July 2003 Articles & Features
By Thomas Marcy | July 2003
When a dashboard oil light comes on, it may alarm your customer and send a chill through you. But knowledge of oil circuit workings can dispel the fears and get the fluid flowing.
Oil is a nearly miraculous substance. In a mechanical device, such as an automobile engine, it reduces friction, cushions impact and seals pressures. It also cleans, cools and protects parts from corrosion. So when a vehicle's oil light comes on, the wise motorist will want to have the problem checked out immediately
That's where you come in. Not to worry, though. For the savvy technician, tracing lubrication troubles can be relatively straightforward.
Three things are needed to supply oil to engine components: First, a pump must be able to do the work. Second, the oil must be the proper viscosity. And third, the output circuit flow must be controlled.
A modern engine's oil pump is a super-efficient, close-tolerance, positive-displacement gear, vane or gerotor pump. Positive displacement means the pump has a specific volume. It thus gushes exactly its pumping chamber's liquid capacity in one drive revolution, and efficiently doubles the flow in two turns. Oil output is proportional to speed.
Pump clearances must be tight. As the pump operates, atmospheric pressure in the oil reservoir, usually the engine's crankcase, pushes oil through the pickup screen and into the pump. Then pump gears, chambers or rotors trap the oil and carry it to the output passage. Because it's a liquid, the oil cannot be compressed, so it squeezes from the pump as more oil arrives.
Now the problem is, since the oil cannot be compressed, the pressure can then skyrocket and destroy the circuit. For this reason, any positive-displacement liquid pump needs a pressure regulator valve to blow off excess liquid volume. Any good, properly installed oil pump can deliver more oil than the engine requires. The pressure regulator valve is set by the engine maker to ensure that the pump provides adequate pressure, with excess oil pressure being relieved. Excess pressure causes misting and oil aeration. Each engine bearing clearance and lubrication spit hole is a calibrated minor restriction as it leaks oil. Each drop contributes to a precise leak rate from the moving parts back to the engine sump. If the pump wears or turns too slowly, if leaks get too large or if the oil gets too thin, the oil pressure drops.
If an oil warning light comes on as pressure troubles arise, hopefully your customer's car has an engine-kill feature, or the driver is savvy enough to quickly shut it down. If the engine is shut down promptly, you now face the problem of pressure failure diagnosis without harming the engine. Don't start the engine! Some say an engine may run for a short while without oil pressure, but it's not advisable to work under this assumption.
First, check the quantity of oil available and its quality. Is the oil clean? Next, check the engine for seizure or internal interference. Use a torque bar and socket to spin the engine using the crankshaft damper pulley bolt. Turn the crank two complete revolutions in the direction of engine rotation. The engine must turn freely. This test will reveal not only engine seizure, but also a jumped timing chain that causes piston-to-valve interference and the oil pump to cease working.
If there's sufficient clean oil in the sump and the engine rotates, connect an oil pressure gauge to the engine's pressure tap. Install a hydraulic tee fitting at the electric oil pressure sending unit tap. Double-check the pressure. Don't rely on the dash oil pressure gauge for a reading; the gauge or gauge circuit may be defective.
Your next step is to disconnect the ignition. If necessary, also disconnect the fuel pump's electric supply. Now remove all spark plugs and lay them aside. Shoot a small squirt of oil in each plug hole to lube the cylinders.
If the engine uses a distributor-driven pump, remove the distributor and use a drill adapter to rotate the pump drive. Often this spinning of a hex head or split tang drive is a good method to run only the pump and check pressure. This is also a good way to prime the entire lube system before starting a newly rebuilt engine.
Also note that you may use an accessory pump to check for oil system integrity. You can artificially introduce oil pressure to the system using an accessory engine bearing clearance tester. To do that, install the test pump and, without cranking the engine, start the external pump and watch the pressure gauge. If there's pressure, it means fairly tight clearances exist in the lube system.
If an oil galley plug popped or any other inadvertent oil drain opened, a gross lack of pressure results. If this happens, you can often use shop air to find the leak. Drain the engine oil and leave the oil pan drain plug removed. Next, remove the oil filler cap. Don some safety glasses and insert the air hose nozzle into the lube system through the oil pressure tap. Use a stethoscope, or hold a heater coolant hose to your ear to determine where the air leak is loudest. Listen at the oil pan drain, filler cap, timing chain cover or lifter galley. Unless you have a borescope to give you a look into remote places, once you hear the leak loudly, you may need to do some engine disassembly to pinpoint the source. But remember, before you go this route, check with the customer for repair authorization.
If your accessory pump method produces pressure, it means the system is intact but the engine's pump flow may be insufficient or blocked. Remove your test pump (if you used one), then the oil filter and place a catch pan under the area. You've lubed the engine's cylinders and primed the engine bearings, so now crank the engine without the spark plugs installed.
