How do I check for TDC???
12-08-2007, 12:00 PM
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How do I check for TDC???
Ok guys, I have searched threads and timing belt write ups, but after my limp home post from last night, I want to be sure that my timing belt hasn't slipped.
Maybe I'm missing something, or just plain stupid, but what do I look for and where? I know that the passenger side cam cover needs to come off, and I'm clear on rotating clockwise with a 27mm on the crank. My problem is what marks should line up to show if I'm still in time?
Maybe it's sleep deprivation from being up and down all night worried about it, but I just can't locate what I'm looking for.
TIA for the help.
Maybe I'm missing something, or just plain stupid, but what do I look for and where? I know that the passenger side cam cover needs to come off, and I'm clear on rotating clockwise with a 27mm on the crank. My problem is what marks should line up to show if I'm still in time?
Maybe it's sleep deprivation from being up and down all night worried about it, but I just can't locate what I'm looking for.
TIA for the help.
12-08-2007, 04:00 PM
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Pictures help a lot.
On the crank pulley/dampre, there are timing marks from about 40 degrees before top center to maybe 10 degrees after. The pointer for the check is on the center cover, on the right side a little as you face the front of the motor. The TDC marks has OiT at the actual spot. The spot in the rotation you are looking for is top center on the compression stroke of the #1 cylinder. The engine is a four-stroke so you have a 50% chance of getting TDC correct on the first try (and, in my experience, a 100% chance of getting it wrong...). Rotate the engine clockwise as you face it. With the upper covers off, TDC on the crank should correspond to alignment of the notch on the back of each cam gear with the little pointer cast into the plate behind the gear. If you are at TDC on the crank and the notches are not visible, you'll want to turn the crank another full 360 degrees of rotation and line up that TDC mark with the pointer again. Do your rotating and aligning carefully, since there's a risk of jumping the belt if you try to go backwards (counterclockwise from the front) with the crank. The ratio is 2:1 crank:cams.
If the cam gear notches don't line up at TDC on either rotation, the belt may have slipped. You may want to get a local second opinion if you suspect slippage, before you break out the crying towels and the credit card. If you have slippage, there's the nagging probability that a piston or two has contacted a valve or two since the engine was run that way. You can test for this by retiming the cams correctly to the crank, then doing a quick low-pressure leakdown test with compressed air in each cylinder after rotating that cylinder to TDC on its own compression stroke. You'll hear any bent-valve leaks without gauges, and be able to assess the work required from there.
On the crank pulley/dampre, there are timing marks from about 40 degrees before top center to maybe 10 degrees after. The pointer for the check is on the center cover, on the right side a little as you face the front of the motor. The TDC marks has OiT at the actual spot. The spot in the rotation you are looking for is top center on the compression stroke of the #1 cylinder. The engine is a four-stroke so you have a 50% chance of getting TDC correct on the first try (and, in my experience, a 100% chance of getting it wrong...). Rotate the engine clockwise as you face it. With the upper covers off, TDC on the crank should correspond to alignment of the notch on the back of each cam gear with the little pointer cast into the plate behind the gear. If you are at TDC on the crank and the notches are not visible, you'll want to turn the crank another full 360 degrees of rotation and line up that TDC mark with the pointer again. Do your rotating and aligning carefully, since there's a risk of jumping the belt if you try to go backwards (counterclockwise from the front) with the crank. The ratio is 2:1 crank:cams.
If the cam gear notches don't line up at TDC on either rotation, the belt may have slipped. You may want to get a local second opinion if you suspect slippage, before you break out the crying towels and the credit card. If you have slippage, there's the nagging probability that a piston or two has contacted a valve or two since the engine was run that way. You can test for this by retiming the cams correctly to the crank, then doing a quick low-pressure leakdown test with compressed air in each cylinder after rotating that cylinder to TDC on its own compression stroke. You'll hear any bent-valve leaks without gauges, and be able to assess the work required from there.
12-08-2007, 06:41 PM
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Garrett PM'd another question, worthy of sharing for future problem-solvers:
Good idea to check both sides, especially since taking the other cover off isn't a big effort once the tools are out and your hands are dirty.
20k is not a lot of miles on a good belt replacement by the PO. While it's possible that a bearing suddenly failed, generally they wear out slowly, with some noises that you probably wouldn't hear. Somewhere in the middle of that wear-out process the tensioner light should have come on, if it's functional. Generally, a bearing failure's first symptom would likely be a belt tracking issue. Pull the air intake hoses off the top nipples on the cam belt covers, and use your beady eyes to see if the belt has ridden forward on the cam gears. Most pictures I've seen of belt installs show at least .100" of gear in front of the belt, so if you can see that gear you probably don't have some mysterious instant bearing issues. This is obviously a lot easier to see with the covers off, of course.
