On Monday, July 11, 2011 at 11:02:13 AM UTC+1, Steve wrote:
Hey all, I have a 2005 3.0l mercruiser sterndrive that is acting wierd. It
will run all day long, runs great, but at the end of the day, it refuses to
idle, it just dies on me. Start it back up and it will run full throttle
just fine, but try to idle and it dies. If I let it sit for a day or two, it
goes back to running normal.
So far I have rebuilt the carb, rebuilt the fuel pump, cleaned the fuel
filter. checked for water in the gas, put another ingnition module in,
replaced the coil. Im out of things to do.
What am I missing?
Thanks
Steve



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Well, sounds very similar to an issue I have at the moment.... so breaking it down
1. Forget the shutter. if it can be traced to "Only" when the engine is hot, then forget this.
2. Same with the electronic ignition components.. i.e. if it is going to fail, you will not be running at any RPM let alone high speeds.
3. Valves... here is one possibility; either adjusted too tight causing them (or at least one of them)to remain slightly open when the components are hot, then returning to running fine when cool again. Add this to cylinder head work having been done, and you have a strong possibility. Before you get all down and deep on compression analysis, try adjusting them to spec.
4. Carb, yes, possibly if you have dirt,water etc floating around in the system, but you mentioned that you had cleaned the system, and I assume you didn't just mean the carb, so unlikely.
5. Cyl head gasket, again unlikely if it is only happening while hot. One point to mention here though: if you are replacing a head gasket I recommend the use of Blue Hylomar (this will take care of any imperfections).
6. Thermostat, unlikely since you have not mentioned any association with excessive temp.
7. Impeller, again unlikely since you have not mentioned any association with excessive temp.
8. So, now to one obscure possibility, assuming all else is good..... tank vacuum; is your fuel tank breathing properly? or is it under vacuum after running a while. I only say this in case your valves are found to be fine. If the breather in your tank is blocked/restricted it will lead you to believe the issue is due to your engine being hot, when in fact it is just down to the running time (fuel consumption/hr) and nothing at all to do with temperature (Since you have not mentioned "High" temp, I am going to assume again the temp is fine). This situation will only show itself after the engine has been performing, so no use trying to sit at the jetty all day trying to troubleshoot, it Must be under load (i.e. using plenty of fuel). again like the valves, once it has been left for some time the situation returns to normal.

So, to summarise; you have 2 possibilities left:
VALVES, OR FUEL TANK.