Knock Knock.. :facepalm:

[Zabo]

Trying to figure out why my LS1 is putting out a slight knock starting around 2K RPM and running up from there. It's not noticeable unless you're listening for it but it's that 'diesel' knock for sure. Gets worse during the cold but I'm hard pressed to think it oil flow related since I've got good pressure and clean oil came out during the last change.

I've also pondered whether or not it's a spark advance issue or just fuel quality with the change in seasons. I normally run British Petroleum (BP) 89 octane but I'm wondering if running 93 could help during the colder months.

Any of you LS heads out there (Revo, Brando) want to chime in I'd appreciate it. I'm just hoping it's bad fuel and the car just being picky..

[SSyanide]

The LS1s usually develop "piston slap" at some point in their existance, but I'd definately run 93 octane reguardless. Also, I know it's a pain , but pull a plug or two and check them out if you haven't changed them in awhile.

[Zabo]

Originally Posted by SSyanide

The LS1s usually develop "piston slap" at some point in their existance, but I'd definately run 93 octane reguardless. Also, I know it's a pain , but pull a plug or two and check them out if you haven't changed them in awhile.

I thought that was an issue on only the early build LS1s. I'll probably pull the plugs this winter after I get my rear end 'readjusted' by my trans/driveline shop. Also have to change out the serp belt.

Both the plugs and the belt are the same ones installed at the factory..

The engine only has 33.5K miles on it so it's relatively 'untouched'. However I know it's 10 years old so maybe winter will consist of some more repair/replace.

EDIT: Google'd 'Piston Slap' and got an audio example; it's not PS..

[Raven02TA]

yep its piston slap all LS based aluminum motors have it, mostly is a cause of the type/quality of fuel being used, you really should be using 93 octane at all times of for nothing other than better gas mileage, your injectors/fuel filter have less of a chance of clogging.

and why does your rear need to be re-adjusted @33K miles.

[Revo1]

Mine does slap a little too, I wouldn't worry too much unless the noise gets intrusive. Mine does it more on the low end tho, mostly noticeable on startup and at idle... How long has it been since your last oil change?

[brandotron]

Piston slap only occurs when the engine is cold. When heated, the rings expand and the slap stops

My LS1 valvetrain chatters like a sewing machine but goes away by 2K rpms

Open your manual or your gas door and look really close. I mean reaaaally close, get down in there. See what that little tag says?
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PREMIUM UNLEADED FUEL ONLY!
You "love" your car, but you're not giving it the factory recommended fuel octane? Sa matta wit' you boy?! Dammit, this hurts my heart. THAT'S why your hot muffler attacked you!


*Sigh* But I digress.
Put 93 in it and the detonation will go away. If not, it may be the knock sensor. If not, it may be bearings.

[Zabo]

Actually.. my door/manual says 91 Bondo. :P So I put 89 more than I do 93 in there (93 to really clean it out).

As for the 'sewing machine' analogy I'm going to guess that it's the valvetrain and not slap. It's not a BAD knock like a diesel-esque sound but it's a chatter in line with 2K-4K. Ran it again today after letting the engine warm up and it was still there but harder to discern over regular engine noise. Maybe I'm just acute to the little noises this mustang eating motor makes. I've never really played long term with a pushrod V8 or pushrod anything. More used to OHC motors so maybe it's just me.

However I'm going to do the next few (4) tanks in 93 and see what that does rather than the occasional tank of 93 (every 5th or so). Also throw some STP or Lucas in the tank again.

Last oil change was this past Sunday, 44 miles ago.

EDIT: the 'readjust' is just a joking term for 'taking it back' since I discovered either the guys crimped the gasket or didn't properly reattach my diff cover. It's leaking a little bit more than the usual 10 bolt weep. Nothing major but I paid good money and have never had a problem with them (they do a ton of specialty contract work for our trucks at work).

[brandotron]

Originally Posted by Zabo

Actually.. my door/manual says 91 Bondo. :P So I put 89 more than I do 93 in there (93 to really clean it out).

As for the 'sewing machine' analogy I'm going to guess that it's the valvetrain and not slap. It's not a BAD knock like a diesel-esque sound but it's a chatter in line with 2K-4K. Ran it again today after letting the engine warm up and it was still there but harder to discern over regular engine noise. Maybe I'm just acute to the little noises this mustang eating motor makes. I've never really played long term with a pushrod V8 or pushrod anything. More used to OHC motors so maybe it's just me.

However I'm going to do the next few (4) tanks in 93 and see what that does rather than the occasional tank of 93 (every 5th or so). Also throw some STP or Lucas in the tank again.

Last oil change was this past Sunday, 44 miles ago.

You're going about your fuel purchasing all wrong. You should be putting premium in yours at every fillup. 91 is premium here in CA so that's all mine eats. The primary function of the knock sensor is to tell the PCM to retard the timing for anything lower than 91 octane. If you're consistently putting lower than that, you're making your knock sensor, which is 10 years old now, work overtime. It could be failing on you, causing detonation (knock) which is bad for your rotating assembly.

I wouldn't worry about Lucas or STP additive in your gas unless you feel like your fuel injectors are gummed up. They're probably not after 35000 miles or whatever.

[Revo1]

Brando, where you gettin 93? Nothin but that 91 swill up here :/

[Revo1]

Just saw the above post.

[Zabo]

Update: Moral of the story..

Kids, if ya have an LS motor or LT motor and it says use Premium..

Use premium unleaded gas!

Knock is gone and she's singin' like a songbird.. no pun intended.

[Fotowns Finest]

would this also apply for the 3800, or does it matter?

[brandotron]

Originally Posted by Fotowns Finest

would this also apply for the 3800, or does it matter?

3800 is designed to use regular. LS's have a slightly higher compression ration and require more ka-boom in their gas.

[Fotowns Finest]

Originally Posted by brandotron

3800 is designed to use regular. LS's have a slightly higher compression ration and require more ka-boom in their gas.

ok sweet, i learned something today