G20 O2 Readings Help!

[lowpass5]

Hi,I'm new to this forum. I have a 95 G20 that won't pass emissions. I was wondering if any of you know the correct O2 sensor voltage for this car. I am reading 0.94 volts fluctuating some, but not significantly. I was expecting something around 0.45 volts. I notice that I get about 0.3V bias with key on engine off. The sensor responds quickly to induced vacuum leaks but returns to around 0.9 even in the presence of a leak. I checked the engine temperature sensor to make sure the computer has the signal to go into close loop - it is working fine. It acts to me like it's not using the O2 to control the mixture, but the vacuum leak response puzzles me. When I pull the hose, the reading drops to around 0.2V, the recovers to around 0.9 and stabilzes. It's less than the 0.94 without the leak, but it seems to be responding. Does this seem correct?The car is not posting any codes. i have had it to 2 repair shops without any results ( but with significant cost). Any advise of a non-suicidal nature would be greatly appreciated.

Bob

[AZhitman]

Hi Bib!

Glad to see you're in Peoria, might be worth your while to roll out to Surprise and let us have a look.

I just happen to have a FSM for a 92 G20, you're welcome to it.

The O2 should oscillate rapidly when functioing correctly... Let me get someone in here with a little more knowledge to help diagnose this thing.

Welcome aboard, let's get your G back in tip-top shape.

[rotorimp]

Your O2 sensor through a chemical reaction produces a electrical signal--(which should be fluctuating pretty rapidly when it is working right.) Your Ecu uses this info to determine mixture. I'm not following when you say "The sensor responds quickly to induced vacuum leaks but returns to around 0.9 even in the presence of a leak" The O2 sensor does not control the idle or leaks--It can effect it. your Idle air control (AAC) does control this. If you would like some help--I'm located in Surprise and could give you a hand with your dilema.

Just some back ground--have you done a good tune up? spark plugs, filters things of that nature? What exactly is it failing for?

[lowpass5]

I have done the filter, plugs etc. The car passes HC and Nx. It barely fails CO. Readings have been between 12.1 and 12.3 against a max allowable of 12.0. Does the car function in closed loop mode at idle? I'm trying to get calibrated on what I should be seeing out of the O2. I checked the engine temp sensor and it functiong properly. The TPS seems a little weird though. I get good linear voltage across the thottle range, but my idle switch does not close until about 1/4 throttle. The book says it should be around 1050 RPM (just off the stop). would this switch position determine if the car is in closed loop? Checking the MAF voltages and the EGR today.

Thanks,Bob

[rotorimp]

From what you have posted it sounds like your TPS is the problem. When your foot hits the pedal that closed switch should go off--I would first set that. Your TPS is a basically a variable resistor and this is the type of signal your ECU is looking for(varying voltage not linear voltage unless you were measuring input voltage). As the throttle plate sweeps it the Ohms increase or decrease (I forget on the SR20's). This along with MAF is how your ECU determines load and-->Mixture . Get some MAF cleaner at Checkers I think CRC makes it and clean your MAF and Adjust your Tps and I think it will be in the ball park.

[lowpass5]

Thanks, I'll try the MAF cleaner. I put a jumper wire across the closed throttle harness connection points to bypass it. It didn't seem to have any affect.

When testing my oxygen sensor at 2000 RPM warm, I'm getting the voltage fluctuation I would expect, but the engine surges significantly in relation to the change in 02 voltage. I would say the surges are about 1 second apart. Is this normal, or an idication that is my O2 sensor responding slowly?

Thanks,Bob

[rotorimp]

If it is seeing too much votage it will try and lean it out--this might be the fluctuation.

Have you adjusted the tps yet?

[lowpass5]

Yes - TPS is set and operating properly. MAFS seems to be in the ballpark as far as voltage readings though it doesn't get over 3V or so at 4000 RPM. I let everything cool down ran it at 2000 RPM again. I would think it would stay in open loop with the temp sensor cold. Even cold I still get the surging. It acts more like fuel starvation actually - RPMs drop dramatically with corresponding drop in O2 voltage then recover. All this at about 1 cycle per second.

I checked the EGR and it doesn't seem to be the culprit - disconnecting the PCV and plugging the inlet doesn't change the symtoms either.

