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Rough Idle & Shimmy – the Ultimate Guide!

by Steve Wilson

(Including information about Catalytic Converters)

Thanks to Johan for examples/pictures on his website – to follow the links hold down CTRL and press the link:

1)     Spark plugs, plug wires, rotor & ignition coils - Clean and inspect.

2)     Check compression – poor cylinder head compression is common for an older car. Big job to fix this.

3)     Use a good fuel-injector cleaning treatment into the tank – take it for a good run.

4)     Change fuel filter.

5)     Remove and clean injectors – might need to replace injectors.

6)     Hose to fuel pressure regulator – might be cracked/worn (buy a new one)

7)     Rubber hose between Airflow Meter and throttle – cracked?

8)     Mass Airflow meter/sensor – if dirty clean it up. Bad Air Flow Meter will cause flat spots in acceleration or erratic spots in the throttle depending on the opening of the airflow meter not usually a complete lack of power.

9)     Hose to air filter – cracked?

10)  Air filter – replace if dirty, check seals.

11)  Distributor cap – if eroded and worn - changed to new one

12)  Check fuel pressure - Replace the Fuel Pressure Regulator. Might need to replace Fuel Pump.

13)  Check and Adjust valves - throttle valves, PCV valve behind intake runner, Idle Control Valve (ICV).

·        ICV Idle Control Valve. Remove and clean it with brake parts cleaner. This regulates the air coming in to your intake manifold when the throttle body is closed and it tends to get dirty, sticky and the motor inside could not turn the valve properly.

·        Idle control valve sometime sticks open – causes issues when in Idle

14)  Throttle Body - Clean it out and replace the gasket (can use gasket material cut to size).

15)  Check all Vacuum lines.

16)  Conduct a ‘leak down test’ on all exhaust valves.

17)  Check for Oxygen (O2) sensor function tests.

18)  Check the wide-open throttle switch under the gas pedal.

19)  Check throttle position switch on the throttle body.

20)  Make sure connections on battery (under back seat) are very clean. Use wire brush.

21)  Check rubber boot between the Mass Air Flow and the DKM.

22)  Check for intake manifold gasket leak

23)  Have the transmission checked for correct functioning – (by mechanic).

24)  Make sure there are no other sources of drag in the driveline.

25)  Checked coolant sensor.

Try the RESET Procedure before all of above.


The reset procedure is billed as a ‘cheap fix’. It does often give good immediate results, however if there is something wrong with your car (any number of issues as listed above) then over a 2-8 week period the car will likely return to its original state.

The reset procedure removes all mean values stored by your onboard computer that control engine timing, fuel use etc. As you continue to drive the car, the stored values ‘average out’ to best suit the engine condition.

If your car has for example a leaking intake manifold gasket or broken O2 sensors, the readings are quickly going to become ‘corrupted’ with incorrect information. This can lead to the engine running too rich or too lean (check your sparks for evidence). Within a short period of time the car will return to rough idle as the underlying problem has not been fixed. That said – the reset procedure can reset values that are no longer applicable. You may have replaced something on the car (e.g. Catalytic Converters) thus causing a sudden change in normal engine running. The readings stored over 10-20 years will suddenly not suit you recently modified car. The car will however read new values in and average out over time. The Reset procedure simply speeds that process up.

The long and short of it is that this procedure can sometimes fix rough idle and transmission timing and seems to improve the cars performance. However with existing underlying issues as outlined above – it won’t be long before the problems return. Perhaps this procedure could be used if you’re trying to sell your E32 ;-)


1.      Disconnect battery cables, first negative, then positive.

2.      Cover and protect the battery posts! Leave no potential of a battery short circuit.

3.      Turn ignition to position II.

4.      Connect both battery cables ("SHORT") for at least 10 minutes. This step drains a capacitor in the ECU/TCU. (Covered the battery with a thick, dry shop towel to prevent actually shorting the battery! – careful of arc welding in my back seat! As the battery was in place, and the cables aren't that long, you could use a very long Craftsman screwdriver and two small vice-grips to actually clamp the negative and positive cables to the metal shank of the screwdriver.

5.      Wait 10 minutes - then ignition off, key out.

6.      Reconnect battery positive, then negative.

7.      OBC says "PPPP", reset time. Start and idle the engine for 5 minutes, then off for five, then out for a drive.

8.      Now, with the car in lowest gear (one or two depending on your make), accelerate until at least 5000 rpm is reached. Allow the car to slow to idle, and then repeat two more times. Let the car idle for 5 minutes. All Done!

9.      Idle should smooth out over 10-12 miles; shifts should be immediately smoother.


SHIMMY @ 80mph – likely culprits

Check the following in this order:

1)     Wheel alignment by a recommended mechanic (dynamic balancing)

2)     Check your brake pads!

