What temperature does ECU switch modes?

[Guest bastaad525]

Can someone clarify this for me... At what temperature does the ECU go into closed loop mode (or is it open loop? I always forget... but I mean the mode where it actually starts reading from the O2 sensor)? Trying to get a better idea of the accuracy/inaccuracy of my temp gauge, and/or possible problems with t-stat or head temp sensor maybe.

 

I was mentioning on another thread how my car runs like crap with the O2 sensor hooked up. This is even after I bought a brand new one... it just doesn't like the O2 sensor being hooked up. So the other day, wanting to check out a few things, I hooked it back up. When I started the motor cold, it ran fine. As it warmed up right to the middle of the gauge (185* indicated) the car continued to run perfectly. Odd... maybe another problem that fixed itself.. now I can run the O2? No such luck... I took a mirror and checked the LED on the ECU... no flashing. So it's not reading the O2... check all the wires, all seems well... O2 sensor is bad already? Only bought it a couple months ago. Pulled it out to see if it was clogged with soot... nope... not at all... actually looks just right. Okay... so, take it out for a drive... get about three blocks down the street, by this time temp had climbed about a needle width (~190* indicated) and then BAM! Car starts running like crap... WTF? Pull over, check the LED on the ECU... sure enough, it's flashing, about once every 5-10 seconds at idle.

 

I've already used a meat thermometer to check coolant temps in the top fo the radiator before... and got some odd readings but the thermometer did consistently show that the in-car gauge was a bit off. But if the ECU goes into closed loop mode at some exact temp, and assuming my head temp sensor is not screwed up, I can use that exact transition point where it goes closed loop and compare to the temp gauge and know a bit better how badly off it is.

[fl327]

I believe it switches when the o2 heats up, on my car I can hear it switching, it goes from a semi misfiring idle to a really smooth one, the 3" mandrel exhaust helps that out.

[Guest bastaad525]

I thought it switched when the head temp sensor reached a certain temp? How does a one wire O2 tell the ECU when it's warmed up or not?

[Sean73]

525- I had identical symptoms to yours and it turned out to be a faulty CHTS. With an ohm meter, check the values of the CHTS on the CHTS, as well as at the ECU plug, and compare with the specifications in the Haynes manual or FSM. The CHTS may be faulty, or the wiring.

 

What happened with mine, is that it would idle perfect until it warmed up. When it warmed up, I disconnected the CHTS and it would idle perfect again.

 

I believe the O2 sensor must heat up to about ~600 degrees before it supplies a useful signal to the ECU. Then, closed loop mode only takes effect during cruise. With your TPS disconnected, the ECU doesn't know cruise from idle, and that's compounding the problem.

[Guest bastaad525]

Sean73 - I have checked the head temp sensor, first by trying 4 different ones, and then finally when I got my hands on a multimeter, I tested it and the connector from the ECU the proper way. Further, your symptoms do differ from mine in one big aspect... if I disconnect the HTS while the car is warm, it will die. This does not change whether I have the TPS or O2 connected or disconnected in any combination (I've tried them all)... unplug that HTS and the motor dies within a few seconds. Regarding the TPS, you are right and a little off... with it disconnected the ECU simply always sees the motor as being off idle, so never goes into idle mode. The only noticeable difference it makes, is that when it's connected, timing at idle is higher, and the injectors run at a different duty cycle, but the problem is when the TPS goes open, the transition to regular duty cycle and timing is very rough, giving a slight hiccup/stumble every time the TPS goes open. I have adjusted it exactly to where goes open the instant the throttle is so much as breathed on. I have tested it as well and it is working fine. This is another one of those problems that seems to be at least somewhat common, I know, for instance, that Jersey from these boards says he gets that same stumble, but he chooses to ignore it and leaves the TPS connected. It bothers me too much though... when the car is cold especially, it can make for very jerky driving when just putzing thru a parking lot at minimum speed.

