Checking for Bent Valves – Broken Timing Belt

1:13 pm How To Auto Repair, Mechanics Tools


© Summary: This article is on how to check for bent valves. Valves get bent most often on vehicles that have had a timing belt break or a timing chain jump a sprocket. Interference engines can allow the valves to crash into the pistons when the timing belt breaks due to the engine’s timing being out of synchronization. Some cars are more prone to having valve damage when the timing is off. We cover some of the more common manufacturers that seem to suffer from bent valves more than others, from timing belt failure. We have some tips on how to check for bent valves with the cylinder head still on the vehicle. Tools that help in determining if there’s bent valves listed also.

Valves normally will not just bend unless the engine has experienced some sort of trauma. A quick look down the spark plug holes with a borescope or video inspection camera may reveal if the top of the pistons have been hit with a piston. One other fairly quick check is to do a compression test of each cylinder while the spark plugs are out. If the timing is off slightly, the engine may still run, however the valve timing will be retarded which will result in low compression readings and low engine vacuum. Backfiring, lack of power and bogging are common symptoms of an engine that is not timed properly. If the valves are bent, they will not seat (close) properly which allows cylinder compression to leak past the valves. A cylinder leak down test can be performed with a leak down tester to help determine if an exhaust or intake valve could have a problem. When the cylinder leak down tester is inserted into a spark plug hole and air is introduced, air escaping from the throttle, indicates a an intake valve leaking; air leaking out of the exhaust indicates an exhaust valve leak. It’s important to understand that if the timing is off (meaning the crankshaft and camshafts are not synchronized properly) low compression readings will be the result. If the timing belt seems loose, the timing belt could have jumped, check the alignment of the timing marks before going any further. Jumped timing is usually caused by worn timing belts, timing chains, guides, tensioners, idler pulleys and even by water pumps that have excessive movement on vehicles that use the chain or belt to drive it (water pump). If timing has jumped one or more teeth, the cam timing will be off enough to cause low compression readings even though valve damage might not have occurred. After verifying the timing marks are lined up properly, a simple tool that can be used for checking for valve problems is a vacuum gauge. The vacuum gauge reading taken from a source on the intake should be around 20 inches of vacuum. If the gauge flutters excessively, either worn or bent valves could be the cause. Pulling a valve cover and looking for full rocker arm movement can also be one way to detect an valve problems. Look for full movement during the rotation of the engine. Cylinder_Leak_PercentageOTC5609. One gauge shows pressure that is introduced with shop air from the air compressor and the other gauge shows the amount of loss. In the example above, the first gauge shows that 28 psi of shop air is supplied to the cylinder and the second gauge is showing that there is a 35% loss of pressure which is acceptable. A small amount of loss is to be expected. If an abnormal percentage of loss is measured it’s time to diagnose and pinpoint where the pressure is going. To determine where a major loss of pressure is leaking; 1. Listen with a stethoscope in the crankcase through the oil fill with the cap removed – this could indicate worn rings. 2. Listen through the throttle plates on the intake – hissing there could be an indication of an intake valve problem. 3. And finally listen for a hissing out of the tail pipe – which could mean an exhaust valve is the issue. This process is repeated for each cylinder until the leak down has been pinpointed. A repair manual can be checked to see if a particular car has an interference motor. Some designs allow for clearance of the valve and pistons even when the motor is spinning with improper timing. Other manufacturers have much closer tolerances which do not allow for clearance – this can allow valve and or piston damage when timing is not in sync for some reason. All Honda engines are interference engines, so when a Honda timing belt breaks it is quite possible that some valves could be bent when a timing belt or a related component fails. Sometimes though, Honda owners can get lucky and not have bent valves. It seems to matter how fast the engine RPM’s were at the time of failure – the faster the revolutions at the time the belt went, the more likely the valves have crashed into the pistons. The Daewoo Lanos seems to have an even higher chance of valve damage when a timing belt breaks. Most of the time ALL valves are bent on a 1.6 when the timing belt breaks on that model. As a side note, the Daewoo Lanos requires a special water pump/timing belt tool for adjusting the tension on the belt. LOCATION OF LISTENING POINTS

