Post
by mat_the_cat » 15 Jul 2011
On a related theme, I have a question about cam/injector pump timing. The engine has one belt at the front which drives the camshaft and water pump, and another from the back of the camshaft which drives the injector pump. Timing is advanced when cold with a manual knob.
A while back I changed the cam belt and tensioner (injector pump belt was fairly recent). I planned to replace the water pump with a genuine VW I picked up cheap, but the pulley was a slightly different diameter, and I could not get the belt teeth to fit exactly without moving the crank or cam. As the pump wasn't leaking and looked newish I left it on there and timing was unaltered. Performance and starting were identical.
Last year I needed to replace the rear cam seal, injector pump belt and some seals on the pump. The pump (Bosch VE) was so far rotated on its mountings that a couple of the slotted holes didn't line up, and hence only being held on by 2 bolts (not 4). Hence I couldn't replace the pump exactly where it came from. It got more complicated when I took the rear cam pulley off to change the seal, and found the pulley wasn't keyed to the shaft in any way - it rather made me wish I'd marked it better!
Anyway, got it started and timed it by ear. However, I suspected it was slightly retarded (like its owner) as it would start instantly with the advance knob out; with it in it would take a couple more revolutions to catch, run a little smokily for a while and I think mpg was slightly down. So I left it pulled out all the time.
I took it for an MOT this year and got them to adjust the timing to the book setting (I didn't have anything to mount the DTI on, and I didn't know the correct measurement). They reckoned it was almost spot on (yay me) but advanced it very slightly. Sure enough, it now sounds knockier. BUT, if I leave the advance knob in, it cranks over for a few revolutions before smokily catching - it didn't do this before I changed the injector belt. Pulling out the advance lever again gives perfect starting.
Not sure of the effect on mpg yet but my guess is it's thirstier - power seems down too, I seem to be in a gear lower than normal. After all that explanation, my guess is that the water pump was changed at some point for one with the wrong sized pulley, altering the cam and injector timing. The injector pump was then adjusted as far as it could go, so it ran OK. I changed the injector belt and set it up reasonably OK. Now the injector pump is timed correctly to the crank, but not the cam.
Does that sound plausible? Seeing as I have to take the cam out to do the tappets, I can check it I guess - the garage will lend me all their tools for checking it (the front cam pulley is also not keyed to the shaft). The only thing I don't understand is that if advancing the timing by moving the pump has made things worse, why does advancing it further with the cold start knob make it better?