Dodge Omni/Horizon/Rampage 1978-1989 Repair Guide

Checking Engine Compression

Print

A noticeable lack of engine power, excessive oil consumption and/or poor fuel mileage measured over an extended period are all indicators of internal engine wear. Worn piston rings, scored or worn cylinder bores, blown head gaskets, sticking or burnt valves and worn valve seats are all possible culprits here. A check of each cylinder's compression will help you locate the problems.

As mentioned in the Tools and Equipment section, a screw-in type compression gauge is more accurate that the type you simply hold against the spark plug hole, although it takes slightly longer to use. It's worth it to obtain a more accurate reading. Follow the procedures below.

  1. Warm up the engine to normal operating temperature.
  2.  
  3. Remove all the spark plugs.
  4.  
  5. Disconnect the high tension lead from the ignition coil.
  6.  
  7. Fully open the throttle either by operating the throttle linkage by hand or by having an assistant floor the accelerator pedal.
  8.  
  9. Screw the compression gauge into the No.1 spark plug hole until the fitting is snug.
  10.  


WARNING
Be careful not to crossthread the plug hole. On aluminum cylinder heads use extra care, as the threads in these heads are easily ruined.

  1. Ask an assistant to depress the accelerator pedal fully on both carbureted and fuel injected vehicles. Then, while you read the compression gauge, ask the assistant to crank the engine two or three times in short bursts using the ignition switch.
  2.  
  3. Read the compression gauge at the end of each series of cranks, and record the highest of these readings. Repeat this procedure for each of the engine's cylinders. Compare the highest reading of each cylinder to the compression pressure specification in the Tune-Up Specifications chart. The specs in this chart are maximum values.
  4.  

A cylinder's compression pressure is acceptable on these cars if it is not less than 75% of maximum. The minimum pressure for these engines is 100 psi. (690 kPa).

  1. If a cylinder is unusually low, pour a tablespoon of clean engine oil into the cylinder through the spark plug hole and repeat the compression test. If the compression comes up after adding the oil, it appears that the cylinder's piston rings or bore are damaged or worn. If the pressure remains low, the valves may not be seating properly (a valve job is needed), or the head gasket may be blown near that cylinder. If compression in any two adjacent cylinders is low, and if the addition of oil doesn't help the compression, there is leakage past the head gasket. Oil and coolant water in the combustion chamber can result from this problem. There may be evidence of water droplets in the oil film on the engine dipstick when a head gasket has blown.
  2.  



Click image to see an enlarged view

Fig. Fig. 2 GENERAL ENGINE SPECIFICATIONS



Click image to see an enlarged view

Fig. Fig. 3 VALVE SPECIFICATIONS



Click image to see an enlarged view

Fig. Fig. 4 CRANKSHAFT AND CONNECTING ROD SPECIFICATIONS



Click image to see an enlarged view

Fig. Fig. 5 PISTON AND RING SPECIFICATIONS



Click image to see an enlarged view

Fig. Fig. 6 TORQUE SPECIFICATIONS

 
label.common.footer.alt.autozoneLogo