Toyota MR2 Message Board - View Single Post

mr2greasemonkey · 05-22-2007, 09:51 PM

Timing of fuel/spark/compression are essential to a good running engine. We are going to discuss the two basic functions of timing. You need mechanical timing and electronic timing.

Mechanical timing: The relationship of the crankshaft to the camshaft(s). There is a TDC mark on the crank pulley and crank sprocket. There is also a timing mark on the cam sprocket(s). A quick check is to line up the crank pulley mark on zero and then take the distributor cap off to see if the distributor rotor is pointing at cyl #1. On a 4 cylinder you can be 180 degrees off so if the rotor can also be pointing at #4. It is best to use cylinder #1 since that's what's going to start the whole process, so rotate the crank till the rotor come around to point at it. Now is a good time to see if the rotor actually moves, in case you suspect a broken timing belt. On a dual cam cylinder head, it is best to remove the top cover so you can verify that both cams are in the proper position. If you have a chain driven engine or one without a distributor you can use a compression tester. Install the compression tester and rotate the engine till you see the gauge move. That will tell you that the valves are closed and the piston is moving upwards ( which is the compression stroke). On an engine like the 4a you can stick a long screwdriver down the spark plug hole and turn the crank till the screwdriver stops moving upwards, that's pretty close to TDC.

Electronic timing: This is where sensors take mechanical timing and use it to fire the spark plugs and fuel injectors. Base timing is what the computer uses as a baseline for ignition advance/retard and injector pulse. If you're base timing ( mechanical timing ) is off then every action there after will be off. This is where a timing light and more sophisticated diagnostic tools come in handy. Always start off with checking base timing. On the MR2's that is accomplished by jumpering terminal E1 and Te1 in the diagnostic box. With the timing light hooked up you should see the advanced timing mark jump back to zero. You should also be able to notice a change in how the engine runs when you jumper it to base timing. It is very important for the computer to see that the engine is at normal operating temperature and is idleing. Otherwise you may not be at true base timing. If your engine is stock then use the base timing specs in the BGB ( or the sticker under the engine compartment lid). If you have a performance engine then you will need to make sure that base timing is where you want it to be at.

If you dont have a timing light I suggest getting one that will show you degrees and rpms. One that is compact is better since it can be hard to get a good look at the crank pulley. I generally try to clean the mark off and paint it. Anytime i have the timing cover off i always paint the crank pulley mark and the degree marks on the cover. It looks good and it makes it easier to see later. My timing light is a black light, makes white paint glow so it's easier to see even on bright days or a well lit shop.

more to come...

05-22-2007, 09:51 PM	#11 (permalink)
mr2greasemonkey Once again....yups Join Date: Sep 2005 Location: Portland, OR Posts: 1,440 Thanks: 4 Thanked 54 Times in 39 Posts iTrader Rating: (0/0% )	Timing Timing of fuel/spark/compression are essential to a good running engine. We are going to discuss the two basic functions of timing. You need mechanical timing and electronic timing. Mechanical timing: The relationship of the crankshaft to the camshaft(s). There is a TDC mark on the crank pulley and crank sprocket. There is also a timing mark on the cam sprocket(s). A quick check is to line up the crank pulley mark on zero and then take the distributor cap off to see if the distributor rotor is pointing at cyl #1. On a 4 cylinder you can be 180 degrees off so if the rotor can also be pointing at #4. It is best to use cylinder #1 since that's what's going to start the whole process, so rotate the crank till the rotor come around to point at it. Now is a good time to see if the rotor actually moves, in case you suspect a broken timing belt. On a dual cam cylinder head, it is best to remove the top cover so you can verify that both cams are in the proper position. If you have a chain driven engine or one without a distributor you can use a compression tester. Install the compression tester and rotate the engine till you see the gauge move. That will tell you that the valves are closed and the piston is moving upwards ( which is the compression stroke). On an engine like the 4a you can stick a long screwdriver down the spark plug hole and turn the crank till the screwdriver stops moving upwards, that's pretty close to TDC. Electronic timing: This is where sensors take mechanical timing and use it to fire the spark plugs and fuel injectors. Base timing is what the computer uses as a baseline for ignition advance/retard and injector pulse. If you're base timing ( mechanical timing ) is off then every action there after will be off. This is where a timing light and more sophisticated diagnostic tools come in handy. Always start off with checking base timing. On the MR2's that is accomplished by jumpering terminal E1 and Te1 in the diagnostic box. With the timing light hooked up you should see the advanced timing mark jump back to zero. You should also be able to notice a change in how the engine runs when you jumper it to base timing. It is very important for the computer to see that the engine is at normal operating temperature and is idleing. Otherwise you may not be at true base timing. If your engine is stock then use the base timing specs in the BGB ( or the sticker under the engine compartment lid). If you have a performance engine then you will need to make sure that base timing is where you want it to be at. If you dont have a timing light I suggest getting one that will show you degrees and rpms. One that is compact is better since it can be hard to get a good look at the crank pulley. I generally try to clean the mark off and paint it. Anytime i have the timing cover off i always paint the crank pulley mark and the degree marks on the cover. It looks good and it makes it easier to see later. My timing light is a black light, makes white paint glow so it's easier to see even on bright days or a well lit shop. more to come...