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A few years back I read about 02 sensors and how you can read them using a Multi-meter. Does anyone know how to do this? I don't want to check the sensor, I want to use the meter instead of a gauge. Joe

You can monitor the O2 sensor by connecting the (+) lead of the voltmeter to the signal wire by backprobing the wire and the (-) lead to ground. There might be another wire if it is a heated O2 sensor which would read at 5 volts.

After letting the engine warm up a couple of minutes readings should be between 0.1 and 0.9 volts.

A reading less than 0.45 volts is a lean condition.
A reading greater than 0.45 volts is a rich condition.

This will let you see if the engine is running rich, lean or just right (going from rich to lean, to rich quickly).

Hope this helps.
Adam

Thanks for the replies, I should have went into more detail. What I want to do is install a 02 sensor in the pipe of my 250. I am still have a vibration/miss/backfire problem at higher rpms. I believe it a lean condition, at low speed it run great, very clean, and responsive. I have a hand heald o2 sniffer that reads from the tail pipe, but I don't believe its acurate when driving due to the way it sniffs. Any air blowing over it changes the way it reads. I have check the aftermarket gauges and they are way to costly for my playing around.

I could just jet the carbs up and see, but that takes the fun out of tuning and checking!

If I did install a sensor, would I have to power it up or do they create there own voltage? And if I read the posts right .1 to .9 volts with .1 on the lean side .9 on the rich side, then .45 would be about right?

Joe

The Oxygen Sensor does NOT require any power from an external source (ex: computer, etc.) unless talking about a fuel injected engine that is controlled by a computer. In that case there might be a 3rd wire that comes from the computer that acts only to heat the O2 sensor up when first starting the engine. I don't think you need to worry about this though.

In your case with a carb. I assume your sensor only has 1 wire which is where the signal will come from when you hook the (+) lead to a voltmeter (ground is obtained through the threads, just like a spark plug. If the 02 sensor has 2 wires, then there might be a ground wire along with the signal wire, but still hook up the (+) lead to the wire that is the signal wire and ground to a good ground.

The oxygen sensor works by a chemical reaction due to the amount of oxygen that is around the sensor. In this way, it acts like a small battery that creats it's own voltage. So a normal 1 or 2 wire O2 sensor needs no external power source because it makes it's own power.

0.45 volts should in theory be the reading that you are looking for using a carb., right in the middle between rich and lean (you might want it slightly richer but something close to .45 or .5 volts). You could then drive the truck around and have a helper look at the voltmeter and see if it goes lean or rich during different driving conditions. With a little experimenting you should be able to tune the engine well. Remember with a carb., there is always a fraction of a second that the engine will be lean due to the time it takes for the accelerator pump in the carb. to send fuel through the intake.

If this were a computer controlled engine, the O2 sensor sends a voltage to the computer and the computer then tells the fuel injectors to either squirt more or less fuel into the cylinders. If the sensor is working correctly on this type of engine the reading will be constantly changing (at least a new reading once per second). The computer is able to recieve and send data quickly enough, so the engine will run rich for 1 second and then lean the next second and then back to rich the next second.

Hope this answers your questions. Sounds like a neat way to tune up an engine. Hope it works.

Adam

One thing I should have mentioned is that it does take a couple of minutes for the 02 sensor to heat up before you get a proper voltage from your voltmeter, so you will have to let the engine warm up a couple of minutes.

You can experiment with the 02 sensor if you clamp the sensor in a vise and hook up the voltmeter to the signal wire and ground on the vise. You can then use a Propane torch and heat the part of the 02 sensor that sticks inside the exhaust until it is bright red. Once it is bright red then wave the torch over the 02 sensor and see what the voltmeter is reading to get a visual idea of what is going on. You can then relate what you see to what is going on inside an engine and maybe get a better picture of what is going on.

Just don't burn any wires or hands.

Adam

This sounds like a cool way to verify a tune up. If I were going to permanently mount this on my truck where would be a good place for the sensor? On the exhaust manifold? Further back in the tailpipe? Does the distance from the engine affect the accuracy of the sensor?

autozone sells a temperature\oil type mixture guage that uses an oxygen senser.like a dash guage.

I think the best place to mount the 02 sensor would be on the exhaust manifold or else on the downtube just below the exhaust manifold. It should be close to the engine, because heat is required to warm the sensor up, but you need it in a place where exhaust gasses will all go by it so you get a good average from all cylinders.

When installing the 02 sensor be sure to put a little anti-seiz on the threads or else it will be very hard to take out.

