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mhfowle 10-04-2006 09:08 AM

Hard Warm Starting Yanmar ysm8
I have a yanmar ysm8 with the following problems: It won't start warm (it starts instantly when cold) and it emits soot out of the exhaust. Also when I bought the boat it had no thermostat and started well when cold as well as after it was run (although after it was run, the engine was still cold due to the lack of a thermostat)

Here's what I've checked. I have replaced the injector, cleaned the mixing elbow, set the valves, checked the delivery valve, battery connections, etc. I haven't checked the compression, but suspect it's good because it starts very well when cold.

2nd, The soot. I'm assuming that the two problems are related. Soot comes out of the exhaust when you accelerate or are running at high throttle. I have hooked the fuel line up to good gas with the same results, so I expect the fuel tank isn't dirty. I tried some high sulfur diesel in the engine. With the high sulfur diesel the boat didn't emit nearly as much soot, but still wouldn't start while warm.

I'm suspecting a few culprits for the problem:
1. ignition timing might be off, I'm not sure because this is unlikely once again because it starts instantly when cold and the engine runs perfect.
2. Lack of ventilation in the engine room. I have a 26ft. s2. The only ventilation is a 3" vent elbow. Does this allow sufficient air flow. I know diesels require a lot of air and even more in my warm climate (I think)
3. My starting battery is a dual cycle 600??CA. Also my batteries are mounted in the engine room (which gets hot. I don't know if heat affects batteries.) Also The battery cables are standard 4 gauge??? they are pretty long as well. Once again I don't know if this matters because the boat starts instantly when cold.


sailingdog 10-04-2006 09:41 AM

Look in Don Casey's books. He has a formula for the amount of ventilation needed based on horsepower IIRC. I don't think a single 3" vent is really enough, but I could be wrong.

Heat does affect batteries, and if they're in a really warm location, the heat may contribute to the electrolyte evaporating...have you checked the water level in them recently??? Also, the electrical resistance of cables generally goes up as the cables get warmer... You should have the shortest run of cables from the battery to the engine as is possible, this is not a place you want slack.

mhfowle 10-04-2006 01:34 PM

As far as testing the battery juice getting to the starter, do you think I could test the voltage at the starter when the engine is cold and again when the engine is warm? Would this reveal if the starting system is to blame??

How much voltage does it normally take to start a diesel (I'm guessing about 13v)

How many cranking amps does the battery need?

With the engine room ventilation: How are sailboat diesels normally ventilated?


Blue Eagle 12-20-2006 05:17 AM

Starting problems
Hi Mike, let's see if we can take this step-by-step"


Originally Posted by mhfowle
I have a yanmar ysm8 with the following problems: It won't start warm (it starts instantly when cold)

This says to me that your problem is specifically related to a change in the engine when it's hot. You don't mention whether it spontaneously stops when hot and won't restart, or just doesn't want to start after you've stopped it and it's warm.

Since there are really only 3 things a diesel engine needs to run happily: Fuel, Air and Compression, let's assume that it's one of those that's causing the problem.

The air I don't think is likely to be an issue unless your engine has some kind of fancy intake - if it's basically a hose with a filter inline somewhere I can't see how heat would affect it.

Fuel however - now that could be affected by excessive heat depending upon how close your fuel lines run to the hot engine. If they get warm enough to vapourise the fuel in them you could get vapour-lock in the fuel system, whereby a bubble of fuel vapour is stopping the liquid fuel flowing. Not likely, but possible. You should also go back over all of your fuel unions and just make sure that they're nipped up tight. Air can get in through the smallest spaces!

Lastly, and I think most promisingly: compression. You say you haven't tested it. I think you should, both hot and cold. As you're aware, when metal gets hot it expands, and I wonder whether your rockers may not be adjusted too tightly - what are the gaps? a good rough starting point should be 12 thou inlet and 15 thou exhaust, when cold - or whatever it says in your manual. remember to re-check the gaps after tightening the screws of course, so you can be sure you haven't changed them in locking off the adjusters.

What state is your oil in and how long is it since your engine was last rebuilt? the oil film may be creating compression when cold that it can't when hot. Do an oil and filter change and see what you get (be sure to put good thick diesel-friendly oil back in).

Lastly, test that thermostat! If everything worked before installing it, pull it out of the engine, put it in a pan, with a thermometer, on your stove and boil it gently until it opens. Compare this temperature to the stated operating temperature, and the temp. stated in your manual for your region. Diesel engines run cooler than petrol generally I think, and I think the 'stat on my Volvo opens at 60 - I'm based in the tropics too.

Soot: Could be a variety of factors: 1) knackered engine oil being flashed off and going through the re-breather system is one. I assume this little unit doesn't have a turbo or supercharger, so we don't need to worry about that?
2) inaccurate metering by the metering unit (wouldn't expect this to be the case in an engine as new as yours though unless it'd been tonked. 3) Choking the airflow of the engine preventing complete combustion. (actually, if the exhaust only smoked as you accelerated, it would have to be the metering unit, not the lack of airflow) What colour is the smoke? blueish-white or black?

The battery has nothing to do with the current problem - assuming it will spin the engine over when it's warm - although, you may be storing problems up for the future if you're gently simmering your batteries and/or if you have long, thin, unsupported cable runs, for the reasons raised before.

Ignition timing is fixed I should think - if your injector pump runs off the camshaft like mine.

hope this helps :)


Goodnewsboy 12-20-2006 06:05 AM

Blue Eagle has covered the bases very nicely. I had a similar starting problem and found it to be related to valve clearances. Proper gaps turned out to be a miracle cure.

erps 12-20-2006 09:52 AM

Our previous boat was a Catalina 34. They had a problem of not starting when the engine was at operating temperature (not turning over fast enough to start). It was because the starter battery cables were undersized and the starter built up resistance as the engine heated up.

I also had a car one that wouldn't start when the engine was at operating temperature (engine cranked over very quickly) The valve lash was too tight, so I was loosing compression.

Faster 12-20-2006 09:58 AM

Mike - it's not really clear from your posts if the engine is slow to crank, or will not "catch" when cranking normally if it's warm.
Since it runs fine once started, I'd say it's more likely the former and as indicated above it may well be temperature related to the equipment you have.

redcorvette1995 12-20-2006 01:45 PM


I had the 12HP version of this engine in my boat and I have worked on them a lot. The nice thing is that they are very easy engines to work on yourself.

I would suggest that your problem is related to compression.

I would start by torqueing ALL the bolts to their specification.
It may just be a loose bolt.
These engines vibrate a great deal and this tends to loosen the bolts more quickly than other engines.

Secondly I would adjust the valve clearance to specifications.

Following that I would change the head gasket.

After that you start to get more involved (rings) and the cylinder liner come to mind.

The engine is raw water cooled so it never gets very hot so it will soot more than most. If you have an after market fresh water cooling system installed this may be the problem in and of itself as the engine was designed for raw water and likes to run cold. This is an engine that you should be able to run at full throttle and comfortabley rest your hand on. It shouldn't get hot. If it is then the cooling passages may be blocked up.
In this engine design the cylinder liner then expands and you can get a loss of compression around the rings.



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