I'm thinking I've got a small vacuum leak under the BAC on my NA 88. (starting fluid method, idle fluctuated when squirted there)

Symptoms:
Bouncing idle when cold
Running rich, 13mpg city, 17.5hwy
'hollow' ??? sound... not when pressing on throttle while driving, or while just idling while driving, only if I'm BARELY on it...

Not saying all symptoms are related, just feel all are worth noting.

Thanks in advance!

BTW, it's just on the other side of the colored plugs that I notice it (I'lll try to get a pic tomorrow)

 

Under the BAC is many things such as the ASV, solenoid rack, purge valve, oil filler, etc. Doesn't matter where the vacuum leak is, you need to fix it.

The Factory Service Manuals for all years of RX-7 (and many other rotary cars) can be downloaded for free here: Foxed.ca - Mazda RX-7 Manuals . They contain all common service information information and more, and are an essential tool for any sort of service work that needs to be performed on these cars.

Additionally, the Haynes service manual for the 1986 thru 1991 RX-7 (both turbo and NA) can be found at most local auto parts stores or purchased from any good bookstore (ISBN number 1 56392 007 7). It is only about $25 and is a great quick reference for things like this. Not only does it contain most of the information in the FSM condensed into an easy to read format, but it includes full wiring diagrams as well.

These manuals will answer 99% of service related questions for the car and if you intend to do your own repair work, you will need at least one of them. Preferably both, since the FSM is a free download. The Haynes is a good quick reference covering 99% of questions, while the FSMs show complete and detailed procedures.

 

Directly below the BACV is the air bypass solenoid valve, which according to the FSM, assists the BACV during cold start-up's as well as bumping up the idle when power steering/AC is in use at Idle.

Ive personally had an air bypass valve fail, and since my S4 NA was running a manual steering rack, and no air con, I removed the valve and capped both ends of where it attached. I like to mention that when the valve failed (and it failed then 'fixed' itself then failed permanently) it caused my idle to bounce cold and hot idle. I didnt notice a lack of mpg's, though that could be several factors (when was the last time the O2 sensor or plugs were replaced?)

 

I had/have the same thing on my 88 vert. Changed plugs, checked vacuum , and removed my seized air pump. I'm pretty sure it is my tps though, before I shelled out the $43 for a banzai tps adjuster, it would dip idle and bounce around. Every now and then it would "fix" itself but it never worked as decent as it does now(still bounces but drives better). So maybe source a tps for cheap, that's where I'm at.

 

And I only get 13-14 city 17 hwy. It just comes with the car, Fc's run at like a 16:1 fuel ratio so the burn pretty rich

 

16:1 is lean. and they'll run stoich if they are working properly

-sean

 

Whoops my bad 16:1 is super lean but if I remember reading correctly, we due tend to run a little rich no?

 

anyone know of the 'hollow' sound when I am barely on the throttle? RB headers with open 2.5" duals btw... sounds like a rapid misfire??? sound familiar to anyone?

 

i wouldn't say it's SUPER lean.. but leaner than you would want to run under load. especially if you're turbocharged.

many fc's do tend to run rich, but i would blame that more on the cars being 20+ years old and fuel economy not playing as important a role back when they were developed rather than any sort of 'natural tendency' to run rich. any old car will develop slight vacuum leaks/compression loss, inaccuracies in the sensors, tired fuel injectors. but like i said, when functioning properly they'll run perfectly normal afr's.

they will never be as fuel efficient as other cars because of how much combustion is actually going on in there. 4-stroke cylinder engine cars will have a 3-cycle break after each combustion cycle in each cylinder. in a rotary when fuel is ignited, there's more fuel waiting to burn right around the corner. x2 for 2 rotors. this is also why the turbine housings on turbo rotaries tend to be huge compared to similarly-powered small engine turbocharged cars. 13b's are physically small and calculated at 1.3L (1308cc to be exact) of displacement, but with 6 combustion faces and no breaks in between, the fuel burned and exhaust produced is astronomical. none of which specifically indicates that the car is supposed to run rich. even though there are many of them happening in a short time, each fuel/air charge will be most efficient around 14.7:1 (other variables will require adjustments of this of course)

so: yes, it's likely that many fc's are running rich. no, that's not 'just how they run'


-sean

 

16:1 could be a totally valid cruise AFR depending on engine setup.

I cruise my bridgeport about that lean.