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Hello,

I've been searching for quite a while now regarding an issue I am having with my 1990 Civic DX. The problem is that it throws a CEL and Code 1 when maintaining constant speed with the throttle almost completely closed, which is usually when driving on a decline. I have replaced the TPS, as the original had some jumping around at the near-closed end. This reduced the problem and improved part-throttle acceleration, but the problem has not been eliminated. What happens as I read the O2 sensor while I'm driving is that at idle it hovers in the .80s, which I assume is normal. This makes for a smooth idle, and is around stoichiometric (oxygen nearly completely consumed). When driving at most throttle positions, except for hard acceleration, the system goes into rich-lean-rich mode, which I assume is typical. However, when the throttle is almost completely closed, it goes back into idle mode, and the oxygen reading climbs into the .80s again. If it stays this way for over 10 seconds (or so), the CEL illuminates, and a Code 1, for oxygen content, is set.

I have read that the fuel injector O-rings can leak, causing rich mixture, particularly at part-throttle. However, first, fuel economy is superb, second the fuel shuts off completely when I coast in gear (throttle closed, O2 sensor reading drops to .00 volts), and third, fuel pressure is within range, and will maintain overnight, so there are no leaks or faults.

The car does not otherwise behave abnormally. My guess is that, unless the car was just designed this way, which I doubt, somehow it is going into idle mode prematurely, which causes the computer to throw a code due to the absence of a rich-lean-rich-lean mixture while driving.

Anyone else have this problem?