Cummins Onan 3.6YFR-7076-AO42U506 Generator Autopsy

Cummins Onan Generator

Cummins Onan Generator crank shaft

Cummins Onan 3.6YFR-7076-AO42U506 Generator Autopsy

No matter how good and reliable a product is, when an unexplained fault occurs the product is then labelled as “crap”.

Here at LAS motorhomes we have come across a good example of this, it was with a Cummins Onan 3.6YFR-7076 (microlite) generator. The customer contacted us and asked if we could have a look at the generator as he could not start it. When the generator arrived the first thing we did was to check the oil level before attempting to start it, it showed that it was below the minimum level. Next thing was to turn the engine over by hand, this revealed no resistance which meant the piston was not going up or down. As these type of engines rely on a splash oil feed to lubricate the crankshaft, it was quite obvious that the crankshaft had broken. We now knew that the crankshaft had broken, but did not know why the owner thought that Onan products were “CRAP”.

As it transpired, soon after the vehicle had been purchased the generator had failed to produce electricity. The answer to the problem was to apparently change the generator with another second hand unit. As this was now the second failure I am beginning to understand why the customer had come to this conclusion.

Quite often you get this reaction with various good products, this is why I have decided to do a silent witness on the engine.

1. We know that the engine was short of oil – but why?

Cummins Onan Generator crank shaft

A good place for the engine oil to leak would be the crank shaft seal, but as you can see in the image it is dry. On the right of the image you can see an oil stain, this has come from a spillage when oil has been put into the generator.

Conclusion so far

An external oil leak was not the problem.

Cummins Onan Generator cylinder head

2. Next step would be to remove the cylinder head, but first we need to remove the valve cover. On doing this we noticed a brown deposit around the exhaust valve spring. Could this be a clue?

N.B. The oil on the right hand side of the cylinder head came out when the valve cover was removed because I turned the engine on its head!

Cummins Onan Generator grey exhaust valve and darker inlet valve

3. With the cylinder head removed you can see the characteristic grey exhaust valve and darker inlet valve. The brown deposits to the right of the inlet valve is carbon, this is a larger deposit than expected but we still need to delve further.

carbon in the combustion chamber

carbon in the combustion chamber

4. Here you can see an abnormally high amount of carbon in the combustion chamber. So far we know the oil level was low when we checked, was this the cause of the broken crank shaft? At the moment we have not got conclusive proof. What we have got though is a very large amount of carbon in the combustion chamber. We need to go further.

5. We now have a choice, remove the sump or remove the end plate! As the end plate is further from the oil level this might tell us more.

Cummins Onan Generator gears

6. As I suspected, the bottom gear that drives the balance shaft (used to reduce vibration), is wet with oil. The next gear up is a light grey colour, this is the crank shaft gear, above that and in a lighter grey is the cam shaft gear. This colouration proves that the engine had been running low on engine oil. My reason for coming to this conclusion is that as I mentioned before, the lubrication system is splash fed. The oil level was that low the splash effect did not reach the crank shaft and cam shaft gear, this starved them of lubrication and cooling (oil cools the engine as well as lubricates), giving the gears the grey colouration.

Cummins Onan Generator con rod

the con rod has snapped

the con rod has snapped

7. Now you can see the con rod has snapped because the big end bearing has seized, when you look at the crank shaft journal you can now see that the bearing surface has picked up.

Conclusion so far

The engine was low on oil which reduced the splash effect on the lubrication system. This in turn, denied the crank shaft big end bearing of oil. The big end bearing now seized to the crank shaft was having an effect on the piston and con rod, it was trying to stop the crank shaft from rotating. This failed, hence a broken con rod.

Now at this point, do we now blame the owner for not checking the oil level, or do we blame Onan Generators in general for using excess engine oil? Anyway, how much engine oil would you expect a 300 cc engine to use, and how long did it take to reduce the oil level causing this much damage? It’s no good looking at the hour gauge because as I said the generator had been changed. We need to look further!

Cummins Onan Generator

Cummins Onan Generator

carbon in the combustion chamber

8. At this point the intake valve has been removed, as you can see a massive amount of carbon deposit on the valve stem. The valve guides are in good health, and so is the valve stem seal, if it had failed it would cause the engine to burn oil. With the large amount of carbon found in the combustion chamber, coupled with the findings of large deposits of carbon on the intake valve, the engine must have been burning the engine oil alongside its petrol.

Cummins Onan Generator piston ring

9. We need to look at the piston condition and the piston ring condition. In the image you can see this being carried out with the compression ring inserted into the cylinder and the gap being measured, all is as it should be.

Measuring the cylinder

10. Measuring the cylinder bore we can see that it is in fairly good condition with the cross hatch marks clearly visible, this concludes that the oil consumption problem was not due to the cylinder bore or piston rings excess wear.

Conclusion

Cummins Onan Generator

Cummins Onan Generator

carbon in the combustion chamber

The good condition of the piston, rings and cylinder bore concludes that they were not to blame. What we do have is a large cylinder carbon content, carbon on the intake valve and a brown deposit around the valve spring, and with no other way the engine could burn the oil as we have examined all the parts which have found to be serviceable.

Cummins Onan stipulate using Valvoline Premium Blue 15w – 40 and this is for a good reason. Synthetic oils tend to be thinner and the engine will burn it, it also tends to leave a tarnish on the cooler part of the engine.

In the case of our Onan engine the inlet side of the engine would be the coolest as it is cooled by the petrol mix. The evidence can only point to one thing, the oil used in the sump was of the incorrect viscosity. The grade of oil used was too thin for the piston oil scraper ring to be effective, it also effected the engines breather allowing the oil to fume more and hence the carbon build up on the intake valve.

So now we have it, Onan products are not crap, and the owner was possibly not negligent. Now we have conclusive proof that the engine damage was caused by the last person who put the incorrect grade of engine oil into the engine.

This again reminds us of what we already know “A little knowledge is dangerous” and in this case expensive.

For more information on new generators and repairs call LAS Motorhomes on 01604 861999.