Engine knock from exhaust when cold

[Therealsmitty]

Hello all, and happy holidays! I could use some help here please;

I recently purchased a 2003 Yukon XL with 140k on the clock, 5.3l engine.
I was a little rushed after looking at many Tahoes/Yukons, especially since my wife was due with our 3rd child in less than 2 weeks. I made one sinful mistake in my purchase... When I went to look at the Yukon it was warm already, supposedly because they needed to put gas in it. All checked out; very very clean, etc. I got cash and picked it up about 4 hours later and started it again somewhat "cold". Again, all was fine.
After sitting for a few days at my house, I started it again. This time I noticed a hollow knock sound coming from the exhaust. Stock exhaust mind you. I have another 5.3l truck that does not make this sound. When you first start it cold, it smells a bit of unburnt fuel, though only for a minute; could be normal. The knock eventually goes away, but it takes a good 10 min. It's a hollow sounding knock, but again, you CANNOT year it in the engine compartment or in the wheel wells or under the truck, only from the exhaust. It doesn't get louder if I rev the engine... might even go away when I do in fact.
Oil was changed by last owner about 2k miles ago with conventional.
Oh, it doesn't take long to turn over, but it doesn't spring to life either the way my other 5.3l does.
Thoughts? I can't seem to nail this one down with a Google search.
Thanks all!

 

[drakon543]

junk plugs? used a conventional oil that was too heavy for the upcoming cold weather. 2k on the oil for conventional oil i dont like to break 3k with regular oil so i would go ahead and do an oil change. checked for anything that doesn't look like it should be in the oil. it could really just need a solid tune up and some good run time. previous owner could of just had it sitting around alot until you bought it.

 

[Big Mama]

Agreed no knock should come out of anywhere start with oil and check plugs

 

[Therealsmitty]

Thanks guys. First thing I'm going to try is a full synthetic 0w-30 just to see if the low cold temp viscosity makes a difference; I would suspect it will. I will update in a few days assuming my local Walmart carries it, otherwise I'll just try the 5w-30 synthetic.
Cheers

 

[SnowDrifter]

I use 0w30 myself. Superior oil IMO.

Could you post a video of the noise?


Piston slap is common


Could be a cold misfire. Are you able to check live data?

Could be something in or around the exhaust itself. Crawl underneath and start tapping with a rubber mallet


Is the knock rpm sensitive?


Have you verified its from the exhaust and not something under the car? A cracked flex plate comes to mind

 

[02Lightning]

Sounds like piston slap to me.

 

[Therealsmitty]

I'll see if I can capture it on video. As far as piston slap is concerned, I was always under the impression that only lasted for a minute or less, not 10-15 like this sound. Is that not right?

 

[Therealsmitty]

Here's a short clip of the sound. Again, you can strictly hear this at the exhaust, nowhere else. And it DOES speed up with the engine rpm's.

 

[iamdub]

Sounds like a mild performance muffler to me. Have you looked at the muffler?

 

[Doubeleive]

ya I don't hear anything bad in that video, my 03 Silverado has a terrible exhaust leak/piston slap or something for the first 2-3 seconds of a cold start and then it's gone until the next day makes it very hard to track down and it's been that way for a few years and hasn't gotten any worse so I just kinda live with it.

 

[dnt1010]

Sounds like someone has gutted the catalytic convertor on this one or it has a mild exhaust leak. Run it by your local muffler shop and have them check the exhaust system. If your not hearing anything under the hood or inside the front fender wells then this is all in the exhaust system. Note: It is important to get any exhaust leak repaired promptly. In certain cases deadly carbon monoxide can get into the passenger compartment.

 

[SnowDrifter]

Sounds like either a rotted out or aftermarket exhaust to me. It will sound different hot vs. Cold. Engine will use a different air/fuel ratio between hot and cold, both for performance at low engine temp and to warm up the cats. Different timing too. You can absolutely hear the difference

 

[Therealsmitty]

Hmmm, I suppose it's possible it could be an exhaust issue; it did pass NV air inspection in Jan of this year, so I don't think it's a gutted cat. I took it in for air inspection here in CO but apparently the traction control won't turn off even though the dash says it's off when you toggle it. For that reason they've not been able to test it yet. I'm pretty sure I know the culprit in that problem, just need to fix it so they can try again.
I have to say, it's a very pronounced knock sound when you hear it in person. When you say it might be an exhaust leak I think you're getting the impression from the recording that it's more of a resonance or hollow sound, but it's not.
All that said, unless it indeed has something to do with the exhaust warming up, it does seem odd that it goes away once hot. I'll be putting in 0w-30 full synthetic tomorrow and will update.

 

[Fless]

@Therealsmitty, this won't help now if you are in the process of getting license plates in Colorado. But for the future if you have to go through emissions testing in Colorado (usually every 2 years with older vehicles), you don't necessarily have to have it tested at an Aircare site and you can skip the traction control, gas cap, etc. issues.

Within certain timing guidelines you can pass a couple of tests at the mobile RapidScreen sites that move around the area. This site shows the upcoming locations:
http://aircarecolorado.com/index.php/rapidscreen/truck-locations/

Last time my Hoe needed the inspection was after my son had used it for a couple of months. He passed by the RapidScreen mobile site a few times over a couple of months, and when the license renewal came in the mail it said the truck had passed and no additional emissions test was needed. Nice surprise! Still had to pay the $25 with the registration but it was a lot more convenient than having to take it somewhere.

Some people intentionally plan their route to go by mobile sites so they won't have to visit the Aircare garages.

 

[Boatguy]

Sounds ok to me. I had a Trailblazer with an early 4.2. That thing had so much piston slap when it was cold, it sounded like a diesel. Dealer said it was normal, and it never gave me problems in the 12 years I owned it. Loved that little SUV.

 

[Therealsmitty]

Fless:

That was my thought exactly, but at the DMV they told me it was to renew only, and that it likely wouldn't work with the temp tags they gave me. I'll have to read all the fine print or give them a call to double check this. We have a Rapid Screen van that's in Castle Rock quite often fortunately. Thanks for reminding me to look into that!

 

[Therealsmitty]

Boatguy: I actually just sold a trailblazer with 170+k on it that indeed sounded like a diesel for the first few minutes when cold. This doesn't sound the same, but it does give me a bit of reassurance nonetheless. Thanks

 

[Therealsmitty]

So I put in the 0w=30 full synthetic and started it up this morning. The sound is a bit more subdued but it's certainly still there. I'm going to have to hope this is indeed an exhaust issue rather than an engine problem. We'll find out with time and I'll check back here for any other pointers anybody might have! Thanks all

 

[sparkvark]

Have you looked to see if you have rusted off exhaust manifold bolts. My 5.3 has several and makes that sound when cold but goes away as it warms up. It has 401,000 miles on it.

 

[gpd4]

Mine has an exhaust leak at the manifold at a cold start. It's irritating, but it goes away when it heats up.