Low power in low RPMs after reconnecting O2 sensor
 

Hey, folks -

I have yet another opportunity to sort out a CIS problem.

Back in late October, I installed a wideband O2 sensor in my '83. The new PLX has a narrowband output that connects to the ECU to provide the lambda signal. The O2 sensor in this car had been disconnected for many years -- maybe a decade.

I did a couple of test drives after installing it and everything seemed to check out ok. I didn't drive the car at all in November.

Last week the sun came out and I figured it was a good day for a drive so I drove to the store about 3 miles from the house (1-way). It was cold -- I'd say about 40F. On the way to the store the car started to wheeze and pop a bit in low RPMs and the car lost power in the low RPM range. Pushing on the accelerator didn't do much and then it was like it would suddenly "catch" and the RPMs would shoot up to where they should be. It did it a couple of times on the way and then when I was in the parking lot at the store it seemed like the RPMs kept wanting to drop. Did my shopping and about 30 min later when I came out the car started up fine and I drove home with no popping/wheezing but it felt like lower power than normal in low RPMs.

This weekend I drove around the neighborhood and the coughing/wheezing was back so I thought this is a not a fluke. My first thought was to replace the O2 relay so I went to the local NAPA store, got a new Bosch replacement (Part number is 0332019150) and installed it today. No change in symptoms.

It seems like I'm looking at a couple of possibilities. It is either (a) the new wideband O2 sensor has created a problem where there wasn't one before or (b) by reconnecting the O2 sensor I have found a new problem. Maybe this is why the O2 sensor was disconnected to begin with? I don't think the ECU uses the O2 sensor on a cold engine though so I'm inclined to believe that the new O2 sensor is not really the problem.

I'm looking for ideas on what to look at next? I could disconnect the narrowband output and see if makes a difference. If it does I will be very confused.

From a quick scan of your link, the wide band thing does nothing with respect to the Lambda box except provide the analog signal. Disconnect the entire unit and see if the symptoms repeat.

All you are doing in effect is unplugging the single wire O2 sensor with respect to engine operation.

If things smooth out, then you know that the problem is isolated to the new box.

You do mention you had to crimp some wires and use little connector pins. Could they be cross talking among themselves?

O2 sensor "closes the loop" once it gets warm enough to send a signal. Should not take long given it's exhaust heat doing the warming.

Did you "mess" with the 12 pin connector on the engine firewall? The frequency valve signal travels through there. Loose? Pin push back?

Thanks, Bob. Comments in-line below.

Planning on doing that today.

I got to thinking about this last night. I was thinking about the differences between the old and new O2 sensors and a couple of things occurred to me.

In my case the original O2 sensor was disconnected. With NO signal coming from the O2 sensor, I believe the proper behavior is that the ECU reads this as an open circuit and goes into its "open loop" operation mode. No surprises there.

Here's my question... Does the wideband O2 create a new problem? Here's what I'm thinking...

In normal operation with the factory O2 sensor, the voltage signal is actually created by the excited exhaust gas. In order to do this, the metal electrodes in the sensor only create a voltage "when the ambient air has a higher oxygen content than the exhaust and the ceramic material is hotter than 575F." (Bosch Fuel Injection book). Obviously it will only send the signal once it's sufficiently warmed up.

The new wideband O2 sensor adds a control module. So I'm wondering if it starts sending a signal immediately at startup or if it takes some time. Is it possible that the wideband unit is sending a signal too early and causing the ECU to move out of open-loop and into closed-loop too soon?

I don't know enough about the ECU logic to know how it would behave.

I was pretty careful with the crimps and they fit into a plastic molex connector so I don't think they are cross-talking. I used a posi-lock connector for the O2 sensor wire to the ECU. I suppose that could be introducing some noise. I will check the voltage on the green/white wire on the test port. I could also disconnect the connection from the wideband module and compare the readings to see if there is a difference.

If I messed with the 12-pin connecter it was by accident. Worth double-checking for sure.

I am wondering if I might have a bad thermoswitch.

If you did not touch the twelve pin, I would let it be. Main thing is continuity between the freq valve and lambda box. A guy in Dallas (older thread) had some corrosion at that pin and in really hot weather the circuit would break and would act like you pulled the O2 relay. Cooled down and all was well. Drove him nuts.

Thermotime switch would spray gas through the CSV if (switch) closed. Normally only when starter is engaged, right? Then should receive no more voltage.

