I'm about to probably buy a 1991 rx7 TII that has an idle problem. if your stopped and its idling under 3k rpm's it will just die. but when driving its fine. Why is it doing that?

 

Probably running too rich or too lean

Too lean:
Vacuum leak, idle mixture adjust, clogged injector(s), low fuel pressure, wrong injectors, faulty anti-afterburn valve, or ?

Too rich:
Idle mixture adjust, leaky injectors, high fuel pressure, or ?

Quick check: measure the O2 sensor voltage; over .45 volts is rich, under is lean. It should be rich at idle (at operating temp), in the range of .5 to .7 v.

 

Hmmm, my narrow band has always read lean at idle. I shouldn't have a vacuum leak though, that I know of.

 

Did you test it at full operating temp? They do not read correctly (false lean) until they reach something over 400F.

The idle mixture is supposed to be adjusted to max rpm per the FSM procedure. That should correspond to an AFR of about 13:1, well rich of stoichiometric. Mine is adjusted that way, and my O2 voltage at idle runs between .61 and .75 volts, with the higher numbers corresponding to having just driven the car. The sensor reads higher voltages in the rich region with higher sensor (exhaust) temperatures.

I did a test the other day and determined that my O2 voltage did not reach normal values until 4-5 min after a cold start (but on a hot day).

If you are seeing lean numbers at idle at full operating temp, then either the gage is wrong or your mixture is lean of the recommended setting. It doesn't sound like you are having any driveability problems, so it must not be terribly lean. Your mixture will probably keep your plugs clean, but you likely are not getting maximum power under acceleration either.

 

The only time the gauge is pegged at lean is at idle or extremely light throttle. When I say extremely light throttle, I mean my foot is just shy of not touching it, but depressed every so slightly enough to keep the car moving. Any more than that, the gauge waffles between lean and rich as normal in closed loop and goes to rich mixture at load.

 

Probably a vacuum leak.

 

That sounds like a vacuum leak to me...

 

Well it must one sneaky vacuum leak. I haven't been able to find the source of it and I have looked and looked for months and cant find it.

 

I suppose if it is not causing a rough idle problem it may not be worth spending too much time tracking down... remember also that the voltage out vs. mixture curve of a std O2 sensor is nearly vertical near the stoichiometric point, so if it is showing only somewhat lean it isn't very much. It might even keep your plugs cleaner...

 

Are you running emissions? Like airpump and ACV? If so, then expect to see lean at idle. Has everything to do with the 02 sensor seeing Port Air at idle.

No emissions? Some kinda air leak. Really bad *** air leak if it dies anytime under 3 grand. Undriveable imho. Maybe I misunderstood the dieing under 3K at idle. Or is it dying and not dieing?