View Full Version : O2 sensor voltage and other problems

01-13-2005, 07:20 PM
Been running super rich on a fresh swap. Haven't even taken it out of the driveway. Trying to figure out why. BTW, it's an S13 SR Redtop with a narrow O2 sensor.

I turned the key to the ON position. Disconnected the plugs for the 02 sensor and checked the voltage at the female connecting points as well as continuity between white signal wire and ECU. There was continuity.

Brown = 12v
White = 23V
Black = ground

How is my ECU sending out 23V to the signal wire? Also, my O2 sensor has different color plugs, Black, Red, and White, but they are a in a different configuration.

They plug in like this.

brown - red (good)
white - black (bad?)
black - white (bad?)

I did not change around the pins on my engine harness. Should I switch them? Is this necessary? Also, I am not throwing any codes. If this thing wasn't working properly, wouldn't I throw a code?

01-14-2005, 08:31 AM
Are you saying you have a blacktop or blacktop harness / ECU? The blacktop signal wire is in the center on the harness. on the redtop (narrow) O2 sensor it is "supposedly" the white wire, which is not in the center. I say supposedly because I have a narrow O2 sensor and a blacktop, and I can not seem to get my O2 sensor to work worth $&%&*@!# I think I originally plugged it into the blacktop harness and the 12V signal going INTO the o2 sensors signal wire, is what killed it. and my A/F guage, come to think of it, since it would have gotten the 12V signal also, where there should only be 0-1V.

As for your ECU sending out voltage over the signal wire, first you need to make sure you are looking at the right wire for signal, then you need to figure out how you wind up with anything more than 12V on anything in a car. The only thing that comes to mind is a bad alternator with a bad regulator, which can spike 16+ volts, but i think something else would have fried in that case already.

01-14-2005, 09:39 AM
It's a redtop with a redtop harness and redtop O2 sensor. I checked for continuity between the ECU and the female plug.

01-14-2005, 11:48 AM
Well first of all, what is the number on the ECU?

second of all, If the ECU is sending out voltage across the signal wire there is something WRONG. either inside the ECU, or somewhere as it makes its way to the plug. Try a different voltmeter, sometimes they do weird things? I had one tell me 7V at my TPS sensor, but when i moved it out of the sun it read 4.5V.

Now, about running rich, even with the O2 sensor UNPLUGGED it should not be running SO RICH as to foul plugs and black smoke. I have not had my O2 sensor plugged in on my redtop OR blacktop for well over 3000 miles, I get 24+ Mpg highway and never backfire on the blacktop (redtops seem to backfire alot more than blacktops? Tuning in the ECU most likelly.)

SO if its running REALLY rich I would not suspect the O2 sensor as the problem, possibly a boost leak, bad ECU (running in safe mode or limp home), bigger injectors (what color are the injectors? should be light purple/blue) etc..

01-14-2005, 02:23 PM
I'm running so rich that all my plugs are black. I fouled my first set and I have to get some new ones today so it can actually start up. My injectors and bluish purple so I know they are the right ones. I tried pulling the fuel rail and hooking everything back up, primed the fuel pump, and they didn't leak. I checked the voltage on the MAFS and it seemed to check out fine. I just have to get new plugs so I can do more diagnosis on the TPS and the coolant temp sensor. I restabbed my CAS too but pulling the valve cover. This crap is tiring me out. Been trying to figure out this problem for a week straight.

01-15-2005, 12:57 PM
First thing to note, is dos it rev past 2500RPM. That said, check the wiring at the ECU for loose or previously spliced connections, the previous owner of the ECU may have installed some wiring. . Now do you know your fuel pressure? The factory regulator can be stuck shut, raiding pressure through the roof, creating symptoms you describe. Next your return line may be clogged, simulating a stuck regulator. I would check the voltage at he maf sensor, make sure its reading less than 1.5V for sure, if the computer sees a bunch of air its going to add a bunch of fuel.

The tps sensor has little effect on fueling at idle, from what I can see on my SR, I noted on a S14 motor it makes a rougher idle when unplugged, but on my redtop / blacktop S13 it made no noticeable difference in idle quality. It could be stuck sending out 5V though, I had one do that, so verify voltages there for sure too.

