Smog technician says my car is running lean - almost failed CO%
 

Max is 1.36, My car measured at 1.34 which he said means it is running lean. Is this true? He said I can adjust mixture via 3mm Allen near the airbox. If I adjust it, can I tell when it's no longer running lean by just my butt meter? My car won't need another smog for two years but he said I should adjust it because when 911s run lean, they run like crap.

CO is a sign of rich combustion in at least parts of the chamber. When there is excess oxygen then the CO goes to CO2.

There is a specification in the spec book for your car that tells you where the CO should be. You can't set it properly without a gas analyzer, so take it to a shop and have it checked. Change the oil, while you're there.

The guy at the smog testing station doesn't know a lot, so ignore what he said.

JR

Were they still using CO as a means to set mixture on the SC's? Modern cars produce undetectable levels of CO because of the catalytic converters. So you need an O2 sensor to detect the excess oxygen.

"Lean"? It's running significantly rich .................. What are the other numbers? Generally speaking, changing the oil won't really affect the CO, but it can make a difference in the HCs.

The Cap'n

Like others have said - High CO% means you are running rich. Adjusting the A/F mixture is quite easy with the right tools. A few months ago I set up on my '79 using the Innovate LM-2 digital A/F meter and passed the smog test easily.

Without an analyzer, check this thread for some ideas:

http://forums.pelicanparts.com/porsche-911-technical-forum/232089-cis-idle-speed-mixture-setting-without-analyzer.html

Yes, even in the later, early Motronic era. You check the CO before the cat...

If I recall, the '78/'79 SC had a CO spec of 1.5-3.5, so it's lean, not rich. That's assuming the guy read it correctly, which I highly doubt. So, he needs to go to a good shop that knows what they are doing, to have it checked and set.

The guy used the wrong spec, so you have to start there. High is only high, if the right spec is used for comparison.



JR

911 CIS Primer - Testing: Mixtures

I've provided a link to a website that provides lots of good CIS system info, including setting the mixture with & without an A/F analyzer.

regards,
al

The rest of the numbers are great. It's just the CO that's on the hairy edge.

http://forums.pelicanparts.com/uploa...1404402999.jpg

Reading that, my guess is that the CO measurement was taken after the cat, which is absolutely meaningless, as far as determining whether the car is rich or lean. The "max" CO value is probably a California regulation that has nothing to do with what a 911SC was supposed to do when new.

Be glad you passed. Worry about it again in two years. If you have any questions about whether your CO is set right, take it to a shop and have it measured. Don't tinker with it in your driveway, it's not worth your time and you'll probably just make it worse. These adjustments are pretty sensitive.

JR

Thanks for posting the printout. I see you're in an enhanced area. While borderline high at the lower RPM test, the CO is fine at cruise, indicating the catalyst isn't working all that well. As it warmed up and the flow got higher, it lit off and did its job. I would have the CO checked before the cat (if the cap can even be removed!), and maybe lean it out just a little. It may be just fine, and the problem lies with cat performance. Note that the HCs drop by nearly half as the RPM increases. Next time you're due for a test, you may find the cat is gonna need to be replaced. Might wanna start saving up for it ..............

The Cap'n

This isn't a substitute for looking about the numbers in a spec book for you car, but it will give you some idea:

http://forums.pelicanparts.com/uploa...1404405953.jpg

Or just make sure it's good and hot when it is tested...

JR

Great advice everyone. Going to save up for a cat, not mess with my settings and not worry until 2016!

Thanks!

Your car used a fairly simple, 2-way cat, not the more advanced cat that was used starting in 1980. Not sure what is available on the market for a car like yours... Just be aware of the difference and the fact that your CIS system doesn't have an O2 sensor and wasn't designed to keep the exhuast gases in the range suitable for running through the later style of cat.

JR

or borrow one for a day - could be a Pelican nearby has one out of the car to "give it a break" :D and you can borrow it.

I like the way you think!

From an excellent SMOG tech, forgot what site it was on so can't give the proper credit.

When I say high, I will usually mean towards the upper limit or over the allowed amount
When I say low, I will usually mean within 5%-10% of the allowed amount to zero
When I say mid, I will usually mean 40-60% of the allowed amount
When I say high O2 I usually mean .3ish to 1% and up
CO is burned fuel
HC is unburned fuel
NO is a byproduct created by high combustion chamber temps. Creates the brown air associated with smog
All the numbers can usually pivot on CO
For the below examples we will assume the CAT is operating properly

High CO means the car is running rich. If the CO is high, usually you will have high HC also as there is too much fuel to be burned by the spark.

