3B cylinder not firing at idle?

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Cylinder #3 on My 1979 B engine doesn't fire at idle but fires fine at about 1,200 rpms and up. No air in the system, injector was tested and is fine. My guess is that i'm just not getting enough fuel pressure at idle for this cylinder - not sure why only this cylinder is not getting it? Checked fuel lines and they look good. Next step is probably to get the injection pump tested. The rig has an aftermarket fuel tank mounted in the back. I wonder if putting a low pressure in-line fuel pump near the rear tank would help? Any other suggestions would be welcomed.
thanks!
 
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Cylinder #3 on My 1979 B engine doesn't fire at idle but fires fine at about 1,200 rpms and up. No air in the system, injector was tested and is fine. My guess is that i'm just not getting enough fuel pressure at idle for this cylinder - not sure why only this cylinder is not getting it? Checked fuel lines and they look good. Next step is probably to get the injection pump tested. The rig has an aftermarket fuel tank mounted in the back. I wonder if putting a low pressure in-line fuel pump near the rear tank would help? Any other suggestions would be welcomed.
thanks!
Sorry, title says 3B but it's a B engine.
 

stevebradford

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Diesel motor cylinders fire off compression. At an idle it’s not compressing the the air fuel mixture like it would be at 1200 rpm. At 1200 rpm the piston is coming up to the head compressing the air/fuel a little more than twice as fast than at an idle. That is giving less time for vapour to escape past the rings or valves allowing it to build enough pressure for combustion. When all four cylinders are firing in the 3b there’s not a very noticeable power loss even with some lower compression numbers.

What makes you think one cylinder isn't firing?

How much blow by and oil is coming out of vent tube?

If you think one cylinder is not getting fuel you could sort of flow test the pump yourself. Take the injector lines off the injectors but leave them on the pump. Turn the motor over by hand and make sure there’s a consistent amount of fuel delivered to each injector from each line. Spinning the motor of by hand would be slower than idle but you might be able to see a fuel delivery problem that also happens at idle. In-line injector pumps on the 3b are almost never the problem.
 
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Diesel motor cylinders fire off compression. At an idle it’s not compressing the the air fuel mixture like it would be at 1200 rpm. At 1200 rpm the piston is coming up to the head compressing the air/fuel a little more than twice as fast than at an idle. That is giving less time for vapour to escape past the rings or valves allowing it to build enough pressure for combustion. When all four cylinders are firing in the 3b there’s not a very noticeable power loss even with some lower compression numbers.

What makes you think one cylinder isn't firing?

How much blow by and oil is coming out of vent tube?

If you think one cylinder is not getting fuel you could sort of flow test the pump yourself. Take the injector lines off the injectors but leave them on the pump. Turn the motor over by hand and make sure there’s a consistent amount of fuel delivered to each injector from each line. Spinning the motor of by hand would be slower than idle but you might be able to see a fuel delivery problem that also happens at idle. In-line injector pumps on the 3b are almost never the problem.
At idle it runs very rough, shakes, and quite a bit of smoke from the tailpipe. I can see that cylinder #3 doesn’t cause any change when I loosen the gas feed to the injector at idle speed but all the others do. Don’t notice much oil blow by at the vent tube. Thanks again for the suggestions
 
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I would pull the injector out and get it tested. And test the engine compression. Or you could remove all the glow plugs and crank it. You should see 4 plumes of diesel coming out of the glowplug holes.

I imagine its makes a bit of smoke under 1200rpm
All injectors were pulled, cleaned, and tested. This was the first thing I did. Yes, quite a bit of smoke at idle due to cylinder 3 not firing.
 
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Dollars to donuts your cylinder #3 has poor compression, it could be worn cylinder/piston, valves, head gasket….
you will need a compression test done ( both dry, and wet) to give you an idea of where to start.
 
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Dollars to donuts your cylinder #3 has poor compression, it could be worn cylinder/piston, valves, head gasket….
you will need a compression test done ( both dry, and wet) to give you an idea of where to start.
Sounds like that’s the consensus, thanks all for the comments!!
 
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I would also say its a compression issue then. A cyl with low compression will begin to fire as the piston ring expands and builds compression, which is probably what happens at 1200rpm.
The smoke is most likely grey and white. This is a sign that combustion is too cold
 
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Better to sort it out, running like this, if this is a compression problem, the unburned diesel will wash out oil on the cylinder, may become worst and worst to a point which need an overhaul.
 
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Dollars to donuts your cylinder #3 has poor compression, it could be worn cylinder/piston, valves, head gasket….
you will need a compression test done ( both dry, and wet) to give you an idea of where to start.
I had the compression checked and the mechanic found that the valves on cylinder #3 needed adjustment. After adjustment, compression was back up and engine ran smoothly at idle. Thanks for all the feedback!
 

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