3.1.1 Oil Dipstick Tube  . . . . . . . . . . . .for leakage from damaged or worn rings and/or cylinder wall.
3.1.2 Radiator Filler  . . . . . . . . . . . . . .for cylinder wall cracks or head gasket leakage.
3.1.3 Adjacent Cylinder  . . . . . . . . . . .for head gasket leakage.
3.1.4 Tail Pipe  . . . . . . . . . . . . . . . . . .for exhaust valve leakage.
3.1.5 Carburettor Air Inlet  . . . . . . . . . .for intake valve leakage.
3.1.6 Fuel Injection Throttle Body  . . . .for intake valve leakage.
3.2.1 Run the engine until it reaches operating temperature.
3.2.2 Remove spark plugs, oil dipstick, radiator cap, air filter from carburettor or, if fuel injected, remove air filter or hose from the throttle body.
3.2.3 Position No. 1 piston at TDC on the compression stroke so that both inlet and exhaust valves are closed. Note: Always rotate the engine in the normal operating direction. To position the piston correctly use a piston position gauge and remove the cam/rocker cover so that closed valves can be confirmed.
3.2.4 Turn the regulator knob fully anti-clockwise. Connect the compressed air, which must be between 45 and 150psi, to the regulator. Turn the regulator knob clockwise until the gauge reads zero (at the end of the yellow ‘SetÙ band).
3.2.5 Screw the cylinder hose into the spark plug hole and then connect to the tester. The amount of leakage will show on the gauge as a percentage loss.
3.2.6 Test all other cylinders, each at TDC, and compare the leakage figures to determine which cylinders are faulty.
3.2.7 If necessary, retest the cylinder(s) showing high leakage. Check the listening points (see 3.1) to determine the cause of the leakage.
3.3.1 If 100% or excessive leakage shows on gauge the cylinder may not be at TDC on the compression stroke. Check to ensure that the valves are closed. Always try to position piston at TDC for uniform results.
3.3.2 If rings are broken or cylinder walls are scored excessive leakage will be identified.
3.3.3 It is important that all cylinders have reasonably uniform readings (as in compression testing). Differences in excess of 15% indicate excessive leakage.
3.3.4 Large engines tend to leak more than small engines.
3.3.5 If leak is excessive on a vehicle with low mileage, piston rings may be stuck. Treat engine with quality tune-up oil for a period of time and then re-test before disassembling.
3.3.6 The lower the pitch of the leakage sound, the greater the leak.
3.3.7 To assist with listening use a length of clean hose, or a mechanic’s stethoscope with the probe removed.
3.3.8 When making repeat tests on the same cylinder, variations in the piston position and engine temperature can cause gauge readings to differ by up to 10%.
3.3.9 If an engine has multiple faults (such as worn rings and burned valves), the tester may indicate only the most serious fault.
Note: There is always some leakage past the piston rings. As a result you will always hear some leakage when listening to the dipstick tube.
The simple Calvan 389-4000 in the video above can also be used to determine if the valves are closed. This is helpful when checking to make sure the valves are closed before drilling and tapping when repairing the spark plug threads. It is recommended to first make sure the valves are closed before using a spark plug thread repair kit to prevent getting metal shavings and other trash down past the valves.


General Instructions -owners/operator’s manual for a dual gauge leak down tester courtesy of Sealey.
6 Responses
  1. dennisb - Auto Tool Sales :

    Date: October 31, 2009 @ 2:35 pm

    The OTC 5609 has instructions included on the inside of the storage case lid. I was unable to find a pdf on OTC’s site, however I did find a great pdf instruction guide for use with a similar tool sold in the UK. It looks to be a comparable two gauge tool that would be used in the same manner. Cylinder Leak Down pdf instructions/directions.

  2. ismael :

    Date: December 17, 2009 @ 12:52 am

    Where I can find a diagram to see how to line up the timing marks a Mazda 626SE v6 1994, for free?


  3. dennisb - Auto Tool Sales :

    Date: December 17, 2009 @ 7:44 am

    Ismael, A search on the Internet using your favorite search engine may lead you to a timing mark diagram for your Mazda. Remember if you come up empty on your search, for a small subscription fee you can access repair information on-line which includes specific data and the time saved is usually well worth the small investment. Just visit our Auto Repair Manual page for more information.

  4. scott :

    Date: December 23, 2009 @ 5:28 am

    I’ve removed the valve cover on my 06 dodge stratus with the 2.4L there is no movement in the rocker arms with the valves released. I can move the valves up and down. Do you think i still could have a bent valves?

  5. dennisb :

    Date: December 23, 2009 @ 9:23 am

    Scott, I’m unsure of what you mean by “valves released.” Most tech’s will try a new timing belt (asssuming thta was your original problem) and see if/how the engine runs. If it runs poorly, then further testing to see which cylinder has problems would be the next step.

  6. Dwayne :

    Date: July 13, 2011 @ 9:29 pm

    2003 2.4 Mitsubishi Eclipse GS,timing belt broke.
    I would like to do a leak down test, and have the tester, but not sure how to do it with the timing belt removed
    I really want to know if I have bent valves before I waste time putting a new belt on
    I removed the rocker cover,and can’t see any bent valves
    thanks for any help

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