Let us know what you find.

What's the "white stuff" at the base of the yellow fuse where you are pulling power? It looks different than the other two.

You may be pulling power from the hot side, though.

http://forums.pelicanparts.com/uploa...1387387819.jpg

Started the car with the dwell meter hooked up. Actron dwell meter. Green wire from meter to green/white wire on test port and black to ground. Cold engine. Ambient temp in the garage was 54F. Started running very lean. 20% duty cycle. Idles rough.

Stopped the car. Disconnected the narrowband output from the AFR system to the ECU. Grounded the green shield wire that goes to the ECU. Started the car again. No change. Disconnected the ground with the car running. No change.

I am pulling power from the hot side. I just looked at the yellow fuse and I don't see it. It might've been glare from the flash on my smartphone.

Just for shiz'n'giggles I swapped the new O2 relay with the old one. No change. What are the odds the new relay is bad?

Does it stink?

I always struggle with this but I think that with a 20% duty cycle the signal from the O2 sensor is saying it is too rich. The frequency valve is being instructed to lean the mixture out to compensate.

I think I remember that psalt says to set it around 35 duty cycle which is a little rich. The Lambda system will compensate and bring it to stioch in closed loop (dithering) but when you got to WOT you have a bit of extra fuel as the Lambda system is disabled.

You can confirm the freq valve is working with the car off. You need to jump a wire among three terminals on the the O2 relay and plug it in.. Let me see if I can find the related old thread.

See post #32

http://forums.pelicanparts.com/porsche-911-technical-forum/559200-911-sc-running-flat-no-idle-2.html

Also, is the O2 fuse ok in the front trunk?

Stinks to high heaven.

According to the Bosch book, a zero voltage output from the O2 sensor signals a lean mixture, which drives the the ECU to compensate by enriching the mixture. The duty cycle SHOULD go to 95%.

To test for the opposite, the book says to use a 2v battery. Connect the battery + to the green shielded lambda wire and - to ground. This simulates positive voltage (rich condition) and the ECU should send a duty cycle of 10%.

My problem seems to be that I'm now sending NO lambda signal to the ECU and I have the OPPOSITE expected condition (low duty cycle reading). I'm scratching my head now.

Either I have a two bad O2 relays, a bad ECU or something entirely different that I haven't figured out yet.

I'll give that a go.

If you mean the dome light circuit, yes it's working.

Confirmed frequency valve buzzes when pins 87, 87b and 30 are jumpered on the O2 relay.

Good - way better than pulling the 12 pin connector.

Low voltage from the O2 sensor is sniffing lean. I think the regular sensor goes up to a max of 0.9 volts when rich.

I am going to find the psalt thread regarding rich vs. lean. I read Probst's book and still had it backwards in my mind a few years back.

Here it is: http://forums.pelicanparts.com/porsche-911-technical-forum/774966-fuel-pressure-fv-anomalies.html

Here is a vid I made of pulling the O2 relay. There are little backfires at the end of the vid. Do you have any of these?

https://www.youtube.com/watch?v=5tdpCYyvqSc

Bob,

That was how it was running the other day. I thought I would try and make my own video and when I started it again the damn thing doesn't register anything on the dwell meter. I've checked and double checked. Something is not right here.

Dammit.

A new symptom. Let the car cool down for a few minutes, double checked all my connections AGAIN. Dwell meter plugged into test port. DVM plugged into narrowband output of the AFR module to see what it's reading. New O2 relay installed. Green shielded O2 sensor wire disconnected.

Started the car and it idled smooth. Same lean reading on the dwell meter. AFR gauge is putting out .645V and slightly fluctuating.

As I'm looking over everything I hear the RPMs change a bit. Look down at the dwell meter and the needle is back at zero.

I'm thinking that the ECU was switching from cold start to partially warm mode.

I also think the dwell SHOULD'VE gone to 50% duty cycle. This is not right. Do I have a bad ECU????

I r stumped right now.

If your car sounds like my video, slow throttle response, etc, the connection to the freq valve is being disrupted.

Check the 12 pin to the right of your left rear shock. Number one connection. Black and red, both sides. May be corroded.

However, you never had this problem before so double check your wiring.

Check the connection on the freq valve too.

Your engine is rich. Not lean. I got a 12 pack that says so. :-)

However, not the issue right now.

Another thought. Hook up that relay jumper you tried earlier and wiggle stuff.