E30 running rich?

My humble guess is that ou have a failed O2 Sensor, and it's feeding more gas than is needed because it always thinks the engine is cold.

When the O2 Sensor is cold, it feeds more gas, kinda like the choke circuit in the olden days of carburators. As the O2 Sensor heats up, it sends a Heated Up Now signal to the computer, and the computer then sets about to lean the mixture. My guess is that your O2 Sensor has failed in a mode where it refuses to send the Heated Up Now signal, hense the mixture remains rich long after it should have leaned out.

Your car is OBD I compliant, this means you need a BMW technician to pull codes to see if the O@ Sensor is reporting that it is slow to heat up. The newer ('96 and later) cars are OBD II, and they give a code for this that you can easily download yourself. Not much point in discussing OBD II with you, you are stuck with OBD I -- as am I.

There is a way to pull codes on your car by holding your tongue just the right angle and switching the ignition ON and mashing the gas pedal just right in a very specific amount of time.

The most likely culprit is the coolant temperature sensor that is feeding the computer. Another possibility is the coolant system thermostat. If the engine is always cold or if the computer thinks that the engine is cold it will feed too much fuel. A failed oxygen sensor can result in a somewhat richer mixture but not to this extent. The oxygen sensor just does not have the authority to enrich the mixture by this amount. On one occasion, my friendly local mechanic destoyed the thread in my 318i exaust manifold while he was attempting to replace my oxygen sensor. We had to put a plug in it. I drove the car for a month without an oxygen sensor until I could find a good time to replace the manifold. The effect on my gas mileage was very minor - only 2 or 3 mpg not the 10 mpg that you are experiencing. The OBC uses the length of the fuel injector pulse to determine how much fuel is being injected and compute the mileage displayed. The fact that it agrees with your poor mileage should rule out worn injectors and faulty fuel pressure regulator because they would enrich the mixture without affecting the injector pulse length.

"MarkS" wrote in message news: snipped-for-privacy@o13g2000cwo.googlegroups.com...

There isn't one, he has no cat.

A worn rotor arm will really kill the mpg on this car, we replaced it and the dizzy cap on my mate's 88 320iA and he got another 5mpg and a LOT more high end power.

I think the coolant temperature sender only operates the guage on the instrument cluster. There is a thermoswitch next to it in the thermostat housing which i think controls the cold start procedure in the computer, which i think I verified as working a couple of months ago, but will check again. As John noted, the car doesn't have an O2 sensor. Thanks for the OBC info, useful to know.

-- Mark

Excellent, I shall definitely try that as I have no record of either being renewed on this car. Thanks.

-- Mark

ps What's the theory behind the link between the worn distributor parts and poor consumption? I assume something to do with a bad spark leading to incomplete combustion?

Thanks again

-- Mark

Further to the above, I have checked the readings from the thermo switch before and after a long run. It read 65ohms when hot, and open circuit when cold. I think this is the correct behaviour, can anyone confirm? Just cleaned the connecter contacts for good measure.

-- Mark

On Motronic engines there's no vacuum advance. It's all controlled by the time the computer fires the coil, the rotor arm covers about a 12 degree arc. If it's worn then it puts the timing off.

I'm not familiar with your exact model of car Mark because I'm in the US with a 318i but I do believe that the fuel injection computer has to have an engine coolant temperature input in order to function properly. My 318i has three engine temp transducers; one for the instrument cluster, the thermo time switch to run the cold start valve, and one for the L-jetronic fuel injection controller. I think your engine is similar. Here's a link to a diagram of the three temperature sensors on the 320i thermostat housing.

link for the L-Jetronic have a book on the Bosch controller and it lists these specs for the temperature sensor:

7000-12000 ohms at 14degrees F (-10C) 2000- 3000 ohms at 68 degrees F (20C) 700-1000 ohms at 122 degrees F (50C) 270-400 ohms at 176 degrees F (80C) Good Luck

another link for the L-Jetronic>

Hi, My car doesn't have the sensor labelled 12 in your diagram, looks like that was discontinued in 1984. I just have the coolant temp sensor (for the guage) and the temperature switch for the cold start, which I think is working correctly. Thanks.

-- Mark