Possible cooling problem when starting and idling

[JHoukes]

Hi,<br /><br />Since a few weeks I have the following problem with my engine.<br />When the engine is at normal operating temperature, it is stable at about 50 degrees Celcius. I have never seen a higher temperature until a few weeks ago.<br /><br />When I start the engine and let it run idle for a few minutes the temperature raises to about 80 degrees and the engine starts to produce a little smoke (which it never does).<br />When I put the engine into a gear and start boating (or hitting the gas in neutral), the temperature immediately drops back to the 50 degrees that is normal. From then on, I no longer experience any problems and I can let the engine idle for minutes.<br />When I stop and start the engine again, the problem sometimes occurs again.<br />A hot or cold start does not seem to matter.<br /><br />The problem does not occur always and I try to keep it to a minimum. I now always make extra RPMs after starting the engine so that the problem does not occur.<br />However, the temperature raise does not seem normal to me.<br /><br />The engine we're talking about is an OMC Cobra 5.8L V8 (1989). I just bought the boat (Bayliner 2455) and it is my first season.<br />Does any one have any idea and suggestions?<br /><br />Thanks in advance!<br />Jarno.

 

[oldboat1]

Re: Possible cooling problem when starting and idling

80 degrees (176F) sounds a little hot, but not too bad. I think the manual on my old 305 calls for a temp range between 145F and 160F. It sounds like you might be due for a new water pump impeller, given the fluctuation in temps between idle and running speeds. Perhaps a new thermostat would be a good idea when the pump is worked on.

 

[JHoukes]

Re: Possible cooling problem when starting and idling

Strange thing is that once I rev the engine the temperature goes back to normal. When back to normal, it can run idle for a long time at normal temperature.

 

[oldboat1]

Re: Possible cooling problem when starting and idling

I'm not quite sure what to make of that either, although it does not sound too serious. You might check the wiring at the dash gauge to make sure connections are clean and tight. At the sender, you might check the connection and also take out and perhaps clean the sending unit to make sure the threads are providing a good ground. If there is teflon tape or something similar to prevent leaks, it would be better to remove it and just reseat the unit tightly enough to prevent leakage. (It's brass, so don't overdo it.)

 

[skeffer]

Re: Possible cooling problem when starting and idling

It may be that when you shut off the engine there is still water in it that is just sitting there collecting heat then when you restart it the sensor it heated from the still water. When you idle it up you move the water and cause coller watewr to enter the system. this is not uncommon in cars. You may want to just start it and watch the temp gauge at idlee it should still cool but will take a little longer as the water is not moving thru the engine as fast.

 

[JHoukes]

Re: Possible cooling problem when starting and idling

It's not the sensor. The engine gets too hot (can't let my hand rest on the engine).<br />When I rev the engine, the temperature drops back to normal again. Cold start, hot start, it doesn't matter.<br />I've had some reactions saying that it might be a semi glogged power steering, a bad impeller or bad risers. I'll check this in the weekend.<br />Has anyone had similar experiences?

 

[b.gagnon]

Re: Possible cooling problem when starting and idling

The impeller could just be worn away. So that when you give it the extra RPM it starts pushing the proper flow of water. I would say change the impeller first. They are cheep, and easy to replace. I would say that the thermostat is ok because the engine is not overheating

 

[oldboat1]

Re: Possible cooling problem when starting and idling

I agree -- sounds as if it's the impeller. At least that is the best guess, and the best place to start -- and a relatively simple fix. Let us know.

 

[JHoukes]

Re: Possible cooling problem when starting and idling

I disconnected the water hose to the thermostat and started the engine with no throttle. There was no water coming out of the hose at all. When I gave little throttle, a large amout of water was pumped through.<br />This indicated that the impeller was not priming at all until throttle was given. Considdering the volume, there is no clogging of the oil cooler or risers.<br />Today, I'll buy a new impeller and the problem will probably be solved. If not, I'll keep you guys informed.<br />Thanks for the helpfull advise!<br /><br />Jarno.