CAMSHAFT PROBLEMS

Diagnosis of Valve Train Problems

If a failure has occurred in the valve train, it is most important to ensure that it does not occur again. Careful checking should take place to pinpoint the true cause of the problem. If the damaged parts are replaced without fixing the cause, needless to say the disaster is going to be repeated. Here is a check list to help diagnose valve train problems.

* Check for unusual wear patterns during disassembly. For example, rub marks on pushrods, wear on rocker balls, wear on valve tips, wear pattern on lifters and cam lobes. Rub marks on spring.

* Keep all parts in order and number lifters and rockers as they are removed from the motor.

* Look at the cam and mark the problem lobes noting which cylinders they are on and which bank if a V8.

* Check if there is a pattern to the problem lobes. Are they all exhausts or inlets ? Are they all from the one bank of the engine ? This can give a lead as to the cause of the problem. For instance if the lobes are all from the one bank, one cylinder head may have been machined more than the other or have spring shims fitted. If the damaged lobes are all inlet or exhausts then further checking of the heads is called for. Look for differences in the valve train such as valve seat inserts, valve guide height etc. If all the failed lobes are exhaust, check the specifications of the cam. If the cam is dual pattern and has more lift on the exhaust then the extra lift may be just over the limit of the valve springs.

* Examine the wear pattern on the UNDAMAGED lobes. Is the wear pattern on the sides of the lobes correct? Is there any scuffing on the sides of the lobe? Does the wear seem confined to the nose of the lobe ?

* Examine the valve lifters. Is the wear pattern the same on all lifters ? Have all lifters been rotating ?

* Check installed height of valve springs. Check open and closed pressure of valve springs.

Lubrication Problems

Lubrication problems are indicated when the sides of the lobe show signs of scuffing. The face of the lifters will also show signs of tearing, rough patches.

The two most common cause of this type of failure are, dry start and excessive slow running.

Dry start this can be caused by failure to use the correct type of lubricant on the cam during assembly, but if the engine is turned over too often before starting the lube will be wiped off. This may result from cranking the engine over to get oil pressure or the engine not starting straight away. Where possible the oil pump should be driven with a drill to prime the engine with oil.

Excessive Idling, The lifter "water skis" over the cam lobe on a film of oil. If the spring loads are high and the engine is allowed to idle for extended periods the oil film breaks down and allows metal to metal contact between the cam and lifter.

The running in period is critical to the life of the cam as the lobes work harden during this period. In many cases of lobe failure the damage begins in the first fifteen minutes of the engines life.

Overloading

This condition is brought about when the load on the cam follower is so great that the oil film between the cam and lifter breaks down causing metal to metal contact and subsequent failure of the lobe, initially at the point of the overloading but as the lifter surface deteriorates the rest of the lobe will be destroyed. Overloading can be caused by the valve spring having excessive open pressure. This often occurs when a spring designed for standard (low valve lift) camshafts is used in a high performance (high valve lift) application. A standard valve spring will have a high spring rate. This means that the pressure rises rapidly over a short distance, a performance spring on the other hand will often have a lower rise rate than a standard spring as it is designed to have a greater travel to allow for higher valve lift. The maximum allowable nose loading on a cast billet camshaft is 380lb, most performance cams operate satisfactorily with far less than this. Race teams spend considerable time testing to find the MINIMUM spring tension as high spring loads waste horsepower.

Valve Train Interference

This is the most common form of overloading and is often accompanied by noisy valve train operation.

This is a situation where something prevents the valve from travelling freely to full lift. The cam lobe will show normal wear on the flanks and a flat on the top of the nose, in some cases it looks like the nose of the cam lobe has been filed. The most common causes for this condition are,

Spring coil bind, wrong spring, shim under spring. The damper and/or inner spring should also be checked for coil bind.

Retainer fouling guide or valve stem seals.

Rocker arm fouling on stud, Rocker slot not long enough, fouling on stud at full lift. It is recommended that roller rockers are fitted to engines with in excess of 0.500” valve lift.

Valve hitting block. etc. During assembly the engine should be rotated to full valve lift and the valve pushed open a further 0.040" to ensure clearance.

Noisy Valve Train

This may be caused by incorrect clearances or mismatched parts. Valve train interference, such as coil bind and rocker arm hitting the rocker stud can often be accompanied by noisy operation. In some cases where a high lift cam has been installed the fitter may not check the lifter preload assuming that it will be OK. Due to the high lift of the cam the base circle size must be reduced and lifter preload adjusted accordingly. If the cam is ground to standard base circle diameter the lobe will protrude above the bearing size and will not fit into the engine. Modern lobe designs use different opening and closing ramps, in some cases if the cam is run backwards it may result in noisy operation (see incorrect profile below). The obvious is often easy to overlook, so check that nothing is hitting the rocker cover or cylinder head. Air bubbles in the oil can also accumulate in the lifters and cause valve train noise. This can be caused by cracked oil pick up pipe or air leaks between pick up and oil pump.

Lifter Alignment in Block

Some engines have misalignment in the lifter bores which can cause an otherwise unexplained cam failure. The machine shops who have jigs to check popular blocks inform us that this is a common problem in several makes and models.

