Hi folks, just today my parent's 1991 Ford Explorer with the 4.0 v6 started making this really loud hissing sound, like a vacuum leak but I can't seem to figure out what exactly is causing it. It sounds like it's coming from the very rear of the upper intake manifold, at the part where the vacuum brake booser plugs in. Theres this very small hose which plugs into the front of that part (facing towards the front of the vehicle) that when touched veels like it's vibrating, and when unplugged the hissing seems to go away. I can't find the manual for it and don't know where this hose goes or whats causing it to be so loud. Some symptoms are a erratic idle (hangs out around 1000 RPM, then drops to around 600-700ish, runs rough then picks back up, over and over). When you unhook the duct from the air filter box to the throttle body the hissing stops and it seems to run just fine. I replaced the IAV thinking that was the issue, but it's still there. Any clues?

 

These engines were notorious for vacuum line leaks. You're just going to have to use good light and find the ones making the noise. One of the problem areas is the line coming up from the transmission modulator valve. When the valve diaphram gets old and starts leaking, it allows tranny fluid to coat the inside of the vacuum lines and they start popping off. One of the ones is the line on the fuel pressure regulator. There is a "tree" of vacuum ports at the rear of the engine where many of these lines hook up. Just follow the lines and you'll find your leak. If there is oil in the lines, change the vacuum modulator valve. ..best of luck.mike

 

i dont know if u said this or not, but did u check to see if there is a vaccuum hose cap missing??? that is possible, it happened to me! and i was REALLY scared at first lol

 

Thanks for the replies so far guys, I figured out where it was coming from. I don't know how I missed it but there's a hose missing from that tap. I looked all around and can't find the hose thats supposed to be attached there. The one is missing is the one on the tap at the rear of the manifold, drivers side (where brake booster connects) and connects to the rear facing (towards firewall), inner most connector. My repair manual is missing in action and since I cant find the hose at all (must have lost it while driving?) so I need someones help in figuring out what it was for. There is fluid, it's hard to tell but I'd guess tranny fluid, so I was thinking this means the vacuum modulator on the trans went bad and it siphoned oil up that line, making it come loose. The engine oil is also overfull buy a fair amount so this makes me think thats what that hose was for. Any suggestions?

 

That sure does sound like the vacuum modulator.
It's still possible that the open vacuum port was just capped (my '92 has one that's capped). Either way, you need to find out where the fluid's coming from (either transmission fluid from the tranny or gasoline from the fuel pressure regulator) and fix it.

 

yeah, when i heard that hissing noise in my 4.0, i located the noise and looked at it and i put my finger on it and i right away realized that it was a vacuum, but then i went home and looked that our 94 explorer 4.0 (exact same everything as mine) and looked at the same spot and found that it was just capped.

also, when my cap was off i seemed to lose power, the truck would basically die after a while w/o that cap on it, anything similar to your truck???
quack