Mystery Vacuum leak

[2stroke]

Ok, I might have a good one for you guys. I've been having an idling problem for about a month now, and am finally ready to try and find it. I just finished putting in new intake gaskets, all three, so I know its not that. I only have three things off the vacuum tree; FPR, brake booster, and a line to one side of my DPFE to keep me from getting a check engine light. None of those seem to leak. How its running is where it gets strange. I have a manual transmission, so no vacuum is involved. Still, I can hear a sucking sound when at part throttle in 1st-3rd gear, but never 4th. Sometimes it comes back in OD, sometimes not. I don't see how it could be transmission related, maybe it just has to do with load on the engine. Still, since I can make the sound with the throttle, and it doesn't seem to pertain to RPM, I would think there is a crack in my intake duct. I can't find one. I've tried two air ducts, and even plugged the hole for the breather tube with no change. I've sprayed carb cleaner around the engine and found nothing. Another strange thing is I can't hear any leaks when I rev it by hand under the hood. It only happens when driving, and it seems to go away with wide open throttle. It has to be a good sized leak because I have my throttle plate stop screw turned almost all they way out, and it will idle at 650 rpm no problem with the IAC disconnected. With the IAC plugged in, it idles at about 900 rpm and unsteady when warm. I mean, what else is there? Its not intake gaskets, its not a hose off the vacuum tree, its not that tube that goes to the throttle body (that I assume is to burn off excess gasoline vapors), and as far as I can tell, the intake ducting is fine. Throttle body and IAC gaskets are new as well. I've never smoked in my life, but I might have to get a cigar and try as Scotty Kilmer does and blow into the brake booster line to find a leak.

 

[Wolfie_85]

Reading the title of your thread, I was just going to suggest that (the Scotty Kilmer method)! That's really all I've got. My dad's 92 had a leak, it was the little rubber connector (I guess you'd call it that. Or boot) at the end of one of the lines at the vacuum tree. Everything looked good upon visual inspection, but with an ear right next to it, it was revealed.

 

[natenkiki2004]

Check the brake booster grommet. Mine was split like crazy and wouldn't hold a vacuum with the engine off. You should be able to come back to the car over a day later and still have vacuum there, you'll hear a hiss as you press the brake pedal without the engine running.

Aside from that, make super sure your vacuum lines are intact. I had all kinds of problems with the factory lines and replacement boots, they all started splitting and causing problems.

For what it's worth, I have a similar issue, I can hear something that sounds like a vacuum leak but I only hear it while cruising and it's coming from a hole in the firewall. Might just be the airflow in the engine compartment. I've also heard of people having issues with their IAC valves causing a whine or hiss, might be something to look at. If your truck idles fine without it, try driving with it unplugged, see if you get the same sound.

 

[2stroke]

I do have the same sound with the IAC disconnected. Personally, I like it connected and idling high better than without it. It starts and idles amazingly without the IAC once warm, but you have to hold the throttle for a cold start, and the RPM's drop way to fast between shifts for my liking. It is certainly not just wind noise, as it goes away depending on the throttle. Its not speed related at all. Other than the cigar method, I think I will buy silicone hose like you, and replace all my lines. One thing I might try is putting a known good hose to my FPR and capping off everything else and running it a while like that. Obviously I wouldn't take it on a road without power brakes, but if it idled down, I found my problem. I never thought of the grommet for the brake booster. If not, I guess I'll be buying my first cigar tomorrow, I hope it doesn't leave too much of a bad taste.

 

[the_don]

I think the sucking sound at about half throttle is the sound of air moving past the butterfly, mine did the same A LITTLE until i dropped a K&N cone filter on the front side of the MAF sensor, now its just horrible loud.

 

[2stroke]

I used to have a cone filter on this, but since got rid of it. If it was intake sound, it would do it all the time. The fact that it almost always goes away in 4th gear is really strange to me.

 

[2stroke]

Well, it turns out there wasn't a "vacuum" leak. I swapped IAC with the parts truck, and the sucking sound seemed to have gone away. I blew smoke into the vacuum tree, and never found anything. I even kept going until it came right out the air intake. I often reset the computer, so codes have not popped up. I've got a few ideas of what it might be, i'll get back if I find the problem.

 

[2stroke]

Changed coolant temp sensors, and almost instantly it idled down. I reset the computer, and it idles right down to 650 again.