Engine problems after touching the battery terminal with the ECM of my 1994-1995 BMW 325i



This is something that happens more often than you may think. We have had customers that ordered a performance chip for their 1993, 1994 or 1995 BMW 325i/525i, and after installing the chip, they have accidentally touched the positive terminal of the car battery with the metal chassis of the ECM, which is a ground. Afterwards, the engine started having problems that will not go away even after replacing the ECM.

The key here is to ALWAYS disconnect the car battery before doing any electrical work. Mostly if working with the ECM, because the battery is in the path of the ECM’s way out. Either inserting or pulling the ECM to or from its bay, there will be a hazard of touching the terminals of the battery with it.

If this has happened to you and your car is having problems with the engine, like needing to keep it accelerated to prevent the engine from shutting off, or if it is very hard to start the engine when cold, or if the engine is consuming much more gas than normal, then do not replace the ECM. Replacing the ECM will not fix the problem. Even when you touched the battery terminal with the ECM chassis and sparks came out, it was another part which got fried when that happened. It was the flowmeter. This part is also known as “air flowmeter” or “mass air flowmeter” (MAF). The ECM could have been fried because of the short circuit, but most of the times, I would say 99%, it is a problem of a fried flowmeter. To do a simple test, just disconnect the flowmeter connector and start your car. If the problems seem to disappear, then you will need to replace the flowmeter only. Below is a picture showing a couple of flowmeters used in the 1993-1995 BMW 325i and 525i. To disconnect the 4-wire terminal of the flowmeter, twist its cover counter-clockwise (like opening a water bottle) and pull it out.

The flowmeter is a device used for measuring how much air, by mass, is incoming into the engine. This information is needed by the ECM for doing the proper calculations of how much fuel to inject in different conditions, to keep a proper air/fuel ratio (AFR) balance.

The flowmeter is located in the air path way, between the air filter and the intake manifold and it has a round connector. If you stand in front of the engine, you will find it at your right, beside the intake manifold. Check the picture below as reference.

Locating a replacement flowmeter at affordable prices can be done by searching eBay or a recycled/refurbished parts shop. A new flowmeter will be expensive and will do exactly the same job as a good, used one. Unlike many other automotive parts, a MAF type flowmeter that is in good working conditions, can be purchased with no worries as it does not have internal mechanical parts that wear out. There are also excellent replacements like the items listed below:

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