OMC 305 OVERHEATING AT IDLE & HIGH RPM

[tyvicire]

I HAVE A '88 OMC 305 I/O ON A BAYLINER. WITH EARMUFFS OR IN WATER, IT HEATS UP AT IDLE OR HIGH RPMS (3200+). THIS MOTOR HAS A 160 THERMOSTAT AND SHOULD BE ABLE TO RUN AT 4000 RPMS AT ABOUT 165-170 DEGREES. THE BEST RESULTS I GET FROM IT IS 175 DEGREES AT ABOUT 3000RPMS. ANY MORE THAN THAT AND THE TEMP GOES UP. I HAVE PUT NEW IMPELLAR, THERMOSTAT, WATER PUMP, MANIFOLDS AND RISERS. I AM AT A LOSS OF WHAT TO TRY NEXT. I AM THINKING THAT I MIGHT HAVE A CRACKED HEAD OR MAYBE SOMETHING SLOWING WATER FLOW IN THE BLOCK. THERE IS NO SIGNS OF WATER IN OIL. ENGINE RUNS SMOOTH. PLEASE HELP. THANK YOU

 

[Walt T]

Re: OMC 305 OVERHEATING AT IDLE & HIGH RPM

Have you verified the water flow from the impeller? You may need an entirely new pump. Otherwise you probably have blockage somewhere. This is common on saltwater boats.

 

[rodbolt]

Re: OMC 305 OVERHEATING AT IDLE & HIGH RPM

hello<br /> not knowing what drive you have I am going to assume its a raw water cooled cobra. there were some flacky problems with the pumps as the units aged. some here corroded enough that we switched to a crank mounted johnson or jabsco and used a thru-hull and strainer. the cobra drive was very popular with the crabbers. it would last longer than the merc but had some quirks. the thing you must verify is water flow. like dieselwalt says. start at the seawater inlet and make sure it has a solid stream at idle speeds. the strangest I ran into yet was a t-stat housing that the corrosion had choked off the water flow and would cause over heating above 3000 rpm. it wouldd help if you can determine if both cyl banks are heating up or just one bank. is this or has it ever been a saltwater engine? its odd to have a problem at both ends of the rpm range. leads me to belive you have a pump intake problem, ie corrosion at the water housing seals on the lower or you have 2 problems .<br /><br /> good luck and keep posting

 

[tyvicire]

Re: OMC 305 OVERHEATING AT IDLE & HIGH RPM

WHEN I HAD THE IMPELLAR REPLACED.THE PUMP WAS REPLACED ALSO.IF IT IS BLOCKAGE. IS THERE AN EASIER WAY TO FIND OUT WHERE ABOUT IT IS WITHOUT TEARING DOWN THE WHOLE ENGINE? THE ONLY PLACES I HAVE BEEN ABLE TO CHECK ARE THE HOSES, THERMOSTAT, CIRCULATING PUMP,AND EVEN THE POWER STERRING UNIT. WHAT DO YOU THINK ARE MY CHANCES OF HAVING A CRACKED HEAD. I HAVE BEEN TOLD THAT THIS IS COMMON WITH THIS ENGINE DUE TO A HOT SPOT IN THE BACK LEFT CORNER. I BROUGHT IT TO A DEALER AND THEY REALLY DIDNT WANT TO MESS WITH IT. THEY TOLD ME TO CHANGE ALL OF THE HOSES TO CLEAR HOSES AND RUN IT AND SEE WHAT THE WATER LOOKED LIKE THEN.

 

[tyvicire]

Re: OMC 305 OVERHEATING AT IDLE & HIGH RPM

THE BOAT IS USED FOR SALT WATER USE. WHEN YOU SAY BOTH CYLINDER BANKS OR 1. IF IT WAS JUST ONE WOULD ONE OF THE MANIFOLDS FEEL HOTTER THAN THE OTHER?

 

[hokey1]

Re: OMC 305 OVERHEATING AT IDLE & HIGH RPM

Iknow this might sound stupid,but are you sure the impeller was installed the correct way? I know from personal experience ther are some repair manuals out there that show and descibe the installation process and if it is followed the impeller will be backwards in its rotation

 

[tyvicire]

Re: OMC 305 OVERHEATING AT IDLE & HIGH RPM

I HAVENT REALLY LOOKED UNFORTUNATELY. I HAD A BAYLINER SHOP INTALL NEW PUMP AND IMPELLAR

 

[tyvicire]

Re: OMC 305 OVERHEATING AT IDLE & HIGH RPM

THE GENTLEMAN WAS SUPPOSED TO BE AN OMC SPECIALIST. HASNT HELPED ME OUT THOUGH. BUT I FIGURED HE INTALLED IT CORRECTLY

 

[hokey1]

Re: OMC 305 OVERHEATING AT IDLE & HIGH RPM

I know before I went through the hassle of pulling heads and clear hoses and crap I would pull the lower end and ck that and make sure the impellar housing was smooth

 

[Lou C]

Re: OMC 305 OVERHEATING AT IDLE & HIGH RPM

The impeller on a Cobra is pretty easy to change, you remove the plastic cover at the rear of the upper unit (3 bolts) and inside is a housing held on with 3 more bolts. When you do the impeller you should get the whole water pump kit and replace the housing, gaskets and wear plate. You have to make sure that the o-ring type gasket that seals the housing is in place or else the pump will pull in air esp when it is on plane and the pump is not submerged. Now for the test, in the OMC shop manual it says that you can run the motor at idle with the hose that goes from the outdrive to the thermo housing disconnceted (temporarily of course!) and hold the hose straight up, it should shoot a stream of water up about 2-4 inches. You should remove your thermo housing and check to see if the passages to the manifolds are necking down with corrosion. If so replace the housing with a new thermostat. There is another test that can be done for head gasket leaks, but you probably would want to have a pro do that one. First make sure enough water is getting in and out.

