Edmunds Answers

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  • avatar zaken1 02/21/10 12:29 am PST

    The crankshaft on a 4 cycle motor in never 180 degrees off between compression and exhaust when the numnber 1 piston is at TDC. There are 360 degrees of crankshaft rotation between TDC compression stroke (which is firing position) and TDC exhaust stroke. Because of this; the timing mark on the crankshaft pulley will be in exactly the same position at both TDC compression stroke and TDC exhaust stroke. So it isn't necessary to know that distinction when setting ignition timing. You can set it at either TDC point. But since the distributor or crank position sensor rotates at half crankshaft speed; that part can be 180 degrees off between TDCs; but the crankshaft cannot. And because of the play in the cam drive system; you cannot accurately static time the ignition on an engine with breakerless electronic ignition. It must be done with a strobe light while the engine is idling. It might require making new marks on the crank pulley, but there's no other accurate method.

    If you for any other reason want to determine whether a particular cylinder is at TDC compression or TDC exhaust; it can be done by one of two different methods. The first, and simplest method is to start with crankshaft in a position where the piston in that cylinder is headed upward; cover the spark plug hole in that cylinder with a thumb, or insert a closely fitting length of rubber fuel line into the opening, and press a finger over the other end of the hose. Then tap the starter motor very briefly (or preferably push a manual transmission car slowly forward a foot or two in second or third gear). If you feel a blast of compression at your finger or the hose; the cylinder is on compression stroke. If there is no pressure in the cylinder while the piston comes up; it is on exhaust stroke. Just stop turning the crank before the piston reaches TDC, and you'll know what stroke it is on.

    The other method of determining the difference between the two TDC positions is to set the piston in that cylinder at TDC, and look at the intake or exhaust camshaft lobe or the intake or exhaust valve spring for that particular cylinder. Be sure to first find out which direction that cam is turning. If the exhaust valve spring is about to become fully relaxed, while the intake valve spring begins to be compressed when the cam rotates any further; the cylinder is at TDC exhaust. If both valve springs are not moving, but the exhaust valve spring is the next spring to be compressed at that point; the cylinder is on compression stroke.

Answers

  • 0patience 02/20/10 11:23 pm PST

    It would be immensly helpful to know what year and model vehicle this is, so we can see how it is set up.

    While we could go on about how you can use a compression gauge to determine when you are on the compression stroke, there may be an easier way if we knew the year and model.

  • zaken1 02/21/10 12:29 am PST

    The crankshaft on a 4 cycle motor in never 180 degrees off between compression and exhaust when the numnber 1 piston is at TDC. There are 360 degrees of crankshaft rotation between TDC compression stroke (which is firing position) and TDC exhaust stroke. Because of this; the timing mark on the crankshaft pulley will be in exactly the same position at both TDC compression stroke and TDC exhaust stroke. So it isn't necessary to know that distinction when setting ignition timing. You can set it at either TDC point. But since the distributor or crank position sensor rotates at half crankshaft speed; that part can be 180 degrees off between TDCs; but the crankshaft cannot. And because of the play in the cam drive system; you cannot accurately static time the ignition on an engine with breakerless electronic ignition. It must be done with a strobe light while the engine is idling. It might require making new marks on the crank pulley, but there's no other accurate method.

    If you for any other reason want to determine whether a particular cylinder is at TDC compression or TDC exhaust; it can be done by one of two different methods. The first, and simplest method is to start with crankshaft in a position where the piston in that cylinder is headed upward; cover the spark plug hole in that cylinder with a thumb, or insert a closely fitting length of rubber fuel line into the opening, and press a finger over the other end of the hose. Then tap the starter motor very briefly (or preferably push a manual transmission car slowly forward a foot or two in second or third gear). If you feel a blast of compression at your finger or the hose; the cylinder is on compression stroke. If there is no pressure in the cylinder while the piston comes up; it is on exhaust stroke. Just stop turning the crank before the piston reaches TDC, and you'll know what stroke it is on.

    The other method of determining the difference between the two TDC positions is to set the piston in that cylinder at TDC, and look at the intake or exhaust camshaft lobe or the intake or exhaust valve spring for that particular cylinder. Be sure to first find out which direction that cam is turning. If the exhaust valve spring is about to become fully relaxed, while the intake valve spring begins to be compressed when the cam rotates any further; the cylinder is at TDC exhaust. If both valve springs are not moving, but the exhaust valve spring is the next spring to be compressed at that point; the cylinder is on compression stroke.

