Diesel Engine Diagnose

[yttrill]

Hello, I have a Kubota Z600 2 Cylinder Diesel powered genset with 6KVa 240V output which is making extra noise. Freshwater cooled. 12 month old head gasket. 12 month old exhaust installation. The engine is overcooled (runs under 35C, gauge bottomed out, keel cooling, water temp currently 15C or so, winter in Sydney).

Whilst running, suddenly there was some extra rattling, noise, and vibration so I turned off. Restarting later, unit still starts and runs but seems extra noisy (its has always been extremely loud).

Some gas appears to be coming out of the corner of the engine between the head and block detected by back of hand, but it's very hard to tell. There are fans in the generator and alternator. No unusual exhaust smell though.

When turning over to start there is a distinct popping sound suggesting some kind of compression leak. With decompression lever open, no popping sound.

Diagnostics done whilst operating: used a torch and lighted cigarette, could not find any issue with head gasket leak. Something looking like a gap in the gasket is visible in the suspect location, but the gasket is black and there is some oil around (and my vision is not good enough to see all that well at close range).

I can think of these possibilities: decompression system, minor sealing issue, one cylinder only. Head gasket failure. Exhaust failure. Exhaust valve seat failure.

Any help with other possibilities or diagnostic procedures appreciated! Thanks!

[model 10]

I would go with the head gasket idea.

[yttrill]

Originally Posted by Guy

I would go with the head gasket idea.

Well that's what I thought myself, but i cannot find a leak with smoke. And I certainly cannot afford to remove the head without being sure because that will destroy the gasket anyhow, and I am too poor to buy another one unnecessarily. Nor am I a mechanic, but I would have to do this myself (and buy a torque wrench, not that i have any specs on the required torque).

The popping sound is quite significant. It is a resonant, which is not how a hissing out of a small leak in the gasket would sound I shouldn't think. The overcooling does suggest a gasket issue but it could be any seal. Or a ring perhaps?

[MarkSF]

Why would you allow the engine to run long term under 35C? This is potentially very destructive.

[yttrill]

Originally Posted by MarkSF

Why would you allow the engine to run long term under 35C? This is potentially very destructive.

There is no regulator. I don't normally need the generator so installing a manually operated gate valve on the input is the only option i can think of and not a high priority. I bought a valve to install but it is the wrong type (with a handle instead of a gate, so it would be hard to adjust and too easy to knock). It's also not so easy to source an appropriate gate valve, the local chandler doesn't have one the correct size. And I don't know where to install it (before the pump, after the pump, or on the output of the engine, and of course there are space restrictions). The pump is an electric Johnson centrifugal engine cooling pump with sufficient capacity to handle my main engine, and it's circulating water through the keel cooling system. Yes, I should fix this problem but it's not clear throttling the water flow would work: it would start too cold and may lead to overheating. A regulator is the solution but one cannot be fitted to the engine due to the way it has been marinised. Originally a shaft driven impellor pump was installed but the engine overheated and we couldn't get it to work so it has been bypassed.

[model 10]

First off you should buy a torque wrench. You might get lucky on a re-torque, but I doubt it.

[transmitterdan]

Originally Posted by yttrill

There is no regulator. I don't normally need the generator so installing a manually operated gate valve on the input is the only option i can think of and not a high priority.

I would think it should have been originally equipped with a bimetallic thermostat. Are you sure there is no place in the cooling circuit of the engine to install an ordinary thermostat? The cooling water around the head should be well over 60C else bad things are going to keep happening I think. This is not something you want to manually regulate in my experience.

Is the exhaust section water cooled or is it dry exhaust?

[yttrill]

Originally Posted by transmitterdan

I would think it should have been originally equipped with a bimetallic thermostat. Are you sure there is no place in the cooling circuit of the engine to install an ordinary thermostat? The cooling water around the head should be well over 60C else bad things are going to keep happening I think. This is not something you want to manually regulate in my experience.

Is the exhaust section water cooled or is it dry exhaust?

The thermostat cannot be fitted. There is a place for it, but a waterjacket is fitted around the exhaust instead. Water must circulate through both the jacket and through the engine in a single circuit, so the thermostat is removed to make a hole.

The system is keel cooled: dry exhaust.

The original cooling system using the impeller seemed to work, the engine would warm up to about 80C. However the cooling started to fail and we could not determine why. The impeller was replaced to no avail. The impeller was on the HOT side of the cooling system. The wrong plastic (cheaper but not designed for hot operation) was used and that size impeller is no longer available with the more capable plastic. In any case impellers suck totally. I hate them. They never last and can cause catastrophic failures, and broken pieces can be hard to remove.

