Duty Cycle Readings- Abrnormal?

jason2guy · 05:30 PM

the motor has been missing at idle when warm. cold start is perfect, doesnt miss a beat. then...something happens and it starts sputtering and running lean. this happened all of a sudden. new injectors, most vacuum leaks sealed so i measured the duty cycle and something very telling happened.
from dead cold the reading, before converting to 4cyl scale was 80% and it was running perfect then it changed to 60% and it started running bad.
also when it was at 80%, if i reved the motor the duty cycle would increase. what is happening?

gazzerr · 12-30-2014, 03:35 PM

Hi Jason,

Check this out it really helped me solve my similar issues. I had vacuum leaks and a bad oxygen sensor. You really have to chase down those leaks.

Start here:

CIS Troubleshooting for Dummies

If you haven't already introduced yourself and posted a picture of your car you need to do that and the help will flow.

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jason2guy · 12-30-2014, 05:29 PM

yup this forum is great. ive been here a while.
cis gurus please chime in

Bob Kontak · Bob Kontak's Garage

Tirwin wrestled with a wide band O2 sensor. In that thread there is matrix which displays what your freq valve Duty cycle should be when temp switches and throttle switces talk to the Lambda box. If you unplug your O2 sensor, the lambda box is blind to how rich or lean your car is running. Try taking readings with the O2 sensor unplugged and see if dury matches the design specs. There are switched that dump extra fuel in past part throttle and when at WOT. Typing from phone.

jason2guy · 12-31-2014, 02:59 AM

the o2 was disconnected. what i later found out, after taking out the lamda thermo switch was that it turns off at 60dgrs, so depending on the temp outside one may not get a real "cold start". if its 70 outside and you start the car it will start in closed loop.
you will still get some enrichment from the 95dgr switch but it will be more lean due to the closed loop

Bob Kontak · Bob Kontak's Garage

Closed loop will only occur after the O2 sensor warms up. Thermo switch only knows one temperature. The temp that it is. It does work at 60f ambient as long as the switch is not over the temp that it stops working.

jason2guy · 12-31-2014, 06:56 PM

these cars have a 1 wire o2, it doesnt have a heater. so that warm up time is determined by that switch. when that switch opens, your in closed loop. there is an in between mode, thats what the switch on the breather cover is used for. when the car is under 35c, the acc enrichment kicks in. i saw it in action, when the dwell mtr was hooked up, during acceleration the duty cycle increased with the rpm's
what im unsure about is why the car runs bad when the duty cycle drops after the 15c switch opens

Bob Kontak · Bob Kontak's Garage

Crap. I was thinking about the cold start time switch. Now it's clearer to me what you are saying. Sorry.
ok going to find that post by Tirwin with the matrix I talked of. Slow going on this phone.

Bob Kontak · Bob Kontak's Garage

I really think you are sucking air. When duty drops from 65 to 50 not enough fuel available to sustain smooth operation. If your O2 was working, it would offset the lean condition to some degree.

jason2guy · 01-01-2015, 01:04 PM

i just noticed youve got an 81, thats cool. ive been running w/my o2 disconnected forever. i remember when it was active i experienced that surging effect at constant speed. what ive done for now is ground the wire going to the switch in question, that has forced the open loop and it runs better. im thinking my FD is bad.