Idles better without O2 sensor

[jwk87]

What are the possible explanations of why my idle and general engine running is better with O2 sensor disconnected. I replaced my O2 sensor a few months ago with a new bosch 3 wire.

Also, I noticed the mixture adjustment screw appears to be adjusted fully clockwise? (rotates counterclockwise but can not rotate clockwise and do not want to force).

The engine idles perfectly until I hook up the O2 sensor. Is it a bad O2 sensor or could there be other issues?

John
87 911 Cab

[DRACO A5OG]

Your DME is laboring via the O2S due to the incorrect AFR, the screw you described. Also insure you have no vacuum leaks but from what you wrote someone tampered with the Air Fuel Ratio adjustments

You need a LM1 or borrow one to dial in the AFR to spec with that adjustment screw (3MM Allen).

If it is clockwise bottomed out, that is it. It is in full rich mode (do not know the tech word for it), this is not good.

If you have no access to a LM1 then use a known GOOD QUALITY Resolution DVM and hook it up to the O2S ( red to white wires and black to signal, or visa versa, voltage will come from the heat of the exhaust on to the O2S) after the car is at full operating temps, hot. Try to adjust that screw out until you get close to 1.00 volts as per Bentley Manual.

After this, you will need to adjust your BASE idle via jumpering B&C. 880 RPMs just a tick below the middle between 1000 RPMs and next mark below (800). Remember our tachs start at 200 RPMs.

Jim

[Lorenfb]

"Try to adjust that screw out until you get close to 1.00 volts as per Bentley Manual."
- DRACO A50G -

Actually, the value of 1.00 is not correct (possible another Bentley error if that's
what was stated). The 1.00 at the O2 sensor would indicate a very rich condition.
The desired value is about .50 volts. The air bleed screw needs to be turned
counterclockwise, i.e. about 3 full turns out. Remember, this adjustment only
affects the idle mixture. One needs to monitor the O2 sensor @ about 2000
RPMs to evaluate the off-idle mixture, again without the O2 sensor connected.

[Joe Bob]

Pull your plugs...bet they are sooty black.

[jwk87]

Thanks for the feedback.

I decided to explore checking the O2 sensor, and with the O2 sensor disconnected I received variable outputs. At first I got .3 volts, but then after attempting to adjust the mixture I did not get a reading, even after returning it to the same position. Are O2 sensors temperamental or go bad easily?

Another question, can a Gunson meter be used to check mixture and get reliable results?

John
87 911 cab

[rusnak]

Your O2 sensor or wiring could be bad.. Is it spliced in (generic 3-wire sensor)?

[jwk87]

It's a 3 wire Bosch sensor that connects to a 3 wire connector. I kept the old one, so I think I will try swapping to see if I get different results with the old sensor. It's amazing how well the car idles without the sensor, not sure what mixture I have but I can't imagine it can be far off. The mixture control screw appearing at near maximum is strange, what can this be a symptom (defective, vacuum leak)?

John
87 911 Cab

[twistoffat]

I just reconnected mine on my 87 carrera. I am getting better throttle response( felt anyway) and improved mileage. You may want to have a read of this thread
Is an 02 sensor necessary?

[T77911S]

Originally Posted by jwk87

Thanks for the feedback.

I decided to explore checking the O2 sensor, and with the O2 sensor disconnected I received variable outputs. At first I got .3 volts, but then after attempting to adjust the mixture I did not get a reading, even after returning it to the same position. Are O2 sensors temperamental or go bad easily?

Another question, can a Gunson meter be used to check mixture and get reliable results?

John
87 911 cab

did you still have a good connection of the MM to the O2?

do these have a throttle position switch?

[rusnak]

It's idling poorly because the CO mixture screw is way, way off, as Draco described. He also described how to adjust it back to spec. I would get a CO% gauge ot an AFR gauge. No guesswork. Ingo recently stated that without the O2 sensor, it will just go full rich but stray signal can cause the dme to intermittantly change the injector pulse length. In other words, the dme does not know if there is no O2 sensor connected. It will react to voltage or lack thereof as if the O2 sensor is working just fine. It is a bad idea to (a) not have one connected, or (b) have a faulty or old sensoe connected.

