How do you tell the difference between "spark knock" and "rod knock" in an
engine ?

'87 Golf, 8v gas, about 180K miles. It has a knocking coming from the
engine under very light load - i.e, when I'm almost coasting on level
ground at about 2000 rpm. Goes away under load.

Am I about to throw a rod, or do I just need to put high octane gas in it?

I've only noticed since the last fillup yesterday, so maybe I just got some
really poor gas ?

On Mar 5, 11:18 am, Fred Mau <fred-dot-... (AT) comcast (DOT) net> wrote:
By the frequency. Rod knock you should be able to feel in your gut.
Spark knock is like muffled ticking.

Peter Wieck
Wyncote, PA

"pfjw (AT) aol (DOT) com" <pfjw (AT) aol (DOT) com> wrote in
news:1173121919.183392.164680 (AT) q40g2000cwq (DOT) googlegroups.com:

This is somewhere in between, it's sort of like the 'clatter' that a diesel
makes but not as loud. Lower in frequency and more hollow-sounding than,
for instance, lifter noise.

I haven't done any serious engine work since the '80s, but I vaguely recall
that it made a difference whether it 'knocked' under light load vs. heavy
load ? Knocking under a light load meant one thing, under a heavy load it
meant another ?

- FM -

On Mar 5, 4:17 pm, Fred Mau <fred-dot-... (AT) comcast (DOT) net> wrote:

Boyoboyoboy.... Engine Noises, my perception:

Knock or Ping (spark-knock, premature ignition): Light diesel-type
noise, somewhat muffled, akin to a dozen (small) marbles in a wooden
cigar box. Actual Diesel noise would be larger marbles, same concept.

Lifter tap: Tap a pencil point on the bottom of a china saucer, with a
thick cloth between your ears and the tapping. Similar in quality to a
mechanical wrist watch sound.

Rod/Bearing/Wrist-Pin knock: Visceral *thrump, thrump, thrump" at low
audio frequency. Akin to a medium-sized bongo-drum at the bottom of a
barrel. Accelerating under load usually less than when decelerating
under 'reverse' load. Best way to diagnose with certainty is to get
the engine warm, accelerate up a pretty steep hill and wind to the
near-top of first or second gear, then let off the gas entirely about
half-way up. If you get a "THRUMP, THRUMP, THRUMP" that you can feel
in your gut or jaw, you have it.

Peter Wieck
Wyncote, PA

In rec.autos.makers.vw.watercooled, Fred Mau wrote:

Fuel knock (detonation) normally occurs under heavy load such as
lugging in high gear, going uphill, accelerating, etc. I can imagine
that some odd control situation, such as an over-active vacuum
advance, could change that, but it seems unlikely.

I would try topping off your fuel with a super-premium for a while
to see if things improve. I expect it won't.

Fred Mau wrote:

Spark knock sounds sort of like pebbles in a large tin can. The
frequency of the sound is high and so is the repitition rate of the
knocking sound.

Valve train noises happen at half engine speed and are typically loud
"ticking" noises. Think sewing machine.

Rod knock happens at half engine speed (low RPM) or engine speed
(*really* bad rod bearing, piston hitting head) or double engine speed
(bad rod bearing, high RPM). It's a heavier sound and the period of the
sound is predictable. Think of someone rapping on the engine block with
a small shop hammer.

Load will not influence a valve train noise. If the ignition is too far
advanced it can influence spark knock. (Were '87's computerized?) It
will influence rod knocks.

HTH.

JRE

could be anything from
piston slap
poor gas
bearing- but normally your oil light will come on first.
lifter tap

my bets on piston slap with that many miles.

"Fred Mau" <fred-dot-mau (AT) comcast (DOT) net> wrote

That is sort of low mileage for an engine. ;-) 180,000 miles. I have
owned VWs with 300,000 miles that were quiet, no bearing or piston
slap noises. Even the Audi engine I put in the Rabbit that had the
slopiest cylinders I have seen in a VW block did not make funny
noises. It was not as bad as the GM blocks I would see in Ohio but it
had a definite ridge that Ihave not seen down here in the south. But I
generaly only work on my own cars now which have regular oil changes
ever 5-25,000 miles. I suppose it depends where you live affects how
ong an engine can last and howlong your commutes are. In Atlanta we do
not have the harsh winters that are hard on an engine. Of course if I
had a daily driver from 1987 the car would have 600,000 miles on it.
My 1987 truck which I retired from daily driver status 17 years ago
has 274,000 miles on it.

On Mon, 5 Mar 2007 21:11:44 -0500, "Lost In Space/Woodchuck"
<newsgroupmail1 (AT) wildblue (DOT) net> wrote:

Well, I figured it out. The rubber rings that hold the rear of the
exhaust in place were gone. At just the right RPM with a light load the
exhaust pipe would knock against the body.

It sure as heck sounded like it was coming from the engine though.




Jim Behning <jimbehning (AT) doesthisblockpork (DOT) mindspring.com> wrote in
news:hooqu2dh64i9dn3jifg2kucl29v9q796hv (AT) 4ax (DOT) com:

On Mar 14, 4:13 pm, Fred Mau <fred-dot-... (AT) comcast (DOT) net> wrote:
Now that is a _GOOD_ outcome.

Peter Wieck
Wyncote, PA