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Thread: Vacuum leak, but not a boost leak? Possible?

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Posts: 1-10 of 27
2012-10-23 01:24:36
Vacuum leak, but not a boost leak? Possible?
So I got something weird going on.

Currently, I'm pulling just 10" of vacuum at warm idle. I'm pretty sure this is low and I should be in the 15-20 area. (correct me if I'm wrong)

I'm at an altitude of 4,400 ft.

Starting up and driving the car around town it feels perfectly normal. It drives fine and even boosts fine. Just less vacuum. Although, there might be a slight bit more lag. On the freeway it feels like I have to put in a hair more throttle to go the same speed. My gas mileage has dropped a bit too.

I checked all my intercooler couplers to make sure nothing is loose and inspected the intercooler for any holes. Don't see any.

So my question is, is it possible to have a vacuum leak but not a boost leak?
I haven't checked vacuum lines and any other hoses attached to the intake manifold. BOV is not leaking.

Any thoughts? Maybe a very small hole that I'm seeing would cause this?
2012-10-23 07:10:51
Im not a boost guru but have had problems with vacuum and boost. And from my findings I have noticed that once you fix the vacuum issue you will see either more boost from the top or have stronger boost to redline. Just my .02
2012-10-24 00:59:05
I'm going to replace all the vacuum lines and hope I come across something. And I'm going to spray some soapy water on all my couplers to see if any leaks are actually present.

(Still hoping for more input)
2012-10-24 01:53:56
If all your lines check out fine then you could have cracked ring lands. They will show up as lower vac on the gauge, you could also have air and/or smoke coming out of the valve cover breather.

A bad vac line wont really be that noticeable under boost as the leak is pretty minor but will show up noticeably on the gauge.
2012-10-24 01:59:04
do a compression test
2012-10-24 04:33:39
Oh another thing that manipulates vacuum readings that I've noticed is the tune, I had two different tunes give me two different vacuum readings also check your intake gaskets if any silicone at all, it will produce a leak. Get a boost leak tester and also use that to check for leaks. Bov could be leaking also. So many things can give that issue. Sorry if I complicating things just want you to make sure you have all your bases covered.
2012-10-24 05:05:42
I Seriously!!!! doubt a tune will cause a different vac reading. I do notice that when the IACV is wide open in the moring that i run less vac for obvious reasons, but besides that theres nothing in the ecu to my knowledge that can effect the vac like your saying.
2012-10-24 05:14:42
Im serious, theres no reason for me to lie about it. Im not the only one that experienced it, the one that shed the light on it was LikeTheMovies on here and I'll have chime in on it. The scenario was the same vacuum was at a low reading I was using a Calum Basic at the time with a t25 driven roller rocker, we were talking about it and he emailed me a secondary base tune to see what would happen. I burned the chips with the "other" base bin and my vacuum improved by a good 5 in-hg it could've been better but I also had a leak under the bov flange and we fixed it together and it improved it even more. I only shared that because thats what Ive experienced at that point in time. The way I see it is, it all depends how stable the tune is at idle and the time for injector opening and closing as this also was a bit screwed up. But again I won't mention something if I know it will derail another fellow forum member from the original problem.
2012-10-24 05:19:14
the only thing that will effect the vac is the IACV bein open, or if its a de/det maybe that other valve under the intake bein open or something but i assure you theres no table in the ecu that controls vac man..

was the first test on a cold engine and the second on a warm engine? that might make a little sense.
2012-10-24 05:22:06
I know theres no table in tuning software to set, both were done back to back at normal operating temperature. Its more so how well the engine operates with different tunes.
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