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Repair Topics / Engine
Compression
Car Engine Compression
Questions and Answers This
category contains featured questions as
examples. They relate to most vehicles.
Question: I read how to fix
my problem (or at least find out what the
problem is) on your site but what do you
mean when you say a cylinder leak down test
must be done?
Answer: After doing a
compression check, if one or more of the
cylinders tests low, then a cylinder leak down
test will help you determine the cause of the
weak cylinder. Special equipment (leak down
gauge) is required which can be purchased at
most auto parts stores. With all the spark plugs
removed, rotate the engine so that the cylinder
to be tested is on top dead center and ready to
fire. This will assure that the valves are
closed. Install the leak down gauge in the spark
plug hole and apply air pressure.
The gauge will register the pressure drop
indicating the percentage of leak. In addition,
by listening at the exhaust pipe or air intake
system or the oil breather you may hear escaping
air, which means one of the components, is
leaking. Example: air out of the exhaust pipe
means the exhaust valve is leaking. By removing
the radiator cap and observing the coolant, you
can tell if there is a blown head gasket and or
a cracked head if there are bubbles.
Question:
1998 Olds Cutlass Ciera mileage: 100,000.
I recently did a compression test on my car
because of poor running and stalling problems,
and because we wanted to determine if the car's
engine performance was good enough to keep a
little longer before getting other things fixed.
We got the following readings, and would like to
get your comments/opinions on them.
Don't know official cylinder number but will
give them from left to right: #1 - 152; #2
- 165; #3 - 152; #4 - 149. Did
notice that the last cylinder mentioned, jumped
up to about 90 psi real quick like the others,
but then took a little longer to get to 149.
Any comments or suggestions? Are these
acceptably balanced numbers or do I have a
serious problem looming? Thanks for your
help.
Answer:
Your numbers are very respectable for a car with
100,000 miles. From what you have
told us, it sounds like the internal engine is
in good shape.
Question:
2002 Chevy S-10 Blazer mileage: 78,200. I
took a set of compression reading the other day,
and need some help analyzing them.
According to what I've read, these compression
readings look good, but I've yet to see an
actual number that would be acceptable.
I just see "lowest cylinder must be 70% of
highest, 100 psi min" does that mean Anything
that falls into that range is fine.
Anyway, here's the readings (took 3 readings
each cylinder, averaged them...all 3 readings
were within +/- 5 psi): Left bank Cyl 1: 163psi,
Left bank Cyl 2: 144psi, Left bank Cyl 3:
146psi, Right bank Cyl 1: 160psi, Right bank Cyl
2: 130psi, Right bank Cyl 3: 146psi. I
then added 3 squirts of engine oil to Rt bank
Cyl's 2 and 3: Rt. bank Cyl 2 w/oil: 135psi, Rt.
bank Cyl 3 w/oil: 140psi. Other helpful
info may be that when I pulled spark plugs, Cyl
2 on both sides were coated with heavy black dry
(carbon?) deposits, and Cyls 3 on both sides
were coated with a good amount of oil.
Cyls 1 on both sides looked to be ok. Any
help on what all this means would be greatly
appreciated!
Answer:
Although the compression readings are not
perfect, they are respectable for an engine with
120,000 miles. The two cylinders that you
added oil to tell us that the rings are in good
condition but the valves may have slight
leakage. Oil and carbon on spark plugs
indicates to us that your valve stem seals may
be allowing oil to get into the cylinders and
foul the plugs.
Question:
2004 Honda Accord Vtec/ 4 cylinder mileage:
56,000. Have a problem with the engine;
mechanic says that I only have 30 psi in first
cylinder, 90 psi in second, and next two are
fine. What do you think is wrong?
Answer:
Have your mechanic perform a cylinder leak-down
test. This will pinpoint whether your
problem is rings, valves or head gaskets.
Question:
1999 Jeep Wrangler mileage: 98,900.
My engine is blowing oil out around the
distributor shaft, oil dip stick, oil pan, and
up into the air filter. A compression check
shows approximately 160 PSI, dry to 175 PSI wet.
All vacuum lines intact and pulling
approximately 20" HG including through the PCV
valve. Would this be a piston and ring
problem or just the head (valve seals) or
something else?
Answer: You may have piston ring
problems. The PCV valve can only handle so
much blow by, the excess will result in the
problem you are experiencing. Try a
cylinder leak down test to check the rings for
blow by.
Question:
1968 Chevy Corvette 327/350, 87,000 miles.
What is the proper procedure to conduct a
compression test?
Answer: Remove
all eight spark plugs. Remove and secure 12-volt
wire from ignition coil. Place compression gauge
in spark plug hole. Crank engine approximately 5
to 10 seconds. Gauge reading should be be
between 135-155 pounds. 
Repair Topics / Engine
Compression
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