Does, can ethanol in gas skew CIS CO readings at tailpipegs
 

Recently had my air/fuel CO reading checked and the Bosch machine was reading quite rich--smell/subsequent lower gas mileage - at 1.04 while specs call for a CO reading /setting between 2 and 4 for a '78 OB at the tailpipe. Motor has been bored to 4.7 litres, heads milled to 9.5 to 1, msds headers, dual exhaust and cats and 84-86 euro 928s cam grind. Could these mods have changed specs or maybe the Bosch analyzer is off ? Expect to try to set again. Any thoughts, comments are greatly appreciated

No mention of ethanol in your test, so why is it in the subject? IIRC my wrench said mine was lean at 1% CO, lifted it to 1.5%, ran much better. 2-4% sounds high in that light, but dont know for sure. Ethanol/methanol need to run MUCH richer than gasoline for proper combustion AFAIK - in pure form, around 7:1 vs 14:1 for gasoline.
jp 83 Euro S (CIS) AT 54k

Ethanol is very far from being methanol.

Ethanol is around 9:1 stoich ON A PETROL SCALE, but this confuses us, so we should use Lambda, which is not changeable.

Methanol is completely different, corrosive, and is around 5:1 stoich

Different fuels have their own stoich, there is no petrol scale. Gas 14.7:1, methanol 6.47:1, ethanol 9:1. Lambda is the ratio of the actual air fuel ratio to the stoichiometric. An engine that is tuned to run on gas will have a lower stoich and run leaner when alcohol is blended in. Cars with computers, O2 sensors, an afm or maf will instantly compensate. Your engine may just need some tuning.

Oops, forgot to mention ethanol again in my message. I'll have to continue to tune and adjust. The fact that ethanol leans out the mixture in a car without the computer,02 sensors,etc. is what is stumping me--because when the machine reads 1.5 or so exhaust is rich enough to burn your eyes. I am thinking the machine may need a tune up also. Specs for '78 is 3+ or- 1 with air pump hose pinched off. Thanks for input guys.

Doesn't it make the mixture more effectively lean as the alcohol contains oxygen?

The alcohol does technically have more oxygen atoms.

CH3CH2OH

vs a nominal gasoline mixture:


C8H18 (tons of other stuff, but this is the basic idea)

Greg, the mixture of a K-Jet system can be adjusted easily. This is all your 928 needs, from the sound of it.

There is a hole in the forward extension of the air cleaner housing, behind the intake plenum (aka the spider body). The hole is at the top of a tube which runs through the air cleaner extension to give access to the mixture adjustment plunger. You need a 3mm (from memory) Allen key, long enough to reach through the air cleaner extension plus another inch or so. Alternatively, you can adjust the mixture with the air cleaner removed, provided that you clean around the sensor plate housing to ensure that the engine doesn't ingest any dirt.

Get the engine to operating temperature (idle oil pressure at 2-2.5 bar, whatever is normal for your car when fully warm). If you are going to remove the air cleaner to adjust the mixture, remove it now, rather than before you warm the engine up.

Insert the Allen key into the top of the spring-loaded plunger (at the bottom of the hole if the air cleaner is in place), and push it down until it bottoms. Ease the pressure slightly and gently turn the Allen key to feel whether the bottom of the plunger has engaged the top of the mixture screw within the sensor plate housing.

With the engine at idle, slowly turn the Allen key clockwise until the engine slows and just starts to stumble (rich mixture limit). Note the angle of the Allen key. Then turn the key anti-clockwise slowly until the engine starts to stumble again (lean mixture limit). Note this position of the Allen key. Note that the angle between lean and rich will be only about 90 degrees; a quarter turn.

The correct mixture is about half way between rich and lean. Ideally, use the exhaust gas analyzer to set the correct CO at idle.

The mixture screw in the K-Jetronic system effectively varies the position of the metering plunger in relation to the sensor plate. Accordingly, any mixture adjustment via this screw varies the mixture throughout the rpm and load range, not just at idle.

I forgot to say: when you are adjusting the mixture, remember "righty rich, lefty lean" (c) Jay Kempf. :-)

Thanks, Glenn. Got it, my concern is the inaccurate reading I got from the exhaust gas analyzer. I'll be trying again on the same and another shop's machine as soon as I can arrange it.

The best tool to use for the CIS is the Gunson Colortune. Cheap, efficient, and very accurate. Tailpipe emissions are affected by a lot of things including Ethanol in the fuel.

Hey! That's cool.

Still raises the issue of seeing the mixture across RPM and engine loading.

I tune my CIS on the dyno with a tailpipe sniffer and a torque graph. Sure, $100 a session but I know what it's doing at high RPMs where a bad fuel system might be leaning out.

That's cool too. What are you going to 'adjust' if your CIS car is going lean at high RPM? As I recall, the CIS distributor compensates already to a richer mixture at higher airflow. Do you modify the control pressure to increase the mixture? Change the shape of the plunger barrel slits? CIS has only one circuit for mixture. The whole thing is calibrated and the CO or mixture adjustment works throughout the range.

A $100 per trip is always going to be better than a one time $25 tool, but the $25 tool will tell you a heck of a lot about the condition of your combustion event.

Yesterday I got onto another shop's sniffer. My initial reading was 10! vs 2-4 as per specs. I Adjusted it slowly (with the 928 specific tool from 928 motorsports) down to ~2.5 now the idle is set correctly and not surging. Car is running well, looking forward to better, normal gas mileage rather than recent 12.4-13.1 mpg. Thanks to all again for commentary and suggestions.