Low CO means the car is running lean. Low CO will also cause high HC as the fuel atoms are too far apart. When the spark happens the fuel burns as a chain reaction. If the atoms are too far apart they will not all catch and will not all burn, leading in raw fuel again going out the tail pipe. You can think if HC and CO as a giant X CO has to be close to the proper volumetric efficiency of the engine to burn as much of the fuel as possible, thereby creating the least amount of HC. As CO crossed the center point going up and becoming richer HC will also increase, same for going the other way back down

High HC alone can usually be caused by the ignition system be worn, plugs wires cap rotor coil, etc.

NOx is created by high combustion chamber temps. If the CO is high, the extra fuel usually cools the combustion chamber and therefore lowers NOx.

If CO is low which indicates lean, NOx will usually be high as the lean mixture heats the combustion chamber.

Ignition timing on all cars except LH 2.4 You can technically 2 deg + and - to work with legally. If you’re close and trying to just squeak by you can retard to timing 2 degrees. This will lower mainly HC and NOx and a little CO, advancing goes the other way.

High CO, HC, mid to high NOX check the O2, vacuum leaks, ignition timing and CAT

Low CO, mid - high HC and high NOX, check the O2, vacuum leaks and the MAF. If you repair something and CO comes up and it still fails, check the CAT

If you only have high NOx, check the EGR and ignition timing, and also check the O2. It’s possible the NOx reduction bed in the CAT has failed, but very unlikely just the NOx bed.

I’ve found not all smog reports have O2 listed on them. Also I’ve noticed not all reports have percentages or PPM listed. This is unfortunate because it makes diagnosis a little easier, hopefully some relation between my examples will help. My examples will all be in percentage and PPM as it is what I am most familiar with. This is important because it can help you determine the condition of the CAT and if you truly have a fuel mixture problem or some kind of outside influence. Low CO readings can be considered good, when in reality they are too low and causing other problems such as high NOx or high HC.

We will again assume the car has a good CAT and no air injection system.

Low CO, high O2 and sometimes mid-high HC and high NOx can indicate a lean mixture.

Normal CO, HC and high NOx, no or very little O2 can indicate a failed EGR system if the car is equipped

There are many more details but since this is a Volvo forum, I’m going to stick to common failures and how to diagnose them. This will be general advise that can probably be used on 200/700/900 series cars mostly equipped with LH 2.2 or 2.4 systems. Some of the advice can also be carried over to K-jet systems.

The common items found on 2/7/9 series cars are a CAT, O2 sensor, MAF, ECT; some cars have EGR, air injection or AIR and TAC (thermostatic air cleaner, that’s the heat door in the air cleaner).

The point of the O2 sensor is to regulate the fuel mixture, but mainly to keep the CAT happy, keep the CAT happy by not dumping too much raw fuel or too much O2 in it. This is why we have the switching O2 sensor, lean rich lean rich. Pump O2 in the CAT to basically charge it, and then dump fuel or harmful exhaust gas into it. The O2 is stored in the car to aid in the burning if harmful gases. Without the O2 the cat will not work to its best efficiency. If the CAT is worn and does not have the ability to store the O2 for its burning process, then you will have excessive O2 content in the exhaust. Also high O2 readings on the 5 gas can be falsely be created by large exhaust leaks up stream of the machines probe. As you think it’s just exhaust leaking out, it’s actually sucking air in at the exhaust leak which can introduce a false reading of O2.

The O2 sensor creates voltage. 0-1 volt. 0 being lean 1 being rich with .45-.5 volts being optimal and the center point of switching. The EFI system operates in a way so the O2 sensor will fluctuate EVENLY between 0 and 1 volt. A good sensor will move between 0-1v 4-8 ish times every second. The proper operation of this sensor is very important as if the computer does not get the proper feedback for what it’s doing, it will throw everything off as the sensor is its only eye.

As the sensor ages a few things happen. A heated sensor will usually have slow heater failure. A heated sensor (3-4 wire) should be at operating temp anywhere from 10-20 seconds for a brand new sensor. Operating temp is average 600 ish deg F. Aside from heater failure the sensor can become biased rich or lean and can stop working all together and sometimes just get stuck at one voltage. Commonly the sensor will become biased, this goes for heater and non heated (1 wire) by biased I mean the sensor will not fluctuate evenly. It may be biased lean and make voltage from 0-.7v. The fuel mixture may very well be over .7v but the sensor is incapable of creating voltage higher due to wear or age. Now, with a center point of .35 due to the biased sensor the computer will start adapting to make the fuel mixture richer to get .7 back up and make the middle point .5 again. This will create a falsely rich mixture.

The other direction a biased rich sensor (which is not as common) may switch from .25-.9v again; this shifts the center point and will force the ECU to create a falsely rich mixture. Also a sensor reading like this may be working properly but due to as unknown fuel source such as a faulty EVAP system can throw the sensor off. But please note, this is not a common scenario for our cars!

