Thermostat Modification V4

[87Aggie]

I've read some posts on this forum concerning the modifaction of the thermostat poppet or something to that effect. I have a '77 140 hp. The modifications mentioned drilling a slot or hole. Is this necessary? If so, what is the purpose and how do go about it?

 

[Jack Shellac]

Re: Thermostat Modification V4

The purpose is to let a little of the hot water from the cylinder heads circulate down to the backside of the thermo and help it open quicker. The OMC thermos don't have this feature, just hot water on one side and cold on the other. It's not absolutely necessary or OMC obviously wouldn't build them that way. The symptom usually shows up as an overheat alarm about 4 minutes after cranking up then everything is OK if you turn the motor off and crank it up 10 minutes later. The 10 minutes gives time for the heat to migrate to the back of the thermo and open it up. BTW, T A&M graduate work in '84 myself.

 

[87Aggie]

Re: Thermostat Modification V4

Jack, I never had a problem with an overheat alarm going off before. I decided to change to thermostats because there was a definite temperature difference in the 1&3 cylinders compared to the 2&4. I could hardly touch the 1&3 heads whereas the 2&4 heads seemed at the most warm to the touch. So, I felt like the thermostat associated with the 1&3 cylinders wasn't working correctly (stuck closed I suppose) This is the first time the tstats have been changed in over 10 years! Has OMC changed the design in any way? Gig,Em Aggies, Don

 

[clanton]

Re: Thermostat Modification V4

Check the pressure relief valves, the little plastic valves stick in the rubber insert. Corrosion will build between the block and rubber insert, causing the valve to stick. Sometimes the guides/ribs on the valve are too large causing sticking, reduce the size by sanding.

 

[Jack Shellac]

Re: Thermostat Modification V4

If they havn't been changed in 10 years, you definitely need to go through the whole thing. Relief valves, like clanton says, and new thermos accompanied by a thorough cleaning of the gunk and deposits accumulated over the years. The water pump needs servicing too, if that hasn't already been done. Use the whole water pump repair kit, not just the impeller. The heads should be hot, but allow you to hold your fingers on them for 30 or so seconds. If you can't, then they are too hot and something is wrong. May be deposits built up in other parts of the engine restricting water flow. This is a regular thing down here in the salt water, but shouldn't happen nearly as quick in continual fresh water use.

 

[steviecops]

Re: Thermostat Modification V4

Hey Jack<br /><br />I can't keep my fingers on the top of my head for much more than a second! I have monitored the temperature with a heat sensor and it didn't get above 63 Degrees C, (145 Degrees F).<br /><br />You must have asbestos fingers if you can touch yours for 30 seconds <br /><br />All the best<br />Steve

 

[87Aggie]

Re: Thermostat Modification V4

Jack and Clanton, I got the thermostat parts in a couple of days ago, I replaced everything last night - for all those that haven't tried replacing the thermostats on a V4 crossflow.. it's a REAL fun job! I got it done however and I'm going to see if that solved the problem of one set of heads getting way hotter than the other tonight. I'm planning on rebuilding the water pump this weekend. Any suggestions? I've got an OEM service manual. It appears the most difficult part is disconnecting the shift cable before lowering and then getting it back in place after rebuild.

 

[Jack Shellac]

Re: Thermostat Modification V4

You did the right thing in getting an OEM manual. They are much better than the generics. It will walk you right through the job. Follow it and don't skip any steps. I used to live in Pine Bluff and, as I recall, the water was pretty hard in the big lakes where you are probably using your boat. You may have some mineral buildups that have to be removed with CLR or something. Yeah, changing the thermos is a juggling job. Do a search on it and you will get some good tips. It's been discussed many times on this forum.<br /> Steve, everything is relative I guess. My fingertips are all callused up from working on OMC motors trying to get into some of the inacessible places and turning fasteners with the tips of my fingers. I probably could't hold the palm of my hand on the head for 30 seconds--maybe 10.