Vac at idle is 14hg.. Low?

[MusPuppis]

Just bought a vacuum gauge and put it on my engine to check my idle. The engine was warmed up and sitting at about 150*. I used one of the smaller ports going to the AC I beleive (I dont have a vacuum distribution block since the engine is in a truck. I switched the 5.0 manifolds stalk out with my truck stalk giving me the branchs I needed). Should I have read the vaccum off a larger outlet? Maybe the PCV or Brakebooster stalk? I used the AC since it was the simplest.

Anyway, It was reading at 14hg at warm idle around 1100 RPM. I idle a little high but havent got my Tach in yet (going in tonight) so the actual RPM is only a vague guess. I'm not real sure what it should be but that seemed a little low to me. Also, I was told that if I had a vac leak I could see some fluctuation in the reading.. How much would be typical? The vac gauge was a little shaky at 14. Moving between about 14 1/2 and 13 1/2. I wasnt sure if that was just natural vibration or something. the guage is a cheap 20$ Advaned Auto Parts affair and having never used one I dont know if a small amount of needle shake is normal or indicitive of a leak.

Engine mods (if it matters much) - .40 over, KB Pistons, ported e7's.

 

[jerry beach]

That is low for a stock cam, I am assuming thats what you have. the vibration in the needle +/- 1" hg is normal, my guage shows that too. Try checking it in a different location. I had a vac leak once and I fixed it whenever I hooked up the guage, I was reading 20 in. hg and steady. I hooked up to a different spot and saw the leak on the guage, felt like a fool. You may have a vac leak, that will cause high idle and low vac. Should read 18-21 in hg on a stock healthy motor. Read all the instructions that came with your new guage, there is good info in there. What make/year truck do you have and is it carbed or EFI?

 

[MusPuppis]

Well, the trucks a 94 but the engine is out of a 92 Stang. EFI and all.

Thanks for the info man, much appreciated. I'll try a couple different locations and see what I come up with.

 

[Michael Yount]

I'm with Jerry - I'd expect 18-20" with a stock cam/healthy cylinder sealing. Port size shouldn't matter - vacuum is vacuum; in fact a smaller port/hose will probably give you a truer reading.

 

[HISSIN50]

hey bud. ill toss my two cents out. a stockish stang motor will put out close to 20" at idle (14-20 is what most folks post). i think i mentioned before that i have 19.5" hg at idle.

right - if one has a vac leak, they can see an idle that is slow to return from higher revs (it sorta' hangs and comes down slow). also a little idle hunting can and usually does accompany it.

if you have doubts about the results with the line you used, i might find another line and try it (and be sure to plug the other receptacle if you unplug something as that alone can create a leak).

good luck MP (and bump).

EDIT: when i loaded this thread, there were no replies. i was doing something else and did not know others had replied. apologies for redundancies in info. my bad.

 

[MusPuppis]

Thanks guys.

I do have a very very small idle surge. Maybe 2-300 rpm swing.

I'm gonna try one more line just to double check but sounds like I have a leak somewhere.

Does anyone have a recommended method for tracking the leak down? I know you can spray carb cleaner, and Hissin and I have talked about that before but Ive already sprayed a good chunk around and not noticed anything.. Granted its possible I just havent hit the right spot.. but if anyone knows a more effective method for finding the leak, let me know.

I'm gonna try and tackle this tomarrow after my tachs installed, which should be tonight.. As soon as my JBweld sets up. (dont ask..)

 

[HISSIN50]

i know Michael and Jerry will good info for you on other methods of finding leaks (Yount is just plain brilliant and Jerry is a creative guy who comes up with all sorts of neat stuff).

 

[jrichker]

Squirt can of motor oil to squirt on the flanges and joints. The engine will speed up and the oil will disappear. The process is messy, but there is no danger of flash fire as there might be with other liquids.