At idle, my vacuum reading is only at -4 psi. Can someone give some explanations? Is it due to incorrect air/fuel mixture?

 

check for leaks.... whats your idle speed?

 

also -4psi??? vaccum is read in inches of mercury

 

I've been debating on posting this but wanted to search a bit more. Now that this tread is up I'll ask. I run 10 in at idle. Have checked all vac lines to the point to get more indepth I'll have to do a hose job. I have check my check valves and they are all functioning correctly. When I pull the hoses off the vac chamber air get sucked in pretty good. (I need to test it with my pressure pump anyone have a link to that) Is there other obvious things I can check before resulting to a full hose job? As a side note, I'm running great boost on my primary, but am having trouble getting the secondary up to full boost even with the AVC-R. Could that be a vacuum issue? Car idles fine at 750 rpm and just had all intake gasket replaced.

Car is stock except boost gauge, AVC-R, and Apexi BOV. Orginal 66K motor.

 

Low vacuum at idle is really only indicative of two things. Either you have very low compression (tired engine) or you have a vacuum leak.

Vacuum leaks rarely allow a steady idle, so if the idle is steady I would have a compression check done.

I have known cars to function perfectly and run for thousands of miles with a solid 10 inches of vacuum at a proper idle (750-900rpm). The engine is most definitely low on compression, but they can last for quite a long time in that condition.

Low compression will show itself as an engine that is sometimes hard to start and floods easily, but if you can get it running usually you are in good shape.

Finally, I doubt anything to do with the "hose job" could cause drastically low vacuum, particularly if the turbos appear to function well.

 

I agree. I would definitely get a compression check.

 

Yes, very true. I don't see this posted any, but I am proof. HP is down some but car still runs ok with mid 6 readings on all faces.

Edit: By: Cpa7man....

 

wow, my wallet is scared guys

 

I'm holding 10 lbs of vac on my autometer boost gauge, and I've got about 100 psi of compression. I was told thats about 7-7.5 compression. Hmm...

 

autometers are NOTORIOUSLY off gauges... im running about 15.5 inches of vaccum on a 10,000 mile motor

 

I'm running an autometer that now runs 1psi when the car is off, but since install i've been running steady at 10in. my AVCR reads about between 250 and 320 mmHg which is about 9.5 to about 13 inches so I don't think the autometer is far off

P.S. RedAnotherDimension sorry for jumping in on your thread like this!

 

4 in. is pretty damn low vacuum, and I agree that it may be a compression issue, but I would at least check the hoses first. 3rd gens are notorious for blowing vacuum lines loose.

And since a compression check in a rotary requires a completely different setup than for a piston engine (read as more rare and expensive), a compression check may cost a bit.

Can't hurt to check the lines first before spending any money, can it? If you do a search on the forum you will find the complete vacuum hose diagram to use as a reference (if you don't already have one). Best of luck.

 

try another gauge see if it's the same. If your boost is weird, it's probably the gauge (or you're running the wrong boost

Otherwise, it's that stuff everyone else said (compression down, vacuum leak)

 

Engberg -

We just fixed a similar problem on a friend's car. He has the same gauge type (also off by 1 psi) and runs about 16-18 inches at idle now, but was only running about 10 before the fix. The car was having trouble getting full secondary in first and second gears. We tracked down one vacuum line, reconnected it and now the car works great getting full boost and power. It was definitely a pain to find and get access to that one hose though.

If you guys want a vacuum line diagram, check out:
http://www.scuderiaciriani.com/rx7/v...placement.html

 

I'm running 16 or 17 while at idle and I just got a new reman installed, trying to figure out if this is correct or not since on previous engine it was always at 15 on idle.

 

SWAT, new engines should pull about 20 at idle. I myself hit almost 20 when my vacuum leak decides to seal up (still need to find the thing!), and normally stay betwen 17 and 18 when it's not. It may not be seeing as much at idle because the components need to break in and wear down a little.

 

Isnt it true that freeing up the intake and exhaust will actually lower vacuum at idle?

 

Bump for more info on mods and lower vacuum #'s I too have an Autometer gauge and it ready 1"mg when the car is off.....

 

Good Question....is it true? I certainly noticed lower vacuum readings with airpump removed.

 

This answer is coming at the end of a late night of fluid dynamics and thermodynamics, so while it's fresh in my mind, I'm also delerious.

The motor should pull the same vacuum no matter what the modifications to the exhaust and the intake tracks. This seems to be making sense to me right now so here goes...

The vacuum is caused by the increasing volume of the combustion chamber as it rotates past the intake port and air rushes to fill the void. From this train of thought, the exhaust won't have an effect on the vacuum reading (at idle mind you) at all. Now, even if you put a resitrction in the intake path, and reduce it by half (before the intake manifold / TB) the motor is going to have the same force pulling air in. Think about a vacuum cleaner. Hmm, not sure where I was going with that. Need sleep.

Hey cool, my comp just switched to day lights saving time.

 

I think having an open intake should lower it some. Think of it as this:
It is easier for air to rush into the intake and fill the manifold because there is less restriction. Therefore, for the motor to pull the same vac with less restriction it would have to suck harder.

Hemlock, I know this is a shot in the dark...but do you remember at all where that bitch of a hose was? My biggest concern is spending a weekend searching every hose and taking **** apart to find it. When I could take it apart in the same time and do the hose job with the same level of work. Do you feel you could have just as easily done a hose job looking for the loose hose in the same time and effort it took to find it?

 

also think about the vacuum too. If you cover the end of the vacuum, the motor starts going nuts and goes into overdrive. Im sure its teh same theory with the motor. It starts sucking harder, because it needs the air.

 

anyone else?

 

Same here. I have 5k on my reman and see 16-17hg at idle. That's with a wide open exhaust.

 

Engberg, that was my car Hemlock was talking about. I'll see if I can explain it well enough.

I had to remove the pressure chamber. It's 2 bolts, right next to the intake manifold, a small black plastic box. The line that came off on mine went from the pressure chamber, over to the turbos. The line popped off the green check valve which is located directly underneath the pressure chamber. If that ends up being your problem, it's a simple fix.

I'm typically now seeing 16 inhg at idle. Before it was 14-15. (And he was thinking of my other car saying the gauge was off). All I know is the turbos work correctly now, and that's all I really care about.