Excessive CO% in Exhaust Gases

[Nicolas Aristodemou]

I have a 1970 280SL auto (US emissions car, emissions system operational). I have an issue with the %CO which I cannot get to drop below 6% (slotted screw at rear of pump fully opened - turned anticlockwise).

Even worse when car is set (using professional exhaust car analyser from a vehicle test station, readings verified though a second instrument) at the 6% CO car stalls when gear is selected within 5 seconds.

In order to avoid this stall, I have to set the %CO even higher, at 7.5% where the car behaves perfectly, no stalls and smooth operation and acceleration throughout the revs range.

I have an 123 electronic ignition installed, properly timed, new NGKs BP5ES, new Bosch injectors. Throttle stop and all linkages adjusted as per "linkage tour" of the technical manual. Cold start valve(CSV) on intake manifold operates properly via time/thermo/switch. Warm Running Device (WRD) operates properly, its filter replaced and operation confirmed during warm up using a stethoscope; operation of the WRD on the FI pump rack was not checked however.

Smoke from the exhaust (blue, black or white) and fuel economy is about 14mpg (imperial gallon) in town (20 l/100km) and about 21mpg (13.5 l/100km) outside the city at about 70mph, which is not bad. These figures in USG are approximately 11.5mpg and 17.5mpg respectively.

Any ideas as to how to get my CO content down to the prescribed limits (around 3.5%) and avoid the car from stalling when gear is engaged? Maybe I would need to adjust the mixture screw inside the FI pump, behind the allen key screw that is effective for all rpm ranges? Even if I do this how would the car run properly at the normal setting (3.5%) if it refuses to run -stalls- even at 6%?

[Cees Klumper]

Others will chime in but perhaps your barometric compensator on the FI pump is faulty and making the engine run too rich at and above idle? You certainly checked most if not all other possible causes. Valve clearances / general condition of the engine?

[ja17]

Most often the engine will run lean when the barometric compensator fails, so I do not think that is your problem. You may be able to lean out the entire range of your fuel injection by adjusting the rack screw or removing a thin shim under the baro compensator. Try the shim removal first. Only do this if your injection system is running rich at all speed ranges after engine is warmed. It sounds like it is. You will be able to adjust the lower range with the thumb screw on the IP afterward. Also make sure your engine compression is good on all cylinders and all cylinders are firing with correct ignition timing. Stalling and bad fuel mixture readings can also be caused by many tuning problems. If your problems occure only on  a cold engine, then WRD adjustments may be required.

[badali]

Were these old cars set up with specific CO2 in mind?  My car runs perfectly fine and it appears that the settings were never readjusted from new since the car had low mileage when I bought it.  When I was at PUB many years ago someone was testing everyone's cars including mine and said mine needed adjusted.  It seems that adjusting the pump causes may problems if not done correctly.  I declined to let anyone mess with it. I say set it to where it runs best and forget about it.  I have not had engine running problems due to fuel injection pump settings.  Only points and plugs in the 14K miles I have put on the car.  My car has 69800 miles on it currently.

[ja17]

Some good wisdom from badali.

[Nicolas Aristodemou]

Thanks for the inputs provided. I will look at the barometric compensator, I must admit I didn't think about it

My valve clearances are ok, recently checked and adjusted and compression good on all cylinders, uniform at around 155psi.

The stalling is more frequent with a warm engine, when cold (presumably richer mixture) it never occurs. It is as if the car is telling me it wants an even richer mixture than the 7.5% CO already set at!

If I find nothing wrong with the barometric compensator I will take the advice and just leave things as they are since with the rich mixture the car runs really well and fuel economy is not that bad.

[Benz Dr.]

What about ignition timing? I think your car would have a vacuum controlled governor and maybe it's not set up correctly. I seem to remember that the throttle valve is closed while at idle and when you put it in drive the vacuum governor opens only the throttle plate. This allows more air to enter and if you don't have a rich enough mixture it will stall.

Not my favorite system to tune. Too many working parts that all have to work or the whole system becomes a mill stone.

[Nicolas Aristodemou]

Yes my car has the vacuum controlled governor as well as a constant speed solenoid currently wired to the AC compressor so it extends when the AC is operational keeping rpm at 700. Currently no AC is used (winter time) so this part is out of the equation.

I think that my throttle vacuum controlled governor is adjusted properly as per instructions in the technical manual.

I have set the mixture even richer, probably around 8%CO - I haven't measured it again but I closed the thumb screw by another 2 clicks from where I had earlier measured 7.5%CO - car runs fine, no stalls.

It is just that I am bothered that I have a higher CO% than specified, but I suppose I would have to live with it as long as the car works fine and needs this richer mixture to operate properly.

Fuel economy is also good I think. There are also unburned hydrocarbons (I get a reading of around 1100) but I suppose it is the effect of a richer mixture.

[Benz Dr.]

What are you using for spark plugs and plug wires?

[Nicolas Aristodemou]

Dr Benz I am using BP5ES plugs and a cable set sourced from SL shop in UK,  OEM number 1501301318 which I believe has silicon leads coupled to 5k plugs.