CONTINUED:

To properly test the sensor you need a time based oscilloscope. But very few of us have that so realistically here’s how to do it. You need a good DVOM with a decent refresh. 2 times per second is probably fast enough. You connect - to ground and + to the sensor. On a 1-wire that’s obvious. On the 3 wire found on later cars it will depend on the brand of sensor you have. But since out cars have a 2 wire connector for the heater and a separate 1 wire for the sensor, this is the one you want. Pull the black cover off and leave the sensor connected, use an alligator clip to connect your meter. The car must be at operating temperature and has to be running for up to a full minute at operating temp to be sure the computer went into closed loop and is using sensor feedback. You want to see the meter read voltage between .1 and .2 volts on the lean side and .8-.9 on the rich side. The sensor will switch faster at higher RPMS. Start at 2,000-3,000 and watch the meter, try to figure the low and high point as the meter is changing. Your goal is the see it pretty even between high and low. Like I said if its .1-.7 its biased lean and could make the mixture falsely rich. .25-.7 isn't great, but it’s not bad as its closer the center point.

You also want to check max and min voltage the sensor will produce. A few ways to do it: One put the meter in the window so you can see it from the driver’s seat. Rev the engine hard, ALL the way to WOT but DO NOT LET IT REV OUT! Only let it rev to 2,500 or 3,500 RPMS! and do it a bunch in a row, kind like your pumping the gas on a carbed car before starting, you want to force a rich mixture. Note how high the voltage goes. Another way is pull a vacuum line and spray crab cleaner slowly into the intake to create a rich mixture. But don't stall it. It should go to a high of .8v or .9 or the full 1v.

Then check the lean side, gun it to 3500-4500 RPM's and remove your foot from the accelerator quickly to slam shut the throttle. The injectors will turn off until its reached 1500 is RPMS. The voltage should drop to 0v or very close to it.

All of the above steps should help you determine how far off the sensor is or if it’s even your problem. If you failed by a lot then you’re looking for something pretty obvious in the readings. If you off by a little, then it may not be so obvious. REMEMBER A BAD COMPONENT CONTROLLING THE ECU CAN AFFECT THE O2 READINGS. If you have a vacuum leak, the sensor WILL be biased lean, or may not switch at idle, and may switch at 2,000 RPMS!

MAF diagnosis is not easy and expensive. If you have no vacuum leaks, the engine is timed properly, the O2 is good, CAT and EGR are good and all other components are working properly, I have seen a worn or dirty MAF cause a lean mixture and cause a NOX failure. Because the MAF is dirty it cannot properly meter the air going into the engine, and therefore throws the fuel mixture off, normally making it lean. You may not see excessive O2 in the exhaust, but you will see a low CO reading!

The CAT should be the last part on your list to replace. The CAT should be a 3 way cat which will take care of HC, CO and NOx. Assuming again you have no vacuum leaks, the engine is timed properly, the O2 is good, EGR are good and all other components are working properly, the CAT can be the culprit. For a failed cat you may see higher than average CO, HC and NOx. You may see mid-high CO normal NOx and mid HC and high O2. High O2 and high CO can indicate a CAT that can no longer store O2 and therefore cannot burn the HC and CO. This is an indicator of a failed CAT. Remember exhaust leaks can also cause false O2.

Air injection system can also cause high O2, but I am not familiar with the air injection systems on 900 series Volvos. I assume they are for warm-up only and should be shut off when warmed up; an air injection system pumping air into the exhaust manifold where it can throw off the sensor will create a falsely rich mixture because of flooding the O2 with oxygen.

ECT's rarely go bad and when they do they make a pretty drastic drivability issue.

The TAC should be closed when the engine is warmed. I’m not sure what this will affect directly then premature failure of the MAF just make sure it’s closed when the engine is warm.

Some of our cars have EGR. EGR LOWERS NOx ONLY!!! To check the EGR system get your vacuum pump and connect it to the EGR valve. Engine running, if you open the valve with the vacuum pump it should start to stumble and possible stall. This means the valve and passages are clean. If it doesn't stall or stumble and the valve holds vacuum, the passages are clogged and will need to be cleaned. If it doesn't hold vacuum, the valve is bad. The easiest way to test the entire system is too tee your vacuum gauge into the line to the valve and engine warmed up drive the car at a steady speed and steady throttle, make sure vacuum comes on and stays on till you remove your foot from the accelerator, or accelerate hard.

Learning how everything works and what causes what takes a lot or experience. I hope this will help you to understand a little more about how the components work and how to repair and adjust things to get the car fixed faster and cheaper. Again, much if this is general info and can be used on many cars. Your situation may differ; take this advice at your leisure