Ok, today I have a good one for you I have been charged with diagnosing and rectifying a no-start issue on an onan diesel marine generator ill be going through all the steps I went through from start to finish.
Beginning with all the steps taken in diagnosing the issue then ill go through what is required to rectify the issue.
I’ll also go through any issues in actually rectifying the issue and lastly ill talk about preventative measures to prevent such an issue from happening in the first place.
Alright, let’s get into it! The issue at hand is the Genset cranks but does not start.
The Genset when cranking starts cranking strongly than the cranking speed drops off immediately.
If you are familiar with some basic mechanics you would presume a flat or faulty battery and this is exactly the first thing I thought of when going through my diagnostic process.
Diagnosing
Firstly always start with the basics battery, fuel, oil, leaks, etc… and always start with the least invasive processes first like pulling hoses off as opposed to dismantling major engine components.
My thought process for diagnosing this issue is as follows –
Because the engine cranks but does not start tells me there is voltage energizing the starting system but from here where I start exploring all the different engine systems that are required for the engine to start.
When you think about what an engine needs to run what do you think of?
You would be correct in thinking it needs electricity/voltage, fuel and air to run, and to expand on this it actually needs 12v dc or better to energize the starter motor solenoid and starter itself.
Also, it needs sufficient fuel in the tank and good fuel pressure in the delivery lines, as well as no air leaks in the fuel system.
Air intake and exhaust are also important and shouldn’t be overlooked, any restrictions in intake or exhaust can hamper combustion.
Just FYI the fuel pick-up for your genset is typically higher in the fuel tank than the engines pickups so if your running low on fuel you can still get home with your start engines but your genset will run out of fuel.
Diagnosing checklist
Data collection
- Take note of engine hours
It is important to take note of engine hours because every engine has an estimated service life and if you reference the engine’s service manual you should be able to find a service schedule.
This schedule tells you what servicing needs to be done and at what hours interval.
The genset was in poor condition for the hours in my opinion, it had lots of corrosion around the engine.
By taking note of the hours you can determine what further works may be required as per the engine’s specific service schedule, eg… valve adjustments, and coolant changes.
- Take note of engine make, model, and serial number
This is important you will need this info to get parts oil etc…
Taking note of these will allow you to find service manuals, assist you with calling for parts and assist you if you call for technical support.
Lubrication
- Check oil level and condition
By checking the oil level and condition you can check for a few things like burnt oil, water in oil, low oil, etc…
Power
- Check battery voltage at battery
You are looking for 12v or better
This will tell you what sort of voltage is available for the genset.
- Check battery voltage at starter motor
You are looking for 12v or better
This will tell you if you have a bad connection between the battery and the genset.
- Load test battery
Load testing your battery will be a more definitive test than testing voltages.
Just having 12v showing on your multimeter or engine gauge is not enuff. By load testing a battery, you rule out any operational issue with your battery like an internal battery fault.
- Check all earth lines and connections
Do not underestimate the importance of a good earth connection a bad earth can introduce all sorts of problems. From the engine not starting to multiple alarms to intermittent issues like the engine starting then suddenly stopping, and random alarms. Generally, if I have an engine issue that just doesn’t make sense I will start by checking my ground connections.
- Check all positive lines and connections
A dodgy positive connection will generally cause an engine to not start or be intermittent with starting.
Fuel
- Check fuel level on gauge
Low fuel can prevent the engine from running.
- Check fuel low-pressure pump operation
A faulty pump will prevent an engine from running.
You can check this by putting your hand on the pump with the prime button pressed, this should result in the pump vibrating as it pumps the fuel, if the pump gets hot or very noisy there is a problem with the pump not pumping fuel.
- Check genset is getting fuel to low-pressure pump
I always pop a hose off after the pump and prime the genset to confirm fuel is getting past the pump.
- Check genset is getting fuel to injection pump
I will pop a hose off where it enters the injection pump to see if it is actually getting fuel coming out there.
- Check genset is getting fuel to injectors
After checking all the other areas for fuel first ill crack the injector lines and make sure sufficient fuel is getting up to the injectors. To do this you will have to loosen off all injector lines and crank the engine, fuel should come out of the injector line nuts. Tighten each injector line nut once the fuel is confirmed at each injector.
- Check fuel governor operation
This engine has an electric fuel governor solenoid so I also needed to check this is operating correctly.
