Close loop and open loop mode question

[VinhZXT]

Hi Guys,

I have a little problem with my 280ZX turbo. I have JeffP's downpipe and a full 3" exhaust and an intercooler on this car and it's running 12PSI. The problem I have is the car takes a long time to go into close loop mode. In addition it goes from close loop to open loop even after the car is warm up. I have a new O2 sensor and that's what I read to determine whether it is in close or open loop. I will check my CHT sensor and O2 sensor again to make sure but if anyone has any idea I appreciate it. Thanks in advance

Vinh

[mario_82_ZXT]

I believe that closed loop on the stock ECU is only under light loads (I think just cruising, no boost) fully warmed up and below a set RPM (which is somewhere around 3.5k RPM). The O2 sensor also has to be fully warmed up since it is a single wire sensor.

 

I'm pretty sure the FSM has guidelines to what should put the ECU into closed loop.

Mario

[Brad-ManQ45]

Good call Mario - I remember that under boost, the manual says the ECU goes into open loop mode, reading from the tables...

[vashonz]

I never messed with the stock ECU too much, But on SDS you set what MAP range and RPM range to run in closed loop. They recommend that closed loop only be used at low map values and ~1500-3500 rpm (basically crusing). If you try to run closed loop under boost or at higher RPMs it can cause bad stuff (14.7 AFR is generally too lean).

 

So basically I agree with brad and mario. It sounds normal to me.

[VinhZXT]

What I am asking is why it takes too long for the O2 sensor to send signal to the A/F mixture meter. I also see the O2 sensor stop sending signal to the A/F mixture gauge after the car was completely warmed up. I don't have this problem with my other 280ZXTs. Once the O2 sensor starts sending signal to the A/F mixture gauge it will continue to do so until you shut the engine down for a long period of time. I will have to double check the O2 sensor and the CHT sensor when I have time.

Thanks for all the replies.

Vinh

[mario_82_ZXT]

Warmed up idling or driving?

 

When I had the original O2 hooked up to MS, the sensor would never get warmed up enough while idling to send a signal to the Megasquirt box.

 

On the other hand a slow reacting sensor can be a sign of an old/clogged/failing O2 sensor.

Mario

[Tony D]

TO solve this once and for all, run a wire from your fuel pump relay, and install the three-wire heated O2 sensor from a VG30 (Z31).

 

The car will be able to go into closed-loop mode after about 30 seconds, and will STAY in closed loop mode MUCH LONGER. The problem is the good flow will let the single wire O2 sensor cool off.

 

This is how I have my MS wired, and this is how I've wired many other ZXT conversions using the stock electrics. I realized long ago that a heated sensor makes for MUCH better fuel mileage in them and never realized why. Someone did some scoping and testing, and found that once the ZXT ECU goes into closed-loop mode, it will stay there at idle and low speeds until the single wire unit stops switching. This usually happens when it cools off. If you run the heated sensor from the 86 Z31T (I think that's the model I specified at Pep Boys) the sensor stays heated, and on closed loop much longer. It screws right in, no problem (same turboon the Z31, slightly different downpipe, but the O2 sensor screws right in to the standard O2 sensor bung like the OEM unit. I will also work the same as your single wire without the heater hooked up, but I'd not run it like that)

 

I know my MS gets a switching signal and I can read voltages at cold idle (er... .9vdc on cold start enrichment...heh!) almost from the time I start it up.

 

Pretty simple test, and only one connectro to swap from the Z31 to plug it right in, as well as a few feet of wire for the heater power and ground leg.

 

Works Great.

 

Godd Luck!

[VinhZXT]

TonyD,

Thank you much for the info. I think you nailed it. I have another 280ZXT with everything stock and it doesn't have this problem. I took the ECU from this car and swap it with the car I am having problem with but it didn't fix anything. So I know it's something unique about this car that cause the O2 sensor going in and out of close loop. It drives fine and all but it is just annoying when the A/F mixture meter doesn't do its sweep. I will definitely getting the heat O2 sensor to try it out. Thank you much for the info.

Vinh