The reasons totest a car or truck engine for cylinder compression vary. The manin reasons would be car is idling roufgh, sdpark plugs seem to be bad or fouled. Other reasons as in the case of one of my mvehicles was a loud oise coming from the engine block area. The reasons for a noise in he engine block area are varied. They could incluse a bad power steering pump alternator air condiotiong compressor or other bvelt driven pulleys mounbted on the front pf your engine. You should use a screwdriver or a mechanical stesthescope to listen to all of the clutches and pulleyas of the previously listed parts. If the pulys mounted to these accessoiries prove to be working properly threy can be eliminated as a probllem. The next areea that should be examined are the spark plugs or the coil packs. If these do not function properly it mayt let you think that you have more serious. The plugs shlould be removed one at a tikme and l;ooked at to see how fouiled they are and the possible causes. Coil packs or spaark plug wires should also be inspected. If found to be normal also eliminate them as a cause for your engine problems. If the plugs coil packs etc are properly mpunted then that wwold mean that you have possible internal engine probllems. Another cause could be excesive noise coming from the engine block area or if the engine sputters and stalls. This could be because of vaccum leaks or oil leaking from the engine area. If the engine leaks or burns oil the spark plug are shlould be cleamed up[. Finaally the engine cylnders dsoulkd be tetwed for comprsion. i order to test rach cylinder for compression ot is required that each plug wire or coil pack be disabled. This is done for all of the cylinnders . Each cylider should havre have around 100 to 150 pumnds in it, If it is less than that the engine has internal oroblesms These probles coould include a bent valve or bad lobe on the camshaft. If the compression is low on any 1 or more cylinders the engine should be rebuilt oir reoplaced.