I have a 2000 Honda Accord Sedan 350,000 miles. I recently go a compression test showed which showed 120, 90, 70 and 60 pounds of pressure in each of the cylinders. Do you have an idea about what the compression test means about the engine?
That refers to how hard each piston is squeezing the air it sucks in. Even 120 pounds is low. The rest are real low. That can be expected with such high mileage. Piston rings seal the air in and prevent it from leaking out into the lower part of the engine where the oil is. Those rings normally wear with age. There's also valves that let the air in and the exhaust out. Over time they too will not seal as well as when they were new.
A compression test is just a fast way of determining the quality of each cylinder and its ability to make power equal to all the other cylinders. Your mechanic can also do a more involved "cylinder leakage" test, or "cylinder leak down" test. That will identify exactly what is responsible for the low compression. They will likely elect to not do that test with the mileage you listed because everything is going to be worn. No professional will do a valve job, for example, and leave all the other worn parts in the engine. This is likely a case where the engine served you well but it's time for a total rebuild.
January, 24, 2013 AT 2:15 PM
So I basically need a new engine?
January, 24, 2013 AT 7:20 PM
It has 350,000 miles. It doesn't owe you anything.
You haven't described any problem or symptom so it's impossible to know what the best remedy is. You also have to look at how the compression test was done and if correct procedure was followed. The same conditions must be present for each cylinder's test. Looking at the numbers you listed, with each one getting lower in order, you'd have to suspect the procedure might be at fault. First of all, a battery charger should be attached to the battery and set to a slow charge rate. The compression test requires cranking the engine at least five revolutions and doing so runs the battery down. By the time you get to the last cylinder, it's going to be cranking slower which will make the readings lower.
During cranking, oil gets scraped off the cylinder walls. That oil helps the piston rings seal and do their job. It gets sprayed onto the cylinder walls when the engine is running at normal speed. When cranking the engine, it's running too slowly to spray oil so by the time you get to the last cylinder, it's going to read lower than normal. Experienced mechanics will check for that by repeating the test on the first cylinder after all four have been done. If it reads a lot lower than the first time it was tested, the procedure is more to blame than the engine wear.
The engine must also be warmed up before starting the compression test. If the mechanic gets interrupted in the middle of the test and the engine has time to cool down, parts contract and don't fit properly. The piston rings won't seal as well as normal.