[Guest bastaad525]

Just for the heck of it, I am going to get another HTS... if I can find one, autozone doesn't stock them, will try Napa today, they usually have Z stuff. Anyways they are cheap so might as well replace it for the heck of it.

[clint78z]

Just a word of warning the Temp guage and the coolant temp sensor connectors can be hooked up backwards rigth by the thermostat. This caused some problems on my first startup on my 78 280z. But I think the ZXT has a different location.

 

To the best of my knowledge the ZXT doesn't use the O2 sensor readings at idle, it probably runs 13:1 from factory. It is up tot the TPS switch to tell the ECCS to jump out of idle mode. The only time it uses O2 info is during cruise (that I am aware of).

 

I would focus on igntion misfire, clogged injector, poor sensor grounding. Good luck, these problems can be a real pain eliminating things one by one starting from easiest to hardest.

[Guest bastaad525]

Can't switch head temp and coolant temp sensors on my motor, head temp has a two pin plug/clip, with the locking metal clip on it, coolant temp is one single pin just a male conector, no clip.

 

ECU DEFINATELY reading O2 sensor at idle, I know this because, as I stated, I've tried all combinations of connecting/disconnecting the O2 and TPS (TPS on O2 on, TPS on O2 off, TPS off O2 on, both off). With the TPS connected and set properly and tested with a voltmeter, making sure the circuit was going 'closed' or in idle mode, and then connect the O2 sensor, within seconds the car starts idling very lopey and badly with lots of misfiring. With just the TPS connected the idle is fine, I would leave it connected but I can't stand the little hiccup every time I first tip in the throttle off idle. Otherwise it doesn't hurt the motors running at all, unlike the O2 which makes it run horribly. I also checked the LED with the motor idling and the TPS connected, and it was blinking, so the ECU definately reading O2 signal even when it knows it's idling via the TPS.

 

I've also made sure I have VERY good ground points for all five of the wiring harness ground wires, either they are hooked to the intake manifold, or to the chassis at an area that I sanded off the paint, so metal to metal. Battery is also hooked to the chassis via a large ground strap, so barring unseen corrosion, all grounds should be good.

 

I know tracking down EFI gremlins on the early ZX EFI can be a super pain, hence why I had given up and just let it run rich all this time... after many days spent fuffing with it I just got sick of it. Except for the sorta crappy gas mileage (17-19mpg's but I do drive the snot out of it) and the slightly dark spark plugs (milk chocalatey color vs. light tan, but not solid black) and lastly, and most importantly to me, the rich smelling exhaust, you wouldn't know there was really a problem because the car seems to run perfectly.

 

I have gone thru the entire ignition system to get rid of any misfires there, cap, rotor, plugs, wires and coil have all been replaced. On top of that I'm running an MSD 6A.

 

The injectors are suspect... I've been debating between pulling mine, sending them out for cleaning, or possibly just buying a set of new ones. Price will range between about $150 to $300 I guess. I've tried running a bottle of techron and also neo fuel system cleaner thru the tank... it seemed to only make things worse. Which makes me suspect even more that I have some injector clogging, seeing as those cleaners probably dislodged some gunk from one place and carried it to the injectors. I have really narrowed it down and am 90% sure this is the cause of my light misfire at idle, but doubt it has anything to do with the O2 sensor problem.

[Tim240z]

Bryan,

I would try two more things:

Run 2 or three more grounds from the engine to the chassis - from the intake to body and from block to body

Try running some seafoam through the injectors. NAPA carries it. It works REALLY well. My wife's volvo 850 was getting a bit sluggish, so I ran some seafoam through the intake (via the brake booster vacuum line with the motor running) and in the tank. Made a world of difference.

Maybe run your tank down to about 5 gallons, and dump in 2/3 pint into it and run the rest through an intake vacuum.

