Ok... after long winter months I took my FD out for a spin yesterday. During winter it had full 3" exhaust installed (previously I only had DP, 2.25" MP and crush-bent catback). Previously I was running about 16inHg of vacuum at idle but now I'm only getting 12inHg at 700RPM. Idle is stable.

Car boosts fine on both turbos and pulls very strong (that MP really did help clear the top end of ignition breakup).

I have seen 12inHg idle before but it somehow sorted it out on its own last year

Now I'm seeing it again, and am looking for some directions for what to check. When I fired her up yesterday I had to recharge my battery and while having the hood opened, I noticed some smoke coming from the LIM/turbo area which stopped about 2-3mins later. Could it be that my UIM/LIM or LIM/engine gasket is shot ? What is that spray you use to detect vacuum leaks - is it a starter spray ?

Could less backpressure (due to full exhaust) also cause lower vacuum ? Engine has about 40k on it and only thing that changed from previous year is the MP (engine wise).

I have searched but I didnt find anything useful except for possible gasket leak

Thanks!

 

Cruiser, I see normally 16/17 inches of Hg of vacuum at idle, but it noticed it also depends on idle speed, my is normally around 800 '/min.
If the speed gets higher the vaccum gets higher, but I don't think a difference of 100 '/min can cause a loss of 4/5 inches of Hg.
My vacuum gets around 20 inches of Hg if I get the A/C on (with a rev speed of approx 95 '/min).
However, that spray used to detect leaks is normally used for pressure leaks, not vacuum leaks, it is usually a bubble soap spray, I think you can find from those shops installing LPG and NGV systems on veichles, but I don't think it will be suitable for vaccum leaks detection, I mean , you risk to get it sucked inside the manifold.

 

Low vacuum is a consequence of leaks or low compression. Check your nipples (tee-hee) covers for splitting. I had this same problem and thought my engine was having problems and it was the UIM nipple. I had used a Autozone nipple cap and it had split...since it was black I could not see the split. It was not until I removed the UIM and removed the cap that I noticed it had split.

Check that first and if not resolved...you can take some soap and water and "baste" it around your mated connections while your helper is gassing the car and look for bubbling around the gasketed transitions.

Hope this helps.

 

more often than not this is a strictly vacuum issue. when you hurt your engine it shows in power and sound and feeling and sometimes my left knee gets to hurtin. yank off your uim, check all your vacuum lines. from what i understand its fairly hard to reduce the compression of out engines in a small way.

 

My guess is a slight vacuum leak somewhere. My car also has low vacuum at idle (around 12 inHg), but everything else is fine including compression. I am not going to worry about it unless it gets worse and in turn easier to find.

You can use carb cleaner to spray on suspected areas. If the idle changes then you have a leak.

 

I do have a leak of some kind somehwere in the LIM area. But its possible its very minor as the materials expand with heat, the smoking stopped

I already ordered a mityvac, hopefully it will help me with diagnosis of such future problems

 

UPDATE: Got it sorted now. 15hgIn at 700RPM.
My throttle body/intake elbow gasket was the culprit. I have seen there was one missing (there was ghetto-silicone fix instead of the gasket) when I did the plug wires job last year. Today I changed the gasket and VOILA. Vacuum is back up. I also did the carb spray trick and couldnt find any vacuum "leaks".

 

How does an intake leak before the throttle body have any effect on vacuum???

I can see it affecting boost, but not vacuum.