A weird problem just popped up recently. When I'm trying to maintain a very low constant speed under very low throttle conditions, the car feels like it's on and off the power rhythmically, every second or so. Under any other condition, it feels normal. Acceleration is fine, idle is fine. On occasion, the RPMs dip on tip-in which may be related. It seems to happen more when the car is cold but is still detectable when it's warm.

The datalog is here. The issue is visible starting at ~50 seconds:



The only unusual things I see are that the exhaust temp sensor keeps flickering and the knock sensor is going berserk but I'm pretty sure it always did that even before. The knock sensor always quiets down as soon as I go WOT.

Any ideas would be greatly appreciated. Thanks!

 

Do the AFR's fluctuate?

 

All I've got in the car right now is the factory narrowband. I've been meaning to either move my wideband over from the other car or just get a second setup but haven't gotten around to it yet :|

 

I've had that happen with the stock O2 sensor before. You're in the feedback loop for the O2 sensor, it richens and leans the cruise mixture to average out at stoich.

If you have a PFC, turn the O2 feedback off and see if it goes away. I've had mine off for years, don't miss it a bit, don't even have a narrowband O2 sensor in my car any more. Still get good driveability and fuel economy too, got 22-23 MPG highway going to and from Deal's Gap this year with a VERY full car and 4.44 rear gears.

Dale

 

^ Yup

A friend had the same problem and it was the O2 feedback causing problems. Turned it off and the problem went away instantly.

 

Good to know, thanks! I will disable O2 feedback and re-test.

 

The feedback entry conditions can be controlled, but for most people it is best to turn o2 feedback off completely.

 

Seems to have done the trick, thanks again guys! Perhaps the heating element gave out which is why the problem only manifests itself when the car is cold.