Philes’ Forum – Summer 2016
by NJ BMW CCA
By Vic Lucariello, Sr.
Hello, bimmerphiles! I hope you are enjoying the hot NJ Summer. I write this at our new place in Colorado. This time out I talk about a problem that I have seen on many OBD II [1996 and newer] Bimmers. The problem presents itself as a misfire, on one or more cylinders, shortly after a cold start. The “Check Engine” indication [Official OBD II Name: Malfunction Indicator Lamp, or MIL] comes on and sometimes flashes. If the engine is shut off and restarted, the misfire is gone and the engine runs seemingly perfectly until the next cold start, or maybe the following cold start. The MIL goes off. Sound familiar?
In some cases, the problem begins with an occasional misfire or rough running after a cold start, with or without the MIL. As the engine warms up, things smooth out. The problem becomes more frequent and severe over time, finally getting to the point where you need to stop and restart the engine in order to clear the misfire and get the MIL to go off.
What the heck is OBD II, you may ask? Well, OBD II stands for On Board Diagnostics, Level II, and it has been federally mandated for about twenty years now. OBD II vehicles have sophisticated software that monitors a large number of parameters that can affect exhaust and/or evaporative emissions. Examples of things that are monitored are:
- The vapor integrity of the fuel tank, lines and gas cap
- The signal quality of sensors such as the crankshaft-position sensor
- How efficient the catalytic converter[s] is
- The response of the oxygen and/or fuel/air-ratio sensors
- Whether the engine is running rich or lean
- Whether the VANOS is properly positioning the camshafts
- How smoothly the engine is running
In monitoring how smoothly the engine is running [This is called the Misfire Monitor, and BMW calls it the “Smooth Running” monitor.] the engine-control computer [DME in BMW-Speak] looks at minute changes in crankshaft rotational speed and acceleration every time a cylinder fires. The DME can detect if a cylinder is not contributing as much as it should, and if the contribution is below a threshold, the MIL comes on. If the contribution is below another threshold, the fuel injector for that cylinder is deactivated in order to protect the catalytic converter from being damaged by unburned fuel. The cylinder will remain deactivated until the engine is stopped and restarted. If the cylinder is still misfiring, its injector is again deactivated and the MIL stays on. If the cylinder contribution is acceptable, the MIL goes off and the cylinder remains in service.
If you have read this far, you are seeing how OBD II monitoring can be involved in the problem we are discussing. But what can cause a cold-start-only misfire on an otherwise perfectly running Bavarian Work of Engine Art?
A lot of things can, but most if not all of them will also affect the warm running of the engine. You may not perceive a problem with the warm running, but examination of the running data [another OBD II feature] in the DME will usually contain a clue. But that is a topic for a future Philes’ Forum.
A common cause of a cold-start misfire, cylinder deactivation and MIL illumination is a sticking valve or sticking hydraulic lifter. What happens is that an intake or exhaust valve fails to close fully, thereby reducing the compression pressure in that cylinder below what is necessary for ignition. Believe me, a lot of parts have been thrown at this problem, both by individuals and shops. Moreover, the problem is difficult to diagnose because the symptom is sometimes so fleeting.
Depending on the severity of the problem, engine disassembly may be required. However, in many cases, the following procedure has been proven to reduce or eliminate the problem, especially if the symptom has recently presented itself:
- Put some Marvel Mystery Oil, CRC Valvekleen, or the snake oil of your choice in your crankcase. Some techs use ATF, but I like the CRC stuff. While you are at it, check your oil level. On Bimmers without a dipstick, this is done via the instrument panel. See your owner manual.
- Warm the engine up by driving slowly and at low RPM.
- Get on the highway where you can maintain a steady speed for, say, a half-hour. This may be difficult and you may have to do this at night or on a Sunday morning.
- Select your transmission gear such that the engine is running at about 4000 RPM or so and you are not exceeding the speed limit.
- After running like this for a half-hour, change the oil and filter while everything is still hot. The idea is not to let the oil cool off.
How to prevent the problem in the first place? I’m glad you asked. Use a high-quality “synthetic” oil of the correct viscosity and change it more frequently than suggested by your on-board maintenance reminder. In selecting your oil, consider using oil right from your BMW dealer. Use an OE-quality oil filter. While used-oil analyses are the only real way to determine the optimal oil-change interval for your particular oil and driving, sometimes it is better simply to put the oil-analysis cost towards an oil change. For some time now, I have been recommending an oil-change interval of, at most, about half of what your maintenance reminder suggests. If you do a lot of short-trip driving, a third of the maintenance-reminder interval may be better.
I hesitate to recommend a particular oil, because seemingly everyone has one that they just know to be the best. But I can tell you that I have had great success for many years with BMW oil, Mobil 1, Lubro-Moly [now Liqui-Moly], and Redline.
That’s all for now, bimmerphiles. See you next time.
Anyone wishing to contribute to Philes’ Forum can contact me at email@example.com. I’m interested in tech tips, repair / maintenance questions, repair horror stories, emissions-inspection sagas, product evaluations, etc.
© 2016; V.M. Lucariello, P.E.