View Single Post
Old 03-30-2012, 08:30 PM   #4993
Mass Out
Joined: Mar 2012
Location: Central Florida
Oddometer: 7
Cam chain issues

Here is a little more on what I've learned about the NX250 cam chain & automatic cam chain tensioner over and above what I detailed in post #4965....

Remove the tensioner and retract the plunger about 3/16" by turning the adjuster clockwise. Hold the plunger firmly in. Place the tensioner (plunger side down) on a bench. Place a 1/2" drive 1" impact socket over the outside (outside is now facing up) portion of the tensioner. Observe how the plunger remains retracted with the weight of the socket. Now do the same test with the socket as a test weight, but retract the plunger close to its most retracted position by turning the adjuster clockwise. You will notice that the plunger spring tension easily overcomes the weight of the socket and the plunger extends.

This test makes clear that the pressure exerted by the plunger depends on the position of the plunger. A more retracted position (new chain) will apply more pressure to the tensioner guide and thus the chain runs with less slack (less noise). A high-mileage worn chain will receive less pressure and will likely whip around and damage the inside of the cam chain cavity or at best will make a lot of unpleasant noise.

I also discovered a few idiosyncrasies about high-mileage NX250 automatic cam chain tensioners (and I think this has already been discussed). The adjuster wants to rotate clockwise (making the chain more loose) in an oscillatory manor during actual running conditions. As soon as I witnessed this behavior, I knew that I had only two options. I would have to buy a new chain and tensioner, or I would need to try the adjuster lock bolt solution mentioned in this thread. I tried the lock bolt, but I discovered that a rough ridge on the end of the bolt was attempting to turn the adjuster clockwise due to friction when I tightened it. I filed the end of the bolt smooth and tried again with a little grease on the end of the bolt for extra insurance. Problem solved.

The next problem was the fact that I needed to know how far to run the lock bolt in. Not enough, and it wont do anything to help the adjuster, Too much and it could break the tensioner housing. It turns out that (at least on my tensioner) the correct amount to thread in the lock bolt is from 1/2 turn to 3/4 turn after first contact with the end of the adjuster. At that point, the plunger is extended outward to its stop, but does not push out with enough force to break the housing. Once you reach a little less than 1/2 turn, pay VERY close attention to a sudden increase in force required to turn the lock bolt. You really need to be paying attention at this point. When you get the bolt right, tighten the jam nut down onto the original aluminum sealing washer (I used a regular washer to protect the sealing washer from the jam nut).

So here is the tensioning procedure I used on this high-mileage chain:
Adjusted the tensioner according to the service manual.
Started the motor with the short stock sealing bolt & washer left out.
Used a 1/4" drive spin handle, 6" !/4" drive extension, 1/4" drive universal, 1/4" deep well socket, and a small #4 slotted head screw bit to reach the tensioner adjuster (a magnet inside the socket will keep the bit from wandering off).
Turned the adjuster just slightly clockwise to get accustomed to the noise resulting from excess cam chain whip.
Turned the adjuster counter-clockwise just enough to stop the sound of chain whip.
Stopped the engine while making sure to hold onto that adjuster setting (remember,a high-mileage adjuster is dancing all over the place).
Ran in the locking bolt and tightened the jam nut as described earlier.

Problem solved (for now).

One thing should be made clear after all of this. I do not think it is a good idea at all to insist on using and old chain and adjuster like I did. Go buy yourself new parts and do the job correctly. You'll have no retarded cam issues and your cam train will be far more reliable, not to mention, you won't have to worry about the chain chewing through from the inside.

Take care....

Mass Out screwed with this post 03-30-2012 at 08:39 PM
Mass Out is offline   Reply With Quote