BMW 3 Series E30

since 1983-1994 release

Repair and operation of the car

+ 1. Maintenance instruction
+ 2. Maintenance
- 3. Engine
   + 3.1.2. Specifications
   - 3.2. Dismantle and capital repairs of the engine
      3.2.2. Specifications
      3.2.3. Capital repairs – the general instructions
      3.2.4. Check of a compression in engine cylinders
      3.2.5. Capital repairs – general comments
      3.2.6. Diagnostics of the engine by means of the vacuum gage
      3.2.7. Capital repairs – alternatives
      3.2.8. Dismantle of the power unit
      3.2.9. Removal and installation of the engine
      3.2.10. Order of dismantling of the engine
      3.2.11. Dismantling of a head of the block of cylinders
      3.2.12. Cleaning and check of a head of cylinders and details of the valvate mechanism
      3.2.13. Repair of valves
      3.2.14. Assembly of a head of the block of cylinders
      3.2.15. Removal of pistons and rods
      3.2.16. Dismantle of the crankshaft
      3.2.17. Intermediate shaft (only M20 engine)
      3.2.18. Cleaning of the block of cylinders
      3.2.19. Check of a condition of the block of cylinders
      3.2.20. Honningovaniye of cylinders
      3.2.21. Check of a condition of pistons and rods
      3.2.22. Check of a condition of the crankshaft
      3.2.23. Check of radical and conrod bearings
      3.2.24. Engine assembly order
      3.2.25. Installation of piston rings
      3.2.26. Installation of an intermediate shaft
      3.2.27. Installation of the crankshaft and check of gaps in radical bearings
      3.2.28. Installation of a back epiploon
      3.2.29. Installation of rods and pistons, check of gaps in conrod bearings
      3.2.30. Launch of the engine after repair and a running in
   + 3.3. Engine electric equipment
+ 4. Cooling system
+ 5. Heating and ventilation
+ 6. Fuel system
+ 7. Exhaust system
+ 8. Transmissions
+ 9. Coupling
+ 10. Brake system
+ 11. Running gear
+ 12. Body
+ 13. Electric equipment
+ 14. Useful tips


3.2.4. Check of a compression in engine cylinders


Results of check of a compression in cylinders of the engine allow to judge a condition of group of details in the top part of the engine (pistons, rings, valves and laying of a head of the block of cylinders). Namely, reduction of a compression can be caused by leakage of combustion chambers owing to wear of piston rings, damage of heads of valves and saddles, progar of laying of a head of the block of cylinders.

For obtaining exact results of check the engine has to be heated-up up to the normal temperature, and the battery is completely charged.

1. Begin with the fact that clear sites near spark plugs for what blow compressed air (in the absence of the compressor blow sites automobile, or even bicycle, pump). It is necessary to exclude hit of dirt in cylinders at measurement of a compression.
2. Turn out spark plugs.
3. Completely open a butterfly valve and enshrine in such provision.
4. Disconnect the central high-voltage wire from an ignition distributor cover, connect it to weight on the block of cylinders. For reliability execute connection with weight by means of the special short-circuiting wire piece with clips of alligator type on both ends. Also will not prevent to remove a safety lock of electronic system of injection of fuel on the assembly block that will provide blackout of the electric fuel pump at measurement of a compression.
5. Insert the compression measuring instrument into an opening for a candle.
6. Include a starter and turn the crankshaft on several turns, monitoring indications of the manometer of the measuring instrument of a compression. On the serviceable engine pressure has to increase quickly. Low pressure after the first piston stroke and slow increase at the subsequent steps of compression indicates wear of piston rings. If after the first piston stroke pressure low, and at the subsequent steps of compression does not increase, then leak in valves or leakage of laying of a head of the block of cylinders is the reason (also formation of cracks in a head can be the reason). Decrease in a compression can be also caused by deposits of a deposit on heads of valves. Write down the greatest value of a compression.
7. Repeat the procedure of measurement for other cylinders, compare results to standard data.
8. Through an opening for a candle enter into each cylinder a little oil for the engine (about three full spray butterdishes), then repeat tests.
9. If after oil introduction the compression raised, then it is possible to draw an unambiguous conclusion that piston rings are worn-out. If the compression increases slightly, then leak happens via valves, or laying of a head of the block of cylinders. Leak via valves can be caused by a burn-out of saddles and/or facets of valves, and also deformation of cores of valves, or formation of cracks on them.
10. If the compression equally low only in two next cylinders, then is the most probable cause a laying burn-out between these cylinders. Emergence in combustion chambers or in a case of the crankshaft of the block of cylinders of traces of cooling liquid will be confirmation of this conclusion.
11. If value of a compression in one of cylinders is 20 percent lower, than in other cylinders and the engine unstably idles, then wear of a cam of the cam-shaft operating the final valve can be the cause.
12. If value of a compression exceeds norm, then the combustion chamber is covered with deposits of a deposit. In this case the head of cylinders should be removed and removed a deposit.
13. If the compression in all cylinders low, or strongly differs for different cylinders, then it is necessary to carry out tests of combustion chambers for tightness for what it is necessary to address in a specialized workshop. As a result of tests places of leaks have to be precisely established and the quantitative characteristic of leak is given.