have cat 312b 3064 engine that has a miss in it when it’s at auto idle, approx 30% throttle. When it’s working it’s not noticeable when it’s at low idle it seem fine but at the 30% range it shakes and sounds like it’s missing on one cylinder. We have rebuild the lift pump, four new injectors, had injection pump off and to the fuel shop, just put new coupler in between the hydraulic pump and engine checked for air in the fuel system return fuel and the delivery side , lift pump pressure is good 25psi approx . We can’t identify a cylinder by itself by cracking injectors , valve adjustment has been checked? I’m stumped a lot of these checks we’re done by cat field service some by us. We don’t know what to check next! Fuel pump guys say it’s meets requirements on bench tests. Any ideas? Thanks hanco
How many hours on it? Maybe the balancer has taken a crap. Fan drive look ok? Just thinking of other things that could be off balance that might create what you’re feeling.
8000hrs .... doesn’t seem to have a “balancer” on it , pulleys on front of engine seem to be just a drive for the belts , nothing there to do what your suggesting. I ask cat parts guy he says it’s just a drive pulley...
That engine might be internally balanced but even so I’d expect something on the crank to act as a damper. What’s your S/N? I’m curious now
8jro1600
Have you checked temp of exhaust manifold where it meets engine. If you have a miss one hole should be colder. I use a non contact thermometer .
Yes they all seem to be close to each other .....
Just asking, have you tried cancelling cylinders while at this 30% engine speed? Full throttle masks an otherwise weak hole with inertia. Having it smooth out at idle is a bit of a head scratcher. Also I would have a competent person pull function at this 30% while you shoot surface temp of exhaust manifold as close to cylinder head as possible. This may reveal your problem. Now Junkyard brought up a good point. I would look at your parts breakdown and see what your pump coupler looks like. Is it a lovejoy, splined hub, dampened disc,etc. I have seen problems with spring dampers failing in drive discs, same with Lovejoy, a piece breaks off suddenly you have a stumble that feels like a miss when the harmonics are right. Just some thoughts Good luck
B model should have an external pump. Crack each injector line at the 30% throttle and see if you can hear a difference. Edit: I thought it was a 3406B. I see its a 312B excavator. So, you have unit injectors. You can still check them the same way by holding down the rocker for each injector and listening for a change.
We have tried checking at low idle, approx30% and 100%. We have checked temps a couple times also, no real difference . Just put a new coupler in this week between the hydraulic pump and engine . But still have the issue ....
Yes we have tried this , can’t pin point any cylinder that’s an issue....
But have you cancelled them out one by one while at 30% under load. For example bucket fully curled in arm curled in over relief. This will load the engine somewhat so that it may make finding your miss, weak hole a little easier to find
Not underload we have not. the “miss” is not there under-load ,but at this point I’m game to try it...
Any progress?
No not really, have tried your idea but didn’t learn anything new. Talk to fuel pump shop they gave me some thing to try on the fuel pump , didn’t work either. Turned a screw in that has something to do with the governor. They called it a bump screw, mechanic called it a stabilizing screw...? I’m thinking I’m going to pull hydraulic pump off and run engine with it off and see what that does I’m lost .....still think it’s injection pump related but trying to eliminate all other options. Thx hanco
Well, that's unfortunate. Let's hope for some results in your next course of action. Although I really don't wanna see you have a pump problem.
You said you replaced the pump coupler and this may be a dumb question, but did you check the motor mounts?
Yes there where all replaced a month ago , they have been check recently still tight and in good shape. Thx hanco
I had some time today so I pulled the hydraulic pump again and ran the engine without it , engine ran the same still have the issue so it’s not hydraulic related. Guess the injection pump is next ... again!
Have you any parasitic loads off front of engine? Fan drive, alternator? What if you were to run without the belts for a couple minutes. Just for testing purposes. Have you any loads off timing case or bell housing? Accessory pump, fan pump? Anything that a core engine would not have?
Yes there is an alternator and air conditioner... not a big issue to disconnect and try , nothing else off bell housing... not sure what drives water pump...
What year is this machine? I don't think you have an injection pump.
It’s a 99 or 2000 , it has a ZEXEL injection pump on it
OK. That's a Mitsubishi engine.
Update.... pulled injection pump again sent it to a different shop and they went through it , found a few issues and fixed them. Machine runs much better now! ..... Hanco
Did they replace the delivery valves and springs in pump? Bob
I don’t know exactly what was replaced but there’s springs in the box of stuff they sent back with the pump , there’s some small shafts, pistons and sleeves if that’s what there called, and the cover off one end
Wow.. sounds like plunger & barrels.?? Did they charge u an arm & a leg?? Those parts aren't cheap. THATS gotta hurt after "just" having it done somewhere else.??
Well it wasn’t as bad as I figured first shop was 800$ and said it was good , second shop was 1850$ ... I don’t feel it was to bad......Hanco
2600.00.?? PLUS u had to pull the pump TWICE{not fun} & THATS not bad?? Dang, we all need more customers like YOU.. Lol Glad u got it fixed..