I didn't used to have the surging. I switched out the MAFS with a rebuilt one, but the shop said the rebuilt unit's voltage was all over the place. I put the old when back in - maybe the computer has adapted to the bad MAFS. I will drive it a while tonight and see if it improves. I really didn't notice any differnece in drivability (or emmisions) with either MAFS.

It's still not throwing any codes. I know the diagnistic is working. I induced a code by pulling the temp sensor - purplexing..... .

[Q45tech]

O2 sensors have a limited mileage life, then switch slowed and slower as the miles accumulate.............why they should be replaced every 60k or 4 years.

Use the Consult to GRAPH the voltage switches and look at the shape of the voltage curves..

Too much CO means not enough air vs fuel or just running too rich [0.9 volts]...................what is IAC duty cycle? Dirty TB?

[rotorimp]

See if you can get to a consult--you will be able to see exactly what the ECU is seeing. I'm in surprise and would be will to put my lap top on it (generic consult--very limited but still functional)--I can get a good idea of what is going on--then again O2 sensors are not that expensive--purchase a new one and see if that fixes your problem (sounds like it might be due anyway)

[lowpass5]

Thanks,

I finally made a little progress last night. I adjusted the TPS again to try to isolate the surge problem. I didn't have a surge problem before I started working through the emissions failures. Sure enough the surge followed the TPS. As I rotated the clockwise or counter clockwise, the surge RPM range would follow. I was able to adjust the TPS to completely eliminate the surge, but it puts me at a point where the idle switch is closed across the throttle range and the idle voltage is way high. I'm guessing the previous owner adjusted it this way to eliminate the surge. I didn't see it with an ohm meter, but I suspect I have a dead spot on the TPS pot. I will look closely at the voltage tonight or tomorrow.

In trying to test the 02 sensor function using the dash light in diagnistic mode 2, I was unable to the the car to cycle rich/lean. Running at ~2krpm, I could get the light to come on (lean) by quickly stepping off the gas, but the car never went into closed loop mode. During this test I did finally log a failure code! I got a cylinder 4 misfire core. I was suspeicious of the #4 ingition wire, but hadn't replaced it yet. The new ones go into toningt. Still don't know if I can get it into closed loop with the TPS adjusted where it is. We may try another run at the Emissions station regardless, but I sure would like to get this thing operating properly.

As frustrating as this problem has been, it's been good motivation to get me back into auto diagnosis and a great way to teach my 16 year old son (it's his car) about cars and about the finer points of cursing and whining.

Thanks Q45tech and RotorImp - I make take you up on the laptop Consult at some point, but would like to get the basics fixed first to see if that cures it. It's great to have experienced resources to bounce ideas against. I really appreciate the help. Any suggestions on the best place to buy a TPS or O2 sensor? Are the generic (reuse your old connector) O2s as good a factory or Bosch units?

[AZhitman]

Try and make a run out to 163rd Ave/Grand on Saturday, I'll be there working on cars with rotorimp and my 15 year old.

[lowpass5]

Thanks, I'll try to do that. Where on 163rd and Grand are you located and what's a good time?Bob

[rotorimp]

Send me a e-mail and I'll give you directions

[lowpass5]

Rotorimp,I sent you an email. I'm [email protected]

Still fighting this monster. It's not the TPS - smooth voltage trasition throughout the range. We'll check the MAF voltage vs RPM tonight. Then I have to figure out how the IAC works - oh joy! Tricky little problem....

Bob

[Q45tech]

IAC is the simpliest part just a ecu controlled solenoid which flutters open, allowing some extra air to bypass the closed TB plate/

The duty cycle is the open time vs closed time 10%>100%..........the valve is paralleled with the manual adjustment path [adjustment screw] with is adusted out 2.5 turns so that the ecu drops the duty cycke to 10-15% at the specified rpm.

100% duty cycle and a properly set manual screw agjustment should be 1300-1400 rpm.

Besides controlling idle under various loads the IAC goes to 100% at 60 mph cruise and steps down as the engine slows down as you take foot off throttle to avoid a stall.

Important for smooth coast down.

[lowpass5]

Thanks Q45tech, very helpful. I think I will check the fuel pressure tomorrow as well. Seems like that could make it run rich if it's regulating high.

It puzzles me why it won't go into closed loop mode. Throttle Position (off idle), and engine temp are the only factors that affect it right? My tach is broken so I am estimating where 2KRPM is. it should run closed loop at any steady RPM setting above idle once the engine is warm right?

Thanks,Bob