I do not have a good solution, other than making a special box containing a regulator. Restricting the flow rate with a gate valve may help. There will be some experimentation required to ensure a good equilibrium temperature is achieved and maintained.

The mechanic seemed to think that since it's an older motor it would be OK to run it too cold as the tight fitting parts are already worn. In any case the temperatures inside the cylinders are in the thousands of degrees and so the heat flow issues will be completely unaffected by the trivial difference between 20C and 80C. The only issues will be related to lubrication and tolerances of moving parts. Remember all engines start running at ambient anyhow.

However this is not relevant to my original question: I need some help diagnosing the current fault. I live aboard and usually rely on solar but it's winter in Sydney and on overcast days there isn't enough power to run my computer, let alone refrigeration. My batteries are screwed too, so i need to recharge them quite often. I need new batteries but I don't have a spare $7K for that at the moment. I have a petrol generator as well, however it's a portable and I can't run it when its raining.

[hellosailor]

One issue at a time. First diagnose your head gasket. There are various ways to confirm if you have a leak between the oil and the cylinders, or the cylinders and the cooling flow, etc. Sometimes pressure tests, sometimes contamination tests...get some tools and run the usual tests, or find a mechanic to come out. Worry about the thermostat and temperature only if you find out that it is a head gasket, as opposed to something worse (like a crack between the cylinders) that may total it anyway.

[shakey doug]

Check Valve gear clearances, bent pushrod, excess blow by. Lubricate rocker & valve stems with w Wd40 or 50/50 Kero & oil then push or hammer down each valve in turn, checking for valve free movement.
A possible remedy for less than the price of a Smoke.

[guyfromiowa]

The 'popping' noise can be caused by either an intake valve that does not close correctly or by an exhaust valve that does not open. The compression pressure is vented back through the intake manifold and air intake. If this is true, removing the valve cover and inspecting the pushrods, rockers and valves will show which is the culprit.

[yttrill]

OK, so the rocker cover is off .. a rod just fell out when i removed it. It appears to be part of the decompression system. Turning over the motor now it is clear the rocker movement is synchronised with the popping sound. However the valves themselves are not visible. The rocker levers operate 4 heavy spring loaded rods on one side and some fine rods (maybe 2mm) on the other (one appears closer to the hole it goes down than the other three).

I can't figure out how the decompression system works. The control level is attached to an rod inside which goes into a hole half way across the rocker cover. There is a flat piece with a nut and bolt through it. The other side has a slot into which the loose rod presumably went. It also has a nut and bolt through a flat section. [Photos later if necessary].

I am guessing the decompression system works by preventing the exhaust valves closing. Is that correct?

I can't figure out it does that. i know it works (or used to) because that's how we used to stop the motor (we're using the throttle now).

It doesn't look like I can inspect the valves without taking the head off.

[yttrill]

Aha! So, the decompression lever is connected to a rod that passes through the rocker cover on one side. There is an oring to act as a seal. Half way across there is a block with a bolt that screws down onto the rod with a split washer. This must be the friction adjustment. There are two sleeves on the rod on either side of the friction adjustment block. The fit into the friction adjustment block, and the entry portal of the control rod.

Mounted through these sleeves are screws, one each, which poke out one side a bit. There is a lock nut on the other side. When the lever is in the open position, the screw heads and lock nuts are directly under two cover plates. Removing these plates would allow adjusting how far the ends of the screws poke out.

The control rod has snapped off at the friction block. This is probably because for over 10 years, the motor was stopped by operating the decompression lever (and turning the fuel tap off). And also because on the far side, the rod and sleeve combo is not supported. It also explains why the decompression level stopped stopping the motor, and we switched to a throttle control.

There is a reason why the throttle wasn't used: this is a 240 AC 50Hz generator that should run at 3000 RPM. There is no regulator, so the throttle was just set to the right place.

If I cut off the control rod and jam the decompression lever in place to hold the oring in the entry hole, I will not be able to decompress, but the motor should run. Having a lose piece floating around inside the motor would make a rattle and surely can't be good if it interfered with the rocker operation (which of course it is *designed* to do! Although I still can't figure out how it stops the valves closing).

However it's not clear I have solved the problem. There's still a popping noise when turning the motor over. Part of the problem is that it actually did run, just didn't sound quite right. If there's a bent rod in there i don't want to break it (because that would probably lead to something else breaking). I may have to live with worn valves and/or valve seats.

[River Cruiser]

Check the valve lash clearance.


Sent from my iPad using Cruisers Sailing Forum

[Steve Bean]

FWIW, the manual for my Universal Model 40 (a marinized Kubota) says not to use the decompression lever to stop the engine. It uses a fuel shutoff.