Low idle knocking

My '93 Accord LX automatic has recently developed a very mild knocking sound
when it is idling at low speed. The instant I press the gas (regardless of
what gear it is in), the knocking goes away. Moreover, I only hear it when
the radio is off and the interior fan is on low running speed. Despite the
knock, the idle speed is normal (~770 RPMs) and it does not feel rough.

A Google search returned a handful of possibilities including carbon
build-up (EGR or otherwise), valves needing adjustment, bad gas, and perhaps
needing a tune-up. FYI, the spark plugs, distributor cap and rotor, and
timing belt were replaced 20,000km ago, it has not had a valve adjustment
since I have owned it (5 years now), and I never put cheap quality gas in
the tank. It currently has 221,000km (137,000 miles).

Any ideas? Thanks in advance.

 

Well you can go under the hood under idle and disconnect 1 spark plug wire
at a time and then reinstall to determine which cylinder may be causing the
knocking. If you pull off number 3 wire and it goes away at least you know
it is the number 3 cylinder area to look at.

I would have a hard time believing any carbon buildup causing a problem at
idle.

I would be worried about pistion slap or a bearing going out.

 

I just fixed a knocking problem. It was the spark plug being loose so I had
to tighten it back. At first I thought it was the valves knocking but with
the help of some people from this newsgroup I got it fixed.

John

 

Well, I did some troubleshooting with a friend today. With me standing
outside, I asked my friend to step on the brake and put the car in gear to
get it to low idle. Before the car was even shifted out of park (and
after), I could hear the sound coming from under the car. This knock was
actually a metallic rattle.

With that said, I am 99% sure it is a loose or rusted heat shield (catalytic
converter heat shield perhaps?). I just find it strange that I could only
barely hear the rattle inside the car only at low idle. Perhaps at higher
engine speeds, the increase in engine noise filters out the rattle?

I have heard suggestions to either replace the heat shield, or just leave it
off. What factors should I consider in making this decision (other than the
cost to replace the shield)? I don't drive the car on grass, so I don't
think I would have to worry about a hot catalytic converter setting things
on the ground on fire when the car passes over them.