Timing belt knock.

[DoctorRobert]

Just bought a '85 300zx...

sounded good when I got it, but when I got it home I started hearing a knocking sound coming from right in front of the passenger side head, under the timing belt over. It only does it off and on. What could this be? Did I just buy a 'clunker'?

Other note... it does not do it when the car is revved up.
Modified by DoctorRobert at 5:14 PM 11/2/2009

[Perrenial Badass]

Directly in front of the cam, or... what? I suppose that it could be a lifter tick, but you'd better pull off the top cover and check that the cam gear bolt is tightened proper and nothing is in there.

[evildky]

if it sounds liek it coming fomr inside the timing cover you need to take the cover off and check the tensioner and belt, do this now before it's too late

[DoctorRobert]

It is certainly coming from under the timing belt cover. I can feel it there when it knocks. But it's on the part of the cover directly in front of the passenger side head. I plan on looking at it tomorrow. I have no experience with the 300zx engines... mainly the KA24DE and L28 engines, so this is all new to me. But the whole timing belt thing worries me and the fact that I'm broke.

Again, it only does it occasionally and and doesn't do it if the engine is revved.

The timing belt was allegedly changed at 120,000 miles... it's at 150,000 now.

[evildky]

just because it was changed doesn't mean they did ti right, they might have neglected to adjust the tensioner properly, or simply failed to torque the tensioner stud nut

[DoctorRobert]

Right, exactly what I was thinking...

once I get the cover off, how do I tell exactly what's going on in there?

If the tensioner is the issue, how difficult would it be to fix?

[evildky]

get the haynes manual or download the fsm or timing belt instructions, they tell you how to measure defeltion, the tensioner basically is a pulley on an offset stud, you just rotate it to make more or less tinsion and the nut lockes it in place, it's pretty simple

[DoctorRobert]

My step-dad and I took the upper timing belt cover off today and discovered what the problem is... the passenger side head cam pulley wobbling. We're not sure if the end of the shaft is bent or it's just the pulley.

On the other hand the timing belt looks great and has great tension.

[evildky]

is the cam gear bolt loose maybe? or the cam gear on backwards? if it's on properly it shouldn't have any play in it, if the pulley rotates independant of the cam you can bend valves

[DoctorRobert]

I'm not sure. I can't move the pulley at all by hand and the belt is very tight...

however, while the car is running the belt is sort of "walking" from side to side of the pulleys, both of them, but the one that knocks against the cover is, like I said, the passenger side cam pulley.

[Perrenial Badass]

Take the belt and pulleys off, make sure they are on the right way and when you reinstall them make sure they are fully seated before you torque down the bolt. Also, check the nose of the cam and the pulleys themselves for damage, warpage, etc.