Intermittent high idle - lambda behavior

schoward · 08-26-2014, 02:33 PM

Recent phenomenon - when warmed up, my SC idle sometimes climbs on its own from 900-950 up to 1100-1200 and sometimes stays there or will sometimes settle back down to regular idle. I had the typical issue with the decel valve (hanging rpm and random idle climb to 2200 or so), but have now disconnected it which gives me a solid idle other than this. Plugged the hose with golf tee. End of decel valve nipple is open.

I run the O2 connected and have observed the above phenomenon with a dwell meter hooked up to the test port. At regular idle, the dwell dithers around 40 (say 35-45). When this happens, I will see the dwell inch up to 45-50, then dwell slowly climbs to about 70 as the idle speed rises to 1100 or so. Often after a little while (1 min or 2) the idle settles back and dwell returns to dither around 40. Sometimes idle wants to stay high for awhile with dwell at about 70. When driving its fine but sometimes when I come to a light idle drops to 950 but then will inch up while sitting at the light. Pull away and all is fine.

Looks like the ECU is sensing a lean condition and trying to richen the mix for some reason. I want to say air leak but this is so random and not the norm. Sometimes can drive for an hour without it happening. Car runs very well. And for some reason, it seems to only do this during the day (?) and only guess is cooler nighttime temperatures maybe changes some parameter (?)...

I have run fuel system cleaner through and cleaned the sensor plate area mechanism with carb cleaner and no change. Plate moves very smooth and nice feeling the motion with my fingers. Swapped in an old decel valve still running it disconnected/line plugged and no change. O2 is only a few months old.

I have pressure gauges but just haven't gotten confident enough (or had the time) to break them out.

Is this normal for the SC ECU/O2? Any ideas?

markivtruck · 08-26-2014, 07:14 PM

those cars are very touchy with any vacum leaks. hoses,intake gaskets.injector o rings insert o rings for injectors, air boxes. have someone do a smoke test to the intake system and see if you find any leaks.

timmy2 · timmy2's Garage

Quote:

Originally Posted by markivtruck

those cars are very touchy with any vacum leaks. hoses,intake gaskets.injector o rings insert o rings for injectors, air boxes. have someone do a smoke test to the intake system and see if you find any leaks.

^^^ This.
As the O2 sensor is new and appears to be working, vacuum leak(s) are quite possible.

Paulporsche · 08-27-2014, 09:29 AM

After checking on vacuum leaks, it may be time to get those gauges out and hooked up. You will want to confirm your fuel and control pressures. If those are OK, look into your mixture setting.

schoward · 08-27-2014, 12:19 PM

Thanks I think i'll start with vacuum leaks. My distributor retard/advance mechanism was changed years ago and bill said 'slow vacuum leak at distributor'. I may start there. Don't recall if that's why pod was changed or if that was something just noted.

I swear - drive it at night and its perfect and daytime (warmer but not much), it does this.