Starting a decompressed diesel

[2Tizwoz]

If an engine is run when the valves are not seating tightly the valves may be burned by leaking unusually hot exhaust gas.

If this should have happened in this case the problem will be obvious and would need the valves grinding and lapping as usual.

Valve decompression gear is not intended for prolonged running and engine manufacturers recommend that the decompression lever should not be routinely used to stop an engine.

My MD7B starts by hand only because the decompression lever permits getting the engine up to starting speed using the starting handle and then quickly turning to full compression when, all being well, it starts.

 

[gandy]

I can't believe the decompressor was fully set. On our MD7A if you decompress and spin the engine on the starter it sounds totally different - no "chuffing" from the engine, and just an even hiss from the starter. It almost sounds like the starter's freewheeling.

 

[KREW2]

From what you describe you were running on 1 cylinder.
Do you have an operators manual for the 2002?
My interpretation of the manual is there are 3 positions of the decrompession lever. This is there guide for hand starting.
Fully compresed is laying flat to the right
Upright is 1 cylinder decompressed
Fully over to the left lying flat is 2 cylinders decompressed.
This will enable you to get the flywheel up to start speed, then move the lever to upright, compressing just 1 cylinder and hopefully starting the engine, with the engine running on 1 cylinder move the decompression lever back to the right compressing the 2nd cylinder in theory this should then fire up.
Well thats the way I read it. Unfortunately I do not know what damage if any is caused by running with 1 cylinder decompressed for too long.

 

[Bilgediver]

Sounds like you started on one cylinder as others have said. The oil in the water is unburnt diesel oil from the injector in the non firing cylinder. There will be no mechanical damage but it is possible that running decompressed IE with the INLET valve held open that bits of carbon got blown back under the seat and trapped there..This is why you should not stop a diesel with the decompressor!!!!! This carbon can start a leak path across the valve seat which may become apparent at a later date.


John

 

[Heckler]

cliff
i despair, you redeem yourself with a bit of common sense and then go and post this loads of codswallop, what you have to realise is that some people on this forum actaully take note of what people say, if you think about it the chap is by the very way that he is posting not a mechanic. he will now have doubt in his mind about his engine. cylinder head off, valves grinding in, really!! he ran it with the decompressor half connected, at worst the valves were being held off the seats a bit, it now runs OK, what is the prob? NON what so ever.
Stu

 

[cliff]

Read my posts carefully, I did say checking the decomp setting was a good idea before moving on to the more expensive fixes (top end overhaul, bores & rings etc). Fairly common sense I would have thought.

I also suggested a compression test (if the injectors were easily removed) before going down the road of lifting the head - Is that not common sense.

Yard bills can be horrendous so better to check everything out your self first before committing to major overhaul / repair.

Reading subsequent posts it does look like there is a possibility that the decomp was not used properly in so far as it was not fully released - perhaps a case for "RTFM" /forums/images/graemlins/grin.gif

I would agree with you diagnosis that if it was mis use of the decomp lever then little or no harm has been done however if the problem is/was due to a leaking (or sticky) valve the problem will surely get worse as time goes on as the hot combustion gasses leak past the valve and erode the sealing surface.

If I was sure I had used the decomp incorrectly and the engine was now starting and running OK I would probably leave well enough alone but if I was sure I had used the decomp correctly I would be looking further than the end of my nose, not burying my head in the sand, and checking the valves. - again fairly common sense
--------------------

"Artificial intelligence is no match for natural stupidity"
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[andy_wilson]

Anyway, having read the rest...

There is only one decompressor lever, it has 3 positions.

Tucked away to the right it is inactive, vertically up decompresses both cylinders, thrown fully left decompresses 1, but I can't remember which one. Seems a bit odd to me but thats how it is. I would have expected it to go both, 1, none decompressed, as a natural progression if used to start an engine with a flat(ish) battery.

I usually start the season, or after a long lay up, with a long burst on the starter to circulate oil fully before starting. The engine tries to start, suggesting that it is possible for the engine to run while de-compressed, but power will be very poor.

There is a risk due to the running whilst partially decompressed. Unburned diesel will wash the cylinder walls and cause wear. It is a common situation where a poor injector pattern can eventually lead to worn cylinders, and also other wear problems due to the oil being diluted with diesel.

After 15 minutes I certainly wouldn't expect your oil to have a problem, but if you knew exactly where on the dipstick the level was before, check it again. If the level is noticably up, change the oil. You are probably about to anyway with the onset of winter.

Can't imagine there to be any more cylinder wear than before on a 15-20 year old engine!

 

[DaveS]

Re: Anyway, having read the rest...

Thanks to all for the replies. There seems to be a fair consensus that the decompresser lever has three operating positions respectively giving 0, 1 or 2 cylinders decompressed, but less agreement about which position does which. I'll have a play with it next time I'm on the boat, turning over the engine by hand with it in each position to feel the respective compression patterns, and report back.

To answer various other points: the oil had just been changed for the winter and its level did not appear altered by the period of peculiar running. There is no handle; I turn it over slowly with a socket on the crankshaft driven by a fairly small ratchet handle. When compression is felt it is sufficiently strong to stop me turning: after keeping reasonable force on the handle for a couple of seconds it then starts to move again. As I said earlier, the operating manual does not describe use of the decompresser.

 

[Billjratt]

The oily exhaust was due to unburned fuel. The fact that it ran at all means one (or more )cyls were not de-compressed otherwise it couldn't have fired. The fuel injected into the de-compressed cylinder(s) was not ignited and so, came out of the exhaust. Much the same effect as having a knackered injector or pump.