3)     Check thrust arm bushings. (Some call them upper control arms). Bushing can go after several thousand km’s

4)     Replace/check tie-rod center, and Left & Right tie-rods

5)     Check Struts / Shock absorbers while your there.

6)     Sometimes (rarely) disks are warped – replace them (machining is just as expensive if not more).

Details about the Catalytic converter:

The catalytic converter is our main line of defense against air pollution, so it's important to make sure it is functioning efficiently and passing exhaust without creating undue restrictions that might reduce performance, fuel economy or emissions. That's one of the reasons for periodic vehicle emissions testing. If the converter isn't working, you won't pass the test.

If the converter is plugged, it will create a restriction in your exhaust system. The buildup of backpressure will cause a drastic drop in engine performance and fuel economy, and may even cause the engine to stall after it starts if the blockage is severe.

The easiest test for converter plugging is done with a vacuum gauge. Connect the gauge to a source of intake vacuum on the intake manifold, carburetor or throttle body. Note the reading at idle, then raise and hold engine speed at 2,500. The needle will drop when you first open the throttle, but should then rise and stabilize. If the vacuum reading starts to drop, pressure may be backing up in the exhaust system.

You can also try to measure backpressure directly. If your engine has air injection, disconnect the check valve from the distribution manifold, and connect a low pressure gauge. Or, remove the oxygen sensor and take your reading at its hole in the manifold or headpipe. Refer to the backpressure specs for the application. Generally speaking, more than 1.25 psi of backpressure at idle, or more than 3 psi at 2,000 rpm tells you there's an exhaust restriction.

If there appears to be an exhaust restriction, disconnect the exhaust pipe just aft of the converter to relieve pressure and recheck the readings. CAUTION: The pipes will be hot so wait awhile for things to cool down. If vacuum goes up and/or backpressure drops, the problem isn't not a plugged converter but a plugged muffler or collapsed pipe. If there's little or no change in readings, the converter is plugged.

Just because a converter is passing gas doesn't mean it is okay. If the catalyst inside is contaminated or worn out, high carbon monoxide (CO) and/or hydrocarbon (HC) readings will be present in the exhaust. If you have access to a high temperature digital pyrometer (or an oven thermometer will do), check the converter's temperature fore and aft. A good converter will usually run 100 degrees F hotter at its outlet than its inlet. Little or no temperature change would indicate low efficiency, or a problem with the converter's air supply. Converters need supplemental oxygen in the exhaust to reburn pollutants, so if the air injection system or aspirator valve isn't doing its job the converter can't do its job either.

Check the air injection pump, belt and check valve. If you suspect that the check valve is allowing exhaust to flow backwards, remove it and blow through both ends. It should let air pass in one direction, but not in the other. Examine the air injection manifold, too, because it tends to rust out and leak air. Check the diverter valve to make sure it is working correctly, too. It should be routing air to the converter when the engine is at normal temperature.

On engines with aspirator valves instead of air pumps, you should hear and/or feel the fluttering of the internal flapper as the engine is idling.


Fouling, clogging, melt-down and breakage of the ceramic substrate inside a converter are common conditions that can cause problems. Plugging is usually the end result of a melt-down, which occurs because the converter gets too hot. This happens because the engine is dumping unburned fuel into the exhaust. The excess fuel lights off inside the converter and sends temperatures soaring. If it gets hot enough, the ceramic substrate that carries the catalyst melts.

The unburned fuel may be getting into the exhaust because of a bad spark plug or valve, but an overly rich air/fuel mixture is another possibility. In older carbureted engines, a heavy or misadjusted carburetor float may be the underlying cause. But on newer engines with "feedback" carburetion or electronic fuel injection, the engine may not be going into "closed loop" (the normal mode where the computer regulates the air/fuel mixture to minimize emissions).

A bad oxygen sensor or coolant sensor may be giving the computer bogus information. A sluggish or dead O2 sensor will make the computer think the exhaust is running lean, so the computer will try to compensate by making the fuel mixture rich. A coolant sensor that always indicates a cold engine will also keep the system in open loop, which means a steady diet of excess fuel. But it might not be the sensor's fault. A thermostat that's stuck open or is too cold for the application can prevent the engine from reaching its normal operating temperature. So if your converter has failed and needs to be replaced, the engine should be diagnosed for any underlying problems before the new converter is installed.

Another cause of converter clogging and contamination is excessive oil consumption. Worn valve guides or seals can allow oil to be sucked into the engine's combustion chambers. The same goes for worn or damaged rings or cylinders. Oil can form a great deal of carbon, and metals present in the oil can contaminate the catalyst. A compression check or leak-down test will tell you if the rings are leaking, while a fluttering vacuum gauge needle will help you identify worn valve guides.