Over the past few weeks I have installed a 85-86 catalytic converter with dual exhaust on my 79 euro 5spd. I have also installed a O2 sensor from 928international. My reason for installing and O2 sensor was to be able to take readings on the mixture and to eventually install a air/fuel mixture guage.

Now keep in mind my car DOES NOT have any type of computer controlled fuel devices. Fuel metering works off of control pressures at cold and warm temps. It is a different type of injection than the 80 and on up. Im not sure if most of you realize that or not. Anyway to my question.

When I hook up a volt meter to the O2 sensor and at idle the reading is supposed to be around 0.60v to 0.65v. My reading is near that but it ranges from 0.60 to 0.80v at idle. Now when I start driving around it drops way down to 0.001v to 0.008v. Is this normal?? If the reading is correct then that would mean that under driving conditions the car is running way, way lean. Does anyone know if Im able to take readings while driving or just at idle?? If you can only take readings at idle then how does a guage work while your driving? Does it use a different type of O2 sensor??

If this reading is correct then I must have a fuel control or system pressure problem. A month or so ago I checked all the fuel pressure specs per the 928 shop manual and everything seemed to be within specs. Maybe I should recheck those pressures again.

Well, hopefully a expert emissions person will speak up. I dont want to hurt the motor running it too lean.

Thanks again

Curt Cook

 

A couple thoughts here.

If your car is a Euro version (mine is an '85 Euro), then you probably have your air pump injecting air into the cylinder heads. On US cars, the air injection goes into the cats, on the Euro cars it goes into the heads. Air injection into the heads is a simple, non-horsepower-robbing way to clean up stray hydrocarbons by causing them to burn right outside the combustion chambers. But O2 sensors work by sensing excess air, right? So you'll want to shut off your air injection before reading the O2 sensor. Simply uhook the vacuum line from the air diverter near the passenger side cam box and plug the line, then check your readings.

Next is the type of O2 sensor you are using. A single wire probably won't cut it - it's too far downstream in these cars to get hot enough. A three-wire can have problems too because it relies on the exhaust system for its ground. And the exhaust system might not have a good enough ground to get the kind of precise reading you want. The best thing to use is a 4-wire O2 sensor, where the heater has its own power and ground, as does the sensor part. But a 3 wire will probably work well enough.

Bryan

 

Bryan, thanks for the pointers. I did have my air pump line plugged so that is was not pumping air into the cylinder heads, but the O2 sensor im using is just the single wire type. bummer cause I just bought it a few weeks ago. Thats 40 bucks shot. I will try and find a three or four wire O2 sensor. How exactly does the power work for the sensor. Does it basically have power when the key is on and stay on until the key is off. Just wondering where I should tie the power into. And what are the four wires for?? One is power, 2 is ground, 3 is for taking the reading and what does the fourth do??

Does anyone have a used four wire O2 sensor that I could purchase??

Thanks again.

 

The good type of O2 sensors are heated.

You see, the sensor only functions correctly in a certain tempeture range. In that tempeture range, it fuctions a bit like a weird battery. The diffrence in the CO in the exaust gass, compared to the outside air causes a voltage to be drawn accrost the element, and that voltage is collected and sent to the computer.

On a one wire sensor, the element is not heated. The ground it through the metal in the exaust, and the signle goes to the computer to the wire.

The 3 wire sensor has two wires for the heating element, and one wire for the signle.

The 4 wire sensor has 2 wires for the heatering element, one sire for the signle to the computer, and one wire for the ground for the signle.

Bosch also makes a "wide band" sensor, which is suppost to be much better, but it requires some more componets than just the sensor it's self.

For the heating element, all you need to do is wire it so it's on, when the ignition is on.

You can get a 3 wire sensor from autozone for 40 bucks. The 1 wire is avalible for $22 from autozone. The other local parts stores were similer.

