79 chrysler ob 115hp overheating

[Thegen]

Hello All Newbie here (to the forums and working on boats)

This may be a bit long but the best I can describe the symptoms so here it goes. I bought this tri-hull runabout with a chrysler 4 cylinder on it, the guy started it (ear muffs) and it sounded fine but when i got it on the water I couldnt get more than 2k rpms out of it. The motor didnt have much pickup and just bogged if I tried to push the throttle up. So I got it back home went back through the carbs and set the float levels as per manual 13/32 top carb was set a 1/4 inch (to lean). I decided to check spark also and had no spark from top two plugs. Well after much looking and making sure things read properly with multimeter, it turned out that the plug wires were shorting to the head. I thought they were glued there but apparently they were melted lol i felt like such an idiot. So i put the boat back out on the lake made sure i was pumping water and across the lake i went at 5k rpms but this was short lived since the temp alarm started buzzing. The motor was so hot i couldnt put my hand on it.

Today i have pulled the thermostat out and put it in some warm water, it opened right up. I pulled the head, gasket seemed fine but to be safe new one on the way. Compression test before i pulled the head was 120x3 and 105on the third cylinder from the top. With the head off i flushed water/air through the head and the block and got water through them both. I dont know what else i am missing unless the head gasket was bad. Also i did replace the impeller, pump housing seemed fine and pump was obviously working. I was getting more water out of the ports on the leg than i had before, I mean I never saw so much water pump out of a boat motor. Is this normal?

Any advise would be greatly appreciated i have read and read forum after forum called many boat shops for advise and im puzzled. Thanks in advance

 

[Frank Acampora]

Re: 79 chrysler ob 115hp overheating

The engine will run uncomfortably hot to the touch. At 160-180 degrees, it is above the temperature of 130 at which human skin scalds. You should not be able to put your hand on it for more than just a touch. However, water dribbled on the head should not boil off.

If you are getting water out the rear hole(s) (idle relief vent) this is correct and if you have cleaned the block and water flows freely, then you are getting cooling water.

It is not common but I suppose the overheat sender can go bad, however, there is no good way to test the sender itself. It is a simple bimetal switch. Try another one if you can borrow one from another Chrysler owner, or, if you are flush with cash, buy a temperature gauge and sender.

 

[Thegen]

Re: 79 chrysler ob 115hp overheating

Thank you so much for the reply.

Well I know for sure that the motor would have boiled water, it was WAY to hot. I should have proboably got an actual temp on it but the overheat alarm was goin crazy at any speed.

Last night i pulled the bypass valve also after reading some more of your other post replies about overheating problems. It was very much stuck in the housing and had a bunch of calcium stuff all over it.

You mentioned a overheat sender, where would it be located and if you could explain how it works that would be great. I am pretty interested to learn as much as i can about these motors for future problems (i know will occur).

Thanks again

 

[Frank Acampora]

Re: 79 chrysler ob 115hp overheating

On your engine the sender is on the head, next to (I think) the number two plug. It screws in and has a 5/8 hex with a cylinder on top. There is a terminal on top of the cylinder with an orange wire attached. As I said before, it is a simple bimetallic switch that closes when it gets hot.

If the bypass was stuck open, that could cause the overheating AND you will still see water coming out the idle relief vents. The bypass opens when the thermostat is closed or when the pump is delivering more water than needed. on later models, it was discontinuedand is not really needed. However, on your engine do not try to plug it--you may restrict the delivery side. Simply clean it off really well, be certain that the spring is not broken and will apply pressure to the plastic button, and put it back in. Also be certain that the cover is not corroded to the point where the spring will not work. When assembling, you should need to push in the cover about 1/4 to 1/2 inch, compressing the spring.

 

[Thegen]

Re: 79 chrysler ob 115hp overheating

So it continues, I replaced the head gasket, made sure the bypass was clear and started the motor without thermostat or thermostat cover on. I was getting water all the way up there and alot of steam too. The engine wants to "buck" and die. It overheated almost instantly (less than 3 min) I cant think of anything else since im getting water to the top of the motor. Anymore suggestions would be greatly appreciated. By the way even after new head gasket the third cylinder was still a little low on compression. Thanks again.

 

[Frank Acampora]

Re: 79 chrysler ob 115hp overheating

Well, I hesitate to mention it because any number of bolts may be corroded and snap off BUT: You may now need to remove the exhaust cover and clean the passages there. You also need to check the stainless plate for perforations which would allow exhaust gases to contaminate and heat the incoming cool water.

