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If I see a low cylinder indication on this quick and dirty test, I follow up with a normal compression test. What I found
had me muttering a few words under my breath. I was kicking myself for breaking the rules, and now I had a major engine fault
to explain to my customer.
Figure 2 I think this is a little low for a healthy cylinder.
Was the original repair necessary? Replacing the ignition wires was; however, the coil was a rushed diagnosis. The low firing
line was a result of low compression – not low spark energy. Remember, the firing line is typically affected by pressure,
gaps in the system and the amount of hydrocarbons available for conduction. The scope was trying to tell me something. I just
wasn't listening, and saw what I wanted to see based on an assumption.
NOW LET'S FIX IT
The next step I take when I have a low compression reading is to perform a cylinder leak-down test. This test uses a tool
called a differential cylinder pressure tester and has two gauges on it. One indicates line pressure (supplied by shop air),
and the other is the pressure being contained in the cylinder.
Figure 3 A 20 psi difference is roughly equivalent to a 20 percent loss of sealing.
When connected and with the cylinder to be checked at TDC of it's compression stroke, the tool pressurizes the cylinder and
will give you a reading as shown in Figure 3. The left side reading is line pressure; in this case, it's 90 psi. The right side gauge reads the pressure in the cylinder:
70 psi. That's a 20 psi difference, or a little more than 20 percent of line pressure. Standard specification is no more than
10 percent difference.
With the shop air line connected, I removed the oil fill cap, radiator cap and air filter housing. That 20 percent of air
pressure is going somewhere, and you can actually hear it escaping. That's the nice thing about this tool. It allows you to
hear if the loss of compression is from the valves (air escaping from the throttle body or exhaust pipe), the rings (air escaping
from the oil fill) or from the head gasket (air escaping from the radiator).
This one was a no-brainer. Air was rushing out of the throttle body with no evidence of air flowing through any of my other
checkpoints. OK, now I've got it: The intake valve is leaking. I got authorization to remove the head, confident that this
was the problem.
With the head removed, I verified the valve was leaking by pouring solvent into the intake port and looking to see if any
leaked past the valve on the combustion chamber side. It began to pour out as soon as the solvent got to the valve face.
Figure 4 With the head redone, there is no loss. But compression is still low. Why?
But because I had been burned on my first diagnosis, I needed to be extra thorough. I also checked the installed valve height
to see if there might be a problem with bent valves or recessed valve seats and found no problems there. I inspected the push
rods for damage, and the cam lobes for wear. While the head was off, I rotated all the cylinders to the bottom of their travel
to look for damage to the cylinder walls. Everything looked good.
I got the head back a few days later and reinstalled it on the Mustang. I turned the key, and the miss was still there.
Peter Meier is an ASE-certified Master Technician, member of iATN and full-time tech in Tampa, Fla. His experience reaches back over 30 years, and his contributions to Motor Age reflect a wide variety of experience with almost every make and model.
Articles by Peter F. Meier
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