Watch the filter feed outlet. There's no compression fighting the starter, so the engine should turn over quickly. You should get rapid oil flow if the pump is operating. If you don't, the trouble is in the pump or the pump drive. Typically, technicians who've worked on older pushrod engines assume the pump hangs from the bottom of the block, driven by a gear and drive rod from the cam. This may not be the case. Many engines today have a crankshaft-driven pump. We've recently seen a crankshaft-driven gerotor pump with a sheared drive key. It was wiped out when plastic timing gear shards jammed it.
If oil flows from the filter housing, however, suspect a plugged filter and filter outlet circuit. Check the filter and note whether a bypass is built into it. Trace the filter output, clearing it out as needed with shop air reversed through the circuit. Engine machine shops use cleaning rods and brushes pushed through oil passages to clear debris.
Servicing the Pump
To service the pump, open the sump to expose the pickup screen. Is the screen at the proper height in the sump? If it's clogged, the screen and pickup must be replaced. Don't just wash it! Washing the screen with solvent or burning away the varnish may loosen mesh wires, which can break loose, jam the pump and cause engine damage. Examine the pickup tube. Look for holes that may allow air to be sucked into the pump, especially on vehicle turns. A leak high on the tube or a damaged O-ring may stop pump flow entirely or cause the pressure to fluctuate.
Don't assume the pump is okay if the screen is clogged; it may be scored due to the lack of lube. Inspect the pump. First, clean it for disassembly. Now remove the pump endplate. Be observant: A '96 Dodge van we recently repaired had a loose pump endplate retaining bolt in its 318-cubic-inch engine that would allow the pump to suck air and lose oil pressure, but only when the engine got hot.
Clean the disassembled pump. Inspect the endplate for wear, scoring and flatness. Also inspect the pump body, gears, rotors or vanes and pressure relief valve assembly for abnormal wear, scoring, corrosion or signs of fatigue. Engines with an integral pump built into the front timing cover may require an expensive rebuild if the body is scored. Although kits are available that increase the pump's capacity on some engines, no machining process may exist to recover the front housing. It's an expensive part, so look at the pump closely!
If the drive rotor or gear has a stub shaft, lift it slightly to check the stub shaft bushing. If the shaft wobbles excessively, the bushing is worn. Remember to remove all parts from the pump. This means take out the pressure relief valve and spring. But be careful! Keep your face clear when you remove the spring-loaded pressure relief valve cup plug.
Look for foreign material in the valve bore and check the relief valve or ball for sticking. Pressure may drop if the pump dumps all the oil back to the sump or returns the flow to the inlet side of the pump. Check the condition on the spring and make sure it's properly tensioned. Spring tension is determined by the spring length when relaxed.
Now put the gears or rotor back into the body, making sure to check the wear clearances. Place a straightedge across the installed gears and use feeler blades to determine pumping unit end play. Check the wall clearances and backlash. If the pump fails these tests, it must be rebuilt or replaced. Inspect the drive and replace any worn parts. Some gears have timing marks; if so, line them up properly upon reinstallation.
Whether you install a new pump or rebuild an old one, prime the pump before starting the engine. Submerge a new or rebuilt pump in the same oil you plan to use in the sump, and spin the drive. Remember that most pumps won't self-prime. Use new gaskets or O-rings during assembly. If the pump is part of the timing gear cover, use petroleum jelly to pack it. The lube seals the tight running clearances and helps create a vacuum at the pump inlet.
Install the new pickup screen and make sure it's properly positioned in the sump for oil pickup. Next, clean the oil pan thoroughly. A pump may fail after an engine has eaten a timing chain or valve seals. The pump then sucks up material after repairs are completed, often in cold weather as the thick oil and foreign material bypass the partially clogged wire filter screen.
Be thorough. Clean the pan and any windage trays, baffles and gasket flanges. Install the pan gaskets or seals, then the pan and torque the bolts to spec. Next, fill the sump with the recommended oil.
The oil you fill the engine sump with should match the manufacturer's viscosity recommendations for the load use of the engine and the temperature ranges expected. Make sure the fill fits the "S" rating needed for detergents and additives. Next, prefill the filter. Lube the spin-on filter seals and install the filter to the proper torque. Some are installed just hand-tight-enough to seat the seal, but not so tight that the next technician can't get it off 3000 miles down the road.
Disconnect your test pump (if you used one) and reinstall the sending unit. Next, prime the pump with a drill or crank the engine over with the ignition and injectors disabled. Oil pressure should build. Recheck the oil level. Lastly, enable the ignition and fuel systems before restarting the engine.
Make sure the idle speed and oil pressure readings are correct. Let the engine run until it reaches operating temperature. The pressure may drop slightly as the oil thins, especially if a light-viscosity oil was chosen. But the pressure should remain within the carmaker's specs at idle, and come up to maximum pressure relief levels when engine speed is increased.
Now raise the vehicle on a lift, if needed, disconnect your oil pressure gauge and check for oil leaks. If there are none, take a test drive to make sure no engine noises exist and the dash oil pressure readings remain good.
If the engine provides adequate performance and oil pressures are at the proper levels, you can breathe a well-earned sigh of relief. And your customer will thank you for correcting the problem before it caused more serious-and expensive-damage.