Assuming a stock '94 RX-7 with all 60 of its vacuum hoses in good shape, what should be the vacuum (in Hg) at idle at sea level?

Ours measures 14" Hg @ 700 rpm, going up to 15" @ 1000 rpm. That seems low to me. (Don't know if all 60 hoses are OK!) Good or bad? Anyone? Thanks!

---- Bill

 

It doesn't sound too bad to me, I think it should be closer to 16-18 though. What are you using to measure this? Also is the car and engine completely stock???

 

700 rpm is a pretty low idle, were you testing it in diagnostic mode? I get around 17-18 at 850rpm, and 18-19 at 1000 rpm (at almost sea level), 15 at 1000 rpm is kind of low, but it could be your gauge or a leak, probably nothing to worry about.

 

when my car was stock at idle it was ~17" Hg @ 900 rpm's. even now w/ a few bolt on's it's still right there. nick

 

Thanks, folks,

The car is stock except for a Bonez DP and cat converter. I tested the vacuum at the intake manifold tap that is normally plugged, on the passenger side of the engine, using a MityVac Silverline Pro gauge/pump. If you have read my other post, you know the car failed CA emissions at 72K miles (high HC) with the original precat/cat setup and fails again with the Bonez DP & cat. I'm just rummaging around trying to find possible causes of the failure besides bad cats. Vacuum leak seems like one possibility. I appreciate your inputs, thanks again.

---- Bill

 

OK, by replacing the Bonez cat with a new Mazda cat, emissions are now in spec. Vacuum (of course) did not change. I have 40 ft of 1/8" ID, 1/8" wall thickness, and 15 ft of 1/4" ID, 1/8" wall thickness Viton hose on order ($561!!) from McMaster-Carr Supply. Will tackle vacuum hose replacement in future when feeling lucky.

---- Bill

 

That vacuum sounds a little low. I have 18-19 in at an idle of 750-800 rpm (hard to tell exactly). It drops to 18 with the A/C on. I have a 95 with 60k. Only downpipe and cat-back.

 

Seems kinda low....mines around 17.5 to 18 at 760k, which is what I set the PFC to idle for. Could be just a loose line or something of that nature.

Tim

 

For the past year my vacuum has read ~16 in hg at idle. According to other posts I read, this sounds normal - however after my last oil change, I've noticed the idle falling at times to around 700 or so and I've seen the vacuum fall to as low as 9 in hg.

Idle speed will sit at this low reading for a while, then jump a bit back to 7-800 rpm and 14 in hg, then fall again. Then after a while it may jump back to 16 inches and a strong idle. Turning on the air-conditioner will steady the idle speed and slightly lower the vacumm reading. The engine is still running strong, but lately I am getting concerned about this erratic idle and vacuum.

BTW, I have not changed the plugs or tuned the idle speed in two years. The fuel filter was last changed 20K miles ago. I plan to do all these next weekend and I am hoping that this will straighten out both the idle and the vacuum reading. (I put another ground strap one the engine today.)

Also, you are all quoting your idle at sea level, any ideas about my vacuum and idle?

How much will an increase of 500 feet of elevation effect vacuum reading?

 

BLKTOPTRVL,

I'm no expert, which is why I posted this thread, but I'm almost certain that 14" Hg at 7-800 rpm is indicative of vacuum leak(s). This engine has about 60 vacuum hoses, and because of the very high underhood temperature of the Series 3, at least some of these hoses will over time become rigid and lose their sealing qualities, allowing air leaks into the intake manifold. From the replies posted here, my impression is that normal vacuum for this engine is 16-19 in Hg at sea level, at normal idle speed (750-800 rpm).

FYI, I believe the vacuum decreases about 1" Hg for each 1000 ft increase in altitude, so for your location about 1/2" decrease would be expected.

Also for general info, I tested a 15 ft. length of 1/4" ID, 1/8" wall thickness Viton hose yesterday and it took all the vacuum my MityVac could make (about 25" Hg) without collapsing.

---- Bill