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Tech Article

BMW E30 3-Series Exhaust Manifold Variations

Last Updated: 07/28/2018
Parts Group: Exhaust manifold
This article discusses the exhaust manifold (and oxygen sensor implications) for the M10-engined E30 318i as sold in the US in 1984 and 1985. If you're not here for the technical analysis but would rather buy a used unit from us, guaranteed to work and fit, then please select the link below:The information herein is based on some online reading, plus my own car and that of a customer.

The 1984-1985 BMW 318i had the M10 engine, fitted with a very sophisticated type of fuel injection made by Bosch, and called L-Jetronic. Some models had an oxygen sensor fitted at the bottom of the exhaust manifold, for closed-loop operation. This means that the oxygen sensor provides feedback to the fuel injection computer as to how lean or rich the mixture in the exhaust is, so as to guide the computer to make compensating adjustments. Accordingly, there are two variations of the exhaust manifold, one with the oxygen sensor, and one without.

A customer of mine lives in California. To pass the state-mandated emissions inspection, he needed an exhaust manifold with the fitting for the oxygen sensor. Sadly, I didn’t have one for sale, so he took the other variation of exhaust manifold (as in, the one that doesn’t accommodate an oxygen sensor) to a machine shop, which drilled out a hole and tapped a thread. Doing so enabled an oxygen sensor could be fitted, and all was well.

My own daily-driver 1984 BMW 318i has the type of exhaust manifold that comes with the fitting for the oxygen sensor, plus it has the oxygen sensor and a wire for the sensor, as part of the wiring harness. That’s good, but if the exhaust gas smell is any indication then my car has for several months now been running on a bad fuel mixture, plus it’s been idling too high. My tech has been chasing this issue for a long time. Today, she did some more troubleshooting: she disconnected the oxygen sensor, and the high-idle problem went away. It turns out that the oxygen sensor in my car is sending bad data that the computer trusts and this causes bad behavior. With the oxygen sensor disconnected, the computer runs in open-loop mode, as in: it works on the premise that there is no oxygen sensor, and so it tries to set the mixture as best it can in that context.

Not that the computer has intelligence (it’s just a deterministic electronic device) but this nevertheless is an interesting analogy to it being more useful to be uncertain, than to be certain yet mistaken.

The 318i cars produced up to 12/1984 use a different exhaust manifold and oxygen sensor than those produced subsequently, and they're not shown to be interchangeable.
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