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Discussion Starter · #1 ·
Hey all, this is something that I've noticed for about three months now and its starting to really bug me. It hasn't been affecting my performance since its only at idle but I want to know if it is a problem waiting to start.
I dont know if its the cold weather or if its a hose that is unplugged or slightly loose then tightens as the engine warms up.

When I first start the car, whether its been sitting overnight or just one hour, my boost gauge reads at -20 in. hg at normal idle with all accessories off. After driving around about 10-15 minutes it reads at -22 in. hg. The reading doesn't fluctuate at all. When it's at -20 it stays there without any fluctuation at all. once warmed up it reads at -22 and stays there without any fluctuation. My idle is stable so I guess I can rule out a vacuum leak which self seals a hose with the heat of the engine. If that's even possible. This was a used block that I bought from my mechanic who I know for 20 years and is fully trustworthy. The motor was was and still is in great shape but this is just bugging me.

My set up is a 95 gsr stock internals with a jrsc at 11 psi very well tuned on a hondata s200b and a hondata intake manifold gasket. No fluid leaks, no weird hissing noises so whatever is happening might be slight. I've checked all my vacuum hoses and everything is snug and there are no leaks, otherwise my idle would be bouncing around since I tested it by loosening a clamp on one of the vacuum hoses. As soon as I loosened the clamp and slightly undid the hose just shy of where it connects to the idle started rising and dropping.

If anyone knows anything else I could look at it would be greatly appreciate it. Ideas and theories welcomed as to what could possibly cause this to happen. Maybe someone can look at their boost gauge and see if the same thing happens to them and it might ease my mind. Thanks in advance.
 

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First you should have address the issue/concern with the mechanic you are talking about that has your trust for 20 years.

He or anyone who knows anything about vacuum in an engine will tell you that that reading at idle is normal. As long as you read boost at boost and it transitions well, the 20-22 in vac is normal.
 

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Discussion Starter · #3 ·
thanks for moving it sak, I thought that I had placed it in the right forum, what a dunce I am for not even thinking about the FI forum.

He did tell me that was normal but wanted to see if I could find out something that some of the technical guru's might know. My mechanic's great but doesn't know some of the specifics as far as what exactly the vacuum should be at starting and warmed up.
 

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As long as th gauge stays still at one condition (idle, etc) and transitions into boost well, then it is not a problem. Even for a stock teg, thats what it should read at idle.
 

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Discussion Starter · #5 ·
on my previous block, before my headgasket blew and the fourth cylinder cracked from it, my vacuum at idle started at 22 when cold and stayed at 22 once warm. But when I changed over to this block it did this from the beginning so I thought something might be wrong. Would that still be something to not worry about?
 

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If it makes you fel any better, I had a JRSC GSR as well as a former TI mod (who had a JRSC LS). We both installed the boost gauge at the same time and we both registered the same 22-20 in vac at idle.
 

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It will fluctuate. I have seem mines dip as low as 24-26 in vac under lifting off the throttle. This is regardless of engine temp.
 

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Discussion Starter · #9 ·
technically the car is supposed to go -4 to -6 inches when you snap the throttle to 2500 rpm and release. As per our manual. I'm talking about idle without touching the gas at all.
 

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Also when the motor is cold, the ECU hold the RPMs higher for some time until you have normal temps.

So the vac reading would be 15-18 because there is more load, but after the temps are normal, 20-22 is perfectly fine.

And dipping is fine as well, as long as it is consistent.

Also notice when you add any load the vac will fluctuate, such as turning the steering wheel (using PS) and pumping the brakes.
 
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