Cobra Overheat saga continues

[sea wolf]

I started a thread here a few weeks ago about an overheat condition in my 3.0 Cobra. Didn't get a response, so I'll try again. It overheats with the lower unit submerged, but runs fine off muffs. I've done the following. Tried 3 different impellers. Replace the following. Water pump adaptor, gasket & o-rings, backing plate, impeller housing, liner, seal & gasket, & backflushed the cooling system. I submerged the drive in a tub of water past the intakes. It wouldn't suck a drop. Hooked up muffs, & it pumped water like it was a brand new drive. When I pulled the muffs, the suction stopped. I pulled the l./u., & split it. I inspeced everything & it all looked good as far as the pickup tube, seals, etc. I even pulled the rubber hose on the pivot housing {not fun to get off} & blew thru it, & changed the 0-ring on the pivot housing. Put it all back together, & I'm at square one. It just will not suck water unless it's on muffs. I am at a total loss as to how to proceed. Does anyone have a clue as to waht the hell is going on here? I don't know what else to do besides replacing the whole drive, which at this point is not an option. Anyone?

 

[a70eliminator]

Re: Cobra Overheat saga continues

Disable the ignition and spin the motor over with the starter and observe the impeller with housing cover off ans see if it's turning? Other than that it sounds like you've done did everything else assosiated, if it runs on muffs then passageways must be clear.

 

[sea wolf]

Re: Cobra Overheat saga continues

Thanx. But, the impeller must be turning as it runs at 160 deg. off off muffs. And the stream out of the t-stat hose is strong.

 

[Don S]

Re: Cobra Overheat saga continues

You have an air leak somewhere between the impeller and the pickup. Where is really hard to say. Things like warped water pump housing from overtorque of screws, seal ring out of place, did you put sealer on the outer seal ring on the pump housing before installing?

 

[sea wolf]

Re: Cobra Overheat saga continues

You have an air leak somewhere between the impeller and the pickup. Where is really hard to say. Things like warped water pump housing from overtorque of screws, seal ring out of place, did you put sealer on the outer seal ring on the pump housing before installing?

Yep, I sealed the 0-ring, & the housing is new. The whole damn water pump is new. I think you're right Don, it is sucking air. But why not off muffs? With the drive submerged & engine running I can feel an air stream coming from the vent tube on the impeller housing. I'm going to try one more thing. Seal up around the adaptor housing with some kind of waterproof gasket sealer. I'm going to start it & spray water around that adaptor, etc. & see what happens. I don't care if I burn up an impeller. At this point, I got nothin' to lose.

 

[sea wolf]

Re: Cobra Overheat saga continues

Well, got home a short time ago & for the hell of it I startred the engine before I did anything with the drive submerged. Walla! It's pumping water like crazy. Pulled the t-stat hose & just about flooded the bilge. I shut it down & reattached the hose. Started the engine & the s.o.b. quit pumping. I sprayed water around the pump housing & it doesn't seem to be sucking air. I also sealed around the adaptor with permatex. Now, I'm going to see if I can find a t-stat hose to relace this one in the hope that there's a pin hole leak somewhere it the damn thing that is sucking air. But, I think I'm wasting my time & money. We'll see.

 

[DBreskin]

Re: Cobra Overheat saga continues

I think you're right Don, it is sucking air. But why not off muffs?

When you're on muffs, there's a pressurized water source pushing water into the drive. When you're not on muffs, the impeller has to pull the water into the system.
If you run the drive in a bucket, be sure to immerse the drive deep enough to submerge the impeller or it may not pump properly.

Is it possible the impeller drive shaft isn't engaged properly at both ends? If I recall correctly it is a press fit into the driveshaft and if it's pressed too deep it may not engage the impeller correctly.

 

[a70eliminator]

Re: Cobra Overheat saga continues

About the only think you haven't mentioned is the water tube that links the upper and lower drive assemblies, I remember there being seals on both ends, the impeller which is in the upper pulls the water up through this tube from the lower, maybe that's where your trouble is.

 

[sea wolf]

Re: Cobra Overheat saga continues

DBreskin, the drive is submerged in a tub of water over the intakes. The impeller does not have to be submerged, only the intakes.

a70, I split the drive & replaced both water tube seals. There's also a guide for the water tube that I didn't replace as I don't have that part. And of course it being a Cobra, which nobody stocks parts for I'll have to order it if I end up splitting this damn thing again.

Today I'm going to change the t-stat hose with some clear tubing & see what happens. What puzzles me is yesterday it pumped water like crazy without me doing anything. Then it quit when I took off & reattached the t-stat hose.

Thanx for the replies.

 

[sea wolf]

Re: Cobra Overheat saga continues

Finally got around to putting the clear tubing on. Started the engine with the drive submerged. Wouldn't pump a drop. I slapped the muffs on & it's pumping like the dickens with occasional air bubbles. Took the muffs off & it continued to pump water with no air bubbles. Shut the engine down, restarted it right away & it continued to pump water. Started it again after a couple minutes, & u guessed it. It wouldn't pump unless I primed it with the muffs on. Why is the pump losing it's prime? I think it's in the lower end, but I'd like to be sure before I tear it down again. One thing I did notice. When it's pumping water with the muffs off I put my hand near the intakes on the drive. I could feel air escaping. I don't know if that's normal or not. I would think not. Opinions?

