Lean mixture measured from exhaust probe, accurate or not?

[jsylvain]

I recently had my 91 318iS tested on a chassis dyno before and after mods (Dinan chip, performance exhaust, K&N stock replacement air filter, fuel filter, fuel injection cleaner, vacuum hoses, etc.). Both before and after, the technician who tested my car told me that the mixture is too lean (13.5 units of air per unit of fuel). In fact, after the mods, the mixture was slightly leaner than before which is expected from a performance exhaust.

After talking this over with a guy at Dinan, he said he doesn't trust mixture numbers if they're measured from an exhaust probe. Does anyone know whether these numbers are somewhat realistic or totally bogus?

Is there a better way to test the air/fuel mixture?

 

[jrt67ss350]

A better way to measure air/fuel mixture is a probe in the pipe ahead of the catalytic convertor (same area as the O2 sensor). Air/fuel mixtures from the tailpipe aren't as accurate.

13.5 isn't really all that lean anyways. Newer cars tend to run leaner rather than run richer.

 

[epj3]

Actually your exhaust manifolds have a hole in each one of them (plugged by a bolt right now) which is ment for a testing device.

Where is your temp gauge riding? If it's closer to the first has than the middle one then replace your thermostat and the temp sending unit. Both are about $17. Mine was lean all the time until I replaced that. Saved gas too.

That's just an idea - but I'm sure the mods have more to do with it.

 

[jrt67ss350]

Mine was lean all the time until I replaced that. Saved gas too.

That doesn't make sense. If it was running lean before you replaced the thermostat and sender, then it would be running richer now and you would get worse gas mileage.

 

[jsylvain]

The temp always sits low near the first white mark on the dash gauge instead of the middle mark where it should be. I've always assumed that this was due to a low temp thermostat. However, when I asked around about the engine running cool and how that affects the mixture, I heard responses that lower temp usually causes the mixture to be richer and would not be the source of a lean mix. Also, I've heard story after story of plastic parts of the radiator intake and outflow breaking due to long term exposure to coolant at normal running temperature. Those two facts make me want to 1) leave the low temp thermostat in place, and 2) check to see if there's another cause of a lean mix (if it actually is lean at all).

 

[jrt67ss350]

The normal operating temp of my E30 (with a known good thermostat, fan clutch, radiator, water pump) is just a hair over the first white mark. As far as I know, E30s don't run at the half way mark like the newer cars do.

And you are right, if the engine is running colder than it should be, it would be a rich mixture.

 

[poboyis]

most newer cars run between 14.0 and 14.5

 

[epj3]

That doesn't make sense. If it was running lean before you replaced the thermostat and sender, then it would be running richer now and you would get worse gas mileage.

Ah my bad. I guess I need to check my fuses

My bad [thumbd]

 

[Chesty Bonds]

The normal operating temp of my E30 (with a known good thermostat, fan clutch, radiator, water pump) is just a hair over the first white mark. As far as I know, E30s don't run at the half way mark like the newer cars do.

Yeah my E30 is exactly the same. I have a Behr thermostat. However, when i'm driving in heavy traffic (lots of start/stop) the temp gauge moves up to just under the half-way mark and stays there until i get meaningful airflow again. Unlike the newer BMWs, the E30 tends to work within a wider operating temp - at least, that's what my temp. gauge is indicating.

 

[bichmgnt]

A good way to check to see if your running the right mixture is to hook up a digital volt meter to your O2 sensor. An excelent anolog meter might work but they tend to become part of the circuit instead of just looking at it. Make sure the engine is warmed up. If it is running lean the voltage out should sit about 0.2 volts. If rich about 0.7 or higher. If the mixture is right it will fluctuate about every second between the two. It won't give a steady output if the mixture is right. Your Bentley manual has all the info on how to check it or go here, click on the E30 Home Page and look for the info on O2 sensors under Engines.

By the way, the ratio for air fuel mixture is just that, a ratio of air to fuel by weight and is actually expressed as 14.7:1. If the engine is running lean then the ratio would be higher than 14.7. A ratio lower than 14.7 indicates the engine is running rich.

If your mechanic explained that 13.5:1 meant it was running lean you should look for another mechanic.

Steve