Ping or preignition. The sound of a marbles rattling around in your engine while lugging, the engine. Known as the high gear, low RPM ping.
This can be caused by any number of things but because our BMW’s are electronically controlled it can typically only be caused by two things. Using the wrong octane fuel or carbon build up in the cylinders.
Wrong octane is generally thought of as using too low of an octane in a high compression engine. But inversely using too high an octane in a low compression engine can cause carbon build-up which in turn results in preignition.
Because we love out BMW’s we want them to have the best but feeding them expensive high octane fuel when they don’t need it is a little too much love.
Specific to the E34 line these models require high octane fuel: 1992 to 1996 530i, 1992 to 1996 540i.
These models do not require high octane fuel: 1989 to 1996 518i, 1988 to 1996 520i, May 1991 to 1996 525i (VaNoS engine).
These models require mid grade fuel: 1988 to May 1990 525i, 1988 to 1990 530i, 1988 to 1992 535i.
Other causes of carbon buildup in the combustion chamber are short drives where the engine doesn’t get to operating temperature or the engine never reaching high RPM’s. Babying your BMW just isn’t a good idea!
Well now it’s too late and you are getting the dreadful ping. These days going to higher and higher octane fuel can be an expensive option but it will work for a while. In a high compression engine octane booster may quiet the problem. For a while.
So the best course of action is to clean the combustion chamber. To get ready wait until your oil requires service. After this procedure an oil change will be required. You will need a 30 cm (12 inch) length of ¼ inch nylon tube and it is best performed on an evening where you have some time the next morning.
Stop by your local GM dealer and pick up a spray can or bottle of liquid ‘Cleens’. I prefer the bottle in a MAF controlled car while the spray is best in a MAP, TBI, or carbureted vehicle.
Liquid is part number 992872 while spray is 993026.
Go for a good drive and bring the vehicle to operating temperature. Do not shut it off when you arrive home for this procedure.
Locate a non ported vacuum line. Here I am using a M50TU as an example and the vacuum line under the PCV line does very nicely.
On the M30 the vacuum line at the PCV is a good central vacuum line.
You may need to add a small length of nylon or similar hose to the vacuum line. I used ¼ inch nylon tubing and inserted it into the end of the vacuum line.
Now bring the engine speed up to about 2000 – 2500 RPM by manually operating the throttle at the Throttle Body.
*NOTE! IF YOU HAVE THE ZF 4HP-22 EH do not exceed 2000 RPM in Neutral or Park!!
This transmission was used on: E23 733i, 735i, 745i, E28 533i, 535i E32 735, E34 530i (1988-1990), 535i. Check to see what transmission your car has if you are uncertain.
You can raise the rear wheels off of the ground and place the car in Neutral and safely exceed 2000RPM.*
Dip the open end of the nylon tube into the open bottle of ‘Cleens’. This is a momentary dip of less than a second. Count to 5 and dip again. The engine will stumble and white smoke will billow from the exhaust. DO NOT let the RPM dip below 2000 RPM. Continue to dip for a second and count to 5 dip for a second and count to 5 until about 2/3 of the bottle of ‘Cleens’ is gone.
Now dip the nylon tube into the bottom of the bottle of ‘Cleens’ and slowly release the throttle until the engine stalls.
Turn off the ignition. Remove the nylon tube from the vacuum line and reinstall the vacuum line.
Wait at least 12 hours or until the next morning.
Start the engine. It isn’t going to like it and may stall once or twice. Plumes of white and blue smoke will billow from the exhaust far longer than you will feel comfortable with. Once the car is idling smoothly and the smoke from the tailpipe is minimal go for a drive to warm up the engine. Take about 20 minutes.
When you get home do the exact same procedure with any ‘Cleens’ that may remain in the bottle.
Now fill the bottle with clean potable water and do the tap procedure you did initially with the ‘Cleens’. Again NEVER let the engine RPM drop below 2000 RPM!
Once the bottle is empty keep the RPM at 2000 until the engine is running smoothly.
Let it idle as you reinstall the vacuum line.
Go for a drive. You may have a ‘Check Engine’ Light (CEL), the result of a code 1222 (O2 sensor rich) but this will settle out quickly. Drop down into a low gear and drive at 4000 RPM for about 10 minutes.
This may seem involved but the alternative is head removal and a mechanical decoking which can be quite expensive.
Consider using a quality fuel treatment like Redline or Techron at every fill and if your owner’s manual asks for 89 Octane USE 89 Octane!!!
Take your car for some high RPM runs on occasion.
The one engine that can use both regular and premium fuel without ill affects is the M50TU; the single VaNoS M50 engines with knock sensors.
If your engine is chipped then use the octane rating recommended by the chip manufacturer.
Article by Shayne Mcguire