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Discussion Starter #1
Hey guys,

In my infinite quest to cure my rough idle, I'm trying to rule out vacuum leaks. I hear a very loud hissing noise behind the alternator but I can't seem to pin it down. I've already replaced all the vacuum lines and smoked the system, and I can't see any leaks. I want to hook up a vacuum gauge, but I can't find any hose going into the intake manifold; all the vacuum lines I see go to the throttle body. The brake booster hose goes into a vacuum tee, which also goes into the throttle body. Anybody know of a good place to hook a vacuum gauge?

Thanks!
 

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Anything behind the throttle blade will see vacuum. Any hose going to the TB plenum will work, since it is behind the throttle blade.

Al
 

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Discussion Starter #3
That's what I thought, I ended up just using the nipple coming off the plenum that goes to the pcv hose (near the IAC valve). The gauge was vibrating from about 19 to 21. Haynes and a few other sources say potential ignition problem when the gauge fluctuates 2 or so up and down. I'll inspect the plugs and wires, I already tested the coil packs and found them to be fine.
 

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I would throw a new set of Motorcraft plugs and wires on there. I have never tested plugs or wires, if you go to that much trouble to remove them, just replace them.

I always remove the alternator when replacing wires. It is so much easier to sort them out neatly and get the looms on there.

Having said all that, what about the hissing sound behind the alternator? That's not bad plugs. Might be the intake manifold gasket.

I have a stethoscope I got from Harbor Freight. For air leaks, I remove the bottom part and replace with a vacuum tee and hose. Then you can listen for leaks.

Al
 

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Discussion Starter #5
Should have mentioned, the plugs are copper and were changed about 10k miles ago. The wires are about a year old. The plugs had barely any wear on them.
The thing that bothers me is the hissing, but if there was a vacuum leak wouldn't the vacuum gauge read far below 19 or 20 in. Hg? My smoke test also did not reveal any leaks.

I'm at my wit's end with this thing. I just checked the motor mounts and did a power brake test and there was no movement at all.
 

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You said you replaced the wires. Are they good quality cables and did you make sure you routed them properly (i.e., just like OEM factory)? Poor quality and/or incorrectly routed ones can result in induced misfiring, for one thing, and can make you think there's something wrong in the ignition system.

Hissing isn't necessarily abnormal, depending on where exactly it's coming from and what it sounds like. I hear mild hissing on my 4.6, with a smooth idle.

Have you checked the IAC valve? It may need replacing or at least cleaning. After cleaning, lube it to ensure the solenoid plunger doesn't get stuck. Last time I cleaned mine, I used this (older can design, though) and haven't had probs yet, and that was years ago.
I would lube it off vehicle and not let the engine inhale lube excess, not that it would necessarily cause damage, but no reason to risk it.

Also, check for leaks around the IAC gasket. If you don't replace the IAC and use a new gasket, take care not to damage the original gasket.

Have you checked for dirty/corroded contacts in the IAC harness connector? Remove/replace the connector a few times, wiggle it and see if it makes the engine stumble (or die). Be careful when handling engine connectors, especially if you have a bad case of plastic embrittlement like I do.
 

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Discussion Starter #7
The wires are definitely routed correctly and are not cut, abraded, etc. In retrospect I should have went OEM but I found a good deal on some NAPA belden powermax cables so I went with those. They have good reviews and seemed like good quality. I just finished looking them over now.

I replaced the IAC over a year ago with a new OEM one, as I had foghorn noise and random stalling. Everything was working good after I installed it. Gasket was new and torqued to spec. I will check the connector, as I also have a case of "anything plastic and unobtanium breaks when you look at it funny" lol.

I disabled one cylinder at a time to see if there was an actual misfire, but each cylinder disabled made the car run even rougher. I'm just getting confused, the vacuum gauge says I may have a ignition issue, but if I had a plug/wire/coil issue I'd imagine it would happen while driving, not just idle.

I'm just going to make a new thread dedicated to this issue, I'll list more specifically my troubleshooting steps exactly.
 
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