Adam

Thanks Adam for the information. It should help and give me an idea if I close. Now its off to find a cheap sensor...... thanks again, Joe

Places like Summit sell sensors, bungs to weld in your exhaust pipe to screw it into, & dash gauges for a readout calibrated for their output. Also, some sensers are 'wide band' type & enable you to see more closely what the mixture is. Most of the tuning will be done within about a 0.1 Volt range so this is helpful. Do a 'net search, there's a lot of info available. Doug

Roger that on the 450-500mv being normal, but that is for criuse - part throttle conditions. Under load, the O2 readings should be up around the 750-850mv range. The EFI folks refer to this as PE or Power Enrichment mode. You need the richened mixture under load to keep exh-temps reasonable, as well as to make good power. To carb folks that's main jet territory. Since you'll probably be using the one-wire (non-heated) sensor, get it as close to the heat as possible. It needs to be hot to work. More than two feet from the exhaust manifold and it will be marginal IMO. If you can't, there is a more expensive 4-wire heated sensor available from GM. The two white wires are for the 12v heater circuit amd the others are signal and signal ground. Application for the heated sensor is 91-92 GMC Syclone & Typhoon Vin "Z" Turbo 4.3 Trucks. There was a NON-TURBO VIN "Z" 4.3 engine that used a single wire sensor, so inspect it when they bring it out. GM & Napa have them. Purists will tell you that the costlier aftermarket wide-band systems are needed, but I disagree. Narrow band will get you in the ball park. mv

Thanks Mike, more good information. Since my engine is a 4.1 (250) does the sensor need to be from the same range of engine, 4.3?? Are they calibrated to the cubic inches? Joe

No, engine diplacemment is not an the issue. If you could keep it hot, a sensor from a V8 could be used on a Briggs mower engine. A Bosch #11027 from AutoZone or Napa. They are around $18. They fit a wide range of cars like GM pickups, Dodge pickups, 87 Honda Accord, etc.
MV

Mike V,when an engine with a traditional down draft carb runs fine during normal driving,but a lean condition is suspected during wide open throttle ,it's the power valve or high speed enrichment circuit that needs attention,not the main jet.

sorry if that was mis-understood. I was referring to using the O2 sensor as a tuning tool. In general, if you go to a larger main jet, you will see higher numbers on the O2 sensor under significant load. I'm not claiming to be an expert on carbs, but I was under the impression that the power valve determines *when* the transtion occurs from low speed to power enrichment, and the main jet determines *how rich* things will be when it occurs. Please tell me if I missed on this. I only pretend to be sure about stuff around my wife <grin>, but she sees right through it. Mike V

This will be boring to some, but interesting to the information-junkies, I've posted a TXT file that contains data collected from a GM computer in a 1/4 mile dragstrip run. It was a high 12 second run with 20PSI of boost. Things to look at and compare are; RPM Throttle position and OXYGN-VOLTS. This is from a turbocharged 4.3 V6 GMC Truck, so the O2 redings are a little on the high side under load to keep from melting the cast pistons. Anyway, it was gathered at approx 4 frames per second, so you can get an idea of how quickly the O2 sensor reacts to mixture changes. Four frames per second isn't fast enough to cover *everything* the O2 sensor is doing, but it definitely gives you an idea of what's happening. O2(OXGN) volts ( is in the 11th or 12th collumn.
See: http://www.enzoco.com/mike/1282.txt
MV

Well I didn't go to the 02 sensor, but will later. I hooked up the tail pipe sensor and covered it with a wind block so driving it wouldn't mess with the results. Its not as acurate as a 02 sensor but did what I needed. Its was way to lean, the three carburators were in the 14.5 to 14.7 AF ratio range at highway speed, they went higher under accelleration. These model H Rochesters don't have any enrichment circuits, the only thing to do is rejet them. As it now, its using way to much fuel. The whole unit is to small for the cubic inch, and it takes to much throttle to keep it running.
I install the stock Mono-jet and intake and went for a drive with in a hour or so of the other intake to keep the same weather and engine temperatures. It was about right, 14.0 at cruize and drops to 13.0 at full throttle. Still on the lean side but much better.

Mike, the drag strip results are pretty cool. You can learn alot from that, I wish I had that kind of inforamtion when I was racing.

Thanks again, Joe.

14.7:1 is right where you want to be at cruise, and richer in higher load situations

Its pretty interesting watching the gauge, you really tell if its running right or not. My three carbs were to lean under a load like pulling a hill or hotrodding. With out a inrichment circuit, they just kept getting leaner the harder I mashed down. The Mono-jet does just what it should, goes on the lean side at cruise and drops below 13 to 1 when needed. Now I just need to replace the Rochester H's with something that actually works! Joe