Timing Belt Cam_Crank Marks Off

[ksdst1]

Hi All,

When I started the timing belt (tensioners, etc) replacement job by lining up the two cam wheel indicator lines to the plastic cover back, and the crank notch below, by manually turning the crack pulley clockwise (btw, it turned easily but then with resistance at times), the two cam lines were close but off, and the crank notch was visible but off to the right at a 2 o'clock position.

Can the cam indicators be close sometimes when the crack notch is not, i.e., did I not manually cycle the engine through enough to get all three indicators in synch eventually? I just assumed the cam lines only match up when they also match up with the crank notch no matter how much you turn over the engine.

Full disclosure is i replaced the head gasket and head (rebuilt head), and the timing belt (tensioners, wp, etc) years ago using great write-ups from here, but couldn't get the lines to perfectly match up then. In fact, I had been very careful to keep the cams and crack from moving during the removal and replacement of the belt. When I started her up, she was running rough but not severe. Drove to a independent Volvo mechanic and said he "got her in time", although I never removed the cover to verify that the marks were in synch.

I've been driving her for years now, idle is good and runs smooth so I never thought there was a timing issue. Now I wonder how much these indicator marks can be off. Wouldn't the engine run rough, maybe backfire, etc., I.E. how much "acceptable" error is there before it noticeably affecting the ride?

I want to retry the job but wanted some expert opinions first just to know where i am.

Thanks..

[erikv11]

A good way to think about engine timing is to always use the crank mark as the reference. When the crank is on the mark, are the cams on or off their marks?

If the cams are off, it will be either be due to (1) mistiming or (2) the belt has stretched. For case (1) it has to be some whole number of teeth of the cam gear. In general, one or two teeth will not kill the engine, but with each notch the engine will run less well. There are many reported cases where someone has run for years off by one tooth on one or the other cams and never knew it. More than two (or three?) teeth the pistons hit the valves and there is engine damage and you immediately know something is very wrong. For case (2) stretching is very common near the end of timing belt service interval, this shows up as the cam marks are "behind" where they should be, by like half a tooth. Engine will still purr if everything else is in shape.

If you decide to check/adjust, use the cam marks to guide when you are getting close but the absolute key is to put the crank dead on the mark and keep it there, and then you can mess with the cams however you want or need to.

[ksdst1]

Thanks erikv11..If i had turned the crack counter clockwise to get it centered on the mark, the cam would have been even more off. Is it possible to turn the crack another 360 degrees and have the cams be in a totally different position, where they might then line up properly? I assumed that every time the crack in on mark, the cams always should be as well. Is this incorrect?

If not, then somehow the engine still runs without a noticed degradation or mistiming. If this is the case, could I have made a mistake (it's been a while) with the placement of the cam shafts when I installed the new head, or is that something that would always be an obvious mistake?

[erikv11]

... Is it possible to turn the crack another 360 degrees and have the cams be in a totally different position, where they might then line up properly? I assumed that every time the crack in on mark, the cams always should be as well. Is this incorrect? ...

360 yes, 720 no: remember that one full revolution of the cams requires two full revolutions of the crank. So after 360 they will be "opposite" where they were at 0.

... If not, then somehow the engine still runs without a noticed degradation or mistiming. If this is the case, could I have made a mistake (it's been a while) with the placement of the cam shafts when I installed the new head, or is that something that would always be an obvious mistake?

Do you remember if you removed the cam sprockets from the cams when you did the head work? The cam notches can be off but the cams themselves are actually on. The definitive way to put the cams in time is by using the slots machined into the back of the cams; the little notches stamped on the cam sprockets can be misleading.

[ksdst1]

Thanks for pointing out that we need two full revolutions for the crank to for one revolution of the cams.

Good question on the second part and I believe the answer it yes! Although, at the time I was trying to be super cautious where I marked the cam sprockets to make sure I bolted them back on in the same positioning. My recollection (a bit faded now) involved putting the cam shafts back on in the right positioning. I remember re-re-checking this but when placing the head on, it seems a bit sketch, I think b/c the valves needed to realign with the lobes on the shaft. In any case, my goal was to extra cautious and follow the write-up to a T.

I'll have to go back and take a look at the head replacement write-up to recall the full process to determine how much variability there could have been with that positioning of the cam shafts.

Can you please tell me if any of this would affect the slots machined into the back of the cams and remind me of how I can see them?

[erikv11]

To see the cam slots you have to remove the distributor cap, rotor and rotor mounting plate for the intake cam, and remove the cam position sensor and its mounting plate for the exhaust cam.

The slots are unambiguous reporters of true cam position, they can not get out of whack the way the sprockets can.

Best way to time the engine (e.g. during head work) is set (always keep!) the crank on the mark, set the cam slots perfectly horizontal relative to the seam between the head and the block, with intake slot offset above the centerline and exhaust slot offset below the centerline. Then timing is perfect to factory spec. Then come around to the timing belt side and look at the sprockets to see if they are marked and positioned correctly. Or to put it another way, then adjust the sprockets so they are marked and positioned correctly. You can also use the triangular pattern of the cam sprocket bolt holes in a similar way to the cam slots, but this is less easy to see.

If you really want to be sure where the timing is right now, you can put the crank exactly on the mark and eyeball the orientation of the cam slots. Those two indicators will always report true timing. It's obviously close because it runs well, but this would give you clarity.

Only turn the crank clockwise with the timing belt on. If the belt is already off then be careful, you are more limited what you can safely move unless the crank is already on the mark.

[ksdst1]

Thanks for all the great info!
I will have to put it into practice in the near future and will re-post with more questions or a nice successful summary!
Happy 4th!