Texasmark
Supreme Mariner
- Joined
- Dec 20, 2005
- Messages
- 14,778
Got the schematic. Thanks,<br /><br />I have a 3 cyl but the process is probably the same; same things to consider, just half the process.<br /><br />On understanding the flow diagram, have a couple of things that aren't clear.<br /><br />1. The role of the pressure relief valve is not clear. If the purpose of it is to relieve system pressure at high rpm's where there is a lot of ram pressure from the water pickup, what happens at low speed?<br /><br />I have proven on my engine with the initially stuck closed tstat that the telltale squirts without any regard for the position of the stat. According to this schematic, that says it gets (has to get) it's water from around the press relief valve.<br /><br />So, That means that the valve opens slightly at like idle impeller pressure and opens more when you are ballin the jack. That would explain a lot.<br /><br />Would also explain the designers intent in the design of the telltale circuit making it a good clue to a bad impeller. I know that's what it's for, but this forces it to play a more positive roll.<br /><br />If it (the impeller) couldn't develop enough pressue at idle to open the press rel valve slightly to get the tell tale squirting (and some much needed cooling spray to the exhaust tubes) it's time to change it. Very clever on Merc's part. Guess the other guys have similar things.<br /><br />Now that makes a lot of sense and answers another question (#2) about cooling vertical outboard motors with flow thru cooling water (as compared to self contained like in a car). I always wondered how they keep the water jackets full of water.<br /><br />So that say that the cooling line going to these 2 places from the cylinder's water jackets is(are) rather small and even though some water is circulated, not a large volume allowing the water around the cylinders to heat to the point where the stat opens.<br /><br />Then when you are balling the jack, the stat must be wide open and that path plus the press relief valve being wide open allows adequate cooling at the higher rpm's. Makes sense.<br /><br />Guess I don't have that many questions after all.<br />-------------------------<br /><br />So, in the case of the initial problem, Mr. Mortey's '88 150, he could have several problems; tstat, press relief valve, or clogged tubes connecting the system as shown on your schematic. So now you have given him places to look for an answer to his problem.<br /><br />I have learned a bunch about Merc cooling systems from this and I am sure a lot of readers have too; readers that may have wondered how it works for a long time.<br /><br />Having made my career as an engineer, I was taught, and learned that you question everything as to what, where, why, and when. Once you have done that you have the building blocks of the (any) system and all you have to do is walk through the process to trouble shoot and fix it. When you come across something that doesn't fit the process, you have found the problem.<br /><br />Thanks, and sorry for the rudeness....but it did get the desired result. Yeah I know.....there are other ways. I have been classified as "Direct" by my ex co-workers because of my ....lack of.....tact. Sorry.<br /><br />Best Regards,<br /><br />Mark
.<br /><br />The cooling of the engine is controlled differently at different RPM ranges. At idle speeds the poppet valve is closed and the cooling is all t-stat controlled. You will see your tell-tale stream increase or decrease when the the tstat opens and closes, respectively. You will still see a tiny stream when the tstat is closed as it has vents to allow some water flow. The poppet plays no role in the cooling of the engine at idle speeds, other than the fact that it must remain closed. If the poppet was stuck open the block would never fully fill with water resulting poor water circulation and hence an overheat.<br /><br />At higher RPM's the cooling of the block is pressure controlled (ie. the poppet). Between 2500-3000 RPM's the poppet is pushed open from the water pressure with in the block. You can actually watch the decrease in the water pressure via a gauge when the poppet opens. This then empties excess water thru the exhaust so water pressure doesn't build up and thus aids in the circulation of water throughout the block. <br /><br />The tell-tale is a indicator that the impeller is operating period, but doesn't really give you a good indication in regards to the health of your impeller. That's why a water pressure gauge is esstenial. Become familar with water pressures at certain RPMs with a known working system, and then from there on in any deviations from a known water pressure will tell you that something is wrong. Like the impeller is getting worn. The poppet isn't opening or closing.<br /><br />Hope this helps.