 

Tim

[Guest SeanyD300zxT]

is it possible that you have the wrong 02 sensor for your ecu.....with the z31 ecu's the oxygen sensor required changes from year to year, and its almost a sport ensuring you get the right one. With the wrong sensor, you end up with very similar symptoms to the ones you're describing. I ordered a 'universal' from napa, was supposed to be the same, just swap the connectors...hit operating temp...and curse and swear cause you think the fuel tank just fell off the way it drives....put the right one in, and everyones happy....might be something to double check...

[Guest bastaad525]

Tim - I had considered adding more grounds... the grounding thing was a HUGE issue with the Sentra I bought a couple months ago... was causing all kinds of problems and giving me check engine lights and a horrible unstable idle, added a few ground wires from the MAF to manifold and manifold to chassis and battery negative to manifold etc. and now no more problems, no more check engine light, and 3-5 more mpg's to boot. I didn't think it'd be a big deal on the Z but now that you mention it I'll try to do this asap. About the Seafoam... so you really like that stuff huh? Pretty interesting suggestion from the guy who says "there's no such thing as a cure in a bottle" seriously though... I have heard others mention that stuff works pretty good as well, and if you're suggesting it I figure it's gotta be good for something, so I'll give it a try. I really can't afford new injectors and even getting them cleaned is kinda pricey... the I/C and FMU install have tapped me out for a while. But I'm almost positive by now that the injectors must be dirty or clogged, as I've addressed just about everything else. I thought the Neo stuff was supposed to be pretty good but it only seemed to make it worse, more misfiring at idle The other possibility by be issues with my cam/rocker arms... I have a lot of reason to suspect this as a possible cause as well, and may need to eventually swap to new rockers and maybe even a new cam to correct it.

 

Seany - *shrug* I've actually tried three different sensors, the one that came with the car, a Bosch bought from auto zone which they let me return, and a Nissan unit. Same results each time. Anyways, I believe all the '79-83 ZX and ZXT's use the same sensor.

[ToplessZ]

Here is a great and inexpensive place to get your injectors cleaned

http://www.witchhunter.com/injectorserv1.htm

Im getting ready to send him my 370cc injectors even though I havnt decided if Im using them or not.

At 12.00 an injector it is way cheaper than new ones!

I hope you find to your engines ailments

[Guest bastaad525]

I hope you find to your engines ailments

 

LOL you make it sound like it's dying of cancer or something really it runs really well, just little bugs here and there but nothing that keeps it from being enjoyable... mostly stuff no one would even notice, just that I'm so darn picky.

 

 

my question still remains, isn't there some temperature point where the ECU 'switches over' to closed loop mode? I could have swore I've read this before but not finding it with search. I remember reading it in regards to using a 160* thermostat vs. a 180 or 190 degree one, that using the 160 would not allow the ECU to go to closed loop, so I'm assuming the 'switch point' would be somewhere in there.

[clint78z]

I think 160F is reasonable for a limit to coolant enrichment it is that on other vehicles and is what I programmed on my EFI.

 

As for closed loop idle (feedback from O2 sensor is used), it is not supposed to happen from the factory. It is only newer cars that do this, it is extremely hard to do. Another reason why I know the ZXT idles open loop is the O2 sensor is single wire, it does not have an internal heater sufficent to keep it up to temp at idle. Perhaps the wiring to the TPS is not showing the idle circuit. You need to track down why the O2 sensor is affecting idle when it should not.

 

Here is a trick to tell if you are idling too rich or too lean for that motor on any AFM or MAF vehicle. Grab a smaller vacuum hose and pull it off at idle, does it idle better or worse? If it idles better you are too rich, and if it is worse you are too lean or just right. What you are doing is leaning out the mixture by adding extra unmetered air to the engine.

 

As for injectors I got a rebuilt set from Neihoff for about $30 a peice. Another way to diagnose a bad injector is to unplug it at idle, if rpm doesn't drop it or it's wiring is bad.

