Starting a decompressed diesel

[DaveS]

On another thread I told the tale of (eventually) fixing a starter motor problem. Part of the difficulty was that I simply ran out of time before I could finish tracing the fault, so had to abandon everything for a fortnight while having the starter motor (unnecessarily) overhauled in the interim. On returning yesterday the actual wiring fault was located in a few minutes, and the starter re-installed with the by-now customary fiddling and bad language.

The starter then worked perfectly - great! But the engine wouldn't start. "Must be cold" says I. Cold start procedure and full throttle. At the 3rd or 4th attempt it fired, but with little enthusiasm. "It'll be better once it's heated up" says I. Anyway, a few minutes later, it ran quite well in neutral at up to 2000 revs and was happy to idle, but putting it in forward gear slowed it right down, and putting it in reverse (Kiwi's v. course pitch) would stop it completely. The exhaust seemed a bit blue and there was a light sheen on the water, both indicating some oil in the exhaust.

With the engine stopped I felt round it to see what sort of temperature it had reached - and then noticed the decompression lever up! A fortnight ago I had been inching the flywheel round while checking / cleaning up the starter ring, which was easier to do with the engine decompressed - and I had forgotten about it. With the lever returned to the normal position the engine immediately started and ran correctly with, as far as I could tell at the pontoon, full power. No blue smoke or oil sheen, and a deeper, less "tinny" sound.

So the two questions are: (1) Is this normal behaviour? (The engine, not its owner! /forums/images/graemlins/frown.gif) I had always understood that a diesel wouldn't start decompressed, but mine (VP2002) clearly does - after a fashion. (2) Am I likely to have done any damage? The total running time decompressed was perhaps 15 or 20 minutes.

 

[ShipsWoofy]

was one up and one down, an engine will run on a single cylinder if the others are in decompress mode.

 

[Searush]

Was it a two cylinder? I found it useful to start my old engine with one cylinder decompressed when the battery was low. All the decompressor does is reduce the compression by allowing some air/fuel to escape (it lifts the exhaust valve). In general this will prevent that cylinder from firing (compression ignition not compressed enough) unburnt fuel will just go out of the exhaust valve. I don't see that causing any damage.
If it was a single cyclinder it is possible that the exhaust valve was only prevented from closing properly rather than fully opened which may, just, allow the engine to fire. Don't worry about it, you have learned to recognised the problem now so you will recognise it if it happens again!

 

[DaveS]

There is AFAIK only one lever (unless another is hiding somewhere round the back) which I assume operates both cylinders.

I have a .pdf of the workshop manual somewhere. I'll have a look.

 

[Appleyard]

If there is only 1 lever then there will only be 1 decompressed cyl. Should not have done any damage.If it has been running fine since you fixed it,then it will def. be ok.

 

[DaveS]

Checking the photos in the workshop manual - it shows the 3 cylinder 2003 - I was right, there is only one lever operating on all cylinders.

 

[Searush]

I recommend that you check your decompressor set up and valve clearances. If the decomp is working properly the engine is unlikely to fire. As it fired, at least one exhaust valve is almost closing, therefore you decompressor lever is likely to be out of adjustment.
Or you could just ignore it as, unless you need the decompressor to stop the engine rather than using a fuel cut-off, it won't matter a d*mn!

 

[DaveS]

Searush (and others)

It is a two cylinder engine but the decompression lever appears to act on both cylinders. Certainly slowly turning the engine over by hand with it up is completely smooth, without any of the compression resistances experienced with it down. If the decompressor did only work on one cylinder I would expect the compression resistances to half in frequency rather than disappear.

So it does look like it was "fully" decompressed.

My gut feeling regarding damage is that I've probably got away with it /forums/images/graemlins/blush.gif, but it would be nice if anyone could provide informed reassurance!

I would certainly recognise the symptoms in future. I don't understand the oily exhaust bit, though. /forums/images/graemlins/confused.gif

 

[DaveS]

That's a good point about checking the valve clearances. I'll add it to the winter "to do" list.

 

[awol]

[ QUOTE ]
I don't understand the oily exhaust bit, though. /forums/images/graemlins/confused.gif

[/ QUOTE ]

The engine ran - therefore there was compression on at least one cylinder sufficient for combustion. The oily exhaust suggests that at least one cylinder was not firing - decompression lifts exhaust valve, fuel injected and exhausted unburnt.
Conclusion is that the decompression lever was positioned such that one cylinder fired, and one did not.

Obviously, having rebuilt a starter motor that didn't need it, you should now strip down the whole engine for peace of mind.

 

[Searush]

So it does look like it was "fully" decompressed.

No, not so. Turning an engine by hand is very slow & produces very little compression due to air escaping around rings, valves etc. Any slight lift (or pitting) of valves means no resistance will be felt. Spin the beast with a starter motor and it is a whole new ball game. Engine turns quite fast & minor leaks round valves & rings are swamped by the speed of pistons. An engine with poor compression will still start & run but will be hard to start, low on power and inefficient ;- co-incidentaly the exact symptoms you initially posted /forums/images/graemlins/cool.gif

 

[boguing]

I vaguely recall that the Volvo 2002 single lever decompresses one cylinder as you move it through the arc, then the second at the limit of travel. This was 20 years ago though.

