Yanmar 3GM hard starting after injector service

[skunther]

My 3GM30F recently had the injector pump and 2 injectors rebuilt and installed by a local shop. Since then, it is incredibly hard to start. They checked the injectors and pump and seem stumped as to what the issue is. Valve lash is fine. Both fuel filters are recently replaced. Once started, the engine idles, runs and cruises fine.

What could be the issue?

[Wotname]

Was it easy to start before this work was done?
What was the reason for having the pump and 2 out 3 injectors rebuilt?
Was any other engine work carried out at the same time?

If it was OK before the work, then the work (or the refitting) is very likely to be the problem.

Have you tried starting the engine at full throttle? The GM series is supposed to (internally) leave the fuel rack fully open regardless of throttle setting prior to start but I wonder (speculating really) if the linkage between the throttle / fuel rack / governor was correctly re-installed; it may be trying to start with the fuel rack in the minimum position if the throttle is at idle if the linkage was incorrectly refitted.

[skipperpete]

Originally Posted by skunther

My 3GM30F recently had the injector pump and 2 injectors rebuilt and installed by a local shop. Since then, it is incredibly hard to start. They checked the injectors and pump and seem stumped as to what the issue is. Valve lash is fine. Both fuel filters are recently replaced. Once started, the engine idles, runs and cruises fine.

What could be the issue?

Was there a reason to get the pump and injectors serviced other than routine maintenance? Try this….flick the stop lever back and forth a few time and when it returns to the “run” position try for a start. If it now starts easily, the rack is sticky and not fully returning to the max fuel stop at shutdown. Also check that the lever is actually not restricted from fully returning to “run” by a badly adjusted or stuck stop cable.
There’s a possibility that the shim pack was altered or changed, this is significant because the height of the injector pump above the mounting face determines the precise point of injection… the timing. Too many shims is late or retarded timing, too few shims advances the timing and might damage the pump internally if the plunger head clearance goes to (or below) zero. Were all the shims replaced exactly as they originally were?
DO NOT mess with the adjusting screw under the acorn nut on the front of the timing cover… its hard to recover the original factory setting once its lost

[Wotname]

If the injector pump was rebuilt, it is possible the original shims are no longer correct. Did the guys refitting the injector pump check and adjust the timing when refitting?

[skunther]

They did not adjust the timing. They used the same shims. The engine does run well once started. Is it possible adjusting the timing would address the hard starting issue and not compromise how it is running once started?

[skunther]

Originally Posted by Wotname

Was it easy to start before this work was done?
What was the reason for having the pump and 2 out 3 injectors rebuilt?
Was any other engine work carried out at the same time?

If it was OK before the work, then the work (or the refitting) is very likely to be the problem.

Have you tried starting the engine at full throttle? The GM series is supposed to (internally) leave the fuel rack fully open regardless of throttle setting prior to start but I wonder (speculating really) if the linkage between the throttle / fuel rack / governor was correctly re-installed; it may be trying to start with the fuel rack in the minimum position if the throttle is at idle if the linkage was incorrectly refitted.

Before the work performed, the engine would start within a max of 5-7 seconds of cranking, without fail.
I always start at 3/4 throttle.
The injector pump had developed a small leak, prompting the service. Injectors were tested for the sake of being thorough while the pump was removed, and 2 of them failed.
While the fuel system was dismantled, I replaced the secondary filter (I had already recently replaced the primary), had the mixing elbow replaced as it was pretty clogged up, and the heat exchanger cleaned.

[skunther]

If the timing is incorrect, could it cause the engine to start hard but still run well once started?

[skipperpete]

Originally Posted by skunther

If the timing is incorrect, could it cause the engine to start hard but still run well once started?

Yes, if the timing was too early (too few shims), but only maybe if late. Its hard to notice the difference once the engine is started although late timing often makes the engine a bit quieter. I’d be looking more at the sticky rack possibility. The test that yanmar suggests before the pump is re installed is to hold the pump with the rack rod vertical and allow gravity to help the rack to slide its full travel…. Then rotate the pump 180° so it slides back again under its own weight. If it fails this test its probable that the governor spring will fail to pull it into the start position at shutdown …..ready for the next start.