 

[sea wolf]

Re: Cobra Overheat saga continues

I've just about reached the end of the line with it. Pulled the drive & split it. I replaced the 2 seals {again} on the water tube & the water passage guide. Put it all back together & it still won't pump water. I took the pump housing off & set the impeller on the shaft. I had the wife turn the engine over & the impeller spins. So, it ain't the water pump shaft. The clear tubing to the t-stat housing is full of air bubbles when running off of muffs. Dunno. I 've been a month trying to figure this out. It just won't give. Thing is, I hate to give up on it, but I don't know what else to do with it. I've owned this boat since '93 & this is the most frustrating issue I've ever come accross. I'm thinking of putting an SEI drive on it, but I kinda hate to sink 2 grand into it just beacause of a friggin' overheat issue. No way in hell will I put another Cobra on it and, I don't know if it's worth it or not to go with the SEI setup. If I bring it to a shop what can they do? I've replaced the whole water pump. I want to sell the boat & get a cuddy. The boat's an '87 19ft. Starcraft Medalist in damn good shape for it's age. This really sucks.

 

[gbeltran]

Re: Cobra Overheat saga continues

You say that the pump itself doesn't need to be submerged, where did you get that info? I cant remember where, but it's either the OEM manual or the Seloc manual I have for my Cobra says that you cant just submerge the intake holes in a bucket to do that test, I know I recently read that because I had an overheat issue (mine was a shirt got caught on the outdrive), after checking everything and replacing the t-stat for good measure I was going to check it that way and the book said no, so I think that the pump has to be somewhere close to the waterline. If you look at that pump design it's really not made for sucking water, it's more there to push it, kind of like a chevy oil pump. Have you tried your test with the boat in the water?

 

[cr2k]

Re: Cobra Overheat saga continues

The pump is not submerged like a MC it is much higher on the drive.

 

[gbeltran]

Re: Cobra Overheat saga continues

I realize where it is, I own one. When my boat is stationary I know it's under, when underway it's probably not but then you have some ram effect from the forward motion helping out. I'll try and find the reference I was talking about when I get home this weekend. I don't suppose that somehow backpressure from the exhaust could be keeping the water from flowing? Maybe the flapper is stuck or came off and it stuck in the bottom of the pipe?

 

[a70eliminator]

Re: Cobra Overheat saga continues

I'm pretty sure that impeller needs to be submerged initially, yes it is out of the water on plane but the pump is already prime at that point.
There's even a purge tube on the housing for air to escape when the drive gets initially dunked.
I can't believe somebody didn't catch it earlier. Your statement about the impeller not needing submerged rang the bell.

 

[sea wolf]

Re: Cobra Overheat saga continues

Thanx for the replies. I was thinking the same thing a few days ago about the pump having to be submerged. The tub I'm using is only big enough the bring the water over the intakes. On the pump housing there is a plastic vent tube. I thought maybe it was sucking air, not allowing the pump to prime. So, I got a small container of water & put it under the tube so it was under water. Had the wife start the engine, & got didly squat. Then I remembered that the tube is a vent which purges air from the system upon start up. That's why it's not supposed to be plugged up. And the impeller is sealed in the housing. It won't get water unless it's sucked up thru the lower unit thru the water tube via the intakes. I'm thinking now that exhaust gas is entering the system. Where or how I don't know. Crack in the lower unit? Beats me.

 

[sethjon]

Re: Cobra Overheat saga continues

Obviously with everything you've done you still missed something. Why not go to a local shop that has a good reputation and tell them the story. They may come up with something you missed. Worst case is you can bring it to them. they can't charge you if they can't fix it.

 

[sea wolf]

Re: Cobra Overheat saga continues

Yea, I guess I missed something. What that is I don't know. Frankly, I'm burnt out working on it & I need a break from it for a few days. But, if I don't get it fixed the boat is worth next to nothing & I want to sell it at the latest next spring. So, I'm going to have to start from scratch & go thru the whole system again.

 

[sea wolf]

Re: Cobra Overheat saga continues

You say that the pump itself doesn't need to be submerged, where did you get that info? I cant remember where, but it's either the OEM manual or the Seloc manual I have for my Cobra says that you cant just submerge the intake holes in a bucket to do that test, I know I recently read that because I had an overheat issue (mine was a shirt got caught on the outdrive), after checking everything and replacing the t-stat for good measure I was going to check it that way and the book said no, so I think that the pump has to be somewhere close to the waterline. If you look at that pump design it's really not made for sucking water, it's more there to push it, kind of like a chevy oil pump. Have you tried your test with the boat in the water?

I read your post again & maybe you're right. Next chance I get I'm going to launch it & see what happens. I also have the OMC shop manuel, but can't find the reference that you referred to.

 

[gbeltran]

Re: Cobra Overheat saga continues

I don't have my OMC manual at my place since it's in my boat, but I do have the Seloc manual, and in the cooling system section under raw water cooling test and under raw water flow test it specifically says
"**Warning
This test MUST be performed with the vessel in the water. DO NOT perform this test with the vessel out of the water with a flushing attachment attached."

It there twice in that section. I'm sure there is a valid reason for it.