Distributor/Oil Pump Gear Failures

Almost all of these failures can be traced to high volume oil pumps, many aggravated by using heavy weight oil which will not pass freely through the oil pressure relief valve. Some engines have different size distributor shafts in similar blocks (usually the difference between electronic and points distributors). Fitting the wrong model distributor can cause the distributor shaft to have excessive clearance in the block allowing it to move away from the cam causing incorrect mesh with the cam gear. Some Ford XE and XF blocks had a slight misalignment in the distributor locating bore in the block. This has caused failure of distributor gears. When fitting a cam to an XE or XF block check the tops of the gear teeth to make sure they have a chamfer. For further information a technical bulletin is available. Note. more bearing failures are caused by the oil pump picking up air than by low pressure. A well baffled sump in a high performance engine is a better investment than a high volume oil pump. The rule of thumb for oil pressure is 10lb per 1000rpm. Top race teams spend many hours testing to find the lowest oil pressure they can get away with as driving oil pumps wastes power. Most HQ racing cars in Australia are using standard capacity oil pumps. In race applications oil pump drive gears can have their life extended by modifying the engine to supply a small spray (0.030" dia. Hole) of oil to the mating face of the gears. Drill oil gallery or distributor to oil the gears.

Scuffing on Sides of Lobes, Scuffing on Face of Lifters.

Lubrication problem.

Likely Causes. Dry start. Cranking engine too long before starting. Insufficient prelube on cam and lifters. incorrect lube on cam and lifters during assembly. Oil pressure took too long to come up.

Localised Wear/ Pitting on Nose of Lobe. Sides of Lobes OK

DULL APPEARANCE ON FACE OF LIFTERS. Excessive nose loading. Likely causes. Valve train interference (something hitting something)

Incorrect springs causing overloading at full lift.

Flat on Face of Lifter, Lifter not rotating.

Likely causes. Lifter bore damaged when old lifter was knocked out. Incorrect radius on lifters. Incorrect taper on cam lobe.

Wear on Edge of Lobe Flanks. Incorrect contact pattern between cam and lifter.

Likely causes. Using old lifters on new cam. Incorrect taper on cam lobes. Incorrect radius on lifters. Cam not located in block correctly, allowing cam to move and not maintain full contact with lifters.

Cam Dowel Dislodged or Broken.

This problem is mostly seen in cams with a single retaining bolt. The dowel in the camshaft is only to locate the gear in the correct place and should not take any of the drive force. If the dowel has come loose or is broken it indicates that the gear has moved due lack of clamping pressure. This may be because the bolt has come loose or was not correctly tensioned. The bolt may have bottomed in the hole. An incorrect bolt or washer may have been fitted. Check for fret marks on the cam and gear to confirm this. Check to make sure the bolt did not bottom in the cam.

Camshaft Breakage

In pushrod engines the most usual cause of cam breakage is overloading on the front of the cam. This is most often caused by timing gears not having enough backlash. Sometimes due to the block being line bored, which brings the timing gears closer together.

Camshaft Breakage cont.

Overhead camshaft engines often suffer from camshaft breakage most commonly toward the centre of the cam. The most common cause of this is running in a cylinder head which is bent. If a cylinder head is bent (usually due to overheating) and has the gasket face machined flat, the back of the head is still bent and the cam must operate through this bend. Similar to bending a piece of wire back and forth the cam will inevitably break. Cylinder heads which are bent must first have the bend straightened before they are machined. This can be done at most engine reconditioning shops.

Poor Performance

This can be a problem in all engines but is more prevalent in overhead cam engines. When the cylinder head or block is machined the distance between the cam and crankshaft is reduced. This can have a dramatic effect on valve timing and hence on performance. To make a quick check on valve timing remove tappet cover and place a dial indicator on the valves of number one cylinder. Turn the engine until the both valves have the same lift on the overlap. Check the position of the timing mark, this should be between two degrees advance and top dead centre. If this is not the case then the timing should be moved to achieve this.

Engine Misfire at Idle

This can be caused by base circle runout (see below) and also by low valve spring tension failing to close the valves correctly. Machining of valve and seats increases the installed height of the valve spring and may reduce tension enough to be a problem.

Overhead Camshaft Engines

Overhead cam engines add a whole new dimension to valve train geometry. Not only can this have all sorts of effects on valve timing, if it is not correct the cam will suffer sure and swift failure.

When setting up an overhead cam with rocker followers it is critical that the contact pattern on the rocker be in the centre of the rocker pad this should be checked by blueing the cam and rocker. If the cam contact runs off the end of the pad it will chew the lobe off the cam within minutes. The contact pattern is adjusted with the use of lash pads and cam base circle diameter. For some engines with high performance camshafts there are rockers available with longer contact pads.

Oiler Bars, when assembling an engine with a cam oiler bar the oiler bar must be cleaned out with a small oil gallery brush. If it is the type of oiler bar that is crimped at the ends, it must be replaced. They re not expensive and can cause a lot of damage and grief should they block. We have seen many engines fail where the bar was washed out and working fine at the time of assembly, only to have a small piece of gunk dislodge from the end of the bar and block an oil hole and cause the failure of the cam lobe. When the rocker cover was removed the oil bar worked fine again. However the burn marks on the cam lobe showed that it had not been oiled at some time.

Rocker Arms. The surface finish on the rocker arm is critical to the life of the cam. The rockers must have a good surface finish if the cam is to survive.

Overtensioning of Timing Belts

Most engine equipped with timing belts are equipped with a spring loaded tensioner for belt tension. If this is not the case it is important to follow the manufacturers instructions for tensioning the belt. Breakages caused by belt tension normally occur toward the front of the cam behind the first bearing journal.