d8pete Posted October 11, 2009 Share Posted October 11, 2009 I have a 1984 R686ST with the 300 and 6 speed and when the truck is under a heavier load/pull or when accelerating hard the engine noise increases quite a bit. Best I can describe it as a general dull pounding noise and is most noticeable at 1600rpm. When empty or pulling on level ground the engine is considerably quieter. The odometer shows about 415,000 miles and at about 1800rpm or better I carry 60 plus pounds of oil pressure hot. When cold the oil pressure is easily over 90 or 100psi. Hot it probably idles at 20 psi and pulling in the low rpms it runs about 30-35psi. So my question is what is normal operating oil pressure for a new and worn engine and how does mack determine when the bearings should be replaced? One friend told me not to worry about the noise and another who was once a Mack mechanic thought I might have a bad or leaking injector causing the noise. Any ideas? Quote Link to comment Share on other sites More sharing options...
HK Trucking Posted October 11, 2009 Share Posted October 11, 2009 Could be a defective or worn out harmonic balancer. A rumble around 1500 - 1600 RPM when pulling is a classic symptom.If that is indeed the problem, replace the balancer with a new one ASAP, as those engines are susceptible to crankshaft breakage if run too long with that vibration going on.. Quote "If You Can't Shift It Smoothly, You Shouldn't Be Driving It" Link to comment Share on other sites More sharing options...
Rob Posted October 11, 2009 Share Posted October 11, 2009 Could be a defective or worn out harmonic balancer. A rumble around 1500 - 1600 RPM when pulling is a classic symptom.If that is indeed the problem, replace the balancer with a new one ASAP, as those engines are susceptible to crankshaft breakage if run too long with that vibration going on..Hey Herb, is there any difference in the cranks from an EM6-300, E6-350, and an ENDT-676? I've got a friend with two R model mixers, (great shape) with busted cranks. I can buy the trucks right and have a couple of cranks from various engines; Don't even ask what Momma thinks!! I know one of the mixers has the dampner comming apart and they kept running it knowing the problem existed!!Rob Quote Plodding along with no job nor practical application for my existence, but still trying to fix what's broke. Link to comment Share on other sites More sharing options...
HK Trucking Posted October 11, 2009 Share Posted October 11, 2009 Hey Herb, is there any difference in the cranks from an EM6-300, E6-350, and an ENDT-676? I've got a friend with two R model mixers, (great shape) with busted cranks. I can buy the trucks right and have a couple of cranks from various engines; Don't even ask what Momma thinks!! I know one of the mixers has the dampner comming apart and they kept running it knowing the problem existed!!RobThe EM6 300 2 valve, E6 350 2 valve and the ENDT676 cranks should all be the same I would think. They're all basically the same engine except for fuel injection settings & turbo differences. Quote "If You Can't Shift It Smoothly, You Shouldn't Be Driving It" Link to comment Share on other sites More sharing options...
d8pete Posted October 14, 2009 Author Share Posted October 14, 2009 HK is there a way to test to see if the balancer is faulty and if it is the balancer wouldn't you hear the noise all the time, not just under a load? Thanks, Pete Quote Link to comment Share on other sites More sharing options...
phantom309 Posted October 14, 2009 Share Posted October 14, 2009 This is just an amature opinion but I don't think you'd hear the bearings unless your engine was really ready to die and then you'd hear them even at an idle.One thing you can try is to get the truck cruising up the road at a steady rpm then try going from coasting to full throttle while watching your oil pressure. The extra pressure on the crank at full throttle should cause the crank to squeeze down on the oil and settle a little deeper into the lower bearing. The oil is fed from the top so that will expose an easier path for the oil to escape and therefore you should see a drop in pressure if the main bearings are worn. I think it's fairly normal to see a drop of a couple of pounds but if you get much more than that it might be something to follow up on. This isn't really any kind of accurate test but if you try it and don't see much of a drop in pressure it might help put your mind at ease.Hopefully someone will be able to answer what the normal oil pressure should be on that engine. I know the Cummins engines run very low pressures similar to what you've described. Quote Link to comment Share on other sites More sharing options...