  • zaken1 02/21/10 3:28 pm PST

    I also need to point out that the determination of the crankshaft position at TDC for either ignition or valve timing purposes is an EXTREMELY critical and precise measurement. It is impossible for all practical purposes to find this point accurately enough, by feeling the piston movement through an object inserted into the threaded spark plug opening. Crank angle errors of 7 to 10 degrees are routinely experienced when attempting to find exact TDC by this method. The difficulty is greatly magnified by the fact that the relationship between the rate of piston movement and crankshaft rotation changes drastically when the piston is at or near TDC. For this reason; it is necessary to hold possible errors in piston position measurement to plus or minus .005" (0.125 mm) when determining TDC. And that level of accuracy can only be achieved by using a dial indicator to measure poston position. So be warned.

  • zaken1 02/21/10 5:53 pm PST

    Since you say that the timing marks are blocked from view by other engine components; it may well be that the shop was unable to adjust the ignition timing after replacing the timing belt. And this could create roughness in the engine. Some engines with this issue turn out to be chronically frustrating for servicing; but there now is a relatively accurate alternative method for adjusting ignition timing. However, it does require having access to an exhaust emission analyzer which measures unburned hyrocarbons, and a vacuum gauge. The principles behind this method are that hydrocarbon emissions become progressively lower and engine smoothness increases as ignition timing is retarded. However, engine coolant temperatures increase excessively and power levels decrease when ignition timing is retarded beyond the optimal point; but the optimal timing point cannot be detected by measuring hydrocarbons alone. This is where the vacuum gauge comes in: If a vacuum gauge is connected to an intake manifold source and monitored while the timing is changed; it will be noticed that manifold vacuum generally increases as ignition timing is advanced, and decreases as ignition timing is retarded. The normal range for manifold vacuum on an idling motor should be somewhere between 17 and 21 inches of vacuum. The way to use vacuum and hydrocarbon readings to indicate optimum ignition timing is to adjust the timing to produce the best combination of low hydrocarbon levels and high manifold vacuum. If the timing is retarded beyond the optimal point, manifold vacuum will begin dropping rapidly, while hydrocarbon levels will not change much. If the timing is advanced beyond the optimal point, hydrocarbon levels will begin increasing rapidly; while manifold vacuum will also increase. The goal is to achieve the lowest hydrocarbon levels, consistent with manifold vacuum levels in the normal range. And those are the indicators of whether the ignition timing should be advanced or retarded from any given point.

  • screwedbygm 02/24/10 3:51 am PST

    Thanks to Zaken1 for three thorougly thought out and presented answers to my question; also for pointing out that my brains were in my butt when contemplating 180 degrees off on the crankshaft between either TDC position when it's a 360 degree change since crankshaft is in same position either way.

    In actuality, I had already resolved the problem myself by using the "looking for compression on the given cylinder with all plugs removed" to differentiate between TDC compression vs TDC exhaust before reading the responses.

    While all of the information is valid, I'm selecting Z's first answer as best and most appropriate.

    Sincerely.

  • woody92 05/25/14 6:02 pm PST

    Ok. This is the absolute easiest method for finding TTDC on almost any motor. Please feel free to share this link with other blogs etc. The 2.5 mitsubishi v6 has the intake plenum over the cylinder head and valve cover where the number 1 piston is. So to use the traditional method of finding TDC on compression stroke you would first have to remove the intake manifold. Remember this. The crank and cam marks will line up on every 2 revolutions of the crank but will only be on the compression stroke every 4 turns. That said here is an old trick for you nee dogs. On the mitsubishi 2.5 v6 the number 1 and 4 pistols will always reach TDC at the same time. This will happen every time the timing marks on the crank line up. The easiest way to locate TDC compression stroke for number one is what I'd called the resistance method. Long story short, do this. Remove the spark plug from the number 4 cylinder. This is the one in the middle on the front valve cover. Now get your breaker bar or half inch ratchet and go underneath the car and insert it on the crank bolt. Start turning slowly clockwise and as you go around every 120 degrees you will be feeling resistance more as each piston reaches and moves past TDC. Now watch as you turn just as your crank timing marks begin to match up and as they pass. Three out of four rotations of the crank ad the timing marks cross you will feel very little resistance or pressure. At the point when number 1 is at TDC pressure or resistance will greatly increase. You have found your number 1 TDC compression stroke.

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