Hey all. I am having an issue where my vacuum seems to get lower when i drive the car (warmed up). I have an AEM X Series Boost gauge to monitor my readings. On a cold start and slowly warming up, my vacuum reads -17 which is right around where it should be. However after i drive the car around for a while, the vacuum seems to drop to -15/-13 for some reason. Lower in hotter weather, what could cause this? I am stumped.

 

Vacuum reading will change based on weather some. But those readings are in line with what mine was with stock cams. With S2 cams its about 7-8 inches of mercury.

Were your readings different before? Any drivability issues? A/C on will also lower the vacuum in while the compressor is engaged.

 

Hey thanks for reply slocho! I believe my vacuum readings were stable before, i have never noticed my readings dropping no matter how the weather was. With A/C on vacuum will be lower, but i rarely use A/C anyways. I honestly thought vacuum readings were supposed to be the same for the most part?

 

Typically vacuum will be a bit higher when cold and lower some after its warm, should be more or less constant but never fluctuate fast during normal idle if that makes sense. A boost leak test wouldn't hurt just to check up on things and maybe cleaning the MAF with some electrical cleaner. If no other drivability issues are present, I would consider that normal and not worry.

 

Yeah my drivability is fine, no issues with that, and i will be doing a boost leak test next week. The vacuum is stable, but like i said it slowly drops down when the car gets “warmed up”. Like for example -17, after 15 min of driving, -14, after an hour of driving -10

 

One more thing, a compression check would be good to. Id say if that and boost leak checks out good, then everything would seem normal. Unless someone else has anything to add.

 

Thanks for the good info. I will update this thread as soon as do a boost leak/compression test or I figure out the issue.

 

I'm no car genius but I can't think of anything that would cause manifold vacuum to change that much with engine temperature. Plus, you say it otherwise operates normally. Failing electronics, on the other hand, might drift once warmed up.