HK Trucking Posted October 15, 2009 Share Posted October 15, 2009 HK is there a way to test to see if the balancer is faulty and if it is the balancer wouldn't you hear the noise all the time, not just under a load? Thanks, PeteThe best way is to borrow a known good harmonic balancer from another engine, put it on your engine, then go for a test drive and see if the problem has gone away.Under a no load condition you may feel a vibration at the critical RPM, but with a load on the engine it is more pronounced.. Quote "If You Can't Shift It Smoothly, You Shouldn't Be Driving It" Link to comment Share on other sites More sharing options...
Rob Posted October 15, 2009 Share Posted October 15, 2009 with a load on the engine it is more pronounced..That would be called uncontrolled harmonics, (vibration) which is exactly what breaks most crankshafts. An inline six cylinder has a power pulse every 120 degrees of crankshaft rotation. These power pulses passed to the crankshaft are amplified in force with the application of the driver's foot asking for more power. The vibration dampner, (harmonic balancer is more of a correct term) on the front of the crankshaft absorbs some of this vibration, therby changing the resonant frequency, the crankshaft is vibrating at. A worn balancer allows this high frequency vibration to go uncontrolled and sets up torsional stresses in the rotating crankshaft. When either the sixth, or 12th harmonic frequency is acheived, crankshaft breakage occurs due to fracture at the molecular level of the steel.Basically the same way a piece of glass breaks due to high frequency soundwaves. Rob Quote Plodding along with no job nor practical application for my existence, but still trying to fix what's broke. Link to comment Share on other sites More sharing options...
Mike Posted October 15, 2009 Share Posted October 15, 2009 The best way is to borrow a known good harmonic balancer from another engine, put it on your engine, then go for a test drive and see if the problem has gone away.Under a no load condition you may feel a vibration at the critical RPM, but with a load on the engine it is more pronounced.. Herb, If the balancer is out of balance then I would think with as many miles as he told us then I would be putting new bearings also into that motor. Since it's probably started pounding at those bearings. It sure doesn't hurt to pull the pan and check the bearings while he's into it. Just my .001% opinion.mike Quote Link to comment Share on other sites More sharing options...
HK Trucking Posted October 15, 2009 Share Posted October 15, 2009 Herb, If the balancer is out of balance then I would think with as many miles as he told us then I would be putting new bearings also into that motor. Since it's probably started pounding at those bearings. It sure doesn't hurt to pull the pan and check the bearings while he's into it. Just my .001% opinion.mike Wouldn't hurt to put new bearings into it. But if the balancer is indeed defective, the problem will still be there.Swapping another balancer onto an ENDT 676 / EM6 300 is easy, just remove the 6 retaining bolts and pry it off the hub, stick the new one on, replace the bolts and tighten them. The pulley is behind the balancer on those engines, so no belts need to be disturbed.. Quote "If You Can't Shift It Smoothly, You Shouldn't Be Driving It" Link to comment Share on other sites More sharing options...
tech_j Posted October 15, 2009 Share Posted October 15, 2009 Just wondering... What is the condition of the mount bushings?Oil pressure is well within spec. for an E6 BTW. Quote Torque wrench? Can I put an air hose on it?? Link to comment Share on other sites More sharing options...
Mike Posted October 15, 2009 Share Posted October 15, 2009 Wouldn't hurt to put new bearings into it. But if the balancer is indeed defective, the problem will still be there.Swapping another balancer onto an ENDT 676 / EM6 300 is easy, just remove the 6 retaining bolts and pry it off the hub, stick the new one on, replace the bolts and tighten them. The pulley is behind the balancer on those engines, so no belts need to be disturbed.. Yes Sir, I agree, If 415,000 miles is correct then sounds to me it is time for a total motor rehab. If problems exist, it normally comes in threes ya know how it goes. Ya install one thing then another pops up. regardsmike Quote Link to comment Share on other sites More sharing options...
micked Posted October 20, 2009 Share Posted October 20, 2009 Sounds like the same vibration i get in my E6-350 just done oil change and found copper inside filter, pulled sump off and main bearing just started to let go.put new bigends and mains in to keep it goin but the vibration persists.I think i'll check the balencer and fly wheel next Quote Link to comment Share on other sites More sharing options...
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