 

[Thegen]

Re: 79 chrysler ob 115hp overheating

ok i pulled the exhaust cover, stainless plate looks good, dirty but no holes or perferations. I also pulled the lower unit off just to make sure the impeller was still in good shape. The impeller looks like its not centered, kinda hard to explain but not all the "fins" look to be bent over equally but the impeller is centered. Could it be that the housing is out of round and im pumping enough water to get to the top of the head where the thermostat housing is but not enough to actually circulate? That would be weird though because of the amount of water i was getting out of the ports on the leg. I think this is really messed up because of the fact that it didnt do this when i was just running on two cylinders. I mean i realize that the other two are going to produce heat too but that much more to steam the water? The water that is coming back out of the ports is really really hot. The miss fire I mentioned earlier seems to have turned out to be one of the plug wires was loose in the distributor. Thanks again for all the help I would just really like to get this figured out so I can actually enjoy the boat for once instead of working on it lol.

Ok just thought i would add that the stainless plate under the exhaust plate was really warped, but there were no holes of any kind in it other than what should be there for the cooling passages. Also could someone explain exactly how this bypass valve works? Im begining to think that it may have something to do with this.

Could this be a bad base gasket also?

 

[Thegen]

Re: 79 chrysler ob 115hp overheating

Anyone please? This is eating away at me, that i cant come up with anymore ideas and would really like to go boating if I can get this figured out. Anymore suggestions would be greatly appreciated, thanks.

 

[Frank Acampora]

Re: 79 chrysler ob 115hp overheating

First the water pump! A brand new impeller is perfectly circular. With age they do tend to take a set. The water pump casing is offset slightly on the drive shaft so the impeller vanes bend on one side and do not on the other. This is how the volume is changed and water is pumped at LOW speed. This is displacement pumping. At high speeds, the resistance of the water bends back ALL the vanes, they do NOT touch the pump wall, and the pump now acts as a centrifugal pump. THIS is why the impeller does not wear out.

The bypass is really not necessary and as i said before, on later Force engines it was removed to save manufacturing costs.

Water comes up the tube from the pump and under the engine block makes a 90 degree turn into the tube with a pipe plug at the end. You can see this plug under the bypass cover. Water then makes another 90 degree turn to go up into the exhaust chest. There is a small hole--about 3/8 inch-- in the wall between between the entry to the exhaust cooling jacket and the bypass valve. When pressure gets higher than the spring pressure, the plastic button is forced back and water flows from the exhaust chest through the bypass and back down into the engine leg, blowing out either the idle relief or the exhaust snout or both.

Because the heat of the exhaust bears directly on it, the stainless plate is almost invariably warped. It will flatten out on the gaskets when you bolt the cover back, even though the center may still be warped.

 

[Thegen]

Re: 79 chrysler ob 115hp overheating

Thanks again for the reply, you have no idea how much its going to help for me to understand this thing. The base gasket that I had asked about was mentioned to me by not one but two mechanics, problem is they mostly work on omc and mercs. So do you think or ever saw a base gasket cause this? This is really weird that im not pumping much water to the top but the first time it overheated it was pumping like crazy. What else do you think could be causing it to overheat? Thanks again.

 

[Thegen]

Re: 79 chrysler ob 115hp overheating

Well I think I found the problem, I pulled the powerhead and pulled the water tube out and it had about an 8" crack in it. I went to lowes and bought a piece of 3/8 copper soldered a coupler to the end of it to make it fit the water pump output, flared the other end to fit the same as original and will see how it works in the am. Man pulling that powerhead was no fun, most definately an all day project. I have worked on plenty of cars, bikes and quads and I think my experience with this will require a little more patience then rebuilding a car motor. Will post if this works out or not so the info is here, I see alot of posts with no end resolve or that just ends. That is really annoying if you ask me, Im not trying to be critical but someone like me who is willing to sit and read a 50 post thread to find out an end result or solution, can get real discouraged when you never see a solution. Anyways I hope no one takes that the wrong way I really dont mean it negatively, just sayin. I want to thank you for all the help you did give, that info was much needed and was put to good use. Thanks again.

 

[Thegen]

Re: 79 chrysler ob 115hp overheating

Well I said I would post the results of my water tube repair/replacement. I ran the boat for a couple hours today and it never overheated once, it was always cool enough to touch the head. After some on the water carb float adjusting the boat came up on plane around 4k rpm and ran great. I think some more fine tuning it will run closer to its rpm range. So if you ever have a bad water tube (wich i couldnt find anywhere) a piece of 3/8 soft copper tubing with a flare on one end and a 3/8 coupler will work. I guess time will really tell but it seems to work. Thanks again for all the help.