'89 240SX, Idle CO way to high (and bad mileage)

[Ovideo11]

Hello,

I've got the following questions, I hope somebody knows the answer!

My '89 240SX (KA24E) is running (very) rich at idle (and as an effect my mileage is very bad). I've got the Nissan Factory Service Manual but I am unable to solve the puzzle.

When I run an E.C.U. number II diagnostics, I notice that the EFI system is going into 'Open Loop' at idle. The FSM states that this is normal behaviour to, quote: 'maintain stabilized fuel combustion'. But in the diagnostics chapter (page EF & EC-28) is says that the leds on the E.C.U. should blink simultaneously at idle (which means that the system is running in 'Closed Loop'). So both remarks contradict each other........?

The question then is, what is the right behaviour? Open loop or closed loop at idle and how can I get the CO back to normal values at idle and improve my fuel efficiency?

Further I noticed that when I disconnect the Throttle Sensor connector (not the idle switch), the system will stay in closed loop at idle. But this isn't a solution because then the engine doesn't idle smoothly and since I have an automatic transmission the AT control unit doesn't 'know' my throttle position which results in irregular shifting behaviour of the AT (and kickdown not working).

I've run all diagnostics on the E.C.U. and found no problem, checked all of my EFI parts, cleaned the MAF, replaced the exhaust gas sensor etc. but without any effect.

Have any of you ever run into simmilar problems?I really hope there that there is a Nissan whizzkid surfing around who knows a solution. My local Nissan dealer in The Netherlands where I live can't help me because the 240SX, being a USA Nissan model, has never been sold in my country!

Thanks for reading,

PaulAmsterdam, The Netherlands

[InsanityInc]

Your TPS is either bad or needs adjustment. Basically, the car doesn't recognize that you are actually idling because the TPS says you are pushing the gas.

[rsmithdrift]

My 300 had that problem and the Nissan pro I took it to fixed it by replacing the TPS sensor. So I'd say that's your soloution.