My car  has an 123 ignition fitted with a matching coil also supplied by 123 together with their electronic ignition. Timing is good, car has very good performance.

I will use my trusted colortune plug this weekend to see the colour of the combustion flame, although I have no doubt that the exhaust gas analyzer from a vehicle testing centre is telling the truth....

[Nicolas Aristodemou]

Today I installed my colourtune plug into cylinder no 5. Combustion colour was more blue with some spots of orange but overall blue I would say. That was inspite of a CO% reading of 7.5

Furthermore, I  went for a long highway trip 105 miles) at a constant 70mph. I filled the tank to the brim before and after the trip. I got 26mpg (imp) or 21.5mpg (usg) which i think is pretty good.

My only concern now  is the occasional stalling at idle with gear selected. i suspect that my vacuum operated pot under the throttle, although adjusted properly (,no clearance at idle in neutral) might not be functioning 100%. When selecting gear in my auto box  my idling speed drops to around 600rpm from 800rpm that the car idles in neutral. Is this normal or does it suggest a defective vacuum pot? when I test this pot manually by applying vacuum to it it does move and can compress its spring in this fashion..... I am puzzled...... I dont want to further enrich the idle mixture to eliminate this stall, although it could be the only way out....

[ja17]

Does your car have the "slip joint" linkage rod going to the venture?

[Benz Dr.]

Does your car have the "slip joint" linkage rod going to the venture?

That's what I was wondering. You have to compensate for extra air when the throttle plate opens.

[ja17]

Yes, the slip joint should be in the compressed condition until engine vacuum drops, then the dashpot moves only the venturi flap open a bit. As Dan mentioned, you must compensate for the additional air so that the engine does not falter.

[Nicolas Aristodemou]

Yes the car has the slip joint between the throttle and the cross shaft. it is correctly adjusted as well, fully compressed with engine warn and car in neutral. I understand the purpose of this slip joint is to provide a small "slack"  movement 3-4mm of one shaft end before the shaft starts to transmit the movement via the cross shaft to the IP. Hence the vacuum pot when energized and open the throttle only admits extra air without fuel.

I have also noticed that the car never stalls when either forward or reverse gear is engaged; it does so only occasionally during deceleration to a stop. It seems that when coming to a stop 4 to 3rd gear change is occasionally delayed (especially when coming to a stop from a higher speed -never in start stop traffic- and the car slows to less than 10mph in top gear (4th) leading to an engine stall since corresponding engine speed in top gear at 10mpg would be 500rpm or less.

 Which in turn makes me think  that is a transmission issue, although I am very happy in general with both my upshifts and downshifts, except this occasional stalling issue....I think I will increase the gearbox  modulator pressure a little bit..... maybe half a turn...... maybe this would help towards solving my problem

[ja17]

Make sure the distributor centrifical mech. and the vacuum mech. in the distributor are not sticking.

[Nicolas Aristodemou]

Thanks JA but the car has 123 electronic ignition. Vacuum operates as it should. I have the US emissions control system with the vacuum valve switching vacuum on and off as required at about 2200rpm

I hope to try today to increase the transmission modulator pressure. Somehow, I feel that with the throttle closed (throttle switch energized) the 10psi transmission  pressure is either not enough or not quite so high to make the downshift from 4th to 3rd gear..... I do not have the banjo fitting with the pressure gauge to check it  so I will increase pressure s little bit and see if things improve.

[stickandrudderman]

HC of 1100 is too high and I'm surprised if your fuel consumption is good with a reading like that.
I would be looking to get that down before concentrating on Co.
I'd start with checking spray pattern of injectors.

[Nicolas Aristodemou]

My injectors are brand new Bosch units installed less than a month ago. You see a high CO and unburned hydrocarbons suggest a rich mixture at idle, as set by the knurled screw at the back of the FI pump. I had to keep this idle mixture setting rich to avoid the stalling.

When cruising in the highway the mixture is not rich since it is set by other settings within the pump. Apparently these mixture settings (medium and high revs) are good and that is why I get good fuel economy in the highway.

In any case, I think that I have finally found the solution this morning by adjusting (increasing) the transmission pressure (screw turned clockwise by almost a full turn). The apparently slightly higher pressure makes sure that 4th to 3rd downshift, as well as 3rd to 2nd have enough pressure  (push) to occur before the engine stalls.

Now all is well. I will slowly -  slowly try to lean the idle mixture now, one click at a time and see how far I can lean before stalling occurs at gear engagement from idle. It will be a slow process, I will keep this forum informed.  I am however, happy that the dreaded engine stall just before coming to a standstill is now a thing of the past.

[Nicolas Aristodemou]

Just an update.... It has been more than a week since my latest adjustments (increase in transmission modulator pressure and gradual leaning, a click at a time of the idle mixture screw).

I am now at an indicated 6%CO - car doesn’t like any leaner idle mixture - have run the car in city traffic for more than 8 hours going to and from work daily and no issues with stalling.

I have therefore concluded that the culprit was the low transmission pressure which didn’t always initiate the down shits into 3rd and 2nd gears when the car was coming to a stop resulting to occasional stalling.