When to rebuild my engine?
Indeed, this is a very common question, and one that is often not easily answered. Obviously, if there is the end of a rod sticking out of your engine block, then the chances are, it’s time for a rebuild!
However, with more subtle noises, broken pieces, and poor performance, the rebuild decision may not be crystal clear. In this section, I will provide you with some questions to ask yourself and some answers to common myths, in the attempt to correctly determine whether your engine needs to be rebuilt.
As with any serious medical condition, it’s always a wise idea to get a second opinion. The same is true with BMW engine rebuilds. Very often, I have heard of unscrupulous (or even over-meticulous) mechanics who have recommended, or even insisted on a rebuild, when not all of the signs pointed in that direction. Keep in mind that no matter how well-intentioned your mechanic may be, he may have a vested financial interest in seeing you rebuild your BMW engine. Of course, he doesn't know that you’re armed with a big tech manual, a good stock of tools and are prepared to do it yourself. Rebuilding engines is a good business for a garage, and will guarantee them at least 50 hours of labor for a complete job. Not to mention parts and machining work profits.
My recommendation is that you take your car to a second, independent mechanic, and pay to have the car evaluated. Have him perform a leak-down test on the engine (see later in this chapter), and let him know up-front that you have a master mechanic friend waiting in the wings to rebuild the engine for you. The goal is to try to get an independent, unbiased expert view of the condition of your engine. Many of the problems with BMW engines can be somewhat subtle, and difficult for a novice to detect and decipher. I’ll give you some hints, tips, procedures, and clues to help you in the following sections, but getting at least two expert opinions is always a wise idea.
High Mileage Engines
Each derivative of an BMW engine has it’s own quirks and problems. Some engines are known for their longevity, and some are decidedly not. Just because your engine has a lot of miles on it, doesn’t mean that it’s automatically time for a rebuild. With proper care and maintenance, certain engines can easily last 250,000 miles or more. Of course, some years have had better track records than others, but the basic rules apply: if the engine was well cared for, and not abused, then it should last a long time, and gradually wear out. In general, the rule of thumb is that high-mileage is not a good yardstick measurement of engine condition. The methods by which the car was operated and maintained during its life affect the condition of the engine much more than the mileage total.
High mileage engines often show signs of their age in compression and leak-down tests, described later in this section. As the engines age and mileage increase, the small tolerances within the engine slowly become larger. While this usually doesn’t result in a catastrophic breakdown, high-mileage engines will gradually see their performance degrade as the mileage increases. Such an engine may be referred to as 'tired'.
Stock engines almost always last longer than modified engines. Higher compression ratios, aftermarket turbos, or superchargers will almost always place added stress on engines and make them wear out or fail quicker. Engines driven constantly on the track may especially show signs of wear. Race engines have such a typically short lifespan that their usage is usually tallied in hours run, rather than miles traveled.
Failed Emission Tests
if you car fails an MOT emissions test....it doesn’t necessarily mean that its engine needs to be rebuilt. In fact, a recently rebuilt engine will most certainly fail the test if it hasn’t been fully run-in yet. The best thing that you can do to get your car to pass the test is to make sure that it is running perfectly. Most of the time an emissions failure car simply has its timing set incorrectly, or has a fuel injection problem.
You must make sure that all of your fuel injection and ignition components are working 100% properly before you can assume that the engine mechanicals may be suspect. A compression or leak-down test should be able to let you know if your failure to pass teh MOT emissions test is caused by internal engine wear.
Poor Performance and Poor MPG
When rings and valve guides begin to wear, the result is an increase in burnt oil inside the engine. Also seen is a decrease in compression. Both will have a negative impact on the power generated by your engine. The burnt oil is a contaminant in the combustion chamber and will interfere with the combustion process. The loss of compression will reduce your compression ratio, and limit the power output of the engine. Both will result in poor performance and poor fuel economy.
However, there are plenty of other factors that can affect fuel economy and power. Most notably, the fuel injection system needs to be maintained in top shape in order to achieve the most power out of your system. Make sure that you have eliminated both the fuel system and ignition system as a potential source of problems before you decide you need a rebuild. Also try to isolate and fix other obscure problems that you might not think of. Improper suspension alignment can seriously reduce power, as can improper tire inflations. Brake problems (especially with the emergency hand brake) can drag on the wheels and create some pretty significant drag.
Strange Engine Noises
Water-cooled engines are designed to expand and contract as they heat and cool. As such, it is very difficult to diagnose strange engine noises that occur when the engine is cold. It is not uncommon for the engine to make some unusual tapping or knocking noises when started stone cold. It’s the strange noises that are made when the engine is warm and running that are the ones to watch out for. All engines also tend to get noisier as they age, and clearances between parts inside the engine become larger.
Engine noises are indeed difficult to hear at times. What may be a loud noise from one area of the engine, may in fact be inaudible from another angle. Sometimes sitting inside the car, you will hear more of the lower-pitched noises, as the higher-pitched ones are filtered out by the cars’s insulation. Closing your eyes when listening to the engine helps to eliminate potential distractions, and allows you to concentrate on isolating the engine noises from one another. An automotive stethoscope is a useful tool for listening closely to the engine. This tool works best when placed against a solid piece of the engine. Local sounds from troubled components can be heard better through the stethoscope because it helps to isolate outside noise. A long wooden dowel is a good alternative to the stethoscope, but be careful not to stick it in your ear, as intermittent engine vibrations can sometimes knock it into the inside of your ear. A piece of rubber vacuum hose will work as well.
There are four basic types of noises that can come from the typical engine. Intermittent noises occur at irregular intervals and seem to have no reasonable pattern to them. An example would be something rattling around inside one of the valve covers. There are noises that emanate with the crankshaft speed, and occur once every revolution. Then there are valve-train noises which come and go once every two revolutions (on a four-stroke engine, the valve train operates at half the speed of the crankshaft). Such a noise would include the rockers and valve noise. This is probably the most common noise heard, and the fix may be to simply adjust the valves.
A common noise to hear is a loud squeaky noise from inside the engine while running. Such a noise can often be attributed to worn alternator bearings or a bad fan belt. Take the fan belt off, and run the engine for no more than 10-15 seconds and see if the noise disappears. If it does, you know the problem is with your alternator or belt system.
Another common noise is piston slap. This is the sound that the piston makes on its power stroke when clearances between the piston and the cylinder are somewhat excessive. It’s a dull-thud clunk that can be heard every two rotations of the crankshaft. Piston slap is most commonly heard when the engine is warming up, before the piston to bore clearances have decreased due to the pistons expanding.
There are a whole host of noises associated with problems such as rod knock, noisy valves, broken rings, chain tensioner failure, detonation, and broken or pulled head studs. Unfortunately, I have discovered that it’s nearly impossible to accurately describe these noises in writing so that someone can diagnose them. The best suggestion would be to take your car to your mechanic and have him listen to the engine. An engine can be loud and noisy, and if you haven’t listened to a whole lot of them, your imagination can get the best of you. Of course, listening to other finely tuned engines in cars owned by your friends will help you with an idea of what a normal engine should be sounding like.