Can Catalytic Converters Cause Startup Noise?

[Steve C]

Hi Eric and everyone else,

Having another problem with my 06′ G35. Over the past month or so it has a terrible startup noise that sounds like a bearing or grinding. It lasts for a few seconds and only usually when the engine has been sitting for awhile. I took it to the dealer to see if it was any major components, since I have an extended warranty through AAA. They said it was due to the CAT heat shield and they still heard an internal rattle from the CAT upon startup.

My CATs aren’t covered by AAA and my car is over 8 years old, so I would most likely have to pay out of pocket to fix this. I did pass emissions yesterday without any problems, so as of now it’s just a nuisance problem.

I’ve done the following:

Changed oil about 5/6 weeks ago and level was slightly low ~.5 qt. I’m pretty sure my engine consumes some oil, but there haven’t been any other issues. Noise was still present.

I replaced the primary timing chain, guide rails, and water pump less then a year ago. So, I don’t think it could be chain rattle unless my oil pressure just is so low upon startup.

I don’t think it could be my P/S pump or alternator since it happens only on startup. AC compressor is out also, since it’s located on the opposite side I hear the noise.

Drive belts are relatively new, however, I’ve debated replacing them soon.

I’m a little leery of their recommendation, since yesterday they tried selling me a 400.00 3rd brake light to fix in order to pass inspection, I declined and haven’t received my safety sticker yet. Upon, getting home I tested the 3rd brake light myself and it works as expected. I’m not saying they’re trying to rip me off and I understand people make mistakes, but something simple like that has me a little worried.

Topic

[Stephen Bowen]

Take a rubber mallet and give the catalytic converter some gentle slaps when it’s cold.

If the external shield is falling off–NAPA sells a H U G E ladder clamp that works fantastic for wrapping around a converter to re-secure the heat shield to keep it from rattles.

If you do the rubber mallet slap and hear internal stuff clunking around? It’s toasted and broken up internally.

This could also be something as simple as a bad exhaust mount, loose hanger, muffler baffle, etc…

Easy thing to do is a cold rubber mallet slap test…(gently!!) and listen for where the noise is coming from. Failing that? Put the car up on ramps or stands and have a buddy start the car/truck up and listen for where the sound is coming from when it’s started.

Just be CAREFUL!! while under a car/truck with it running!!

S-

[Steve C]

Thanks Tedy, I’ll probably try the mallet test and see if anything is clunking around. I did fix a lot exhaust problems I had a few months ago (y-pipe, mid-pipe, and rusted flanges), so I’m not sure if that’s that the case.

If the CAT was toast though would I still be able to pass emissions?

[college man]

If it is a heat shield you can put a clamp on it or remove it.
usually a heat shield does not make a grinding noise.

http://www.ericthecarguy.com/faq/diagnosing-noises-in-your-car?start=7#ExhaustNoises

http://www.ericthecarguy.com/faq/diagnosing-noises-in-your-car

[Steve C]

They did put a clamp on it and the dealership the noise was coming from within the CAT. But, I only hear the noise on startup. That’s why I’m confused if the internals of the CAT were not functioning wouldn’t I fail emissions and have a problem across the board. Not just when starting my car up after sitting for a few hours.

[Stephen Bowen]

[quote=”coffeysm” post=111646]Thanks Tedy, I’ll probably try the mallet test and see if anything is clunking around. I did fix a lot exhaust problems I had a few months ago (y-pipe, mid-pipe, and rusted flanges), so I’m not sure if that’s that the case.

If the CAT was toast though would I still be able to pass emissions?[/quote]

Each state is different for the requirements. CT for example does the exhaust ‘sniffer’ as well as Mass does a emissions exhaust test. (used to live in both states)

NYS outside of the city–parts have to be there. NO CEL, even a P0420 converter code can tank the inspection.

In regards to the comment about the brake light. Do what I did for our VW. Pick and Pull. The 3rd brake light normally goes for under ten bucks. (Trick is to get there just after they put a car/truck in the yard–the ‘flippers’ are wicked quick for nipping up anything they can flip on eBay or Criegslist.)

As far as driving with a dead converter? We’ve all gotta breath. Plus living near Syracuse, NY? On a hot summer day you can cut the smog with a knife.

S-

[Bryan Carter]

[quote=”coffeysm” post=111649]They did put a clamp on it and the dealership the noise was coming from within the CAT. But, I only hear the noise on startup. That’s why I’m confused if the internals of the CAT were not functioning wouldn’t I fail emissions and have a problem across the board. Not just when starting my car up after sitting for a few hours.[/quote]

A catalytic converter works by chemical processes, not mechanical ones. So something in your cat can be mechanically loose, but still remain functional. But I’m not sure if that’s the source of your noise. Is your car an auto or a manual? How many miles are on it? And where are you hearing the noise coming from?

[Steve C]

My car is an auto with about 138K miles. The other thing I thought is that it could possibly be the starter not disengaging from the flex plate quick enough. The noise usually comes from the passenger side engine compartment. The starter, power steering, and alternator are located in that area and it’s only after sitting for period of time.

[Stephen Bowen]

You know…a starter with a sticky bendix would sound like a higher pitched gear whine sound. It would “Weeeeeeeeeeee” for about 5-10 seconds or so depending on how long it takes the bendix to retract.

McWicked is right. The converter is a process of chemical reaction and heat. Not mechanical. I still think if you drove the car on ramps and catch it in the act….

S-

[george gonzalez]

A cat with loose insides usually has a mild rattley clang-clang clang kind of sound, and it doesn’t go away.

If it’s more of a buzzy sound and goes away in a few seconds, that’s often a starter gear that’s slow to disengage. That can be due to just lack of lubrication but often it’s due to worn or bent gear teeth. The good news is that on most cars the starters are not hard to change, two bolts and two wires and $120 for a rebuilt one.

[Author]

Replies