Hi All ! Need some help troubleshooting this issue. Searched the forum but didn't find anything closely related.
Building a 99 hoping to join the NorCal spec Miata crowd this year.
Was hoping first posts would be of joy and excitement related to the first race weekends.. unfortunately the garage section it will have to be !
Just installed a new engine. Previous engine ran fine when removed, just old and keeping aside as a spare.
New engine went in without issues. Started fine other than idling high but adjustment of throttle cable and idle screw fixed that.
AFR at start looks fine, however after a few minutes my AEM wideband shows the AFR drifting to lean until the gauge cant even display anymore. (18+AFR !)
Sounded strange to me so first thought there must be an issue with the wideband.
Happened to have a spare Wideband kit so swapped the wideband 02 sensor and then the whole gauge itself but no difference.. it looks like it actually reads correctly. FYI Sensor is located on top of mid pipe around where the transmission fill plug is.
Since it appeared to start acting up while the engine is warming, I replaced the temp sensor at the back of the head. No change.
Thought maybe a big old vacuum leak, sprayed Brake cleaner around all the potential locations I could think about and no luck, vacuum hoses, intake, injectors, no change in rpm or signs of leak.
Only thing I changed during the swap other than the engine itself (long block including intake) was the primary oxygen sensor which was completely rounded off by the previous owner and positively now one with the pipe. Procured a used pipe and installed new OEM sensor. Using Racing Beat extension to reach the plug in the front of the valve cover (Ex California emission car).
Could it come from that if it is defective ? Once the ecu goes to close loop idle, does it use the o2 readings ?
Still have the secondary o2 sensor plugged in to avoid engine light but zipped tied to the PPF by the trans.
Otherwise everything looks ok to me. Checked all the hoses. plugs, contacts. Engine runs fairly fine, can hear a very slight change in tone as it warms up but nothing dramatic. Revs immediately if I touch the throttle, which I avoid to do much until I can figure out what exactly is happening here. I'm not going to put any load on it if it really idles at over 18 AFR...
I mean, would it sound fine idling at that kind of AFR, or run at all ?
Once fully warmed if I turn it off and then back on hot, wideband shows AFR restarts around 14.7 and then again progressively drifts back to oblivion lean.
Fuel pressure is at 52psi. Idle at around 850. Timing at about 14 with the jumper on. When jumping GND and TEN idle barely raises.
Going to smoke test for leaks tomorrow evening, but would appreciate any tips or leads as to what to look for next !
This is starting to be a bit of a puzzle to me.
Attached is a log of the wideband reading on the top of the graph and rpm bottom for context. cold start. AFR crests at about the time the coolant temp passes 160.