As far as noise, smoke, etc., the belt jumps I've heard of here have resulted in instant poor performance. Belt failures result in the car not running at all. No noise or smoke reported that I can remember.
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While it's possible that you have a belt issue, it's not the highest likelyhood. Conservative folks like myself will suggest that you check/confirm the belt before you do --possible-- additional damage. It's precautionary. The 90+% probability is that you did something as part of the last repair/service that's manifest itself a little later. My general troubleshooting method is to check on what I did just before the pain started. Undo that if possible and see if the pain stops. Surgery is the last option, generally. After anesthesia anyway.
I think in your other post you mentioned replacing a bunch of electrical stuff like caps and rotors. A rotor not installed correctly will break, a coil wire not secured into the cap will drop out, a plug wire or three installed incorrectly will cause a miss. With two distributors and coils, one can drop out and leave the car running poorly. Plenty of other "in the area" opportunities to disturb things too. Double-check your previous work as part of the timing belt check, and maybe you'll spot something that was overlooked the first time.
If you haven't done so already, get in touch with Jim Morehouse for a set of manuals and tech bulletins on CD. Find local listmembers who can add a set of eyes to your diagnostic process too. Maybe a belt tension tool is in the neighborhood and available for loan to you while yours is shipped from 928 Int'l.
Originally Posted by gf261
...
I would guess that I do need to check both sides since slippage could have occurred on either cam? I'm hoping that it's something simple, but the belt was last changed by the PO around 20,000 miles ago. I never got a belt tension light, but I guess the cam pulley bearings or the like could have wobbled enough to slip a tooth, I'm not sure. In the event that there has been contact, would there be some noise, smoke or the like?
I would guess that I do need to check both sides since slippage could have occurred on either cam? I'm hoping that it's something simple, but the belt was last changed by the PO around 20,000 miles ago. I never got a belt tension light, but I guess the cam pulley bearings or the like could have wobbled enough to slip a tooth, I'm not sure. In the event that there has been contact, would there be some noise, smoke or the like?
20k is not a lot of miles on a good belt replacement by the PO. While it's possible that a bearing suddenly failed, generally they wear out slowly, with some noises that you probably wouldn't hear. Somewhere in the middle of that wear-out process the tensioner light should have come on, if it's functional. Generally, a bearing failure's first symptom would likely be a belt tracking issue. Pull the air intake hoses off the top nipples on the cam belt covers, and use your beady eyes to see if the belt has ridden forward on the cam gears. Most pictures I've seen of belt installs show at least .100" of gear in front of the belt, so if you can see that gear you probably don't have some mysterious instant bearing issues. This is obviously a lot easier to see with the covers off, of course.
As far as noise, smoke, etc., the belt jumps I've heard of here have resulted in instant poor performance. Belt failures result in the car not running at all. No noise or smoke reported that I can remember.
------
While it's possible that you have a belt issue, it's not the highest likelyhood. Conservative folks like myself will suggest that you check/confirm the belt before you do --possible-- additional damage. It's precautionary. The 90+% probability is that you did something as part of the last repair/service that's manifest itself a little later. My general troubleshooting method is to check on what I did just before the pain started. Undo that if possible and see if the pain stops. Surgery is the last option, generally. After anesthesia anyway.
I think in your other post you mentioned replacing a bunch of electrical stuff like caps and rotors. A rotor not installed correctly will break, a coil wire not secured into the cap will drop out, a plug wire or three installed incorrectly will cause a miss. With two distributors and coils, one can drop out and leave the car running poorly. Plenty of other "in the area" opportunities to disturb things too. Double-check your previous work as part of the timing belt check, and maybe you'll spot something that was overlooked the first time.
If you haven't done so already, get in touch with Jim Morehouse for a set of manuals and tech bulletins on CD. Find local listmembers who can add a set of eyes to your diagnostic process too. Maybe a belt tension tool is in the neighborhood and available for loan to you while yours is shipped from 928 Int'l.
12-08-2007, 07:34 PM
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I've got the WSMs, and they've helped immensely, especially with the repairs I had to make. What has me befuddled is that she ran just fine for roughly 100 miles or so before this happened. I did peer down into the vent holes, and my pulleys seem to have roughly the distance you described visible in front of the belt. I checked the coil cables and wires, and everything seems to be in order there as well. I also replaced the ignition electrical switch, but again, it was working just fine too. Work necessitates heading out of town tomorrow, so it'll be next weekend before I can check the cam alignment. I will retrace my steps again though when I start tearing it apart.