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BMW Rough Idle & Hesitation – Rich Mixture

April 22, 2010

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

car year: 1989
car model: 325i
My car is running extremely rich and has a terribly rough idle with extreme hesitation when applying throttle. I have replaced the AFM, O2 sensor, tested the ICV,replaced the TPS, replaced the engine temperature sensor, and replaced the intake manifold gasket. I am aware of a valve cover leak but it is because one stud is stripped. I do not know if that would be the cause of these symptoms, but I would like to know if you could help me out with this problem. I’ve spent days trying to figure out the problem and I cannot seem to find the problem. Please help me out.

If you are certain that the poor running is due to a rich mixture (black smoke from the exhaust and/or spark plugs carboned up and fouled quickly), we would consider the following points.

Items that you have already replaced, that could cause a rich mixture (we will not talk further about these, since you have already replaced them):

* Engine Temperature Sensor
* Air Flow Meter
* O2 Sensor
* Throttle Position Switch (not very likely, but possible)

Further areas to consider for a rich mixture:

* Fuel Pressure Regulator- The regulator could be keeping the pressure too high or it could be leaking fuel into the vacuum line. Remove the vacuum hose and inspect for fuel. There should be none. If there is, replace the regulator. To further test the regulator, connect a fuel pressure gauge to the pressure fuel line, at the fuel rail. This will be the hose that connects directly to the rail (the one that connects to the bottom nipple of the regulator is the return line). Remove the hose and install a “T” between the nipple on the rail and the removed hose end. Connect the gauge to the “T”. Check the applicable Bentley repair manual for specific pressures. With this noted, most ’80s and ’90s BMWs will run in the mid to high 30PSI range with the engine idling. Pressure will typically rise 5 to 10 PSI if the vacuum hose is removed from the regulator. If the pressures are notably higher than this (verify the proper pressures using the Bentley manual), the regulator may be faulty.

Click below for fuel pressure testing gauges:

* Restricted fuel return hose/lines – In conjunction with testing the regulator, we will test the fuel return. Remove the return hose from the regulator (engine not running). Apply low air pressure to the hose (with the gas cap removed), about 20PSI (you could also blow into the hose, but we don’t recommend this). As the fuel is pushed back into the tank, you should eventually be able to freely blow through the hose. If not, there is some form of restriction in the return lines.

* Cold Start Valve & Thermo-Time Switch- NOTE: not all models have cold start valves. The 325i/is/ic models, such as yours, do not. Skip this step. The cold start system could be opening the valve or the valve could be leaking. Remove the fuel hose from the valve and plug the hose. Run the engine to see if the conditions are improved. If so, either the valve or the thermo-time switch is faulty. Reconnect the hose and unplug the electrical connector from the valve or the thermo-time switch. If the engine continues to run better, replace the Thermo-Time Switch. If not (but it did run better with the fuel disconnected from the valve), replace the cold start valve.

* Fuel injectors -You may have one or more leaking fuel injectors. If only certain cylinders are running rich (as shown by the spark plugs), this is a possibility. If all cylinders are running rich, this is not as likely to be due to leaky injectors (all of them would have to be faulty).

In all of the above tests, when running the engine to see if things have improved, remember that the spark plugs must be, at least, moderately clean. If the spark plugs are heavily fouled, the engine will run poorly even if you’ve cured the original fault.

Finally, if everything tests and diagnoses as OK, the Engine Control Unit (ECU) may be faulty. This is far less common than mechanics or other parts replacers may lead us to believe, but it certainly can, and does, happen.

Click below for fuel system parts (regulators, injectors, ECUs, etc.):

One Comment
  1. Tony Townsend permalink

    I had this same problem on my E30 and it turned out to be corroded ignition wires. Another possibility.

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