Techni-Tips – July ’87
Q. I have a 1983 320i and the car seems toping most of the time. It pings at high RPM and low RPM . I’m now using unleaded supreme gasoline. I’ve had the car tuned tofactory specs., but this doesn’t seem to help. Canyou shed any light and offer solutions to my problem?
Brad Smith, San Clemente, CA
A. Preignition or ping has been around in engines for a long time. The ‘ping’ occu rs as the piston is traveling upward on its compression stroke and the ignited fuel pushes against it . The sound produced varies from a light rapping to a harsh ping. It usually occurs under low rpm / high load or high rpm / high load conditions. If left untreated, severe pinging will cause excessive overheating in the combustion chambe r and bu rn valves and / or pistons.
The major factors affecting ping are:
- Fuel octane rating – the lower the octane the faster the burn. A slower, controlled bu rn is preferrable to a quick burn. Therefore, the higer the octane rating the better.
- Combustion chamber temperatures – fuel is more volatile when hot. The hotter the outside or u nderhood tem peratures, the easier for pinging to occur.
- I gnition timing – the m ore advanced the timing is the more apt you are to hear pinging .
- Compression ratio – the higher the compression the more volatile the fuel becomes; thus the easier it is to Carbon build-up on the top of the piston or underside of the head also contributes to raising the compression.
- Fuel mixture -the leaner the mixture the hotter the burn, therefore inducing
The solution to curing an engine t hat pings requires altering one or more of the five factors influencing preignition. By far t he easiest method to try is switching to a high grade premium fuel. Some gas additives will give an increased octane rating and, at the same time, help to reduce carbon build-up inside the combustion chamber. When using a gas additive as a carbon blaster, I recommend driving the car at 4,000 to 4,500 rpm intermittently for five minutes at a time to create enough heat in the combustion chamber to do its job. This can be achieved easily in 2nd gear.
An easy way to reduce pinging is to retard the timing. Ifyou set the ’83 320i timing mark at about 2600 rpm, this should help. The drawback in retarding the timing is that in so doing you affect the power output of the engine and reduce the mileage. Nevertheless, no ping is preferable to ping.
A rich mixture (one that has a higher concent ration of fuel per given volume of air) will burn cooler, thus helping to reduce pinging. A mixture too rich or too lean causes other performance and drivability problems. Your engine is designed to operate in a very narrow fuel/ air mixture ratio. Partially plugged fuel injectors (refer to June article of Technitips) may also cause pinging. A good fuel injection flush or replace ment of the injectors may help in this case.
As you can see, the causes and cures for preignition pinging are nu merous and varied. Sometimes it simply requires playing with the variables a little to see what works best for your car. Other times, it’s best to take it to a specialist who u nderstands engine dynamics and have the problem properly diagnosed .
Q. I have a newer type BM W with service indicator lights. How accurate are they in indicating service?
Celeste De Voise, Redmond, WA
A. The lights• you are referring to have been on most BMW’s manufactured since 1982. From my personal experience, I don’t know yet how well they operate in indicating oil services. They seem to indicate the need for engine inspections (tune-up services) at about the proper intervals.
The service indicator lights operate from a series of inputs from your car and your driving habits. The inputs include: engine temperature, vehicle speed, tach / fuel economy guage, engine temperat u re, t he number of starts, mileage of trips etc.
I’ve always been of the opinion that the oil change interval should be 3,000 miles. I’ve seen what happens to the upper end of engines (specifially the rockers and shafts) when the engine oil was changed regularly at 7500 miles. The rockers and shafts, which are located within the head, function to operate the valves. Dirty and contam inated oil takes its toll quickly on these components. In my opinion, changing t he oil every 3,000 miles is the best insurance to engine longevity regardless of what the service lights indicate.
This article was originally published in the July 1987 edition of Zundfolge