[skunther]

Originally Posted by skipperpete

Yes, if the timing was too early (too few shims), but only maybe if late. Its hard to notice the difference once the engine is started although late timing often makes the engine a bit quieter. I’d be looking more at the sticky rack possibility. The test that yanmar suggests before the pump is re installed is to hold the pump with the rack rod vertical and allow gravity to help the rack to slide its full travel…. Then rotate the pump 180° so it slides back again under its own weight. If it fails this test its probable that the governor spring will fail to pull it into the start position at shutdown …..ready for the next start.

If there is a “sticky rack” problem, wouldn’t that be addressed by simply starting the engine at full throttle every time?

[skipperpete]

Originally Posted by skunther

If there is a “sticky rack” problem, wouldn’t that be addressed by simply starting the engine at full throttle every time?

Nope, the stop control lever is the only way to move the rack with the engine stopped, the speed control lever is arranged to resist the governor weights, its a very delicate balance and on 3GM’s where stable rpm is really important ( generator version) the governor cross shaft has needle rollers not bushes to further reduce friction.
Check the valve clearances again and ask the pump tech if he changed the shims and checked the timing.
How fast is the cranking speed?
There’s still a possibility that you have air getting in on the suction side, most 3GM’s don’t have a return line, only a leakoff line from the injectors and if you were getting fuel drainback through the lift pump valves, that might explain the hard starting.

[skunther]

I didn’t quite understand the drain back scenario you mentioned. You mean fuel draining back toward the lift pump? How would that occur?

At one point, the shop said there was a faulty injector causing fuel to leak into the cylinder while at rest, causing unburnt fuel out the exhaust and affecting starting. They addressed the injector and said that issue was resolved. I don’t want to muddy the water, but am not sure if it might be related.

[skunther]

The service manager mentioned that the lift pump may have been previously modified by another shop to order to adapt it to the low compression in the cylinders and make the them perform better. He said the rebuild was to spec, and any modifications to the pump would have been lost, possibly explaining why it now starts hard.

Has anyone heard of such a scenario? Does it sound reasonable?

[skipperpete]

Originally Posted by skunther

The service manager mentioned that the lift pump may have been previously modified by another shop to order to adapt it to the low compression in the cylinders and make the them perform better. He said the rebuild was to spec, and any modifications to the pump would have been lost, possibly explaining why it now starts hard.

Has anyone heard of such a scenario? Does it sound reasonable?

NO, it does not.

[Wotname]

Originally Posted by skunther

The service manager mentioned that the lift pump may have been previously modified by another shop to order to adapt it to the low compression in the cylinders and make the them perform better. He said the rebuild was to spec, and any modifications to the pump would have been lost, possibly explaining why it now starts hard.

Has anyone heard of such a scenario? Does it sound reasonable?

Did the service manager say 'lift pump' or did he say 'injector (injection) pump'?

Either way, skipperpete is totally correct.

Originally Posted by skipperpete

NO, it does not.

However if the service manager did say 'lift pump', then he is an idiot as well as being misleading. There is nothing in the lift pump that can be modified.

[Wotname]

Originally Posted by skunther

Before the work performed, the engine would start within a max of 5-7 seconds of cranking, without fail.
I always start at 3/4 throttle. .......

Just getting back to this piece of 'evidence'. 5-7 seconds is a long time to crank a 3GM30F in order to get a start, even if ambient temperatures are below 50F (10C). If the ambient is above 60F (15C), I would expect a start inside 3 secs and with a low hour engine, inside 1 sec.

I mention this as it indicates to me there are other issues at play here that have not been rectified thus any additional problem induced when servicing and/or refitting the injection pump (timing etc) would exacerbate starting time in an already compromised engine. I.E. say a timing issue which might only make a small difference on an otherwise good engine could make a significant difference if there are other underlying faults.