Compression Test ~ 1988 Chevy Nova/Toyota Corolla

Question

I have a 1988 Chevy Nova/Toyota Corolla I-4 4ALC and just performed a dry & wet compression test. My readings on all 4 cylinders were all about the same.. 1st turn over (puff) ran about 80 psi and 4th turn over ran at about 100 psi.. I then performed a wet compression test and got the same readings. I checked the shop manual and saw that readings should be between 124 - 160 or something very close to that. I'm simply trying to get a few opinions on why the compression could be so low even after performing the wet test. Car has 126k miles, smokes a lot, uses about 1qt of oil every 150 miles, but strangely it runs good!! Could this be piston rings or valves? Any suggestions?? Thanks!!

Answer 1

if you get the same reading after putting 3 squirts oil in the cylinders, then the rings are not the problem causing the low compression.

Not to say that the rings aren't an issue, but not the cause of the lower than expected compression.  The oil rings are probably stuck to the ring lands allowing more oil to remain on the cylinder walls.  What's important to remember is that all of the cylinders should read the same. The lowest reading should not be less than 70 percent of the highest reading.  No cylinder reading should be less than 689 kPa (100 psi).  This spec was taken from a 1998 Pontiac Grand Prix just as an example.  There are two other important base mechanical diagnostic tests to use.  The first being a cylinder leak down test.  This is where you use a cylinder leakage tool to apply a regulated amount of air pressure into the cylinder with the piston on TDC with both the intake and exhaust valves closed.  The cylinder leakdown tool has a gauge on it that will measure the amount of air escaping from the cylinder.  If you can hear the air hissing in the exhaust pipe, you know it is coming from the exhaust valve.  If you can hear the air hissing in the throttle body, it is coming from the intake valve.  If you can hear it in the crankcase, then it is going by the rings.  The third test is a running compression test.  This is where you install a compression gauge in one cylinder at a time and run the engine with the spark and fuel disabled from the cylinder being tested.  I don't have the specifications or expected results with me to pass on to you but I have included a link to a web page that describes what a "Dynamic" compression test is and the expected results.  BTW, the proper way to perform a static compression test is to bring the engine up to normal operating temperature.  Remove ALL of the spark plugs.  Disable the fuel and spark systems to all cylinders.  Prop open the throttle blade with a screwdriver or a block of wood. Crank the engine through at least 4 compression strokes in the cylinder being tested.  Check and record the readings on the gauge at each stroke.  Remove the compression gauge and repeat for all cylinders.  

When the compression measurement is normal, the compression builds up quickly and evenly to the specified compression on each cylinder.

When the compression is low on the first stroke and tends to build up on the following strokes, but does not reach the normal compression, or if the compression improves considerably with the addition of three squirts of oil, the piston rings may be the cause.

When the compression is low on the first stroke and does not build up in the following strokes, or the addition of oil does not affect the compression, the valves may be the cause.

When the compression is low on two adjacent cylinders, or coolant is present in the crankcase, the head gasket may be the cause.

Answer 2

It could be any of the pistons, ring, or valves.. or it may be all of them.. if its not leaking water out or into the oil.. then that crosses out alot.. but youre using oil.. so.. just get the heads/head checked and check out the combustion chambers.. pistons.. etc..

Answer 3

Could be either/both. With that amount of miles, even well-maintained metal components are going to be a little bit worn. While it's slightly more likely the rings/piston walls are a little worn, only a tear-down and inspection would give you a definitive answer.

Answer 4

Yeah, you are looking at mostly rings and valve ring leaking. Time for an over haul.