Rod Bending ~ weak rods /poor fuel/ bad injector timing - PeachParts Mercedes-Benz Forum

layback40 · #1 01-29-2011, 12:18 AM

For some time have been looking over information on the 350 'rod bender' & other information on high speed diesels that have suffered rod bending.

It is generally accepted that the primary cause of the problem with the 350 was weak rods.

One issue that is often raised by MB & others is why did it only happen to a proportion of the motors & only in the USA? There are still 350's running with the early weak rods & not in need of replacement. Both in the USA & other parts of the world.

In other parts of the world the 350 motor with the early style rods were used without problems & during MB testing in Germany & other parts of Europe prior to the release of the motor MB did not see the problem.

The answer appears to be related to the diesel sold in the USA & the statistical variation of injector timing and rod strength that occurs in the manufacturing process.

The fuel;
The main property determining measure used is the Centane Number.
USA produced diesel is known to have a lower value than the fuel produced in the EU/ Middle East/Africa/ S. America/ Asia, basically the rest of the world.
What does Centane Number measure?
The following is from BP & explains it in simple terms;

What is cetane and why is it important?
Cetane number is the key measure of diesel fuel combustion quality. The number relates to the ignition delay - the period that occurs between the start of fuel injection and the start of combustion. Good quality combustion occurs with rapid ignition followed by smooth and complete fuel burn: the higher the cetane number, the shorter the ignition delay and the better the quality of combustion.
Conversely, low cetane number fuels are slow to ignite and then burn too rapidly, leading to high rates of pressure rise. These poor combustion characteristics can give rise to excessive engine noise and vibration, increased exhaust emissions and reduced vehicle performance, with increased engine stress. Excessive smoke and noise are familiar problems associated with diesel vehicles, particularly under cold starting conditions.

Basically what we had with the 350 was a motor that was fine with high Centane Number fuel & potentially a recipe for failure on low Centane Number fuel. That is the main reason that the 350 motor did not fail in other parts of the world.
You will note that the lower the Centane Number, the slower the fuel is to start to burn, some ill informed individuals argue that one should advance the injection timing to compensate for this. The problem is that is only half the picture, along with delayed ignition, the fuel burns many times faster. This is more significant. The flame velocities with low Centane Number fuel puts the combustion of those fuels into a category of explosions. Even though the fuel may be a milli second or so later to start burning, its combustion is complete many milli seconds before a higher Centane Number fuel. So we have a situation that the fuel may be burned even prior to TDC. This combined with the PVT effect of the compression, results in higher combustion temperatures & extream loads on rods/pistons /rod & wrist bearings. we could replicate this problem with other diesels by simply advancing the injection timing a bit.

If you wish to kill a 61x, 602 or 3ltr 60x thats one way of doing it. Probably the rods will survive but something else will fail. Could be the head/head gasket/rings/ rod bearings/pistons. Those of you in the USA have the fuel that will do it. Interestingly the latest Porsche & VW top end diesel 4WD's call for higher Centane Number fuel than the norm in Europe. It is likely that a detuned version would be needed in the USA or we may see a 350 type repeat. BP are marketing a high Centane Number fuel in the EU for them.

What we all should learn from this is that if you advance the injector timing on any diesel motor including those made by MB, & use low Centane Number fuel like that sold in the USA, you greatly increase the risk of premature motor failure.
If you set your injector timing for maximum combustion temperature, you are probably having complete combustion before TDC some of the time.

The second part of the 350 story is more intellectually interesting.

Why didnt all 350's in the USA fail?

There are two main reasons for this.

1/ the rods are manufactured with normal manufacturing tolerances. Half are going to be lighter (weaker) than the median, the other half, stronger.
The rods are graded by weight so that any motor tends to have rods of very similar weight. This results in some motors being more susceptible to bending than others. You may note that MB fixed their problem with the 350 motor in the USA by using stronger rods. If some one was lucky enough to get a 350 with the original rods that were on the heavy(stronger) end of the production range, they may have not experienced the problem.

2/ Injector timing;
The motors leaving the factory typically had a tolerance of +/- 1.5 deg on the injector timing. This meant that some motors ex the factory were much more susceptible to the ill effects of low Centane Number fuel. Those motors leaving the factory with retarded injector timing may be less likely to bend rods.

Putting these 2 together; If you had a motor with the heavy end of the rod variation & on the retarded end of the injector timing, you were luck & probably would get many 100k miles with no problem. If your motor had the light end of the rod variation & 1.5 deg advanced ignition then you may have been one of the first back to MB with a problem.

To try and vary from manufacturers recommendations with regard to injection timing for what ever reason is a very foolish practice, particularly in places like the USA that have low Centane Number fuel.

If any one has any credible information that contradicts this please provide a link.
I know that we have a few members with university training in reaction kinetics, thermodynamics & combustion theory. I note that when this sort of thing is discussed, they stay right out of it. Probably they realize that many of those commenting do not have sufficient technical back ground to be able to make proper informed comment.