Why do you have to loosen the cam sprocket?
A good question and exactly what I was asking myself before I did the belt change I posted the link to above. On the older engines (like Felicity, our 1999 Panda Parade 999cc Fire) the cam and crank pulley/sprockets were keyed to the shafts so could only be fitted in one position. So why did they change it? I can only guess that it was in the interests of more accurate cam timing?
Obviously if you have keyed pulleys then the pulleys can only be "meshed" to the shaft they run on in the one position so, if there is a slight difference in manufacture between the old belt you are removing and the new belt being fitted (say in length for instance) there is no easy way to compensate for this and cam timing will be compromised (a longer belt will retard valve opening and a shorter belt will advance it). When building up high performance or race engines a lot of time and effort goes into precisely "dialing in" cam timing to maximize cam performance.
So, if we look at doing Felicity's belt (where the pulleys are both keyed) synchronizing the timing is achieved by lining up the timing mark on the crank pulley with a corresponding mark on the front of the crankcase (alright you clever guys, it's actually on the oil pump isn't it) Then checking to see if the cam pulley mark is lining up with the mark on the cylinder head (it may be half a turn out as the crank goes 2 revs for every one rev of the cam) and with both marks lined up you slacken the tensioner pulley, remove the old belt, fit the new belt (we'll leave water pumps etc out of it for this consideration) and retension the tensioner. Largely speaking that's the job done?
The problem with this is that if there is a difference between the old and new belts you can not compensate for it unless you go to extra ordinary lengths (offset dowels/woodruff keys and the like) so you just have to live with it as it is. Until recently, on "cooking" every day runarounds I really don't think this has much effect. (especially on petrol engines - diesels like things to be a bit more exact) - We've lived with Keyed pulleys and chain sprockets for many years - However:
With our more modern engines a great deal of effort goes into maximizing fuel usage and minimizing emissions. Cam timing is a big player in this respect so the more accurately you can "dial in" your cam timing so that it meets the designed parameters the greater will be the benefit. If you have a "free" cam sprocket (some even have a free crank sprocket too?) which is not keyed to the camshaft such as our later model Fire engines have (both our Becky 2010 Panda 1.2 and my boy's 2012 1.4 8 valve Punto have free sprockets) Then, if you slightly slacken (maybe 1/4 to 1/2 a turn just enough to allow it to "slip" freely on the cam journal) you can then use a timing tool kit to lock up the camshaft itself and the crank shaft in their optimum positions, remove the old belt, fit the new one and apply tension with the tensioner BEFORE then finally tightening the bolt which holds the cam pulley in position on the end of the camshaft.
Doing it this way guarantees (well maybe not if you include idiots in the calculation) that the relationship between the cam and crank positions will be exactly as was intended by the engines designer. An additional benefit is that it makes it a little easier to actually fit the new belt as the cam pulley can be rotated to any position which allows the teeth to engage. Also It's really impossible to be just one tooth out on the timing which with keyed pulleys was always a possibility. (Because with a keyed pulley you had to sometimes "nudge" the cam either slightly clockwise or anticlockwise to get the teeth to line up with the belt's teeth. If you were inexperienced - or just in too much of a hurry - you might end up out by a tooth).
So, being aware of all of this, when it came to doing our Panda and Punto belts I found myself thinking "Modern manufacturing standards are very very tight indeed so I bet there's not even a nat's whisker of a difference between a Gates, Delphi, Continental or who ever - genuine Fiat even - which is probably made by gates or one of the others anyway". So my thinking continued : If my supposition is correct then simply renewing the belt should not change the relationship between the crank and cam should it?
As detailed in my link, I set out to investigate my hypothesis and changed the belt on both engines without slackening the cam pulley bolts on either. Before starting I checked that the timing was correct by fitting the timing tools (which it was) and after I'd finished and briefly run the engines, I rechecked by fitting up the timing tools again. On both engines the tools just slid into position as nice as you like - The timing was spot on on both and they drive exactly as they did before the change.
Another pointer to there being no disparity between the new and old belts is that I intentionally did not perform a "Phonic Wheel Relearn" on either engine and neither engine has lit it's engine check light or thrown a fault code. This would seem to be further proof that the timing has remained the same. In fact I might go so far as to say that if you do slacken the pulley bolt there is a chance that you will unintentionally introduce a slight difference into the can/crank angular relationship greater than that due to any manufacturing difference between belts? I've asked a number of small garages around me as to whether they slacken this bolt - none do - and say they get no problems with check engine lights illuminating. I'm beginning to suspect that a check engine light illumination is actually more likely if you do this task "by the book" and slacken this bolt?
Finally an additional factor for me is that I am a long term subscriber to the view that "sleeping dogs should be left to lie" In respect to this task that relates to several issues for me.
The bolt that holds the cam sprocket in place is VERY tight and is a Torx on the later engines. There are posts on here about the difficulties some have had with it. Damage those splines and you're into a whole new ball game! (the 1.2 is tightened to 70Nm with the VVT pulley on the 1.4 Punto being first to 20NM then turned a further 55 degrees). Also on the VVT pulley there is a blanking plug which has to be removed to gain access to the main retaining bolt. Quite a bit of oil spills out when you remove it which is annoying if you aren't ready for it but you have to consider that it seals with a rubber ring. Are you going to replace this ring on reassembly? My dealer didn't have them on the shelf.
The cam has to be held still whilst doing this so you don't loose it's relationship to the pulley and you can't rely on the cam locking tool to do this as you will likely snap off the lug on the back end of the cam itself. You need to remove the cam locking tool before either slackening or tightening this pulley bolt. On the VVT engine there is also a nice little pulley locking block which screws to the front of the head to stop the pulley moving but even a very slight movement of this block when under pressure will introduce a variance. (probably not a factor, but it could happen).
Of course if the last belt change was not performed with due diligence and the cam was not set up correctly by the person doing it you will have to slacken this bolt to get things back to where they should be so I would always advise using a timing tool set to check an engine you don't know. After that, when you know for sure that things are set up spot on, you can do a belt change without removing the cam cover and just tippexing the cases, pulleys and belt. Makes the whole job really quite a bit simpler and quicker.