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BMW and MINI – Battery Still Going Dead After Replacing Alternator

Aug 5 14

This answer is applicable for many BMWs in addition to the one listed below.

Dear Otto,
I’ve been having some issues with my 98 328i.  A while ago, it started missing and chugging once in a while, then would be fine for a while.  Oddly enough, this seemed to happen more at night.  Then a couple weeks ago, the battery went dead.  I got a jump and it ran fine.  Now, it may run fine for who knows how long or it may just not start right after shutting it off, but it will always take a jump and then seem to run fine …. with the exception that the missing and chugging seems more common.

Since the CHARGE (or BATTERY, Otto) light has never come on, my mechanic suggested that the battery was weak and I should replace it, and I did.  All seemed to be fixed (both the missing and the dying), but after a week or so, the same problems started to return.  This time, he checked the output of the alternator and said it was putting out, but seemed to be a little low.  He said that this could be why the battery was sometimes charged and sometimes dead, depending on how much electrical power I was using (lights at night, blower motor, etc.).  He also said that the engine missing may be due to low battery voltage, due to the alternator not charging enough.

Well, he replaced the alternator and the problems returned almost immediately.  I need some help!  I have a new battery and a new alternator, that still shows borderline low on output ….. and all of the same original problems.

All of your symptoms tend to point to a possible common issue on older models.

*  Intermittent engine missing

*  Intermittent dead battery

*  Engine missing and dead battery more common when electrical accessories are being used (lights, blower motor, radio, etc.)

*  Charging warning light does not come on

*  Alternator tests OK, but maybe a bit low, in car, but tests fine on a test bench.

These symptoms tend to point to a possible engine and/or battery ground problem.  If the grounding cable/strap that runs from the engine to the chassis, or the ground cable from the battery to the chassis is corroded and does not have full electrical conductivity, the alternator will not be able to deliver its full output to the electrical system and the battery.

Test the engine ground as follows:

With the engine running; use a set of jumper cables to ground the engine to the chassis, by using just one of the cables to connect from a bare metal part on the engine (alternator housing or mounting bracket, oil filter housing, motor mount, etc.) to a proper chassis ground (the provided ground jumper terminal or a strut tower nut or other clean and bare metal point).  If there is a spark when connecting the jumper cable, the engine ground cable is likely corroded (or missing) and needs to be replaced (and the terminal points cleaned).  You can double check this by testing the alternator output voltage with the jumper connected (vs. without the jumper).  If the output voltage is higher with the jumper connected, again, the engine ground is at fault.  Note that the cable may look visually fine at first glance, but may have internal corrosion.

On most BMWs, the engine ground cable runs from the left side of the engine (often the metal motor mount arm, alternator mount, alternator housing, or oil pan) to a point on the engine bay chassis.  In replacing the ground cable, clean the mounting points to bright metal and apply the DeoxIT compound to prevent future oxidation.

Please call your BavAuto Advisor at 800-535-2002 to order the proper engine ground cable for your model.

Click below for DeoxIT contact enhancer:

Bavarian Autosport

BMW and MINI – Engine Will Not Start After Running Out of Fuel (Gas)

Jul 31 14

This answer is applicable for many BMWs in addition to the one listed below.

Dear Otto,

I ran out of gas in my 2004 330ci.  After getting a couple gallons of gas and pouring it into the tank, the engine would not start.  I went and got two more gallons and added it, but the engine will still not start.  How much gas do I need to put in to get it started?  This seems really odd, since I can normally run it for 10, 20 or 30 miles after the reserve warning light comes on, before the “range” shows 0-miles.

We suspect that you may now have a failed fuel pump.  The fuel pump on later model BMWs and MINIs (as well as most other cars) is mounted inside the fuel tank and actually uses the fuel to cool and lubricate the pump.  Consistently running the fuel very low, will limit the amount of fresh cool fuel to keep the pump cool and lubricated …… accelerating the wear and shortening the life of the pump (this also applies to earlier models that have external in-line fuel pumps).  It’s likely that this last episode of running out of fuel was the “straw that broke the camel’s back”.  This ends up being fairly common, especially if the pump has a lot of miles of use or is often run with less than 1/4 fuel level.