 

[DaveS]

I agree entirely with your last point.

On the issue of turning over manually, let me try to clarify. When turning the crankshaft over manually with the decompresser lever in the normal position a distinct resistance is felt for part of every revolution. With the lever up this resistance is not felt. My conclusion from this is that both cylinders are being decompressed. If one was still compressing normally I would expect to feel resistance every other revolution - but I don't.

 

[DaveS]

Ah, that's interesting. I didn't know that. I seem to be using this phrase a lot recently. /forums/images/graemlins/blush.gif Always delighted to learn, though. /forums/images/graemlins/smile.gif This is IMHO the real value of these forums (fora?): the many who are happy to freely contribute knowledge easily compensate for the few who sneer at others' mistakes. I firmly believe in openly declaring both successes and cock-ups where the story might be of interest or use to others.

Back to decompresser operation! This could be easily confirmed by feeling the compression pattern with the lever in different positions a la my previous reply - I'll try that when next aboard.

Of course it's not helped by the fact that the operating manual barely mentions the decompressor and the workshop manual just tells you how to remove it!

 

[PeterGibbs]

Rest easy. Diesel engines are made tough to operate at high compressions - nothing you have described sounds like mortal damage.

The decompression lever may not be set to completely release the valves - or may have been knocked off by you to partially decompress - hence your experience.

Starting from cold with decompression applied is a good technique to take the pressure off the starter motor: this bit of kit definitely will not suffer extended use without expensive consequences!

PWG

 

[boguing]

Actually, on further reflection. I'm sure I'm right. It wasn't 20 years ago, it was about five, and my technique for cold starting with flattish batteries was to do the cold start procedure with throttle and T handle, decompress both and get it spinning, then knock the lever back to half to start on one (and half the number of compressions for the starter/flywheel to cope with), then when it fired, knock the decompression off completely.

I only sold that boat five years ago, but had forgotten it was a 2002.... Previous 2002 experience was actually nearer thirty.

The engine I'm talking about was an early '80s model. Dunno if they're still the same?

 

[DaveS]

Thanks for that.

I think awol is right: the only sensible explanation for the oil is running on one cylinder. I was having difficulty seeing how that could be, given the evidence of complete decompression, but I think boguing has supplied the answer. If the lever was fully over and decompressing both cylinders when I was turning by hand, but in the half-way position and decompressing one while cranking all might be explained. In describing the position of the lever, I used "up" as a shorthand for "not in its normal position". Was it in fact actually up or fully over when cranking? I simply can't remember: until a few minutes ago I believed that either produced the same effect. So why move it between these positions? Absolutely no idea and I can't remember doing so. Boat gremlins or onset of dementia?

On the relative fragility of the starter, it did occur to me that although this time the problem was easily fixed, a genuinely failed starter in a remote spot could be a serious embarrassment, so I now have a starter included in the slowly-growing collection of strategic spares.

 

[cliff]

[ QUOTE ]
When turning the crankshaft over manually with the decompresser lever in the normal position a distinct resistance is felt for part of every revolution.

[/ QUOTE ]Wow, you can actually turn your engine over with the decomp lever in the normal position? I would be looking at removing the head and checking the bores and re-seating the valves. If that does not cure it then engine out and new rings, re-sleeve etc. Again it depends on the speed you are turning the crank - maybe if you were inching it round little by little you could get her to turn through a full revolution.

Sounds to me like you are ready for a top end job at least.

If you can get the injectors out you could do a compression test on each cylinder to confirm before further dismantling and further loss of beer tokens
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"Artificial intelligence is no match for natural stupidity"
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[Superflid]

2 days ago I was giving a mate a hand to start a Petter single, which had stood outside for at least 3 years without being touched.
Turning it with the handle we could feel very little compression and assumed the rings had stuck in the grooves, I'd taken the valve cover off to see if the valves were moving. As the Health and Safety inspector would get his arse kicked if he set foot on Dave's place we were free to devise a plan......

We got another identical engine (these are both stationary jobbies, not boat) and set it in line with the non-runner, ran a long "V" belt between the 2 then started the "good" engine, once that was ticking over nicely we pulled the 2 motors apart until the belt tightened and both engines were turning - then the decompressor lever (see, there is a point to this.....) was turned and the second engine fired up!
This was left running for a couple of hours to give the rings time to loosen up, however, once the engine was stopped there was still almost no compression!
After some kicking and poking we realised that the decompressor was not letting the valve close completely, a bit of adjustment gave us the compression we wanted!

The relevance of this to DaveS's problem?

With no compression (not enough to feel while hand-turning it) the motor still started and ran.
There would be no damage to your engine as others have said.
Cliff might be right and it could need an overhaul, but check the decompressor adjustment first!

 

[cliff]

[ QUOTE ]
Cliff might be right and it could need an overhaul, but check the decompressor adjustment first!

[/ QUOTE ]Very good point about the adjustment of the decomp lever - start with the simple (cheap) fixes and slowly work up to the more expensive fixes /forums/images/graemlins/grin.gif
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"Artificial intelligence is no match for natural stupidity"
sailroom <span style="color:red">The place to auction your previously loved boatie bits</span>