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Discussion Starter · #1 ·
I have posted on this before and the “4BT hard knock” thread got me thinking. First let me describe. If my engine is at idle, the tensioner is moving 1/4”. To clarify, it is a 1/4” blur. On the other side where the belt goes from the crank to the first idler, I would say the belt bounce is about 1/2”. If I shift it in to gear, but don’t add throttle, that bounce is 3/4” or more. If I add some throttle, the bounce is gone at the free belt, but the tensioner is still a blur.

It drives fine, but I’ve always felt this is not right. In the “hard knock“ thread the OP mentioned “breaking the fuel lines to check injectors”. Could someone explain this? If one of my injectors was not firing, would that cause the belt bounce. Is it possible to have a bad injector and the truck drive this good?

If I “break“ the fuel line, how do you keep it from shooting fuel all over. Or do I not understand the term?
 

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I would think the tensioner should stay pretty stable when running. Lots of movement would seem to indicate either a weak spring in the tensioner or a belt that is a little too long or a bit of both. Breaking the fuel line is when you loosen the nut on top of the injector. That usually done when you're trying to get the air out of the lines, And yes, if your injection pump is functioning properly, it will squirt all over the place. Can hit the hood. Must be careful as that fuel is under over 3500 PSI. It could shoot through your skin. You normally would only have those nuts loose for a brief instance. If one injector is not firing the engine would run pretty rough. If in doubt, pull the injectors and have them pop tested. Over time, the internal springs in an injector relax so and pop pressure can vary between each injector. The closer they are matched to the same and correct pop pressure the better the engine will run. VE pump injectors pop at 245 bar which is 3553 PSI and P7100 Pump units pop at 260 bar which is 3771 PSI. To get the pressure just right there are some micro shims in the injector body that you can add to get it just right. Those little sucker are pricey and there are 30 different thicknesses of them. Testing of injectors doesn't cost much but the rebuild can seem expensive and those little suckers are part of that.
 

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Discussion Starter · #3 ·
Do you think the injectors can cause the belt bounce?

So your saying to test them on the truck, I should just loosen the nut like I’m bleeding it? (I’ve done that on lots of diesels.) Basicly, I just start the motor and loosen each one till it runs rough and tighten it again. If one doesn’t run any rougher, it’s bad. (I know pop testing them would be the correct way) I don’t know anywhere around me that would pop test them. Is that something common?
 

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Testing injectors is done in a shop. Each injector is tested on a machine to see what pressure it fires at. You can buy your own tester for around $75-100 and check then yourself. If they are off be any significant amount then the rebuild is normally done in a shop. Most usually trade in their old ones for rebuilt units because having them rebuilt sometimes costs more. Never know what you're going to get into, Rebuilt injectors will usually hold their tolerances better because the internal spring has relaxed some. For example, the P7100 injector is part 3802547. New ones cost a little over $200 each. Oregon fuel injection sells the rebuilt units for $84 each plus a $30 core which you get back so long as your old ones are rebuildable.
 

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Discussion Starter · #5 ·
One other thought. Ten years ago when I got my 4BT, it had a bent crank pulley. At that time, people were saying get a 6BT crank pulley because it has an active ballancer on it. So I got one. Since then I’ve seen a couple posts where people say the 6BT balancer is tuned for the 6 and doesn’t work on the 4BT. So my double question, Does it work at all on a 4BT? And if not, is it detrimental on the 4BT. Could it be causing some belt bounce.
 

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To answer you question the 6bt harmonic balancer has little to no effect on a 4bt. Won't do any damage but doesn't help either. The 4bt doesn't have harmonics like a 6bt. Cummins offered a balancer for the 4bt but it helps by adding mass to the crank. That thing weighs about 28 lbs. It is a fluid type balancer and there are aftermarket versions made by Fluidampr which are basically the same. The OEM Cummins units were mainly used on marine applications. We have a few members who use these but most have found them quite expensive. The Cummins units are around $600 or more and the Fluidampr about the same depending on which model of theirs you use. They make them with and without the drive pulley.
 

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Discussion Starter · #7 ·
I wonder if “locking” the heavy ring to the inner hub on a 6BT pulley would be a benefit to the 4BT. You would get the weight and no rubber bounce. I think I could weld some bridges across the rubber or maybe run bolts through.
 

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I don't think a totally hard setup like that is the best idea. The fluid type units are filled with some type of material that can move around inside the housing. Probably a lot more flexible than that rubber ring on the standard type. Most 4 cylinder engines have similar problems with vibration. On gas engines they often cure it by placing a counter rotating shaft inside the engine to offset the vibration. Works pretty good. Cummins did something like that and had a unit that bolted to the bottom of the engine block. Worked perfectly but had a limit of 2000 RPM. They used them on some generator applications where speed was limited to 1800 RPM. I have only seen a couple of those and they were expensive. Cummins part 2831353 is one used on the QSB4.5 and parts 3907369 and 3915763 are others that were on the 3.9. Those units weren't really practical for road vehicles due to the fact they hung down under the engine and required a different oil pan. Below is a photo of one. As you can see it looks simple but that sucker has a lot of parts.
 

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