[Lorenfb]

"Ingo recently stated that without the O2 sensor, it will just go full rich"

That's inaccurate! Read the other thread where this was fully refuted and explained.

Bottom line: Please, let's not provide mis-information that's repeated from other
mis-leading threads which causes the thread starter to waste time and money
on the wrong solutions.

[r_towle]

On the Motronic 3.2 the O2 sensor is not being read at idle.
The idle air bypass screw adjusts idle speed and the Idle control valve keeps it at that speed....the Idle stop micro switch tells the ECU to activate the idle circuit.

If its bouncing up and down or running rough at idle, you need to verify those systems are functioning.
Also look for and repair any vacuum leaks.

A simple tune up and valve adjustment also does wonders for idle.

Rich

[rusnak]

Scarceller did mention closed loop idle a few months ago, and I think this is why base CO% is associated with a bouncy idle.

Loren, you are reading sloppily today. I did not say the dme goes into "default mode", I said that it continues to see a signal, or lack thereof. I found that fact to be enlightening - in the "other" thread.

[Lorenfb]

"On the Motronic 3.2 the O2 sensor is not being read at idle."

- r_towle -

That's not correct! The DME ECM is in closed-loop at idle, i.e. O2 sensor is being read.

"Ingo recently stated that without the O2 sensor, it will just go full rich"

- rusnak -

Which is incorrect!

[DRACO A5OG]

I (actually Bentley does, not the first time ) stand corrected .50 Volts on O2S at operating temps.

Thanks Loren.

I still remember you when I first joined PPB and you directed me to clean my ICV.

Thanks agains

Originally Posted by Lorenfb

"Try to adjust that screw out until you get close to 1.00 volts as per Bentley Manual."
- DRACO A50G -

Actually, the value of 1.00 is not correct (possible another Bentley error if that's
what was stated). The 1.00 at the O2 sensor would indicate a very rich condition.
The desired value is about .50 volts. The air bleed screw needs to be turned
counterclockwise, i.e. about 3 full turns out. Remember, this adjustment only
affects the idle mixture. One needs to monitor the O2 sensor @ about 2000
RPMs to evaluate the off-idle mixture, again without the O2 sensor connected.

[rusnak]

Originally Posted by Lorenfb

"On the Motronic 3.2 the O2 sensor is not being read at idle."

- r_towle -

That's not correct! The DME ECM is in closed-loop at idle, i.e. O2 sensor is being read.

"Ingo recently stated that without the O2 sensor, it will just go full rich"

- rusnak -

Which is incorrect!

Loren, with no signal from the O2 sensor, what would your DME do? Something different from everyone else's I take? Are you raising an issue over a default map, or what? Are you saying that the DME has a plausability check?

[Joe Bob]

When my 02 went on my DME/Motronic 3.6, the engine surged at idle. Plugs were sooty black. Replaced sensor and plugs cleared up.

[DRACO A5OG]

Originally Posted by Joe Bob

When my 02 went on my DME/Motronic 3.6, the engine surged at idle. Plugs were sooty black. Replaced sensor and plugs cleared up.

Yeap, that is what will happen without a O2S, our Euro Brothers will run rich a bit

[jwk87]

Follow-up on this thread;

I was able to check CO emissions by using a Gunson gas analyzer and determine that with the O2 sensor unplugged, I could adjust CO levels to about 2%. This reading was repeatable (measured thru inspection port) with the mixture adjustment screw set to about 2 turns out. Also, Idle speed was now about the same, with O2 sensor connected or disconnected.

I next measured CO levels (after adjusting) thru the inspection port with the CO2 sensor connected. The level was now at near 0 indicating a extremely lean condition. The o2 sensor appears to be leaning out a normal mixture for some reason? The O2 sensor read a steady voltage (0.6v) using a basic ohmmeter (don't have an oscilloscope). Could not see wavering voltage, but this may be that the measurement was done with O2 sensor disconnected and/or due to measurement device. Interesting that without the O2 sensor connected, opening the oil cap lowers idle and leans mixture as expected, but no effect is seen with idle when the O2 sensor is connected and cap removed.

I'm assuming I will need to replace this relatively new sensor as a next step, but does anyone have any other suggestions or issues to consider??

Thanks,

John
87 911 Cab.