[Guest 2slo4u]

Bastaad, I believe I have the same problem as you but with a different cause. I have done the ecu and mas swap and have the hesitation right at about 1000rpm. I have changed:

-New chts

-new plugs, wires, cap, and rotor

-new pcv

-new vacumm lines

-Brand new turbo injectors (stock 280zx turbo)

-no injector inline resistors

-New grounds here and there and large braided one at battery to body

-all injectors have direct connectors, no plug ins

-chts is soldered directly to harness

 

From a cold start, mine has to be cranked over usually twice to stay running, then for the rest of the day it will crank first time. It runs perfectly from 1500rpm up. I have O2 sensor output voltage while driving and it is well within spec. It does idle kinda rough and misfires and the O2 reads very lean when it is idling. My plugs are also perfect color and never get black or sooty. This seems like the opposite to your rich running problem, just wanted to add some info. for you to compare to.

 

Brian

81zxturbo

95slobaru

[Guest bastaad525]

Clint - yeah see I thought the O2 was supposed to do nothing at idle as well. I thought the function of knowing when the motor is at idle or not, was soley a function of the TPS. I have adjusted the TPS perfectly with a voltmeter, and I know the ECU is getting signal from it and reacting, as you can hear the injection cycle 'switch over' as soon as you touch the throttle, as well as feel that little hiccup, and if you're watching timing, you will see it retard by a few degrees. So the ECU is definately running in 'idle mode' with the TPS connected, it knows what's up. And if I leave it at idle, TPS connected, and hook up the O2 sensor while the car is running (must be fully warmed up as well), within a few seconds the car will immediately start to idle much worse.

 

About testing the idle mixture by pulling a hose... I've done this one a few times, and yeah pulling any hose off results in a much rougher idle, although idle goes up, it gets very lumpy and lots of misfiring. I even tried pulling a hose and then blocking off the port mostly with a finger so VERY LITTLE air was allowed to bypass, still a noticeable detriment to the idle (by the way, I was pulling off the hose to the turbo wastegate). So you'd think I was running too lean at idle, yet with a wideband O2 plugged into the downpipe, idle air/fuel ratio sits at around 11-12:1.

 

 

2slo4u - mine starts instantly when cold, though it idles a bit low and lumpy, I believe this is partially if not completely due to the fact that I'm running the '75 intake manifold, and don't have ANY of the cold start equipment in place... so effectively I have no choke to keep a high idle when cold. It usually starts right up when hot as well, sometimes it takes a couple seconds. With the TPS and O2 disconnected, it runs near perfectly at idle and all the way up, but has the slight misfire from idle up to about 1500rpm, but doesn't have it under load, only in neutral free revving.

 

And I pulled my spark plugs again last night just to see if anything had changed. This was after the car had idle for a bit, I always let it idle down for a couple minutes to cool the turbo off. Well... they looked the same, a nice milk chocalatey color... just a bit darker than you'd probably want to see, but definately NOT black or sooty, which I would expect from 11:1 air/fuel ratio. I keep forgetting to take a pic but I'm sure you guys can imagine. But again, if I try to lean out the mixture at all, either by tightening the AFM, or lowering fuel pressure, or pulling a vacuum hose, the car runs noticeably worse.

[clint78z]

The TPS I believe is just a switch as opposed to a potentiometer style, at least that is how my old non turbo was. Mabey reread the manual to see if it was setup right, seems funny that O2 is affecting idle. Never hurts to recheck. With the car idling do a drop cylinder test, pull each injector wire off while idling write down the new rpm on the tach. The cylinder with the lowest rpm is the one that is casuing problems.

[Guest bastaad525]

yeah TPS is just a switch, on N/A's it's a three position switch (idle, WOT, and open which is in between the two), on turbo's it's just a two position (idle and open). I set mine so that it is 'closed' at idle, and with the slightest bit of pressure applied to the throttle (enough to raise the RPM about 25rpm or so) it goes open. You can HEAR the engine 'switch over' but I also did verify with a multimeter.

 

I tried your drop cylinder test and they all affected the RPM the same.