Can someone please explain to me how a vac leak can cause a very rich condition?

 

depends on where and when is it leaking
if u got TII and have leak after turbo, it it will be rich when boosting (engine sees less air than what is it measured at AFM)

 

More specifically, how can a vac leak cause a rich idle?

 

Vacuum is being monitored by the ECU. Like hIGGI said if any sensors like the MAF (mass air flow) and or MAP (manifold absolute pressure) sensors dont get the correct reading, the ECU cannot calculate the right fuel needs. This would throw off your A/F ratio which in your question, make it run rich. But if you were running rich at idle after the engine has already warmed up, you might have a O2 sensor problem causing your engine to stay in open loop control. I could explain what open loop is but you didnt ask that so im gonna shut up now.

 

That doesnt make any sense to me. If you have a leak after the MAF youre going to run lean because the engine is pulling in extra oxygen that it doesnt know about. I could see if it somehow tricked one of the other sensors... but that just doesnt make any sense. You should throw a DVOM on your 02 and see if its switching or not. It seems like a rich condition at idle would have to be something else beside a vacuum leak.

 

My bad, RX*TooNER is right. I typed this out too fast without thinking, I meant that vacuum can effect your A/F ratio. A vacuum leak would actually cause the opposite a (lean mixture).

 

It's my understanding the O2 sensor doesn't affect idle.

My car won't idle properly (too low) and I smell raw gas in the exhaust. It seems to be dumping fuel in there. My injectors were recently cleaned (fyi)

The symptoms are that of a vac leak, but I can't explain the rich condition which is why I asked.

 

Even a perfectly tuned/ perfectly idle adjusted stock RX7 runs way rich at idle- my O2 almost pegs to the "lean side", yet verything else fuel-scheduling-wise on the car is top notch, netting 27.08 mpg on the last fillup...

Is that what you mean, asking WHY we run so rich at idle, or do you think something's wrong with your car?

 

Well.. If it wont idle maybe you do have a vacuum leak somewhere. Do you still have the vac rack and all the emission stuff?

 

one wire narrowband O2 will not show anything real at idle, it has to be heated up to atleast 600C (EGT) to start showing something trustable