 

[tyvicire]

Re: OMC 305 OVERHEATING AT IDLE & HIGH RPM

THANK YOU ALL FOR YOUR REPLIES. ONE MORE QUESTION.... WATER COMING OUT OF ENGINE....THIS IS A RAW WATER COBRA BY THE WAY... ANYWAY, THE MANUAL SAYS THAT WATER IS DISCHARGED THOUGH THE IDLE EXHAUST RELIEF WHICH POINTS TO THE SLOTS ABOVES THE PROP. I NOTICE MOST WATER COMING OUT SOMEWHERE IN THE AREA WHERE THE ANODES ARE CLOSER TO THE TRANSOM. I REALLY DONT RECALL SEEING WATER COMING OUT OF THE SLOTS

 

[Walt T]

Re: OMC 305 OVERHEATING AT IDLE & HIGH RPM

Thats normal at idle. The outlet hose from the pump should move a LOT of water at idle. Check that first. I doubt you have a crack. Plugged head gasket is possible.

 

[tyvicire]

Re: OMC 305 OVERHEATING AT IDLE & HIGH RPM

TO CHECK FLOW FROM PUMP, I WOULD HAVE TO CHECK THE HOSE CONNECTED TO THE THERMOSTAT THAT WAS REFERRED TO EARLIER?

 

[Lou C]

Re: OMC 305 OVERHEATING AT IDLE & HIGH RPM

That test you need a helper to do, and you would warm the engine up on the hose, shut it off, remove the hose from the OD to the thermo housing, and just start the engine for a couple of seconds is all it should take (realize when you do this, cooling water is NOT going into the thermo housing) to see if you get a good stream of water shot up out of that hose.<br />BTW, get an OMC shop manual, they are better by far than the ones from aftermarket, and cost about the same. You can get them from a Johnson/Evinrude dealer, it has a whole section on the cooling system.<br />You may have the aftermarket style exhaust bellows with the slits, in that case most of the water will exit from the bellows (you will see it if you look up between the upper unit and the transom housing. If so that is normal.

 

[tyvicire]

Re: OMC 305 OVERHEATING AT IDLE & HIGH RPM

I went ahead and ran the boat and of coarse it did not overheat this time. never does when I want it to. I took the hose off of the thermostat housing and it was flowing out about 2 inches. During the process of warming it up, it only got up to about 140 degrees but I did notice that one of the risers felt hotter than the other. If I had to guess, about 20-30 degrees warmer. When looking towards the back of the boat, it was the left side that was warmer.

 

[tyvicire]

Re: OMC 305 OVERHEATING AT IDLE & HIGH RPM

The last time I ran the boat out of water with muffs. The temp went straight up to 200 degrees which is redline on this boat. I couldnt get it to go down and shut motor off. Flirting with redline has been a common thing for this boat. I am at a loss?????????????

 

[hokey1]

Re: OMC 305 OVERHEATING AT IDLE & HIGH RPM

Might not be a bad idea to connect another known good temp gauge

 

[Lou C]

Re: OMC 305 OVERHEATING AT IDLE & HIGH RPM

Well it sounds like you do have enough flow from the impeller but you still have to look at the passages in the thermostat housing, make sure the belt for the circulating pump (water pump on the front of the engine) is tight. Now about the risers, I have heard it common for the left side as you face the engine from inside the boat (driver's side, starbord really) can run a little hotter. On mine which is an 88 Cobra with the V-6 and the one-piece manifold/riser, the riser sections are the same temp at idle (barely warm), if you feel the exhaust pipe after the riser on the starbord side it is a tiny bit warmer that the port side. I have heard it is possible for the gasket inbetween the riser and the manifold to be installed wrong, and that can block some of the water flow and cause overheating. Another possiblity. You might want to see if you can borrow one of those infared heat measuring guns to see if your temp gauge is right. I know that the voltmeter on my dash is about 1.5V low because I have checked the batt with a digital voltmeter, so they are not always accurate. <br />Lastly, I have heard it is possible for there to be a problem with the tube in the lower unit that the impeller sucks water through, the seals sometimes leak and the flow drops when you need more cooling.

 

[tyvicire]

Re: OMC 305 OVERHEATING AT IDLE & HIGH RPM

I am sure that the gauge is is at least close to being correct because 1 time when it got up past redline (around 210-220) while in the water I had to shut it down and it did in fact freeze up. I was able to free engine once I got it home by taking the plugs out and spraying a little WD-40 in each hole and turning it over a couple times. As for the manifold gaskets. I am sure they are fine as well. Do these symtoms lean more toward bad flow or the head??

 

[tyvicire]

Re: OMC 305 OVERHEATING AT IDLE & HIGH RPM

The passage ways through the thermostat are also fine. I checked that about a month ago when I replaced the thermostat. The circulating belt is also tight. There really was not much corrosion on the thermostat housing or the manifold ands risers. I went ahead and replaced the Manifolds and risers anyway but didnt notice a difference in temp. I also ran boat in water without thermostat and it took a little longer but eventually got hot.