shake too. It's had a slightly rough idle for a couple years, but the steering wheel/seat vibration started around the same time that the noise did.

This car was perfect up until 5 or 6 weeks ago (161K trouble free miles). I used to ask my truck why it couldn't be more like the Vibe. Guess who's running better now?

[x_mods]

Originally Posted by JeniorNV

You know what else is ironic? I'll be using a vehicle that's throwing an intermittent CEL as my control vehicle.

Perfect

Originally Posted by JeniorNV

It's very possible that the noise and the CO aren't related at all. Question: If there is a problem with the secondary cat that isn't tripping a CEL, what are the chances that the car will pass smog?

The secondary cat is past both O2 sensors, so I don't think that will ever throw a code if it is bad? But depending on how strict your state is, I wouldn't think it would pass smog with a bad cat of any kind..