engine temp rises when going fast..why?

[stunod12]

Hi all,<br /><br />Took a 60 mile run today, (twin 318s striaght inboards)and noticed that my port engine runs hot when over 3000 rpms.<br />For example, I ran the boat at 3500 rpms and the port engine started to over heat. I then ran the boat at 2800 rpms for 40 miles and the engine was fine. As soon as I increase over 3000rpms, the temp starts to climb rapidly. From 160 to 200 within 1 minute. I back down the rpms, and the engine runs 160-165.<br /><br />Question:<br />IS it the Manifolds and risers?<br />Is it a sticky terminstat?<br />is it the water pump?<br /><br />Thanks in advance,<br />Lou

 

[jomac]

Re: engine temp rises when going fast..why?

stunod12 The mechanic in me would say to start with the timing advance in the distributor of the hot motor, some where vacuum & some are mechanical either can stick and cause that problem at close rpm levels But check all u mention plus your fresh water pick up. Stay good luck

 

[Boatist]

Re: engine temp rises when going fast..why?

Stunod<br />I will bet on manifold/ riser. What you decribed is exactly what happends when the manifold rust and scale starts to flake off and plugs holes between manifold and riser. In saltwater normaly in the 4 to 6 year range. If that is what it is you can clean out rust and scale from holes and will be ok for a while but next thing you will see is when manifold rust thru and water starts to run to exhaust valve and cylinder. Then when start motor if lucky will miss on one cylinder until get water out then be ok until you shut down. Do you run in saltwater and how old are manifolds?? If it is rust plugging manifold output then time to change manifolds and risers.

 

[William B Arthur]

Re: engine temp rises when going fast..why?

Could be a bad fuel pump on that motor, unless you have one that is shared between the two like an electric.

 

[stunod12]

Re: engine temp rises when going fast..why?

well the engine was rebuilt in 98, so if they were changed too, than they're 5 years old.<br /><br />Also, the engine that ran cool all day used less gas? ie: I Traveled 60 miles with one engine running cool at 3100 rpms and the other engine running warm (168ish)at 2800rpms. At the end of the day, I had 1/4 tank more on the 3100 rpm side! is that normal..and why?<br /><br />P.s <br />also,when I brought the hot running engine up past 3100 rpms, as the temp climbed I quickly brought the ilde down to 2500 and I watched the temp gause move down Just As Fast as I moved the throttle. I thought the temp gauge might be sticky so I tried it up on the flybridge and it did the same on those gauges too. <br /><br />Can Temp change that quickly?

 

[MrBill]

Re: engine temp rises when going fast..why?

Boatist is on the right path...head straight toward issues that restrict or impede flow of coolant. Fuel and ignition related issues couldn't impact the heat transfer rates in your engine to the magnitude described. Check all the potential coolant flow related items, engines all run hotter at higher RPM. With time a constant, a faster running engine needs to dispel a greater amount of heat energy, and the cooling system needs to be able to proportionally keep up...yours is not. Typically, manuals have a "running hot" check list which can be used as a diagnostic. Too many to type here without more specifics on you exact engine, etc.

 

[William B Arthur]

Re: engine temp rises when going fast..why?

"I will bet on manifold/ riser. What you decribed is exactly what happends when the manifold rust and scale starts to flake off and plugs holes between manifold and riser. In saltwater normaly in the 4 to 6 year range. If that is what it is you can clean out rust and scale from holes and will be ok for a while but next thing you will see is when manifold rust thru and water starts to run to exhaust valve and cylinder. Then when start motor if lucky will miss on one cylinder until get water out then be ok until you shut down. Do you run in saltwater and how old are manifolds?? If it is rust plugging manifold output then time to change manifolds and risers."<br /><br />How would you go about checking that? Do you have to pull the exhaust manifolds off the motor to do it or could you leave them on the motor? I mostly work on cars, so this type exaust system is new to me.

 

[Dobs]

Re: engine temp rises when going fast..why?

Well after my first "experience" with the restricted water flow thru the manifold/riser...let me tell you what I was told...<br /><br />With engines running, place palms on risers...should be "about even temp" between the two, and warm to the touch...not hot.<br /><br />My manifold "scaled" and clogged the exhaust system resulting in riser overtemp and subsequent "rubber tubing" overheat (read LOTS AND LOTS OF SMOKE with an excited wife to add to the noise level). I got lucky..it didn't catch fire, and it cooled fine at idle...so I managed to poke my way back to the marina. FYI...the engine temp never climbed out of the normal temp...it was JUST the manifold and riser overheating.<br /><br />A local mechanic pulled the clogged manifold, unplugged it, and put her back on. She runs nice and cool now, and the otherside is now the "warmer" of the two. He also showed me what happens about the 5 year point to exhaust manifolds in salt water..how they wear thin from scaling and have the potential to "fail" and allow water into your engine. I am now looking to replace both manifolds to save myself any worry for the next 5 years <br /><br />So take the advice from these guys, and either pull them yourself, or have a mechanic do it (wasn't hard...watched the guy work on mine) the first time for you.<br /><br />Dobs

 

[Boatist]

Re: engine temp rises when going fast..why?

Wild Bill<br />Two ways to check if manifold / riser.<br />Pull the Riser off and check water output holes in manifold. If plugged up clean out. Also check the rise holes. N0 need to pull manifold unless you are going to change them. If the problem is a plugged manifold or riser then a good Idea to replace. Remember the next thing that happens is Salt water in valve train and in cylinders which can equal new block and heads.<br /><br />Another way to test is to pull the hose between the block and manifold and run over board. If engine cools down fast probably a plugged manifold or riser. Can only run this way for a short time as have no cooling to exhaust. Water will be hot so be careful.