Hello all,

I have mostly had experience with overhead cam engines so I am not very familiar with hydraulic lifters / cam in block engines and wanted to clarify some things about them.

Basically from what I understand the lifter rides on the cam lobe and transfers the lifting motion of the cam to the pushrod, and on to the valve. The two types of lifters I've seen are solid (with manual clearance adjustment) and hydraulic (which adjust clearance on the fly).

From what I understand the clearance in the valvetrain changes because the block and head expand as the engine warms up which causes the camshaft and rocker to get further apart and would change the valve lift.

With solid lifters you would adjust the valves so there is some clearance when they are cold that is taken up when the engine warms. If adjusted too tightly, the valves could be held open on the base circle of the cam and never close completely, which is likely to cause misfiring due to lack of compression, if adjusted too loosely, they are likely to be very noisy because of the impact that occurs as the slack is taken up, also with a loss of cam lift, and reduced cylinder breathing.

Am I correct on those points?

So with hydraulic lifters it seems that they run in lifter bores which are fed by a common oil gallery which is toward the end of the oiling system of the engine, so low oil pressure or volume problems show up as lifter noise on these engines?

The basic idea with the hydraulic lifters seems to be that they have an oil feed hole that allows the gallery to pressurize the body of the lifter and expand it when the lifter is down, and then as the cam starts lifting it a checkvalve closes and prevents the oil from draining out, making the lifter turn "solid" as it works against valve spring pressure to open the valve.

The clearance that increases as the engine warms up is taken up filling the lifter more and more. The lifter also drains down slowly as oil bleeds through the .0001 clearance between the body and plunger, as the valve spring tries to crush it down so if the vehicle is parked and left to cool off overnight the valves adjust so the clearance is proper for the cold engine start the next day.

That's how it works as I understand it, now on to malfunction. Malfunctioning lifters are often noisy, sometimes referred to as "collapsed", which I take to mean the check valve is not working and the lifter is not holding the oil or not filling. Maybe there is no oil pressure there to fill it? If the lifter collapses isn't that the same as a loosely adjusted valve? there will be a loss of cam lift?

Another failure referred to as a "stuck lifter" I think means that the lifter plunger and body has seized together and no longer adjusts to fill the slack correctly, so you could have a lifter that is "pumped up" to the level of a hot engine and stuck there which then holds a valve slightly open when the engine cools off. Or if the lifter stuck at the cold expansion then you would get that tapping noise that doesn't go away after warming up and maybe a slight loss of cam lift?

Another issue is with valve float - at high rpms with worn out valve springs, the lifters could pump up too much because the valve spring has weakened too much and can't keep the lifter down - a competition between the oil pressure trying to expand and take the clearance up and the valve spring trying to keep the lifter against the cam.

If there was a lifter problem affecting the cylinder would you definitely see it in the compression reading? I would think that a cylinder would still seal properly but might not pack as much air in with reduced valve lift?

Am I understanding this correctly or have I got a big misunderstanding?

Andrew Libert
Technician
General Motors
Ann Arbor, Michigan, USA