C-C_Rider

I happened to be talking to a mechanic friend of mine today regarding my rough idling problem and he suggested testing the O2 sensor. Apparently you disconnect the wires and run a test wire to the sensor, holding the other end of the test wire in your fingers while with the other hand touch the positvie terminal on the battery. The car should increase in RPM's and the opposite when you touch the negetive terminal. Have any of you ever tried this before? It sounds goofy to me but seeing as I'm no mechanic I just had to ask...

Bruce '89 s2

Tis a good thing we're talking 12 volts here.

C-C_Rider

I told ya it sounded goofy to me Bruce! But, all in all... worth mentioning

MHT

that will test the car's ability to respond to a signal but will tell you nothing about the sensor itself. You grasp the signal wire to the ecu in one hand and touch the other hand to the pos batt. cable, this sends a rich signal to the ecu, you then ground the signal wire to the ecu. Ideally you would have a voltmeter hooked up to the O2 sensor wire and would watch the voltage swing from low to high as you changed from pos. to neg.

C-C_Rider

Thanks for the reply, do you know of a credible way to test the sensor itself?

Terry Jamison

I watched as the mechanic checked my O2 sensor recently. He disconnected the sensor lead at the plug on the firewall. I have a California emissions car, so the O2 sensor is heated and has 3 wires. He hooked up his multimeter to the lead and read the milivolts generated by the sensor. In my case, the readout was constant close to 1. According to the mechanic, this was opposite of what should have been. Without the sensor sending a signal to the DME a rich fuel mixture is commanded. In this rich environment, the O2 sensor should generate a voltage such that the DME leans the mixture. I don't know the specific readings. What I do know though is that the car ran rough with or without the O2 sensor connected. So, I spliced on a $39 Bosch 3-wire universal O2 sensor. The gas smell in the exhaust went away. The engine smoothed out. Unfortunately for me, I was too late. The overly rich mixture fried my catalytic converter. I had to replace it before I could pass smog.

C-C_Rider

Some very good info Terry thanks, sorry for your misfortune though. I'm thinking that my rough idle problem could very well be related to the sensor then as the car is burning rich, hence the reason I have a black soot like material on my tail pipe.

I wonder if it's right to assume that if I disconnect the sensor and the car still idles rough that the sensor is bad then? I've replaced so many other things in the last 2 months I am running out of options.

Terry Jamison

The first thing that I noticed was a decrease in gas mileage.

As far as disconnecting the connector to check it's operation is concerned, I'm not sure. It seems to me that this is hit or miss. There would be some times when a rich mixture is needed at idle. I noticed the difference in idle only after having had replaced the sensor.

Terry Jamison

Some time ago, there was a thread on this board that covered the O2 sensor at length. One of the posters actually worked for Bosch with O2 sensors. I don't recall his user name. Perhaps he'll read this and respond. He had some really good info. I'll search the archives.

C-C_Rider

Thanks again, I appreciate your comments...

C-C_Rider

Where can I find an O2 sensor for the best price?

Charlotte944

Testing O2 sensors is easy, all you need is a digital volt meter.

Set the meter to read the 2 volt scale. Connect the negative lead to a good ground point (I use the negative battery terminal).

For a late model car you can "back probe" the sensor by pushing the tip of the positive meter lead into the back of the sensor connector. The black lead is the sensor output, the two white leads are 12 volts for the heater.

For early cars I made a jumper with a male bullet conncetor on one end, a female bullet on the other, and an alligator clip in the middle. To connect the meter, disconnect the sensor at the fire wall, install the jumper, and use the alligator clip to connect the positive meter lead.

Now you can start the engine.

Once the sensor is hot, raise the engine RPM to about 1200 or so. If the sensor is functioning properly the voltage displayed by the meter should vary across 0.5 volts.

To get a technical description of the O2 sensor circuit in the DME, go here:
http://frwilk.com/944dme/ and then click on "Lambda Inside."

To get inexpensive O2 sensors, go to NAPA and ask for either a Bosch 11051 (Single Wire), or a Bosch 13913 (three wire heated). Prices are between $30 and $40.

BTW: Your mechanic's "trick" is pretty slick. In essence he is using his (or your) body as a resistor. When you touch the battery with one hand while touching the sensor input to the DME with the other, the DME "sees" either a positive or a negative voltage.

If that voltage is above 0.5 volts the DME thinks the mixture is rich, if the voltage is below 0.5 the mixture is lean.

BTW: When you replace the sensor, do not solder the wires, use butt splices. The reason for this is O2 sensors work by measuring the difference between the amount of O2 in the exhaust gasses and the outside atmosphere. Since the sensor samples for O2 via the sensor wire, soldering the wire can seal the insulation to the wire strands, which blocks the flow of outside air to the sensor.

C-C_Rider

Very informative! Thanks Cliff! Does it matter which Bosch sensor I choose, 1 wire vs 3 wire? What do you suggest?

Thanks again!

Robert D

Hmmm. The soldering could be the reason for my strange idle. With the O2 connected, the car's idle cycles from spec to lean and back again. The ISV is new, and works (I can hear it). Without the O2 connected, the car idles perfectly but does the stumble when you let off the gas and drops rpms down to 200 sometimes....Another problem I've noticed is with the A/C on when I start the car, with the O2 connected, the car sits at 200rpm trying to get up but eventually dies (happens when the car is cold). Any thoughts regarding?

Charlotte944

Which sensor to get depends upon where you live. California residents MUST get a 3-wire sensor because that is how the California emissions laws are written.

The best advice I can give is to replace the sensor with the same type that is currently installed.

For the enviornmentally sensitive owner, I recommend a 3 wire sensor. With a heated sensor the DME can go to closed loop control earlier in the warm up cycyle, there by conserving fuel and lowering emissions.