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Question: Light Valve Chatter

air1jpg

Well-known member
Joined
Jun 4, 2007
Messages
394
Location
Louisville, KY
Corvette
1996 Polo Green LT1 Coupe
I am hearing a little valve chatter when the engine is overnight cold on light acceloration. The chatter will go away when the engine reaches about 120 degrees. Could it be a small problem? ;shrug
 
Knowing the temp effects it, the ECM is not in its normal operating area until around 150+, so fuel or ignition could be the source. It might just be the summer fuel causing a mild knock until its hot.
 
Could me running 93 octane have anything to do with it? The LT1 engine only requires 87 octane. I put 93 in it because I believe the LT1 engine with the ECM can take advantage of 93 octaine gas.
 
It could....

I saw a tech article one time that explained what happens when hi-oct fuel is burned in an eng designed (low compression) for low oct or 87 oct gas.
I forget the details but the bottom line was that it can have an adverse effect due to the extended burn time of the 93 gas.
All hi octane gas does is burn longer. 87 pops like a firecracker and has a short combustion. In low compression engines that can mean that exhaust valves are opening at a specific time and if there is 93 still burning or expanding then that can cause some clatter, raised emissions, and even burnt valves if run too hard.

I wish I knew were to find that article, it was very informative. Basically it says to run whatever the Mfg recomends unless its been modified to run a hotter fuel.

All gasoline is born the same. Refineries add a chemical to slow the burn time of some fuel and call that "premium" or 93. The longer the explosion the longer it applies pressure to the piston. That works as long as there is not a valve opening during that burn time. In motors that are set up for Premium gas, more power is developed from the greater continued pressure on the piston dome from the longer explosion/burn time.
 
A cold motor is not too likely to knock spark, except possibly with 87 gas. The timing retard response for knock detection tho' is FAST, even with old GM computers. It should almost not be heard, unless it's valve/lube issue.

To confirm that it's NOT knock, fill it with 93. 93 won't knock cold unless the heads have been shaved, and the squeeze is higher than stock.

Try putting in a quart of 20 - 50 weight (Synth), after draining a quart. See what happens...
 
Fuel

This comes from a owners manual PDF for a 93.

Fuel​
Use​
premium ,unleaded gasoline rated at

91 octane​
higher, You may use

middle​
grade or regular unleaded
gasolines, but- your
vehicle may not
accelerate as
well. The gasoline you use

should meet specification​
As STM

D4814 inthe​
US. and CGSB 3-5-92 in
Canada.
These fuels should have the
proper additives,
so you should not
have to add-
anything’ to the fuel.

In​
the US. and Canada, it’s easy to be
sure. you get the right lrind
of gasoline
(unleaded). You’ll
see “UNLEADED”

right​
QII the pump. And-only unleaded
nozzles
will fit into your vehicle’s filler
neck.
Be
sure the posted-octane for premium
is at least
91 (at leatit 89 €or middlegrade

and​
87 for regular). If the octane

is​
less than 87, you may get a heavy

knocking​
noise when you drive. If. it’s
bad enough, it
can damage Your engine.
If you’re
usingfuel rated at 91 octane or

higher​
and you still hear heavy

knocking, your​
engine needs service.

But don’t​
worry if y ou hear a little
pinging noise when
you’re accelerating
or driving up
a hill. That’s normal and

you don’t​
have to buy a higher .octane
fuel to
get rid of pinging. It’s the heavy,
constant
knock that means you have a

problem.​
Fuel​
Capacity:
20 U. S. Gallons
(75.7 L)
. Use unleaded he1 only.

24
 
The problem is very unlikely to be detonation.

The problem is also unlikely to be "valve clatter".

I suspect it's "cold piston knock" and, unless it's really loud or it does not go away once the engine reaches normal coolant temp, it's unlikely to be a problem.

Change from a 5W30 to a 10W30 synthetic oil then road test the car to see if the noise changes.
 
The problem is very unlikely to be detonation.

The problem is also unlikely to be "valve clatter".

I suspect it's "cold piston knock" and, unless it's really loud or it does not go away once the engine reaches normal coolant temp, it's unlikely to be a problem.

Change from a 5W30 to a 10W30 synthetic oil then road test the car to see if the noise changes.

Yep, there was a big deal made back in that time frame about piston slap with some owners trying to sue GM about "decreased value" due to the noise. Most all 350s in that era make the noise when cold. Used a GM crate motor in my son's 97 pick up and it still has the slap noise when cold.
 
I do recall something from way back when, regarding "piston slap" and that GM dismissed it as a non-issue.
 
Thank you for the replies, I guess this isn't a big deal. The noise I am hearing is not very loud and is only on acceloration when cold.
 

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