OMC Cobra 4.3 over heating. New impeller. Still overheats.

[curt_irocz]

So I bought a 89 four winns with a cracked 4.3. I replaced it with a mercruser 4.3. It runs well but over heats in water. 200-220 degrees. I replaced the old melted impeller. And it still over heats. Check impeller again and it looks perfect. The exhaust manifolds get very hot. Could they be plugged? Or the flaps broke? When on the muffs it pours water out the exhaust bellow like crazy. So I figured they can't be plugged. Before I put then on I blew them out and knocked out all the lose rust. What could it be? It also runs cool on the muffs. Thanks a lot!!

 

[indybleck]

Re: OMC Cobra 4.3 over heating. New impeller. Still overheats.

Take the hose off that comes from your lower to the circulating pump and check to see how much water you're getting from the impeller, this should help in determining where your problem is, if your getting go water flow from the impeller, then look further downstream. Did you pull the pet cocks out and drain all the old stuff out of the block? I once had the hose from the lower to the engine go bad and had to replace that it was hard to see had to run on the muffs and look for a leak and it wasn't much but it made a difference.

 

[Bondo]

Re: OMC Cobra 4.3 over heating. New impeller. Still overheats.

So I bought a 89 four winns with a cracked 4.3. I replaced it with a mercruser 4.3. It runs well but over heats in water. 200-220 degrees. I replaced the old melted impeller. And it still over heats. Check impeller again and it looks perfect. The exhaust manifolds get very hot. Could they be plugged? Or the flaps broke? When on the muffs it pours water out the exhaust bellow like crazy. So I figured they can't be plugged. Before I put then on I blew them out and knocked out all the lose rust. What could it be? It also runs cool on the muffs. Thanks a lot!!

Ayuh,... Welcome Aboard,... Did ya find ALL the pieces of the old impeller,..??

Did ya replace the Whole impeller Kit, or Just the impeller,..??

 

[curt_irocz]

Re: OMC Cobra 4.3 over heating. New impeller. Still overheats.

Take the hose off that comes from your lower to the circulating pump and check to see how much water you're getting from the impeller, this should help in determining where your problem is, if your getting go water flow from the impeller, then look further downstream. Did you pull the pet cocks out and drain all the old stuff out of the block? I once had the hose from the lower to the engine go bad and had to replace that it was hard to see had to run on the muffs and look for a leak and it wasn't much but it made a difference.

I'm going to put it back in the lake and check out how much water flow I'm getting. The block is clean I just built the motor. Thanks

Ayuh,... Welcome Aboard,... Did ya find ALL the pieces of the old impeller,..??

Did ya replace the Whole impeller Kit, or Just the impeller,..??

the old impeller melted but was all there. I replaced it as a whole kit. Housing and all.

 

[curt_irocz]

Re: OMC Cobra 4.3 over heating. New impeller. Still overheats.

So I took it out on the lake. It's run a hot. 200 degrees All the time. I brought it back home The motor was still at 200. Hook it up to the hose and it drops to 100 while running. Cools right down. Like I said. Brand new impeller.

 

[curt_irocz]

Re: OMC Cobra 4.3 over heating. New impeller. Still overheats.

Pulled out the new impeller and it looks like the housing is already starting to melt??? What is going on??

 

[Reinell-BRXL-191]

Re: OMC Cobra 4.3 over heating. New impeller. Still overheats.

well, I hear a few issues... first off, when on the muffs, the engine should still run at 160 or so.. so if it runs at 100, the thermostat is bad... Now, it sounds to me, as if when in the lake, the impeller is not priming causing your over-heating issue, when using the muffs, you are talking 25-45 psi forcing the water in to the lower unit housing, where it is easily fed to the impeller housing, and it picks the water right up.. in the lake, it has to suck it up.. there could be many causes for this... check the vent hose off the impeller housing, and make sure it is not blocked.. then determine is there is a blockage down stream by removing the feed hose to the thermostat housing and see how much water flows out of there... somewhere in the system, something is causing the impeller to not prime easily... and get inside that thermo housing and check that t-stat... sounds bad to me..

 

[curt_irocz]

Re: OMC Cobra 4.3 over heating. New impeller. Still overheats.

Well it doesn't seem to be getting water to the thermostat when in the lake. And even on the muffs the the water stream is maybe 1/2 inch high. Also I did notice the impeller housing vent got plugged last time I used it. Maybe that's why it melted?

 

[sea wolf]

Re: OMC Cobra 4.3 over heating. New impeller. Still overheats.

