Some thoughts on hydraulic lifters & on oil consumption

February 1, 2009 | By Richard Prince

Question:

I’ve been enjoying your magazine the past few months but have some different opinions about a couple of answers that were given in the “Mechanic on Duty” section regarding hydraulic lifters and oil consumption. Myths are common about “lifter pump up.” The “piston” in most hydraulic lifters measures .625”, meaning it has an area of 0.307 sq. inches. If you have oil pressure of 50 lbs. per sq. inch, the maximum force the plunger in a lifter could exert is less than 16 lbs. Lifter pump up can occur if the valve spring has less pressure than the piston in the lifter can exert. This can be from valve spring wear, spring damage or valve seat recession. Pump up can also occur during extreme high engine RPM where the valve spring tension is not adequate to return the valve to the seat or harmonics in the valve train occur. The hydraulic lifter will then do what it is designed to do, make up the clearance and hold the valve open until the spring pressure bleeds the oil back out of the lifter. Most hydraulic lifters used in pushrod engines have about .250” plunger travel. Some late 1960s or early 1970s Mopars had lifters with only about .125” of travel. There are numerous engines that do not have any adjustment for the hydraulic lifters. As long as there is enough plunger clearance in the lifter to account for expansion and contraction of the engine, the hydraulic lifter function is perfect. There is a common practice of setting hydraulic lifters with a 1/8 to 1/4 turn compression only. This is used to help keep the valves from hitting pistons during pump up from over revving the engine, and to reduce the time it takes for the oil to bleed out of the lifter after over revving.

The bottom line is that as long as the lifter plunger does not contact the bottom of the lifter or the retaining clip at the top, it will perform as designed under normal engine use if proper valve spring pressure is maintained.

On the topic of oil consumption, there are some basic facts that are often overlooked by experienced mechanics.

1. You can have good compression rings, which yield a good leak down test and the engine can still use oil because the oil rings are not doing their job.
2. You can have bad compression rings, which will yield blow-by, and the oil rings can still do a decent job of keeping oil from migrating into the combustion chamber.
3. If you see visible smoke from the exhaust system the oil is being pulled into the combustion chamber through the rings, intake guides, PCV system or intake manifold and burned in the combustion process.
4. If you have excessive oil consumption without visible smoke or leakage, the oil is most likely being pulled through the exhaust guides when the exhaust valve is closed, which has a negative pressure behind it after closing. When oil enters behind the exhaust valve, the high temperature vaporizes it so you do not see any visible smoke like it creates when it goes through the combustion process.
5. A failsafe method of determining if the rings are the culprit for high oil consumption is to look at the top of the pistons. Ring induced oil consumption will show a clean area around the edge of the piston of about 1/8” to 1/4”. Normal carbon will usually cover the rest of the top of the piston. Borescopes are common tools that can go through a spark plug hole to see the top of the pistons very clearly.