Overhead cam engines come in all sizes and configurations. Some have single overhead cams, some have dual overhead cams. Some have belt drives, some have chain drives, and some use both. Some overhead cams operate the valves directly and some use followers. But despite their many differences, one thing all overhead cam engines share in common is a need for good upper valvetrain lubrication.
When the camshaft is mounted in the cylinder head rather than the block, it increases the distance between the valvetrain components and the oil pump. It takes plenty of oil flow to keep the cam and followers lubricated, so anything that slows or restricts the flow of oil to an overhead cam will cause trouble. The oil also has to be clean and be the correct viscosity to maintain oil film strength between the cam journals and head, and the cam lobes and followers or valve buckets. Cam journal clearances must also be within wear limits to maintain good oil pressure, too.
The most common problems that afflict overhead cams, therefore, are usually lubrication related.
Most newer engines specify 5W-30 oil for several reasons. One is that the thinner oil improves fuel economy. The other is that it flows better at low temperatures, which reduces the lag time it takes oil pressure to reach an overhead cam when a cold engine is first started.
If a late-model OHC engine is filled with a heavier viscosity oil, say 20W-50 during the summer, the oil may be too thick when cold weather arrives.
This could delay oil pressure reaching the cam and upper valvetrain components, resulting in valve noise and possible cam journal damage. A 10W-30 viscosity oil should provide good lubrication most of the year. But for subzero weather, a 5W-30 would be the preferred lube. For really cold climates like Alaska (or Chicago), a 0W-30 synthetic might be even better. Some European automakers now specify 0W-30 as the recommended oil for their motors for year-round driving.
The important point here is to follow the vehicle manufacturer’s oil viscosity recommendations. If you don’t know what kind of oil a customer’s engine requires, check the dipstick or the owner’s manual.
Regular oil changes are especially important on OHC engines that subject the oil to higher temperatures, such as turbocharged and supercharged engines, performance engines and those with known cooling problems in the upper cylinder head area. Many OHC heads today run much hotter than their pushrod counterparts, and subject the oil to very high operating temperatures. Consequently, some of these engines are experiencing lubrication problems when oil change intervals are stretched out too far.
Oil neglect results in viscosity breakdown, oil oxidation and sludging. Sludge is a lousy lubricant and can plug up oil screens, oil galleys, hydraulic lash adjusters and followers, to name a few. Several vehicle manufacturers have experienced severe engine sludging problems with extended oil change intervals, so that’s a valid reason for continuing to recommend the traditional 3,000-mile oil and filter change.
OHC Cam Bore Damage
The most common problem in OHC engines is worn and damaged cam journals in the cylinder head. In most late-model OHC engines, no cam bearing inserts are used. Instead, the camshaft runs directly on machined bearing surfaces in the head. The aluminum casting provides a good bearing surface, but also requires a steady film of oil between it and the cam to prevent wear and metal-to-metal contact. If the journal runs out of oil or the oil is dirty or oxidized, wear, galling and seizure can occur.
Some engines where OHC cam journal bore wear is often found include the Ford 4.6L V8 (the wear is often worse on the right head), 135 and 153 Chrysler engines, Neon 2.0L DOHC engine, 22R Toyota and Saturn single and DOHC engines.
When an OHC cam journal runs out of lube, it isn’t pretty because there are no bearing shells to spin and absorb the damage. It either chews up the journal in the head, or binds the cam, which may cause the cam or cam drive to fail. We’ve seen OHC cams that have snapped in two or have sheared off the drive gear because a journal seized.
The cure obviously isn’t cheap. It usually requires removing the cylinder head(s) and either repairing or replacing the head(s). There are a number of ways to salvage a damaged head. One is to line bore or hone the OHC cam bores to accept a new cam with oversized journals (provided one is available for the application). If the head has removable cam journal caps, the face of the caps can be ground down and the bores machined back to standard size to restore the journals. If there are no caps, the cam bores can be enlarged to accept bearing inserts (if inserts are available).
Another problem to watch out for is a bent cam. Mishandling a new camshaft or pulling an engine out of a car using the OHC cam as a handlebar for the engine hoist chain hooks can bend it. It only takes a few thousandths of an inch of misalignment to create a bind between the head and cam. This may prevent the cam from turning in the head, and will likely cause accelerated wear in the cam bores or bearings.
To check cam straightness, place the cam in a pair of v-blocks (one at each end), then rotate the cam with a dial indicator placed on the middle journal. If runout exceeds specs, the cam is bent and needs to be straightened or replaced.
Watch for Overheating
Overheating can be damaging to any engine, but especially OHC engines with aluminum heads. Aluminum expands at a much higher rate than cast iron, so if the engine gets too hot, the head tends to swell and bow in the middle. This may damage the center cam journals or cause the cam to bind or seize.
Overheating also will cause an OHC head gasket to fail. As the head swells from the heat, it smashes the gasket. When the engine cools, the gasket may not retain enough thickness or elasticity to prevent oil, coolant or combustion leaks.
If the head gasket has failed, check the straightness of the face of the head. If it is out of specifications, the head will have to be straightened and/or resurfaced.
