Rough Idle, Stalls and Poor Gas mileage

I have a 98 ZX2 DOHC 107K. My car has poor gas mileage, idles rough (progressive ok 10 min then rough), and stall smostly driving slowly or stopped. When it stalls the check engine light tries come on. Computer read at two different site and there are no codes. Drove with the MAF sensor disconnected and not much of a change in idling. Cleaned MAF anyway. Changed 02 sensor in engine. No change as well. Replaced fuel filter and changed spark plugs and found probable valve cover oil leak into first cylinder. IAC replaced some improvement but idle/stalls has gotten worse since I turn the heater on with the colder weather. PCV and hose replaced.

No sound of possible vacuum leaks. Any ideas? :?:

Going to need more than just a simple code scan. Somebody is going to have to check the actual scan data, specifically look at the temp inputs and see what the long term and short term fuel trim values are, then report back to us. Also, tell us what the Baro. value is, sometimes cleaning the MAF is not always effective. Plus, once you change something you need to disconnect the battery to dump the volatile memory or the computer will still be operating on it’s previous assumptions.

Also, not every vacuum leak will be loud enough to hear. I just fixed one that netted me a 2 MPG improvement - a tiny little hose up on the firewall.

With the years and mileage, do the usual - visually inspect for cracks - replace if seen, spray, listening for changes in the engine… those type of things.

Be persistent, and you’ll figure it out.

Good luck

Weak fuel pump?

check that pcv hose running from the pcv valve to the underside of the intake manifold. i’ve seen the elbows rip or hoses collapse, causing idle issues.

Don’t forget to check the Throttle Position Sensor, not just with a voltmeter, I recommend using a live scope to check for signal dropout.