Runs hot while on "through prop" exhaust, runs cooler if switched to &

[lenny2113]

Mercruiser 5.7L, Alpha 1 drive.<br />This is what I noticed while operating at cruising speed at 3000-3500 RPM:<br />When I switch to "through prop" temp gauge reads higher temperature then if I swich to "though hull". <br />I have "silence choice" exhaust system on the boat. Any ideas why this happens?<br />Thanks!

 

[rodbolt]

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

most likly cause more water can flow with less resistance thru the hull. could be caused by a degrading seawater pump. does not matter how much the pump is pumping all that goes in must be able to get out. when was the last seawater pump service? when going through the prop the exhaust tends to try to pressurize the seawater system. so the seawater pressure has to overcome exhaust pressure. how much temp fluctuation are we talking ?

 

[Laddies]

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

You must have a vacume leak in the pump system and with the pressure of the exhaust around it the pump is sucking air---Bob

 

[lenny2113]

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

I would say about 30-40 degrees difference. <br />Reads 160-170 through hull, reads 200+ through prop. Makes me concerned to see 200 on the temp gauge...<br />I change pump impeller every season. Pressure test every season also. Both drives held 15psi for 10 hours without a drop. Both temp gauges show higher temp. when exhaust is switched to "through prop".

 

[trog100]

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

i think rodbolt has the obviouse answer.. something is restricting the flow out thru the drive.. or to put it another way its easier for water to go out thru the hull than out thru the drive.. so flow is better and engine runs cooler<br /><br />course is could be down to weak pumping or a genuine restriction in the drive that shouldnt be there..<br /><br />its probably impellor replacement time.. if that dont cure it u have to go the other route.. restrictive drive outlets..<br /><br />something is wrong with the general system flow.. it should be the thermostat that governs engine temps not which way the water goes out..<br /><br />trog100

 

[lenny2113]

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

When I changed impellers, they show minimal wear on both drives. It is my choice to get a new one every season. I just do not like to pull the boat out of the water in the middle of season. Water intake openning was clean and I see a fare amount of water flowing when I swich to thru hull.<br />Puzzle...

 

[lenny2113]

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

restrictive drive outlets

How do I test it? <br />Oh, when I had the drive split for impeller change, I took a flash light close to prop hub. Was able to see light looking down into the lower unit... Would this indicate "clear drive outlets"?. Thank you!

 

[rodbolt]

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

hello<br /> is this fresh or seawater cooled?<br />and either way 200 degrees is to warm. not critical but warm. about what year is the set up and are the seawater pumps belt driven or in the drives?<br />and about what year package are we dealing with ?

 

[lenny2113]

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

This is sea water cooled. When you say sea water pump, please correct me if I am wrong, you are talking about rubber impeller and the pump housing both of which sit on vertical drive shaft inside of lower unit. Water pump impeller and housing is replaced every year. The package is alpha one which is probobly from early 90's.<br />Both engines are pre-86 model (2 peice rear main seal). One is rebuild from last year, the other is a 2 years old rebuild.

 

[rodbolt]

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

when was the last riser change?

 

[lenny2113]

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

Risers and manifolds are 2 years old. Boat used in salt water.

 

[kdmiller8251]

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

Correct me if I am wrong, but from the way it sounds you have two engines in your boat, hence two drives, and both are showing the same temps for thru hull and thru prop. So what are the chances of both having the same problem?? I would say pretty low. Unless you are performing the maintenace wrong on both or installing the parts wrong on both.<br /><br />Has it always run that warm? maybe its normal for your setup.<br /><br /><br />Tallman

 

[trog100]

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

i dont understand the two engines having identical overheating problems situation..<br /><br />i think there is coolant flow problem.. but to have identical problems on two independant engines dosnt really seem plausible..<br /><br />wrong maintainance carried out on both engines or drives is a possible explanation.. how long as this situation been going on.. did it just start happening on its own or is it related to some other event..<br /><br />trog100

 

[Mamier]

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

Hey sorry to interupt your string...but I just posted a question asking what silent choice is. So as i understand you, you can switch back and forth from thru hub and thru transom. I just bought a 1985 Formula with a 260 and "silent choice". So, is this correct? Thanks man.

 

[lenny2113]

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

Mamier <br />You got it right on "silent choice". Good luck with your Formula.<br /><br />Guys, <br />This is a problem that was happening last season. The boat is not in the water yet. I asked this question to make sure I am not missing something important before I put her in the water. <br />Now, that I heard some opinions, I am leaning more and more towards "exhaust pass was partialy closed by barnacals.<br />I hope to check it next week. Working on getting the boat in the water... <br />Thank you for your feedback!!!

 

[trog100]

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

Re: Runs hot while on "through prop" exhaust, runs cooler if switched

Lenny2113.. i spose barnacle infestation could happen to both engines at the same time.. the local conditions is something both engines have in common..<br /><br />trog100