hot engines don't fry O2 sensors do they?

mark '87 930 · mark '87 930's Garage

I replaced my O2 sensor a couple of months ago cause I fried it when removing it from my stock headers.... All has been good since then, except for this morning. I took it out and when I was in third and starting to hit boost (under more load than 1 or 2) the car starting to bog down again like it did when my older O2 sensor was fried!

The only thing that has happened since yesterday morning was that the car car hot yesterday afternoon when I was coming home in traffic, hotter than regular driving. It almost got to the second white line on the temp. gauge.

Would my O2 sensor have fried due to the hot engine?

If so, are the true Porsche O2 sensors more durable? More so than the aftermarket ones?

2.7RACER · 08-20-2006, 11:14 AM

Mark,
No sweat. The O2 sensor is designed for the heat. Way more than the second line on your gauge.
Just to be sure, you can check the O2 at the ECU under the seat. Pin 24 to ground (pin 16 or 17). Should fluctuate from .1 to .9 volts once the engine is running and warmed up.
The book says you need a breakout box. I use small wires inserted into the conecctor and then reconnect to the ECU. Not simple, but it works.
Just in mine yesterday checking CHT, MAF, Altitude relay, etc.

mark '87 930 · mark '87 930's Garage

I don't have an ECU, the turbo's had none in those years... I do have a cheap A/F gauge that I plan on hooking up to it tonight to monitor if there is something going on with the O2 sensor.

Jim Smolka · 08-20-2006, 02:01 PM

Back in the day when I was an electronics tech for Bosch's O2 sensor line, the furnace used to test the sensors was setup to run at over 900 degrees C. When the sensors came out of the oven, they were glowing orange. Most of the times when a sensor fails it is due to contaminates on the inside of the tip of the sensor.

mark '87 930 · mark '87 930's Garage

Thanks, It hasn't done it since and I put an A/F meter on it and all seemed ok!

Maybe the car was jsut cold or something, I also reconnected the O2 sensor, who knows!

2.7RACER · 08-20-2006, 06:42 PM

Mark,
Whether you have an ECU or not the method for checking the output of your O2 sensor is the same.
Once it's warmed up, it should read .1 to .9 volts. Usually fluctuating. To test slip a small wire into the connector that you can reach with a meter once it is reconnected.
This will tell you whether the O2 is working or not.

ianc · 08-20-2006, 08:24 PM

The sensor does not even have to be connected to the harness to test. Just drive the car until it's at operating temp, then test the voltage across the pins at idle with a VOM. As mentioned, you should see .1-.9 volts. If not, there's an issue with it.

ianc

mark '87 930 · mark '87 930's Garage

I did do that yesterday evening and all seemed fine... so It may have been a fluke or something else!

Thansk for all the input.

javadog · 08-21-2006, 06:00 AM

Quote:

Originally posted by mark '87 930
I don't have an ECU, the turbo's had none in those years...

Well, I'm glad your car is running better but I thought I'd mention that the 930 does have an ECU, if it came equipped with an O2 sensor. It lives under one of the front seats and takes the input from the O2 sensor and adjusts your fuel mixture during normal driving. It's not like a Motronic system, but it is a computer.

For what it's worth,
JR

mark '87 930 · mark '87 930's Garage

That's news to me.. I knew there was some kind of box that the O2 sensor plugs into but unclear of its whereabouts and function... I'll have to look at it further on the diagrams to see what t controls. I'm not sure how it would richen or lean out the mixture as all works with vacuums!

Do you know if there is a relay on this thing as well?

javadog · 08-21-2006, 07:17 AM

Yes, there's a relay in the same area.

On a closed loop CIS system, the O2 sensor sends a voltage signal to the ECU which in turn changes the duty cycle of a frequency valve that is plumbed into the lower chambers of the fuel distributor. At a certain duty cycle (say 50%) the flow of fuel from the lower chamber keeps a certain pressure differential between the upper and lower chambers. If the computer increases or decreases the duty cycle, in response to a measurement from the O2 sensor that the mixture is not optimum, the net result is a change in the pressure differential between the two chambers that resultes in a change in the fuel flow to the injectors. Thus, the mixture changes.

In operation, the mixture is constantly changing from slightly rich to slightly lean, and back. The average mixture is close to what it needs to be to burn most cleanly.

It's hard to describe any CIS system in a few paragraphs. I'd recommend that you buy one of the many books available on Bosch fuel injection systems.

Cheers,
JR

mark '87 930 · mark '87 930's Garage

Ironically, I have a chapters certificate which I intend on using for just that...

I purchased awhile back this inexpensive A/F tool (http://www.autospeed.com/cgi-bin/browse.cgi?product=888800044)which I put together and connected the other night after I measured for voltage... it works (but it's slow as I don't have a wideband O2 sensor). i just wanted to get an idea, and it does work.

I'll pick up the book and do some more reading. Unfortunately winter is around the corner.. but that will also give me some time to learn more about the system, and as well check all of my CIS components.