Air Fuel Ratio

[Jonny B]

Jim Villers was kind enough to lend me his air/fuel meter. I put in use today. Here are the results so I am asking for some help on interpretation and potential next steps.

After warm up, at speed, the meter is bouncing between 15.2 to 16.2 %. At idle it is running at 21-22%. At about 35-40 mph the ratio drops a little to 14.7 +/-. I did stop and dial in two more clicks at idle on the enrichment crew (clockwise as you look at the back of the screw). The car appears to be running reasonably, still some shake and the gas mileage coming back from Blacklick was around 15. We are planning to get the injectors checked, and I know I need to get the valves adjusted.

Are these readings in the right ball park???

[mdsalemi]

Jonny,

JA did the advanced version of the Blacklick air/fuel mixture (CO) test on my car after you left.

It works on a simple principle: absent external forces/controls, air and fuel will want to burn at the "ideal" or (remember this word from chemistry) stoichiometric ration.  All the linkages and adjustments provide these external forces and controls.

Warm the car up.
Separate the linkage above the FI pump.
Open the butterfly to add more air; if the RPM goes up, you are too rich.
Open the throttle to add more fuel; if the RPM goes up, you are too lean.
Adjust the idle richness with that screw you talk about.

DO THE SAME TEST at midrange (1800-2000 RPM) and at high range (3500).  Do it by holding the air and fuel linkages together as you press them down.

On my car, the idle was perfect mix.  Midrange and High range: too lean.

Note: On nearly every car I've seen this test done, something isn't right.  I suspect many cars have out of kilter fuel air ratios.

It's on my list to get in the pump and richen up the entire pump, and then lean it back at ldle.

Worth a check on yours.

[bpossel]

Jonny,

Your c0 values seem high???
I thought the values should be more in the 5% (or less) range, give or take...
See attached .pdf

Bob

[JimVillers]

The gauge that I loaned Jon measures air/fuel ratio.  Normal "power" occurs in he 13-14:1 air to fuel.  Idle and cruising can be leaner; 15:1 or greater.  You should not be lean under power (greater tan 15:1) as you will have a tendance to burn valves or aluminum.

Jon ... The real value is to focus on flat spots while driving and to determine if it is caused by too much or too little fuel.  then make adjustments within that range.

[mdsalemi]

Because there can be a variety of circumstances that alter the "numeric" state of perfection, Joe's test is ideal.  If you strive for some set of numbers you may find these numbers don't work ideally for your car, your engine, your circumstances.  While this isn't scientific, I can tell you that someone "tuned" my air fuel (so they say...) mixture with a CO meter and that gave me an overall lean condition.

There is an overall rack adjustment in the FI pump for rich/lean across the entire RPM/load range; then there are several screws for ranges (high/medium); and the idle adjustment which is external.

I suspect the best thing to do is adjust your car for the ideal, best running condition for your engine and your circumstances, and whatever the numbers are, they are...

Bob Possel: those numbers Jonny quoted are not CO %, but air/fuel ratios, I believe.  That's why they sound very high.  In a perfect world under perfect conditions, the ideal air/fuel ratio burns all the fuel and nearly all the oxygen and leaves a CO in the % that you are expecting.  See the links below for some interesting information.  Many race cars and motorcycles have these gauges built in as dash gauges.

Some info:

http://www.autofamille.com/stoichiometric-af-ratio-meter
http://www.rbracing-rsr.com/downloads/RSR_O2_install.pdf
http://en.wikipedia.org/wiki/Air-fuel_ratio_meter

[JimVillers]

Michael ..... Your references are for narrow band sensors that are non-linear co2 sensors.  The gauge that I loaned Jon uses a wide band sensor that accurately measures unburned fuel in addition to unburned oxygen.  It accurately measures the air/fuel ratio from about 10.0:1 (very rich) to 20:1 (very lean). 

I have found that using a wide band gauge for tuning greatly improves the process ofer the seat of the pants method.  Using accurate measurements you can both reproduce the setting and also allow you to set it where you want it.  You always need to verify tuning by driving.  The purpose of tuning is a better driving car and the wide-band gauge is an excellent new tool.

I have a friend that swears that tuning the ignition "by ear" is better; I prefer a timing light.     

[glenn]

Brings to mind the old country saying  'Are you going to believe me, or your lying ears.'

[mdsalemi]

Brings to mind the old country saying  'Are you going to believe me, or your lying ears.'

Hey, that's a good one.  What country?

Hey, if you are running rich at 10:1, or lean at 20:1, my suspicion--just a wild hunch here--is that you don't need a [expensive and sophisticated] gauge to tell you.  Just a guess here, though.  The narrow band is a tool of the racing trade, working with installed o2 sensors; the wide band a diagnostic tool of high performance tuning.

