Missed some of this, but for about 4 years, while I worked at a
Rover dealership ('87-93), we had a cam wear problem on 3.5L engines. It
dated back a ways (prior to the USA intro), according to many sources, and
was common on any engine with the "detox" or "low lift" cams (the NAS
'87-88 3.5L had the low lift cams). To a lesser degree, the other 3.5Ls
were affected as well. A lot of cams were warranteed on 3.5L Rover engines
from '87 until the last of those vehicles went out of warranty.
We discovered the problem while repairing a rash of intake manifold
coolant leaks. The bad lobes were often readily visible. After that we
started checking when the engine was down. The earliest cam failure I saw
was at 1,100 miles but I have torn apart about 50 NAS 3.5L engines in my
career and only a handful did not show abnormal wear.
The 3.9, 4.0, 4.2 and 4.6L engines seem immune to the problem to a
large degree, due to better designs and manufacturing. In the course of
doing an article on cams a few years back, I queried 8 cam manufacturers,
including some in the UK, about the problem. I was told it was due to two
situations; the small diameter of the cam (which leaves less surface area
when the profile is ground) and poor heat treating. Lack of maintenance,
internal coolant leaks or subgrade oil made the problem worse. The surface
area problem was solved by using more of the material in the blank when the
cam was ground and the heat treating was improved.
As to checking, the bad lobes can be identified visually but if you
don't know what to look for by experience, you will only spot it when it
gets very bad.Tthe best method with an assembled engine is via a dial
indicator. You will need to remove the rocker arms to do this, as the
lifters will bleed down via valve spring pressure and give inaccurate
readings. You can use the pushrods to measure from. Put the cam on the
center of the heel to zero the dial and then rotate the engine to the peak
lift point and record the results. Test each lobe a couple of times until
you can dublicate the results exactly each time. Since the hardening is
little more than .010-012" thick, if the readings vary more than that, lobe
to lobe, you are into the soft stuff somewhere. Also, you can compare the
reading to the lift specs in the manuals. If you have a cam out, you can
mike the lobes, compareing differences between the lobes or to a new cam of
the same grind.
As to checking timing chains, I tested the cam timing of about 30
vehicles in the course of my Rover dealership work. Any time I had the
front cover off and had a few extra minutes, I checked with a degree wheel.
I was curious as to why one engine could be a rip-snorter and the next a
dog. I found that only five or six of the engines I tested were at, or
close enough to, the factory spec. About a third were advanced a few
degrees (up to about 4 degrees- making them the rip-sporters) and the rest
were retarded (the dogs) to varying amounts (up to 8 freaking degrees).
Going farther, I checked to find where the errors came from and found that
every cam I checked was dead on and the end result was that the timing
gears were the cause. The keyways were machined incorrectly. After that, I
did not trust the factory timing gear. The few (maybe two) I replaced under
warranty (no, they would not replace the units for timing errors unless the
engine was grossly effected), I checked for accuracy and swapped sets with
the parts dept to get a good set. If the customer was buying, I installed a
Cloyes True Roller set. I know of one customer who could feel the
difference, seat of the pants!
Hope this helps.
Jim Allen
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