Cat 312B engine spins wrong way at load/idle

[digger_hunter]

Hi Guys,

Has this ever happened to you? We have a Cat 312B excavator which we bought from some not so darling brothers! Proper scammed us. Anyways the engine and pump appear to have been overhauled with a brand new Zexel injector pump and all. However something weird happens to this engine when you apply a load at low rpms. I mean lets say you set the engine speed to 1-3 and try to do a spot turn, the engine sorta gets loaded up and then its sputters a bit and theres exhaust gasses been blown out the air filter! I checked and couldn't believe my eyes when i observed the cooling fan juste stutter and spin the other way round. DOesnt last much long like that, cuz it sputters a few seconds and dies.

But does anyone know whats wrong with this or how to fix it? The guys who sold it to us won't answer our calls as they probably knew the issue so they tried a new injector pump and just sold it off to gullible us!

Thanks

 

[lumberjack]

I'd check the pump's timing.

 

[Bob/Ont]

I think the main hyd pump isn't unloading enough at low RPM and if you demand flow from it the load can kick the engine over backwards just like stalling a dump truck and have it roll backwards in first.
Later Bob

 

[lantraxco]

Bob, just curious, wouldn't the hydraulic system normally have load checks in the valve stack that would keep oil in the system from flowing back from the valve bank to the pumps? Or am I hallucinating?

 

[overworked]

I would think it would pull oil from return/cooler and tank to pump or probably cavitate. Should start at pump timing like lumberjack stated.

 

[Bob/Ont]

Bob, just curious, wouldn't the hydraulic system normally have load checks in the valve stack that would keep oil in the system from flowing back from the valve bank to the pumps? Or am I hallucinating?

You are right. Was just thinking about what could cause Blow Back and reverse the engine. It has a new injection pump on it which may or may not be timed right. Just thought they might have changed the wrong part.
Later Bob

 

[digger_hunter]

You are right. Was just thinking about what could cause Blow Back and reverse the engine. It has a new injection pump on it which may or may not be timed right. Just thought they might have changed the wrong part.
Later Bob

Hey guys. Think we found the issue. Looks like the seller,darling sons in WA sold us a piece of junk and got away with It. Basically the engine has sucked in loads of water as there was loads of mud in the air intake etc. This probably messed up the injector pump which is why they replaced it. We found that the electronics settings were all changed to stop the engine from idling so they knew the problem was there. So as soon as we changed the settings to standard the problem came back. Mind you we found it only when we tried to get the thing to auto idle.

In the end we had to change the timing on the injector pump as lumberjack suggested and that solved the issue. But we could still see some water on the engine dipstick so these guys hadn't repaired te engine at all. Just changed the pump. But I am curious how the engine spinning the wrong way affects the main hydraulic pump? Would it cause any damage?

 

[Steve Frazier]

I don't know about the hydraulic pump but it will definitely cause damage to the engine over time. When it is running backward so is the oil pump so you have zero oil pressure to the bearings.