Firstly I checked the service manual to find out how exactly the governor worked then proceed to test it by applying a voltage to it with it removed to see its operation.
- Check fuel pressure
By putting a fuel pressure gauge on the engine you can see if it is getting sufficient fuel pressure.
Compression
- Carry out compression test
A compression test can tell you if the cylinders are making adequate pressure. If one cylinder is lower than the rest by more than 10% then you have an issue with that cylinder. This can be due to all sorts of things like holes in pistons, failed rings, sticking or broken valves, etc…
Intake and exhaust
- Visually inspect intake for broken components or restrictions
A restricted intake will prevent the engine from getting sufficient air to fill the cylinders therefore not being able to make efficient combustion.
- Visually inspect exhaust system externally for signs of corrosion and possible restrictions
Looking around all the metal exhaust components if you see bad corrosion externally chances are internally is going to be much worse.
- Check exhaust system internally for restrictions
By pulling the exhaust apart you will be able to see clearly if there are any restrictions or blockages in the system.
- Inspect exhaust muffler box for any restrictions
By popping a hose off you should be able to see into the muffler box to see if there is anything going on in there like melted fiberglass etc…
Diagnosis findings
- Genset battery has been replaced and charge checked, 14v charging voltage and also load tested and it was within spec.
- Checked the voltage at the starter motor and it was 14v.
- Checked voltage at starter while cranking and voltage drops to 10v (within acceptable range)
- Checked fuel delivery at the injection pump and it has plenty of fuel coming into the pump.
- Checked fuel delivery at the injectors while cranking and found very little fuel coming out (this could be due to slow crank speed)
- Removed fuel governor solenoid and tested its operation and found it working correctly.
- Checked voltage at fuel solenoid wires while cranking and have 10v while cranking engine.
- Installed fuel solenoid and gave it battery voltage straight from the battery while cranking the engine and found no change.
- Engine intake valves and manifold are caked in oily sooty muck.
- Checked engine block earth straps for continuity and found them to be in spec.
- Electrical circuits look very rusty.
- Engine compressions within 10% of each other.
- Engine turns over fast with glow plugs out.
- The exhaust mixer is very rusty.
- Bolts seized in the exhaust mixer had to remove the exhaust manifold.
- Exhaust manifold to the exhaust mixer is totally corroded shut.
The Issue
So long story short, the problem with this genset is the exhaust manifold and exhaust mixer are both so heavily corroded the exhaust port has completely closed up so no exhaust would have been able to exit the engine.
This explains why the intake is oily, the exhaust would have been forced past the rings in the combustion chamber because it would have nowhere else to go.
This also explains why the starter was struggling to turn the engine over And why the engine turns over easily with glow plugs out.
How to rectify the issue
Replace the damaged components and replace all gaskets necessary.
This means replacing the exhaust mixer and exhaust manifold. As well as replacing the engine coolant.
Actually rectifying the issue
Replaced damaged components and the engine started straight up
The blocked exhaust manifold and exhaust mixer were replaced along with any other fittings that were needed to make these work like bungs, water elbows, gaskets, etc…
Engine coolant was also needed after installation.
Difficulties along the way
Access around the genset was very tight inside the bilge.
As with most vessels, these days every inch of bilge space is usually utilized. This is great for you the boat owner because this means you are getting as much useful equipment as possible onboard your vessel but as a mechanic, this sucks because it makes moving around the bilge very difficult.
Preventative measures
Regular servicing and following manufacturer-recommended service schedule as well as doing regular checks of the oil level, coolant level in the header tank as well as the overflow bottle, visual inspection of the heatexchanger end caps for external corrosion, inspecting genset tray for any oil or water leaks as well as keeping an eye on voltages both 12v DC and 240 AC.
Will give you a very good overview of engine condition.
Another thing to look out for is if the genset becomes extra smoky it may need its valves adjusted.
If you follow the engine’s service schedule as well as regular servicing and the aforementioned checks you should increase engine reliability and be able to prevent any issues from becoming major issues.
Conclusion
As with most diagnostic jobs, it is not always immediately clear what the issue is so it is so important to start with the basics no matter how silly or mundane it may seem because by skipping the basics you are potentially overlooking vital diagnostic information that could point you straight to your problem saving you hours in searching for the issue.
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