Is there a way to test the Oxygen Sensor? If it helps, I do have a high impedence VOM. The car is a 1981 N/A.

Thanks,
John

 

If you can one of your leads into the 02 wire connector and then ground the other lead with the meter set to DCV you should get a reading of sometihing around 800mV. If the 02 isn't functioning properly then you're reading will be off by a few hundered mV. Now keep in mind however that a off reading may not neccessarily mean a bad 02 but actually be the 02 reading a rich or lean condition. But either way you'll know that something ain't right. With the meter hooked up, slowly raise the rpm's, if the 02 doesn't respond then it needs to be replaced.

 

Thanks - I'll give it a try. Also, do you know where the ECU is located? I need to do some testing there but can't find it. What happened is that yesterday I took my "Z" for its two year smog test and it failed as a "gross poluter" and runing rich. The FSM lists some tests but in unclear as to where the ECU is located.

Thanks,
John

 

OK, I checked the voltage at the O2 sensor and only got 365 mv. This was with the O2 sensor connected to whatever it normally connects to, i.e., I did not disconnect it from the harness. I got some change in voltage as I increased RPMs but nothing over 400mv. I still want to check the light on the ECU. If the O2 sensor is functioning its supposed to flash at least five times in 10 seconds. The problem is I still can't find the ECU

One more thing - I disconnected the battery's neg. cable. I read somewhere that's supposed to reset the ECU. Nothing about it in my FSM so it might just be an "urban legend" but I'm trying it anyway. Will leave it disconnected for one hour and re-measure the O2 sensor voltage.

 

Your ECU is under the passenger seat. Remove the door threshold trim, and then the passenger seat. The trim has 4 screws, and the seat has 2 bolts, and 2 nuts. Then take it out, and the ECU will be right there on the floor. Hope this helps, and let us know how it goes from there.

 

Ok a couple things. WildmaN, if your ECU is under your passenger seat than someone at some point moved it there. It shouldn't be there. Under the seat is, I believe the ACSD controller, but I could be mistaken on that. The ECU should be behind the driver side rocker trim piece. It's the lowest panel against the side of the footwell toward the door. It's held in place with one or two screw. To check the light you'll need to remove the lower dash panel that contains the foot lamp. Also two screw. The light you're looking for is on top of the ECU as it's installed in the car. You can slide a piece of paper above the ECU and try to catch the light that way or use a mirror (which is the better way). If you're only reading 400mV than you've either got a bad O2, a bad O2 circuit, or you actually do have a lean condition. My money is on the fact that you need a new O2 or refresh the wiring because if your car were actually running that lean then you'd probably have burnt up a couple of valves by now or maybe even popped a whole in one of your pistons. Oh and one more thing, you did let the O2 warm up first right? Give it a few minutes to warm up before you try to take a reading. And disconnecting the ECU is supposed to reset trouble codes that may have been triggered by an ECU. The L28 doesn't throw trouble codes and I don't think it has any AI capabilities to reset either. But it couldn't hurt either.

WildmaN, I'm not saying your ECU isn't where you say it is, I'm just saying that that's not where Nissan put it.

 

Well, my attempt at resetting the ECU had no noticable effect. According to the guy who smog tested me I'm running rich. This is probably true as the inside of my tail pipe is black with soot as are my spark plugs. Oddly, the car runs like a bat-outta-hell, but I'm certain its rich. I'll look for the ECU tomorrow and post whatever the light is doing. At this point I believe the O2 sensor has, in fact, gone south.

Thanks to all.
John

 

Doh I stand corrected, my mistake. I for some reason thought the ECU was under the seat. Yeah your right it is the ACSD controller. Well thanks for telling me where it is. It will be good in the future. After all I am still learning as I go, I am by no means an expert on these cars yet. But I am still trying to be, but I have learned a lot. And sorry man, for steering you in the wrong direction.
Now fairlady, if you don't mind me asking. What does the ACSD controller do?

 

Leeky injectors will make you run rich as well, oh and a week spark will too, oh and a dirty or cloged air filter will too.

 

I think it's actually "ASCD": Automatic Speed Control Device. Basically a fancy way of saying Cruise Control.

 

If you're running rich then you have a bad O2 sensor. 400mV would indicate a less than ideal situation, on the lean side. So here's my theory: The O2 is defective therefore sending a V signal to the ECU that indicates to it that the engine is running lean. So the ECU then try's to compensate for the lean condition by dumping in more fuel. The problem is that the motor is not actually running lean, the O2 is just faulty so in the end, with the computer dumping the extra fuel you end up running rich. Too rich and you start to lose power and your fuel economy goes down the $#!%er. So go get yourself a new O2 sensor since you now know you're not running lean, which means the O2 has got to be bad. You can get a Bosch O2 sensor without the connector for like $25. The connector adds like $15-$20 though if you want it but you can cut the old one off and solder it to the new O2 sensor. When you change that thing you'll notice a world of difference since yours seems to be so far off the mark.

Oh and WildmaN don't sweat it. Hey I told you it was the ACSD, which Heat Rave R proved me wrong on that one. It is ASCD, and he's right

 

I put the 240sx on an 02 wide band yesterday and I am running 2 points lean almost through the whole RPM range

 

JFairladyZ, I reasoned the same that the sensor was falsely reporting lean and the ECU was compensating by enriching the mixture. So, I bit the bullet and bought a new Bosch sensor from Autozone (~$35.00). Now, my problem is I can't get the old one out. Its 22mm but my deep sockets aren't deep enough. I have a 22mm combination wrench but there isn't enough space to use it. Any ideas? I even thought of breaking the stem on the old sensor so my deep socket would fit, but that seems a little too much like a "shade tree mechanic."

Anybody done this that can help?

Thanks,
John

 

I used a combo wrench to get mine off. Rooms limited and if it's stuck in there good chances are you're going to come out with a scraped set of knuckles. Try spraying down with some WD-40 or the like and let it sit for a while. Then try to break it loose. Wrap your hand in an old towel or tshirt if you're worried about it. You can also try hitting the wrench with a mallet. The sharp blows might break it free. If you gotta remove something to get to it then remove it. I know the brake shield and exhaust shields can get in the way of a larger wrench. If its the angle you cant deal with then you can try it from under the car too. A good blow with a hammer to the wrench after a few minutes of WD-40 always does the trick for me though If all that doesn't work then cut the old one down like you said and use a rachet. What ever works. But like I said I used a combo wrench and a hammer. Good luck.

 

Oh, and when you install the new sensor use some anti sieze so next time it's an easier fix. And remember if you smash the stem off you still have to put the new one in with a combo wrench.

 

Sounds like a winner. I'll have to wait til I can talk a friend into helping cuz I had a heart valve replaced in July and I'm restricted to limited physical effort for another month. Trouble is "friends" seem to have something else to do when we need them. Myself, I never say no to friend or neighbor unless it involves $$. Oh well.

 

We got a lot of members in SoCal. Give a holler and I'm sure somebody around there would be willing to help you out. I would but I'm a little over 2000 miles away. I miss Cali, the humidiy out here in VA is killing me

 

I'm thinking if I put the car on jack stands I should be able to get under it and wail away with a mallet. If that doesn't work, I'll scream for help. I still have two weeks left before the registration expires. With gasoline pushing $3.00/gal I really want to keep the Z on the road. My other car is a GMC Sierra 2500 4x4 that gets 13 to 14 mpg on a good day. I have a 60 mile comute to work, so I'm really motivated.

Thanks for all the ideas.
John

 

First, the good news. Replacing the O2 sensor is a piece of cake with the correct tool. I went back to Autozone and found that, for a nominal charge, they rent a tool that works like a charm.

So, what's the bad news? Now the bugger is running lean. Previously, the spark plugs had black soot on them. Now, they are white instead to the coco brown I'd hoped for.

I'm about to give up and let the car become a trailer queen, maybe find some solo events in the So. Cal area.

Thanks guys for all the help but looks like this one is beyond me ability.

John

 

Did you check the voltage again? What's it reading now? Don't give up so easily, you can still get through this. After all you got all us here to help you out.

 

Hopefully this drama will benefit others who might run afoul of the California Smog *****. I forgot to check to voltage but will do it this afternoon. I might also go ahead and have the POS re-tested. Two years ago the car passed smog with flying colors. The only changes I've made were plugs, plug wires, and a new clutch. I know the plugs are the correct heat range so the whiteness must be due to a lean condition. BTW, when I test drove it yesterday it had noticable better acceleration. If I had the $$ I'd go for a SBC/T5 swap.

 

OK, I finally got to check the voltage output from the new O2 sensor, however it was fluctuating too rapidly to read with my digital VOM. It seemed to read between 400mv and 700mv, but I'm not certain. I'll try to borrow an analog VOM. That would probably give a more stable reading. The car is now runing better than ever with great acceleration. It just won't pass smog.

 

Exactly why won't it pass smog?

Is the Nox too high? Hydrocarbons?

Could be as simple as a bad cat.

What gas or gasses are reading too high on the report?

An extreme rich or lean burn will wipe out the cat in no time... not to mention that if it's still the OEM cat in a 24-25 year old car... I'd be amazed if it wasn't toast.

A good way to see if the cat is working (Will give you a good idea anyway), is to see the temp difference just before, and just after the cat. If you have access to a hand held digital thermometer gun you can do this. Let the car warm up to operating temp. Point the gun at the inlet to the cat and see what it says. Now, compare that to the what it reads when pointed at the outlet of the cat.

If it's working (Lighting off), the temp will be significantly higher coming out than it was going in... like a hundred degrees higher. If the temp stays unchanged, drops, or only gets a little higher, you got a cat problem.

If you determine the cat to be bad, don't get a cheap one. We in the industry call them "One day miracles". It might get you passed this time, but in two years when it's time to smog it agian, it'll probably be dead.
One of the things that makes a cat work is Platinum... how much platinum do you think is in a hundred dollar cat vs one that cost 250?
Also, you can upgrade to a three-way cat (The OEM was only two way) and it will burn much cleaner and run better.

Rod.

 

Any suggestions on a decent 3-way cat for a Turbo? Random Tech?

 

Take Rod's advice on the cat. Those things can get damaged very easily. Especially considering your O2 was bad. As far as the rapid fluctuation between about 400 to 700. Thats spot on. The ecu switches back and forth between lean-rich-lean-rich-etc... to keep the mixture balanced. The rapid fluctuations just means the O2 is up to par and working. The Voltage range is perfect for factory set AFR. So don't sweat it empty. If you failed the test let us know why. I'm not a smog tech but I can tell you what would cause certain readings to be off (sounds like Rod could too ).

Oh and A word of WARNING: DO NOT USE AN ANALOG METER ON THE O2 CIRCUIT. The voltage is too low in that circuit and the analog will put a draw on it. The DVM's are self sustaining and don't have a noticeable draw.

But take it from me your O2 is working (based on what you just said) and your car isn't running rich or lean, it's right where it should be (again, based on what you just said). Like ROd said, could be the cat, could be the EGR, could be a vacuum leak.

A quick rundown(and this is just basic):
NOx=Bad EGR (Controls combustion temps, high combustion temps cause NOx)
CO=Most likely the cat. Could be something else, but it's most likely the cat.
Hydrocarbons=Incomplete combustion. Weak spark, too rich (700mV is not too rich).
CO2=SPOT ON BABY, keep farting flowers and pissing water

Thats just a basic list and it gets more in depth but until you know what your readings are it's impossible to tell whats wrong.