Vacuum Leak/Whistle During Acceleration - 94 4.0 xlt

[Explorin'VA]

I changed my spark plugs last week and after getting them on and taking it for a spin I noticed a loud whistle coming from the drivers side of the engine and huge power loss while accelerating. I figured I knocked a hose loose so I referenced the vacuum diagram and searched for anything that didn't look right. I capped a couple hoses and replaced a couple that looked worn, but the leak didn't go away. I sprayed every hose I could see with carb cleaner including the intake manifold gasket and didn't notice any change in idle. Then I went and bought a nasty cigar to use to blow smoke into the manifold and smoke only came out from my intake filter and a cluster of small hoses on the passengeres side by the A/C stuff, so I taped the hose connector that was leaking, but the whistle/leak still remained. I cleaned and inspected the pvc valve, which made it run well but didn't get rid of the leak. (gonna replace the pcv valve tomorrow)

Finally I found this cap in my driveway where I changed the plugs at.

I didn't see any hoses that this could go to. I feel like this missing cap is what's causing the leak. The previous owner made a mess of all the hoses and wires. Anyone know where this might go?

truck is a 94 explorer xlt 4.0 v6

 

[awood]

http://www.explorerforum.com/forums/showthread.php?t=359540

 

[Explorin'VA]

That's a thread I referenced when looking for all my hoses. I've inspected every hose shown there with no luck. Anyone know where that cap might go?

 

[Xeek]

the only place with caps is the octopus on the intake. if you arent missing any then its just an old one that fell down and eventually made its way to the ground. looks like it might have a hole dryrotted through anyway.

octopus: fpr, cruise control, brake booster, tranny(metal line), and a line that goes to the ac stuff.

ac stuff(comes out on the other side of the coil pack): 'T" connection, left goes to wireharness going into firewall. top goes to another 'T', one goes to the stock air box and the other goes to the submarine lookin canister on top of the ac box.

 

[malohnes]

I had a similar experience and I found that the grommet to the brake booster was deteriorating and a small crack developed when I was servicing the engine form removing and installing the hose to the intake. Made a loud whistle exactly as you describe.

If the leak is big enough it will affect idle, if that is not happening, then look for a leak with out revving the engine may be fruitless.