If your car is good enough all around, with no vacuum or exhaust leaks, all electricals 100%, known and good engine condition, known and good fuel quality and all the other parameters that affect performance known and controlled, (and in our cars that includes things like performance of the FI system and injectors) and you want to tune your car for peak performance, well this is one tool to do it.  (This is the reason why you see these devices used as precision tuning devices for high performance engines and racing--not a general tool of diagnostics.  When you have a well running engine and car, they will help you tune it for peak performance.)

I'll let you know when I'm at that point.

[jeffc280sl]

Just received my Innovate LM-2 air fuel ratio tuner.  Need to buy another cable for rpm input and an sd card to record data.  So my readings so far are at approx rpms.

warm idle              22
1300 to 2000 rpm   12
above 2000           14.7

I have experienced sputtering when accelerating in the low rpm range.  Once I'm up over 2000 or 2500 the car runs great.  This data indicates that I have a lean condition at idle followed by a rich condition in the lower rpm range.

Once I get the cable issue sorted I should be able to drive the car and record air fuel ratio and rpm data and then display/graph the information on my laptop with the included software.  With some more data I can start working of fine tuning the air fuel ratios.

I've wanted a tool like this to help understand the barometric cell shims.  My car seems to have thicker shimming then others.  I have not touched/adjusted that stuff in the 15 years I have owned the SL.

[jeffc280sl]

FYI Found a conversion table for co and afr.  Here is a sample

   co           afr
  3.5          13.20
  4.0          13.00
  4.5          12.83

[JimVillers]

Jeff .... Do not worry about the A/F at idle or just reving the engine.  Watch the numbers as you drive the car, specifically when it stumbles or there is a flat spot.  The gauge will tell you if is too much or too little fuel.  My experience is that there was too much fuel and leaning made the car run better.  Every car is different so use the instrumentation to help you solve your issues.    My target was 13:1 to 14:1 during  both light and heavy acceleration.   While crusing at a steady speed, the A/F can go higher.

[jeffc280sl]

Thanks Jim,

Just got back from a 20 minute drive.  I had the lengthen the o2 sensor cable.  I ordered the rpm input cable and  sd card yesterday.   Soon I will be able to plot rpm and af at the same time.   This will help show the car under load as I accelerate going through the gears on my manual transmission.  I was able to get a pretty consistent af ratio of 19 crusing on a level surface at 2500 rpm and 60 mph.  When hard accelerating from 3000 to 5000 rpms the af is in the 15 range and the engine feels very strong.   I think this will be a usefull tool.  In addition to the stumbling my car idle varies from a warm to hot engine.  Sometimes it will be 1100 and other times 700.  I've done everything there is to be done with the linkage and I'm comfortable with that not being the problem.  The WRD works.  The tts and temp switches are new and working.  I have the 123 ignition system, new plugs, newly adjusted valves, no vacuum leaks, new fuel filter, good fuel pump.  I've started to look at the barometric compensator and now that I can take performance data I may run some tests with different shims.  Previous owner had 4+ mm shims. I don't know if thats right or wrong.   I haven't touched that or the wrd shims.  Should be able to record some data next week.  I'll post what I get.

[jeffc280sl]

Jim,

Do you know where I can find factory lambda or co data for our cars fuel injection system?  I know I've seen it somewhere.

Thanks

[JimVillers]

Jeff .... I have attached a graph from my Weber technical book on setting up Weber carburetors.  The injection system should be similar.

It sounds like your injection pump is too lean.  Full throttle should be about 13, cruising normally is about 15 or so.

[jeffc280sl]

Thanks Jim.  I agree the fip is too lean.  Going to record some as is data for a baseline and then do some tuning.

Jeff

[jeffc280sl]

Taken delivery of an air fuel ration tester, model LM-2, from Innovate Motor Sport.  I installed a manifold air pressure and throttle position sensor so that I can display/record real time data on my lap top.  The map sensor is hooked up to a the rear of my manifold that previously had a plug in it.  The tps is installed where the emissions on/off switch was located on the throttle body.  No permanant changes were made and all can be easily removed after testing.  The idea is to duplicate the load testing specified by MB in the BBB.  These test can be done on a dynomometer or on the road.  Tests require specific rpm ranges and map pressure or engine loads.  Preliminary testing indicates that my car is very lean everywhere but at idle.  Seems prior owner mechanic had some idea about this so shims were added to the baro compensator.  In order to adjust at idle my idle knob was 32 clicks from full rich.  This is 4 clicks from being out of the leaning range.  So I'm near the limit on trying to enrich the mixture by shimming the baro.

After looking at fuel flow rates after the filter, ignition timing, wrd, csv and baro I'm ready to start making changes inside the fip.  Floowing that I will record data on a test run and then decide where to make additional changes.

