After rebuilding my engine last week, which was prompted by a carbon-damaged exhaust valve, the cam timing slipped after running for about five minutes.

I have the proper timing tools, and followed the timing chain sprocket torque sequence exactly as the Bentley manual specifies. I thought I had everything buttoned-up. I filled up the fluids. And as I was bringing the motor up to temp to top off the coolant, the motor started knocking horribly on the intake side so I quickly shut it off. It was late so I called it a night.

The next day after work I checked for codes. None to be found. Started it up and the knocking began immediately so I switched off the motor. I took the valve cover off, set the crank to the 90 degree timing position and both cams had slipped out of phase. So I unbolted the sprockets, turned the cams so they fit into the timing tools, put new bolts into the sprockets and tightened to spec. Once I had the cam cover and all the hoses and wires hooked back up I started the motor again. It ran fine again for about five minutes then started knocking again. This time on the exhaust side.

I haven't pulled the cover again to check but it seems obvious the cam timing has slipped yet again. Where am I going wrong? The one-time use sprocket bolts aren't expensive but they do take time to come in the mail and I've already been without my car for nearly five months.

 

Check the vacuum pump to make sure that it is not seizing up and causing the cams to slip. Only idea I have.

 

I failed to mention i installed a brand new vacuum pump, so that's probably not an issue.

 

where are you located?
before you tighten the sprocket bolts, leave them a bit loose where they can rotate but not fall off the cam. set the timing again and tighten the tensioner FIRST. keep an eye on the ex/in cam sprockets. once tensioner is tight, tighten the cam bolts. good luck.

 

Near Erie.

That's the process I've done the last two times. Once with just the tensioner and once with the factory tension tool. My next set of bolts will arrive tomorrow. I'll meticulously go through the process and add a bit of thread lock and mark the position of all the components with a dab of paint. This shouldn't be so difficult. I must be missing something somewhere but I don't see where.

 

make sure you position the intake sprocket similar to the picture in the manual.

 

I've had so many issues with this dumb **** bmw cam designs now I set timing, remove tools and hand rotate the engine at least one revolution and verify timing again

 

Yep. I cranked around by hand four times (a real pain in the *** with new rings and valves - tons of compression) and checked with the tool. Everything is great until it warms up then the timing slips. I'm wondering if i have a bad VANOS unit.

 

Your torque wrench is working correctly? Have you tried using a different wrench for final sequence to verify NM reading.? Everything you have mentioned sounds correct so far.

 

I'm going to try another torque wrench too this time. I also picked up a tiny inch-lb wrench so I don't have to best-guess the bolts that are less then 20 ft-lbs.

 

Also remember, when rotating the assembly by hand, only rotate in the CW direction --- CW when facing the chain end of the engine.

 

You say you torque but you don't mention the final angle torque using the angle gauge. Are you doing this too?

 

I did a little experiment tonight. I locked the crank in the 90 degree timing position. I broke both the intake and exhaust cam sprocket bolts loose, loosened the cam caps, and torqued down the cam caps to 84 inch-lbs (7 ft-lbs). Then I put a wrench on the end of the intake cam and it rotated freely. Then I put a wrench on the exhaust cam and it was locked solid. That could be the reason my timing was slipping. I gradually reduced the torque on the exhaust cam caps until it moved as freely as the intake. That turned out to be 20 inch-lbs. It's running fine now. I ran it for a half-hour before I left the shop and then drove it home with no problems.

My guess is my cam has a slight bend in it. Just enough that the cam caps can't be torqued to spec without binding. I'm going to run it as is. As long as the cap screws don't back out, it should be fine. I'm pretty paranoid about the chain tensioner, and I pull the cam cover off every six months to check it's condition. I'll just add checking the cap screws to the list.

 

Does it make sense to replace the vanos cam sprocket? A bent cam shaft.....really? I have never heard of that possibility. Thankfully you have the tools and the experience to stay at it and find a solution. If you were paying retail repair rates - this could get ugly. Good luck!