Hi guys, I ordered the new tensioner from Amazon last month, and here is my two cents after working on my 2.2 liter Ecotec L4, 2007 Cobalt:
When you take the old tensioner out, it's very easy for the timing chain to skip teeth at the crank end because the chain will probably be under tension between the intake cam sprocket and the clockwise-turning crank gear. By taking the tensioner out you are releasing all the tension on the exhaust side, and it only takes a very minor disturbance for the chain to skip teeth as it's pulled toward the intake side over the crank gear. I learned this the hard way.
The only way to be 100% sure that you don't skip time is to take the engine timing (i.e. front) cover off before you change the tensioner, and mark the exact current position of the chain on the crank gear for reference. This is a big pain in the butt however, so if you don't want to do this I would highly recommend the following procedure for no-engine-cover-removal tensioner replacement:
1) Jack up the front passenger side securely and remove the wheel.
2) Remove the plastic splash guard to access the crankshaft pulley (harmonic balancer).
3) Use a 13/16" socket wrench to rotate the crankshaft slightly COUNTER-CLOCKWISE. You only need to rotate just enough to move the chain tension from the intake side to the exhaust side, so do this by feel. You will feel a bit of resistance when any slack has been taken up from the exhaust (tensioner) side.
4) Now that the tension is on the tensioner side, this will actually help in removing the old tensioner. Use a 32mm socket to unscrew the old tensioner. A 1.25" socket will also work. If the socket has trouble getting a grip you can do as I did - have the chamfer cut off your 32mm socket so that it engages immediately. I took mine to a machine shop and they simply cut the chamfered part of the socket off with an angle grinder while the socket was chucked and turning in a lathe. This is not needed for new tensioners which have a much bigger head.
5) Unscrew the old tensioner very carefully, and pull it out gingerly to try and avoid any parts staying behind. If not all the parts come out then put a fairly strong magnet into the tensioner hole to retrieve them. This worked for me. If you are unlucky and tensioner parts fall beyond reach, then you will unfortunately have to start removing some engine covers. I don't think this is likely to happen though.
6) With the tensioner out, don't be doing anything to jar the timing chain or tensioner guide. This just increases the chance of skipping time at the crank end. And this is exactly why I recommend the new tensioner be installed ALREADY EXTENDED (next step). Whacking the chain and tensioner guide with a tool to release the tensioner spring is the last thing you want to do, because the chain does not have enough (and even) tension at this time!
7) Take the new tensioner and press it hard onto a block of wood to release the spring and extend it. Yes, they all have a spring and it's crucial for it to extend. Mine took a couple of tries and a surprising amount of force to unlock, so I'm glad I didn't try to do it from the top with the valve cover off. I know people will say that the tension needs to be set in-place, but it's just a spring people. The spring alone gives enough tension for assembly and cold starts. Supposedly the tensioner will also extend a bit more once the engine is running due to oil pressure.
8) Insert the already-extended tensioner into its hole and screw it back on. When it makes contact with the guide, you may need to push it in at the same time as you screw it in, until the threads catch. Torque to 55 ft-lbs and you're done!
If at any time you think that the timing chain may have skipped teeth at the crank end, don't try to start the engine! You will need to remove the engine front cover and valve cover to check the timing. Since you already have access to the crank pulley, you can try carefully turning the engine over clockwise by hand with your 13/16" socket to check that it turns freely (no piston-valve contact). This is best done with spark plugs and serpentine belt removed plus car in neutral so that the crank turns more easily and you can immediately feel resistance. When in doubt, do the extra work and open the engine front cover and valve cover to check the timing. You can bend valves even when turning by hand. If the engine runs but you now get a P0016 OBD code, then time has slipped slightly at the crank end, hopefully not enough to damage the engine.
Good luck!
Max
P.S. Checking the timing is not 100% straightforward when doing it visually. But the basic procedure (with valve and timing covers removed) is to get the INT (diamond) timing mark to the 2 o'clock position on the intake cam sprocket by carefully turning the crank shaft by hand. (If you feel resistance caused by piston-valve contact you need to remove the timing chain and re-do the timing, knowing that valve damage may already have been done). At this point the exhaust cam sprocket EXH (triangle) timing mark should be at the 10 o'clock position and the crankshaft gear timing mark at the 5 o'clock position.
Without removing the chain it will not be possible to get the coloured links to exactly the right spots, so just get them sort of close and count chain link offsets. Remember that each link is worth 2 sprocket (or gear) teeth, so a timing mark can theoretically fall either in the middle of a link or between links. Going clockwise around the chain, the uniquely coloured link corresponds to the intake timing mark, and the two matching coloured links correspond to the crank shaft timing mark and the exhaust timing mark respectively. If the uniquely coloured link is not close to the intake timing mark, just keep turning the crank clockwise by hand until the intake INT (diamond) timing mark is again at 2 o'clock, and the uniquely coloured link is somewhere close. Now let's say that you get the uniquely coloured link to be 5 links clockwise of the timing mark. Look at the crank end, and the first matching coloured link should be exactly 5 links clockwise of the crank gear timing mark as well. Ditto for the EXH (triangle) timing mark. Whatever offset you see has to match all the way around.
If the offset does not match and you need to fix the timing a bit, you can do so at the crank end (the cam-to-cam timing is highly unlikely to slip in this procedure). Remove the tensioner again so you have some slack, and carefully work the chain over the crank gear teeth to fix the timing. The idea is to achieve the correct timing mark offset all the way around. For reference, the distance from the uniquely coloured link to the first matching coloured link (going clockwise) is 27 links, so however you count them there have to be 27 full links from the INT (diamond) timing mark to the crank gear timing mark. Similarly there are 15 links from the second matching coloured link to the uniquely coloured link, so there have to be 15 full links counting from the EXH (triangle) timing mark to the INT (diamond) timing mark. There are timing diagrams online that show this. In my case the uniquely coloured link was grey, and the two matching coloured links were red.
