Glend wrote:
That's a shame, and there are no aftermarket alteratives for the rockers.
As Geordi noted there are rocker sets available from the UK, but I think there is a secondary manufacturer for rockers. I have seen sets that did not come from the UK, that while dimensionally identical, appeared to have come from a different manufacturer than the VM, down to the markings on them.
crd260 wrote:
okay, so your basically saying the badly worn rollers on all the majority of my rockers are due to simultaneous valve hits on pistons due to the timing belt skipping a tooth?
I'm saying the "wear" isn't wear at all, but rather the bearings failing from a piston impact.
crd260 wrote:
I understand how it could skip "back" one, but how exactly would it slip "forward" (back into it's original position)......
I mean... the crankshaft has all the inertia when the engine's running....
Here's my logic on it, the crank is turning CW(as viewed from the front), the long stretch of an untensioned belt between the IP and the crank can flop around a bit when we have an old stretched belt. Now the crank/pistons all do have inertia, but under some conditions, where the vehicle is running/driving the compression works against the belt turning, the belt "skips" or maybe "floats" at bit on the crank, some valves hit and all of a sudden the crank is stopped a bit, the belt then has an opposite force working on it than before, and the belt "skips" back.
Maybe think of it this way, the belt slips forward, and then the impact force slips it back....make sense? That impact of valves hitting the head holds the crank in place(albeit briefly) and the crank realigns at the right point.
The only reason any of that is possible is that the belt is old and stretched(it is noticeably easier to reinstall a used belt than a new belt for example).
crd260 wrote:
if the rollers on the rockers usually last fine like you say, and it's common to find sudden damage to multiple rockers on these engines, the only answer I could think of would be belt stretch.
Diesel's run high compression, and therefore the pistons run very close to the valves...
So much so, that maybe just retarding the timing a tiny bit due to belt stretch could cause it.
Think about it.... There is a LOT going on with that timing belt... compared to say a common DOHC gas engine, the belt is twice as long.
two idler pulleys, the tensioner, the water pump, the high pressure pump...
Maybe theses engines run the piston to valve clearances so low that at full redline with all the mechanical draw from the two pumps and two cams that the belt stretches enough to cause an impact....
Maybe....some sort of crossover of valve float and high RPM on a stretched belt. That doesn't explain why it would happen while sitting at a stop light, now its still hard for me to fathom the conditions while idling at a red light that would cause a stretched belt to slip like my theory states, but the idea of high RPM valve float/belt stretch doesn't work there. It also doesn't work in the other conditions people have noted(coasting downhill at higher speeds for example).
GreenDieselEngineering wrote:
The bearings in the rocker arm rollers are sub standard. The high levels of soot in the oil is most likely leading to the bearing wear. Get rid of the soot and use a slightly heavier oil and the problem should go away. These rocker arms have a much lower failure rate in the export market (outside north america), mainly due to different oil specs and less use of EGR.
Once the rollers have too much play it is possible for a rocker to jump and fail or sieze and fail. Hopefully your head is still in good shape.
While I agree with your first couple of points about wear, I still don't see how the wear explains how an engine can be running so good one moment, and then so poorly on ALL of the cylinders. Yes if a rocker somehow came off of the valve I could see that being a problem, but in two cases now I have pulled covers where all 16 rockers were still in place.
Is there some way that the bearing surface can wear and wear and wear, until it reaches a point where suddenly a "race" or something gives out, the bearing collapses, and we get a suddely collapsed roller, and a suddenly much worse running vehicle.
The only other thing I can think of to throw in the mix is that when I see the damaged rockers the exhaust side is always worse than the intake side. They tend to be very uniformly "worn/collapsed" I would feel like if wear only was responsible for the damage it would be consistent across intake and exhaust, not noticeably different on intake, and uniformly across intake and uniformly across exhaust.
This is all interesting discussion, for me it would be interesting to tear apart several 100k mi + CRDs, mine for example has been running delo 15-40 since 30k with no EGR, if wear is a mitigating factor, how would an engine with 120K worth of 15-40 oil have compared to an engine using factory oil specs this whole time.
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