We recommend replacing the fuel pump and this will likely cure your no-start issues.  Remember to replace the pump seal ring and have fresh hose clamps available to replace the original factory crimp clamps or rotten old hose clamps.  The Bentley Publishing repair manual (for the applicable BMW or MINI model), will cover the fuel pump replacement.

Fuel pump:

Hose clamps (search “hose clamp” after clicking link):

Bentley Publishing repair manuals:



Bavarian Autosport


BMW or MINI Spark Plug Fouling – Misfire Fault Codes P0300, P0301, etc.

Jul 30 14

Are you having trouble with random or specific cylinder misfire fault codes?  These OBD-II fault codes can often be troublesome and difficult to track down and eliminate.  OBD-II fault codes for ignition misfires are commonly displayed as shown below:

P0300 – Random misfires; this is indicating that the engine management system is detecting misfires intermittently on multiple cylinders.

P0301 – Misfire on cylinder #1

P0302 – Misfire on cylinder #2

P0303 – Misfire on cylinder #3

P0304 – Misfire on cylinder #4

P0305 – Misfire on cylinder #5

P0306 – Misfire on cylinder #6

P0307 – Misfire on cylinder #7

P0308 – Misfire on cylinder #8

P03xx – Misfire on cylinder #xx, where the cylinder number is the last two digits of the code.

Today’s engines are designed to run within very tight parameters in order to satisfy emissions requirements.  Small variances in system functions can trigger fault codes through the OBD-II (On Board Diagnostics, v-2) monitoring system.  While there may be genuine problems within the engine management system,  keep in mind that short-trips, slow driving and prolonged idling can compound cylinder misfire issues, by not allowing the spark plugs to work at the higher temperatures required for “self-cleaning”.  In these cases, an otherwise properly functioning engine can generate misfire fault codes due to minor spark plug fouling.  Therefore, engines that see these types of conditions may need more frequent spark plug changes.

If you have fault codes for specific cylinder misfires, the first step is to swap the ignition coil assembly from the indicated cylinder, with the coil assembly from another cylinder.  Next, clear the fault codes and drive the vehicle.  If the fault code now comes up indicating the new cylinder location (the cylinder that the coil was moved to), you can assume that the coil assembly is at fault and replace the coil.  If the original fault code returns (same cylinder as the original misfire code), you can assume that the fault involves this specific cylinder.  The fault could be a fouled or worn spark plug, fuel injector problem, vacuum leak at this cylinder or a mechanical problem within the cylinder (leaking intake or exhaust valve, carbon build-up, etc.).

If the fault code returns to the original cylinder, after the coil swap and code clearing (as noted above), the next course of action should be to inspect the spark plug.  The plug may be worn or fouled, causing the misfire.  If the spark plug looks visibly fine (if not, replace it), swap it with another cylinder, similar to the coil swap.  Clear the fault codes and see if the fault returns.  In the same manner as the coil swap, test, if the fault code moves to the new cylinder, replace the spark plug (in fact, you should just replace them all, in this case).  If the code returns on the original cylinder, further testing and diagnosis will be required in order to determine if the fault is caused by the fuel injector, a vacuum leak or an issue within the cylinder.

If fault codes for random misfires are present, this could be fouling or worn spark plugs, arcing coil to spark plug connector boots or weak coils in addition to other engine management system faults.  If the spark plugs have more than a few thousand miles on them, it would be wise to pull them and have a look at the electrodes and the center electrode insulator for wear or a buildup of deposits.  If in-question, replace them, especially if the vehicle sees a lot of short trips and/or idling time.

Spark plugs and ignition coils:

Fault code readers and reset tools: