greythunder3

so after changing the coil packs, plugs, and injectors, my misfire on number 3 been absent, so far. the new problem, is the idle is around 1000 rpms in neutral and park.

over the past few years i have noticed a sucking sound from under the hood. i just assumed it was the air getting sucked into the airbox. it has gotten louder and louder again, over the years, now you can hear it with the hood closed with no problem. when the engine is shut off it is last thing you hear, a second or two after the engine stops. i have looked all over for vacuum leaks, even spraying almost a full can of brake parts cleaner at all the vacuum and intake connections while the engine is running to listen for a difference in idle. nothing.

the most telling test was removing the intake pipe from the throttle body, and placing a hand (my neighbor's) over it while running. drastic suction, but nothing changed (except he had a semi palm hickey). every other car i have had would have dropped drastically in idle and struggled to stay running or died.

correct me if i am wrong, that means air, and a lot of it, is getting into the engine somewhere else between the throttle body and the head.

my next step should be the removal of the manifolds to look for cracks or breaks the gaskets, right?

boxburn

Just a thought, have you tried disconnecting all of the vacuum connections from the intake manifold and blocking them off? The vacuum lines on these things can go everywhere and make it difficult to spray test everywhere. Blocking evrything but the throttle body off would be the best way to isolate the sound to confirm if it's manifold related or vacuum related.

I think, but can't say for sure that the heater controls on a 97+ are still vacuum operated. If they are, can you continue to select different vent settings for a while after the engine is off? I know on my 95 I can continue to operate my heater controls for maybe a minute or more. What I'm getting at is, is it possible that your vacuum reservoir is cracked and letting air in? Your comment on hearing it for a second or 2 is telling my it's vacuum system related as it's the only thing capable of sucking air once the engine has shut down. There are also check valves that could fail but you wouldn't hear a noise from them, however an air leak + a failed check valve could let air get in through the vacuum system.

How well does your brake servo assist? Press the brake pedal before starting the engine and then start the engine with your foot still down. Does the pedal sink? Let off the brake pedal, let it idle for 10 seconds and shut down the engine. Give it another 10 seconds or so and press the brake pedal. Can you feel any servo assistance on the first press?