How do you tell the difference between "knocks" ?

By the frequency. Rod knock you should be able to feel in your gut. Spark knock is like muffled ticking.

Peter Wieck Wyncote, PA

" snipped-for-privacy@aol.com" wrote in news: snipped-for-privacy@q40g2000cwq.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 -

innews: snipped-for-privacy@q40g2000cwq.googlegroups.com:

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

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.

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.

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 wrote in news: snipped-for-privacy@4ax.com:

Now that is a _GOOD_ outcome.

Peter Wieck Wyncote, PA

Oh, _those_ knocks. Yes, it often sounds like it's coming from the engine, because in fact the place where it's knocking is often where the cat is (because it's sort of bulbous and there's not a lot of room there). Kinda a terrifying sound, too, because of the way you can sort of but not quite feel it, almost the way The Bad Knocking is.

There might be more exhaust system suspension parts missing up around the cat -- some models had a hook there too. Worth a check, because the connecting flanges can come under some stress if the suspension isn't all quite right.