[erickmarciano]

purchased a zeitronix a/f meter . with this meter you do not need the RPM cable it is included and works really well . did you put a hole for the 02 or use a clamp. by the way at idle you can not get a good reading with the clamp on the tip of the exhaust .
Thanks

[jeffc280sl]

I use a clamp in the exhaust pipe to hold the o2 sensor.  As far as I can tell it is accurate at idle.  The o2 sensor has a preheating element that requires high current (cigar lighter) to get up to temp.  It will not provide data until it is up to temp.

[erickmarciano]

tuners will not use a clamp to take idle readings since it is to close the the end of the pipe and ambient air can affrect the reading .  was told this many time for my Porsche 930

Thanks

[jeffc280sl]

I heard that mentioned but I've not been able to find any comparison data.  Any idea how far off they say it is.  I took an 18 inch piece of exhaust pipe with a 1 5/8 inch od and welded a bung 6 inches from one end.  It fits tightly in the tail pipe and does not require a clamp to hold it in place.  It looks awful but I have 12 inches of exhaust pipe sticking out after the bung to minimize contamination.

[glenn]

What happened to lambda of 14.7 used on the Bosch CIS?

[jeffc280sl]

I've been tuning my fip for the past three weeks.  I've made atleast 30 test runs after adjustments in an attempt to understand what effect the adjustments have on afr so that I could tune my car to meet the MB road test specification.  The BBB sets the following exhaust emission specifications for my car.

Full load road test - third gear from 3K rpm up -              13.77 to 12.64 afr

Lower part load test - third gear-1500 rpm - 11.81 inHg-    14.54 to 14.11 afr

Upper part load test - third gear - 3000 rpn - 11.81 inHg-   14.72 to 14.11 afr

Tuning is challenging and tedious.  That's maybe an understatement.  It starts with having to remove the fuel shut off solenoid to better access the rack and white/black screw adjustments.  Some SLs have two solenoids on the back of there pump.  After taking it off and on a couple of times I realized fabricating a cover plate was in order.  Helps a lot.  The rack screw is pretty easy to get at and adjustments are straight forward.  The white and black screws are a pain to adjust  You need to remove the idle thumb screw cover plate and allow a considerable amount of oil to drain.  It goes all over the place and you will make a mess.  I used a small adjustable mirror mounted to an extendable pole to see the screws.  Now the you can see them but the image is upside down and backwards.  Adjusting them is tedious because of the need for good lighting and a steady hand on your screw driver.  Turning the screws requuires skill and you need to count clicks and turn the screws in the correct direction.  Its very easy to get mixed up.   So you need to be careful and keep a record of the changes.

I got to where I could change the rack in five minutes and the white and black screws in 20 minutes.  But I had lots of practise.  Any way it can be done and I improved the upper range afr to where it now meets specs.  It was very lean and as a result of the changes the throttle response is fantastic and the engine is much stronger.

I'll post some more information about my experience over the next week or so.

[jeffc280sl]

Here is the setup I used to see manifold air pressure and rpms so that I could run the partial and full load tests.  Regulating rpms at a specific load is easier if you can find the perfect hill.  If you can't the parking brake can provide an additional load.

[jeffc280sl]

Just looked at some of the data charts of my afr testing.

Here is a comparison of the average afr from the original fip settings and todays "tuned" settings.

         rpm   1000   2000   3000    4000

Nov 6          14.4   16.4    18.6    17.1

Nov 25         13.5   13.7    14.3    14.9

The comparison data is taken at a map of 12.5 inHg which represents a lighter load than the MB spec of 11.81 inHg.  I just thought it was a good way to explain the starting condition of my fip.  Way too lean!

[jeffc280sl]

For those interested in tuning their fip I suggest that you purchase a portable wide band air fuel ratio meter with data logging capability.  In addition to the meter you will need to weld a bung into your exhaust system for the modern O2 sensor.  An alternative to welding a bung in your exhaust pipe is to buy an 18 inch piece of 1 5/8 in od exhaust pipe and weld a bung about 5 inches from one end.  This can be slipped inside one of the rear exhaust tips on the back of your car. 

Make sure to buy an afr meter with the capability to accept analog inputs for recording rpms and manifold air pressure (map).  The rpm pickup can come from the negative side of the coil.  You will need to buy a modern map sensor to measure air pressure in the intake manifold.  In addition to power the map sensor needs a vacuum tap in the intake manifold.  I had a plug in the rear of my intake manifold which I used to mount a fitting for the vacuum hose.

After you have the afr meter inputs available you can begin to monitor the afr of your engine.   As mentioned earlier under this topic MB has a very 4 specific CO specifications.  Two of the specifications require you to place a load on the engine during the test.  This load requirement can be read by the map sensor.  These tests require you to establish a specific load and rpm readings.  Once these measurments are achieved you can read the afr and compare that to the MB specification.  The idle and full load test do not require map sensor inputs.

With this data recorded you can compare you actual fip performance against the MB spec and tune your pump if needed.