 

Thanks, does anyone have part numbers for the 3 or 4 wire sensors?? Sounds like thats what im shooting for.

Will the sensor work while the car is driving or does it still have to be ideling to adjust air/fuel mixture.

 

I belive the 3 wire O2 sensor has a Bosch part number of 13913.

Enjoy.

 

If you go with a 3-wire, which I would since the 4-wire sensors are expensive (the latest technology, you see), make sure your exhaust system is grounded well. More precisely, make sure the groud for the measuring device (multimeter, air ratio meter, etc) uses the same ground as the O2 sensor - the exhaust system.

You're going to have to build a wiring harness for the O2 sensor and measuring device. You can power it however you want - having it come on with the ignition is probably a good idea.

You can measure your CO at idle and in all driving conditions with the O2 sensor. Obviously you can adjust the idle mixture at idle. Mixture modifications under load/while driving are trickier on a CIS car. You're best off making sure there are no vacuum leaks and just letting the CIS system do its thing.

Bryan

 

Thanks again Bryan for your help. I will get and install the three wire sensor soon. and will install it per your last message. (grounding to the exhaust.) that I should be able to do relatively easy.

I will post again after I finish the install.

Curt

 

Curt,

FYI, my '85 was doing the same thing. Turned out to be a bad computer (not helpful in your case).

Is it possible that you have a major vacuum leak?

I'm just guessing that your one wire sensor is working fine. It works fine at idle, it seems, so I would imagine it would work better off idle. More hot gas is getting to it to keep it warm.

good luck, keep us informed.

 

I have suspected a vacuum leak but have searched for it and cant seem to find anything. When the air box is on everything is pretty quite and the engine runs smooth. So if I have a vacuum leak it must have affect off idle. I will be working on the car again tonight to check timing belt tension so I will again search for any vacuum leaks. I will also pull a couple plugs to see if it is indeed running lean. Also will check some fuel pressures.

 

A couple of ways to check for a vacuum leak is to spray starter fluid around suspect areas and see if the idle goes up suddenly.

Another way is to blow air into the intake, like 5 to 10 psi. The crank has to be rotated until you don't hear any air leaking through valves. Use a stethescope to listen around.

 

Starter fluid is a BAD idea!Its way to flammable to be spraying around a running engine.You can try spray carb clean but even that has gone up in flames a few times in my life when used on a running engine.Last time I used it I blew the dist. cap off of a V 12 jag engine!

 

Thats how I check the plug wires, squirt a little starter fluid around! JK!

Just have to be reserved about it

 

Last night I started replacing vacuum lines that I thought were a problem. Well I ended up pulling off the entire intake to replace all the lines. Just about every hose was hard a brittle. Dont know if one was leaking more than the others but Im sure all of them combined make up the problems that I have been having. For now I left of the decel valve unit and didnt hook up the auxillary air valve with its hose routing to the cold start valve. All those hoses need to be replaced because they pretty much broke apart when removing. So I need to get come custom bent hoses and redo those lines. but for now I installed the rest of it and put it back together. Besides having to warm-up the car a little bit due to the cold start valve in the in-op mode the car drives great. Very responsive and no hesitation. I believe its running a bit rich because now its not sucking vacuum from other sources. I didnt have time to re-test the mixture last night but I will get to it this weekend sometime.

For all those who are a bit nervous about removing the intake dont be nervous. I had no problems at all removing it to get to all the vacuum lines and hoses. I only spent about 3 hours from start to finish, however, I didnt spend much time cleaning up things because I am gonna have to pull it all back apart in a month or so to do cam cover gaskets and I want to remove the fuel distributor unit and do a really good job of cleaning the valley and the entire top of the motor. When I do all that stuff I plan on sending out the intake to have it powder coated. and if its gonna be torn down that far I will probably put a new timing belt and water pump in it. so that will be my next major repair coming in the next few months.

Im pretty confident that my vacuum problems will be fixed by the end of this weekend.