Well it doesn't seem to be getting water to the thermostat when in the lake. And even on the muffs the the water stream is maybe 1/2 inch high. Also I did notice the impeller housing vent got plugged last time I used it. Maybe that's why it melted?

If the vent is plugged the pump won't prime. Also, make sure that the 0-ring is in the groove on the impeller housing. If it's not the pump will suck air, causing an overheat.

 

[pruffo]

Re: OMC Cobra 4.3 over heating. New impeller. Still overheats.

If the vent is plugged the pump won't prime. Also, make sure that the 0-ring is in the groove on the impeller housing. If it's not the pump will suck air, causing an overheat.

Just had the same bproblem with my 1996 4.3. I changed the raw water pump impellar and the circulating pump and new thermostat. Ran ok on the hose but hot in the wtare. The problem was the copper tube in the out drive. The tube had a 3/8" corrosion hole in it and the pump was losing prime and sucking in exhaust. On the hose the water is pushed up into the pump but in the water it couldn't creat enough vacuum to pull in sea water with the hole in the pick up tube. Hope this helps.

 

[curt_irocz]

Re: OMC Cobra 4.3 over heating. New impeller. Still overheats.

If the vent is plugged the pump won't prime. Also, make sure that the 0-ring is in the groove on the impeller housing. If it's not the pump will suck air, causing an overheat.

It still over heats with the vent clear and the o ring in the grove. On this omc the whole out drive is under water while idling so even if it had a leak it would just suck water. And it still over heats just idling in the water.

Just had the same bproblem with my 1996 4.3. I changed the raw water pump impellar and the circulating pump and new thermostat. Ran ok on the hose but hot in the wtare. The problem was the copper tube in the out drive. The tube had a 3/8" corrosion hole in it and the pump was losing prime and sucking in exhaust. On the hose the water is pushed up into the pump but in the water it couldn't creat enough vacuum to pull in sea water with the hole in the pick up tube. Hope this helps.

Yes. This is what I'm looking for. How can I check this tube? Split the lower unit?

 

[Idlespeedonly]

Re: OMC Cobra 4.3 over heating. New impeller. Still overheats.

You will have to remove the whole outdrive, then split them. Since you should be in fresh water the tube will probably be ok. The seals on the ends of the tube may be faulty though.

 

[ljsweeney]

Re: OMC Cobra 4.3 over heating. New impeller. Still overheats.

On this omc the whole out drive is under water while idling so even if it had a leak it would just suck water.

If there are air gaps in your cooling system, you would experience the exact symptoms you describe. It doesn't matter that the o/d is under water or not, the 'pull' of the impeller is just not strong enough to overcome the gap.

Yes, to check your water tube (between upper and lower units), you will have to drop, then split the o/d. As idlespeed pointed out, the tube itself should be O.K. (they have been known to fail in fresh water, though), and the seals should be replaced (you're gonna be in there anyway, may as well). With the overheating you've describe in this thread, I would not be surprised if the water tube seals weren't partially melted.

I had the same issue on my Cobra o/d last year, and was only able to resolve it by resealing the lower.

Good luck.

 

[curt_irocz]

Re: OMC Cobra 4.3 over heating. New impeller. Still overheats.

Ok so I'm going to pull the out drive out ( already have had it out when I replaced the engine). But get this when the boat is at like 10-15 mph. Not on full plane. It will cool down and run cool. It's very strange.

 

[Idlespeedonly]

Re: OMC Cobra 4.3 over heating. New impeller. Still overheats.

With a new impeller, boat completely in the water, drive completely down, idling in neutral how much water flow do you have?

 

[curt_irocz]

Re: OMC Cobra 4.3 over heating. New impeller. Still overheats.

With a new impeller, boat completely in the water, drive completely down, idling in neutral how much water flow do you have?

Zero. None.

 

[Idlespeedonly]

Re: OMC Cobra 4.3 over heating. New impeller. Still overheats.

Have you checked the oil cooler for a blockage?

 

[curt_irocz]

Re: OMC Cobra 4.3 over heating. New impeller. Still overheats.

Yes I have. I can see straight threw all the little holes. And have blown water threw it both ways. Did the same with all hoses.

 

[curt_irocz]

Re: OMC Cobra 4.3 over heating. New impeller. Still overheats.

Here is what I found when I dropped the lower unit. The water pick up tube grommet is melted. But I pressure tested it and it doesn't leak.im Going to replace them and hopefully fixes my problem.

 

[curt_irocz]

Re: OMC Cobra 4.3 over heating. New impeller. Still overheats.

Fixed. Boat runs 150 all day. It was the intake tube grommet wasn't installed right by who ever took it apart before.