I have a 99 Accord EX, with 113 k, V6

I notice a a kind of thunking noise that emanates from the rear of the motor. It seems to be more pronounced when the throttle backs off.

I have had issues with multiple cylinder misfires and just cleaned the intake manifold port that delivers air when the throttle body is closed. The codes dont come back quickly after I clear them. I dont think that had anything to do with it but it is in the recent history.

I replaced the idler pulley and drive belt recently; not related but it elimiantes a source of noise.

I adjusted the valves about 15k ago.

I used a vacume gauge and got 21 lbs rock soild at idle; no fluctuation but when I goosed the throttle the vacume actually increased? it shot up to like 40. I thought my gauge was bad because I wondered how could vacume go above 21lb? One of those charts said it can be the sign of an exhaust restriction.

It seems to idle fine when cold but after it runs for a while like a half hour the idle tends to fluctaute more.

I ran a can of Seamfoam through it for the heck of it.

I checked the spark plugs and they seemed ok: nice greyish brown color.

The car starts right up and runs great at high rpms.

It almost sounds like a minny backfire that emanates from the rear exhaust manifold but I dont see any exahust smoke back there.

I am kind of wondering if my cat maybe is starting to go out and causing too much back pressure when the throttle closes?

If the cat is clogged or going out you would get a code from the aft 02 sensor; wouldnt you?

One thought is maybe the cat is partially restrcited and as it heats up it gets worse. At idle when the exhaust stream is weaker the gas cant push through as well causing too much back pressure however at higher rpms the gas pressure over comes the resistence better.

Any ideas?

 

Me, again I can't hear it....I'd be leaning towards the ignition system, I know you looked at the plugs but testing wires and cap and rotor?

Next on the list, egr passage.

 

The front and rear engine mounts (#5 & #6) are vacuum-operated. Maybe something flaky is going on with that? They're supposed to go SOFT when you're idling in "D" or "R" but maybe they're switching when they aren't supposed to?