We worked on our BMW 328xi and fixed many problems. Even with all this work we still had a nagging problem. On startup, the BMW would surge.

Cold Start Engine Surging

This car would start and surge. The surging was present below 300RPM’s and would make the car lurch when it was really bad. Like a shudder as you drove. Very annoying problem! What are the first steps we took to diagnose this problem?

Diagnosing the problem

Smoke Detection Test

The first thing we looked at for this surging problem was air leaks. Unmetered air entering the engine can mess with the air-fuel mixture. Seals, plastic intakes, hoses, o-rings, there are many ways for parts to fail. The best way to check for air leaks is with a smoke detection test.

A smoke machine creates light smoke. You can see the leaks if you see the smoke escaping. Smoke escaping means air can get in, this can cause a lean condition in an intake situation. Smoke detection is good for many systems, but do not use it in your air-conditioning system. It is not compatible with the smoke.

Removing the Intake manifold

When this engine had no air leaks, we decided to dig in a little deeper. We went deeper by removing the intake manifold to see if the valves had carbon build-up. Originally, when we bought this car it had a PCV, positive crankcase ventilation problem. Oil was getting into the combustion chamber, it was very ugly. We had a giant cloud of smoke when taking off, not something you could miss. This is what lead us to think there was carbon build-up. But, as it turns out it was very clean on the intake.

Faulty Fuel Injector Causing Surging?

The surge could be caused by a fuel injector not supplying the correct fuel charge. After the engine work, we got a hard code about an injector. And, even though they were new we had a faulty injector. To test if this was the injector or further down the line, we moved the injector and the problem moved.

Finding the Fault

To find our surging problem we were going to have to look harder. We had tested the Air Mass Meter. The meter was functioning and it did not have a fault code. But everything was pointing to it being the problem.

The Air Mass Meter

The air mass meter reads the incoming air by heating a wire. It records the temp and how many amps heat the wire. As more air flows in the temperature goes down and it requires more amps at the wire. We took a reading with our bavarian technic software to see what the meter was doing as the engine was running. Then we changed out the meter. It is a simple job with easy access.

Testing the unit again showed very different results. The air it saw was way higher. The air mass meter turned out to be the source of the surge. To complete the testing we drove the car. The surging was gone, but the real test would be the cold start the next day.

Cold start

The next day it was cold, really cold. Good time to test the repairs.

With a successful start-up, we are happy with the repair. But now the clutch is slipping! Oh no, it is the case with older cars. We will be documenting the clutch repair to show you how you can keep your older BMW on the road.

If you need to change out the fuel filter in your BMW, check out our “replacing a fuel filter” We can save you doing it twice as we did!