Basically fueling is controlled by only a few things. Fuel pump provides fuel volume, which obviously isnít an issue for you, pressure is controlled by a return style regulator, which could cause high pressure, then injection quantity is controlled by injectors, when properly sized and properly supplied with pressure, the only thing left is the ECU which gets its info from a base map and is told by the maf sensor how much fuel needs to be injected.

01-15-2005, 02:34 PM
OK, thanks for the thorough response. Iíll tell you exactly what is happening and everything Iíve checked.

My car is running super rich. Fuel is shooting out the exhaust at idle and through out the entire range. It wonít go above 4500 RPMs because it gets bogged down with so much fuel. Another weird thing is that Iím hitting full boost with no load on the motor. Iím not sure if the fuel is spooling the turbo or if the combustion is occurring with the exhaust valves open. This leads me to think that my timing is off so I realigned the CAS with the valve cover off. I brought the car to TDC on its compression stroke. I made sure that the two silver links in the chain were lined up with the cam sprocket. I also placed a paper towel in the sparkplug hole to confirm that I heard the compression stroke. I then aligned the CAS the way itís supposed to be by using the base marks and whatnot.

Next thing I decided to do was check all my voltages at idle and with the engine at 75 degrees Celsius at the sensor and at the ECU.

MAFS = 1.8v (normal for S13 SR MAFS)
O2 sensor = .9Xv(reading correctly because of the rich conditions)
TPS = .5v (normal for the throttle body being closed)
ECT (engine coolant temp sensor) = I think it was at 1.5v (gotta double check)

I then proceeded to switch to a stock fuel pump and that did nothing. I also pulled the fuel rail out and hooked everything back up, primed the pump, to see if the o-rings were bad on the injectors. Everything checked out fine. I am going to check the return line for blockage today but I donít think itís clogged. If I remember correctly, it felt like there was less pressure in the line then the sending line which means that itís not backing up. I think Iíll just unhook the return line and have it return to a bucket to see if that helps.

01-15-2005, 04:55 PM
If you are hitting full boost at no load, i would recommend checking and rechecking timing.

01-16-2005, 05:09 AM
I've checked and rechecked timing. I had my friend who works at a shop that does SR swaps all the time check my CAS alignment. He said that it was fine. He is convinced that it is a MAFS or ECU problem. Could the fact that my harness and TPS are from an automatic have any effect?

01-17-2005, 04:30 PM
I strongly believe its an ECU problem. My car acts the same way as you described, becoming bogged down with too much fuel. Granted I upgraded to larger injectors and a Z32 MAF and a Power FC, but the way the car was acting I thought it was timing as you did. I checked and rechecked the timing, but it was not the problem. I finally began to slowly lean the fuel maps out by RPM and BINGO! no more bogging when the ecu tells the injectors to start dumping more fuel. So I would say that your ecu for some reason is running in safe mode or maybe a connection is loose and the ecu is just dumping a lot of fuel to be safe. Its definately a fuel delivery problem when the ecu tells the injectors to start dumping the fuel when your boost comes on. I hope this helps.


01-21-2005, 10:15 AM
It does sound like wiring / ECU related. a bad mafs wouldnt read the correct voltage, because then it wouldnt be bad. although if it revs past 2500 you know the ECU sees the mafs, which makes me wonder why if it sees the mafs does it dump so much fuel? perhaps it has been reprogrammed, or worse, you have incorrect injectors? the colour sounds correct, however, so I doubt thats it. have you opened the ecu to see if there is a daughter board inside? im not sure if the SR ecu has a memcal (limp home memory) but if it does, it could be reading from that instead of the actual ROM base maps.

but it doesnt make sense if the ECU is seeing all the right sensor inputs, why then does it want to dump fuel? again, this sounds ECU related, somehow. but then, I cant count the number of times people have pointed at the MAFS or ECU when it actually wasnt. or to quote david vizard, "99% of carbeurator problems, arn't." in other words, just because the tune it wrong, doesnt mean the carb is bad. or in this case, ecu. At least I *think* david vizard said that... Hrmm.