tyro

Hi-

I wish I could find the post, but someone recommended an inexpensive O2 sensor from eBay. (should have known)

I bought one, and I'm thinking it's junk - supposedly a Beck/Arnley model.

The problem is that the car is running *very* poorly with this sensor connected:

1. cold car, acceleration is garbage unless at WOT
1a. if I tried to open the throttle, sounds like backfire/miss/not enough of something
2. hesitation also present during steady-state driving.

With sensor disconnected the car runs very well. I'm not going to spend the time to test this sensor if the replacement is $50.

Unfortunately I hope I'm not being quick to blame the sensor, because I replaced the sensor at the same time when I also did the venturi delete/vacuum mod - no leaks that I can find, vacuum is 16in @ idle.

Thanks for reading my novel..

Zero10

If it runs better with the O2 sensor disconnected then something is very wrong.
Perhaps somebody else here can tell you what sensors are ignored when the O2 sensor is disconnected, and through that you can find the source of your problem.

All of the above aside, I never run anything other than Bosch O2 sensors, and for the price of a universal one (~$35) I don't see any reason not to.

tyro

I'd read some other posts where users had been running with the sensor disconnected for some time..I don't plan to do so.

I'll have to find a Bosch 15726.

Also, I'm not sure if this is correct, but it came from "The 944 Motronic DME" by FR Wilk:

If the Lambda sensor is disconnected, the Motronic gets it fuel levels from the map only. It never just "goes rich to protect the engine" which is another famous and incorrect saying. When the Lambda is disconnected, it uses the fuel map.

A lot of posts have said that the DME goes full rich..I'm not sure what to believe, but someone who has a website dedicated to Motronic has a better chance in an argument!

Theo951

The DME O2 input signal comparator delivers a mid level signal to DME when O2 input signal is missing.
This is approx. 0.5 Volt DC. You can measure this signal when by running engine you remove the O2 & measure between chasis & singal input from the connector comming from DME.
The fixed mapping table is swithed only when this signal is grounded. Is not only enough to remove the O2 sensor. However If car is running well without it, your mapping as vacum lines are OK, ONLY YOUR SENSOR IS BAD.
With a runnig engine check is the O2 can deliver 0.2 to 0.8 V DC in 300 milliseconds interval.
A portable scope is a good tester for this.
You may also check the heater wires if the sensor is heated up. There should be few Ohms resistant between them.
A generic Bosch sensor is a good choise if the price for the original is high. However you must connect the 951 connector on it without soldering the wires.
Good luck
Theo

black944 turbo

I will have to disagree a bit Theo. I have a similar problem and I have changed my o2 sensor 3 times now and all of them where good and it still does a similar thing. My car runs perfectly but when you are on boost lightly say 5psi or less it surges a bit. My car has all new vacume lines and a rebuilt throttle body and it has been tested for boost leaks. I have not run the car without the O2 since I went back to the AFM but I will assume that it will probably run better.

Zero10

IIRC it ignores more than just the O2 sensor when running from maps.

ShApE

I thought that 16in. was pretty low?

SchnellerUmsetzer

When you install a new 02 do you have to reset the Co2? Considering that the throttle body was rebuilt?

tyro

Back from autocross..the car runs great with the sensor disconnected!

Is 16in vacuum too low? Keep in mind this value is coming from a mityvac brake bleeding tool..

black944 turbo

Schnell what do you mean by reseting the CO2? Idle is set properly, the AFM is set properly, and the TPS is ok.
Thanks, Richard

TurboTommy

I've had the same problem off and on (depending what mod configuration I was on at the time) over the course of the last ten years or so. To this day I still don't know why sometimes there would be hesitation when the O2 sensor was in the loop
The only thing that I have observed was when the base maps were a little on the rich side (like, in the 12s or 13s air/fuel), as compared to when the O2 sensor is connected, the hesitation is not there.

Theo951

In case that the O2 sensor is replaced & the car is still hesitates on close loop mode, you may consider that the output signal the O2 delivers is wrong ( does not corresponds the actual stich values)
The cause or wrong signal delivery can be addressed in 2 faults.
1) The cross pipe were the sensor is connected or the sensor threats does not have low (must be less than 1 Ohm) resistance to the chassis ground resulting voltage drop & DME reacts wrongly.
2) There is a leak on the exhaust pipes from headers / cross pipe ( all before O2) causing wrong measurements to O2. Because of this O2 delivers wrong signal output & your DME can not keep the needed 14,7/1 Air Fuel ratio in loop mode.
The above assumptions are based on the statement " car run perfectly without O2 sensor"
Here I have a screenshot of the AF/R of my car in Idle. See the sinus waveform
The narrow O2 is providing to DME.
Red is TPS Open blue is RPM dark blue is MAF out.

tyro

Vielen dank, Theo.

Grounding situation may be the issue, I do have temp/fuel indications that bounce around from time to time.

I'll be checking tonight..

black944 turbo

I have the same type of thing with grounds. Where do I check those? Thanks

tyro

Got a nice Bosch 15726 in, car runs fine.

Not sure why I just didn't notice it, but the ceramic in the 'cheap' sensor was in many pieces..

Caveat Emptor!