OHC Cam Drive
OHC timing belts have a nasty habit of snapping unexpectedly at high mileage. Heat and friction weaken the cords that reinforce the belt, and the risk of failure goes up sharply after about 60,000 miles on most OHC engines built before the mid-1990s. That’s why most vehicle manufacturers recommend replacing OHC timing belts for preventive maintenance at 60,000-mile intervals.
Starting in the mid-1990s, OHC belts made of more durable materials began to appear with 90,000- to 100,000-mile replacement intervals. These new “long life” belts are made of a special high temperature grade of neoprene called “highly saturated nitrile” (HSN). HSN belts extend belt life up to 50% or more, and reduce the need for periodic belt replacement. But no belt lasts forever, so follow the OEM-recommended replacement interval.
A broken timing belt can cause a lot of damage in an engine if the valves hit the pistons. So if the engine is an “interference” application, replacing the belt at the recommended interval will greatly reduce the risk of bent valves or shattered pistons caused by a belt failure.
Check It Out!
If a visual inspection of an OHC timing belt shows the belt is glazed, has missing or damaged teeth, cracks or fraying, the belt must be replaced. Don’t delay because there’s no way to know when the belt might fail.
A timing belt also may have to be replaced if it is making objectionable noise. Check pulley alignment and belt tension first. Also, if the belt shows sign of physical wear, check the condition of the pulleys. There should be no nicks, rough spots or other damage that could chew up the belt. If a pulley is worn or damaged, it must also be replaced.
If a timing belt has snapped, the engine won’t run because the camshaft won’t rotate when the crankshaft turns. Consequently, you won’t find any compression or vacuum. If the engine has a cam position sensor, a cam-driven distributor or an ignition pickup that triggers off the cam drive, you also won’t find any spark – which can make diagnosing a no-start confusing until you realize what’s going on.
A quick way to confirm a broken timing belt on an OHC engine with a cam-driven distributor is to remove the distributor cap and see if the rotor moves when the crankshaft is turned by hand. If there is no distributor, remove the oil filler cap or a valve cover and watch for cam or valve movement when the crankshaft is turned. You also can remove the timing belt cover and check the belt.
Sometimes a belt will jump one or more teeth, throwing off cam timing. If this has happened, the engine may still run, but valve timing will be retarded causing low compression and vacuum readings. Ignition timing will also be retarded if the engine has a cam-driven distributor. If the belt is loose and you suspect it has jumped timing, check the alignment of the timing marks on the cam drive and crankshaft. Also check the teeth on the belt for wear or damage, and the belt tensioner adjustment.
On OHC engines where the water pump pulley tensions the timing belt, a failure of the water pump shaft bearing may cause enough loss of tension to allow the belt to jump time. So any time you’re replacing a failed water pump on such an engine, don’t assume cam timing is correct. Always check the timing marks to make sure the belt hasn’t jumped time. This is especially important on engines that have balance shafts. A slipped belt can throw off balance, too, creating annoying vibrations and harmonics.
Before you remove an OHC timing belt, rotate the crankshaft until the timing marks indicate top dead center. On most engines, you want the No. 1 piston at TDC on its compression stroke (not exhaust stroke) to get proper timing alignment. Many engines don’t have any external timing marks, so you’ll have to remove the timing belt cover and line up the marks on the camshaft pulley(s).
On some vehicles, you may have to use a special tool to hold the crankshaft and/or camshaft(s) in position while the new timing belt is being installed and tightened. On GM’s 3.0L DOHC V6, for example, GM says to use a crankshaft holder tool (J42069-10) and camshaft holders (J42069-1 and J42069-2) or something equivalent to prevent movement while the timing belt is wrapped around the pulleys and tightened.
Some engines, such as GM’s 3.4L DOHC V6 and Chrysler’s 2.0L SOHC Neon engine, have a hydraulic tensioner that uses oil pressure to keep the belt tight. On these engines, the tensioner must be prepared before it is reinstalled by draining out the oil, fully retracting the plunger and refilling it with 5W-30 motor oil.
Handle with Care
When installing a new OHC timing belt, use care not to nick, twist or squeeze the belt excessively while you’re working it into place. Squeezing or crimping a belt to a small radius may damage the internal cords. Timing belts do not stretch, so never attempt to force one around a pulley. If the belt won’t go on, something is misaligned or misrouted, or you have the wrong belt for the application (it happens!).
Once the belt is in place, make sure all the timing marks are in alignment, then install or adjust the tensioner so the belt has the correct amount of load. Automatic tensioners and hydraulic tensioners will apply just the right amount of pressure, but if you have to set belt tension manually, don’t overdo it. Excessive tension puts added stress on the belt and pulleys and can lead to premature belt failure. Follow the vehicle manufacturer’s recommendations for belt tension and use a belt gauge to be accurate.
As a final check, rotate the crankshaft twice and recheck the timing marks to make sure they are still in proper alignment. If everything appears to be okay, replace the belt cover. Then place a label on the engine indicating the belt has been replaced and write down the odometer reading. This will alert other technicians that this service has already been performed.