Mercury switch and Overheating

[Boatnut50]

I recently bought a boat that had a 1986 Mariner, 135 HP motor with a bad cylinder on it. I intended to scrap the motor, but after we looked at it a little closer, it was so clean that I decided to try to rebuild it. We got the machine work done and new parts and got everything back together (our first time rebuilding a boat motor). When it came time to start the motor it took us a while to understand the manual about timing, and then when we finally got it to fire we weren't getting fire on all the cylinders, and it seemed that the missing cylinders were actually moving around. For some reason after studying the schematic, someone suggested that we disconnect the "mercury" switch. After that all cylinders started firing and the motor actually started with almost no problem, although idling a little too high (1200 rpm). When it did start it ran for less than 30 seconds and we didn't see any water from the tell tale hole so we shut it down and put in a new water pump. After getting it started the oil alarm started going off so we topped off the tank on the motor and that seemed to cure that problem. The next thing that happened was the temperature alarm. We put in new thermostats when we rebuilt the power head and had just installed the new water pump(which the old impeller was actually shreaded, and not even connected to the shaft, we then felt pretty good that we found what caused it to mess up that cylinder the first time). The motor will not run very long now without setting off the temp alarm while there is plenty of water coming from the tell tale hole and you can touch the heads and block with your hands. The water is warm and the motor is hot to the touch, but you can hold your hand on it without getting burned. Also, the temp gage on the console doesn't say hot. My main questions are.<br /><br />1. What does the mercury switch do that causes the plugs not to fire, and do I have it installed wrong, or could it be bad. After rebuilding it we might have oriented it wrong.<br /><br />2. I am at a stand still with the overheating problem. How can I test the alarm or actually find out what is causing the overheating problem.<br />(or could the 1200 rpm idle be contributing to the problem)<br /><br />Any help you guys can give me would be greatly appreciated.

 

[The Marine Doctor]

Re: Mercury switch and Overheating

The switch is to stop the engine from starting or running in the tilted position.<br /><br />Does the engine actually get hot?<br /><br />The V6 product does not pump out the tell tail or discharge holes when you first start it.<br /><br />TMD

 

[Boatnut50]

Re: Mercury switch and Overheating

I don't know if the engine is actually hot when the alarm goes off or not. Like I said, we can still hold our hand on the motor, or the head and not get burned. The gage reads cool, but the alarm goes off so I don't know which one to trust. I'm planning on getting one of those melt sticks to test the temp and see if it is really hot. Any recomendations of what temp stick or where on the motor to check it. Also, we know this engine overheated before, enough to mess up a cylinder. Could this have damaged the sender or the alarm in some way.

 

[Boatnut50]

Re: Mercury switch and Overheating

Well to follow through on this post I thought I'd let everyone know where we are in case someone else has the same problems. The mercury switch that was giving us the problem was installed with the wrong orientation. We had it too high which was causing it to stop the engine. After figuring out where it was supposed to be the problem seems to be cured. The overheating problem seems to have cured itself. The motor was started over the weekend and ran on the hose for 20 minutes without an overheating alarm. The temp gage rose higher than it did the other night but gave us no alarm. The highest the gage got was about half way up (no numbers on gage, just hot and cold). I'm hoping that the alarms we were getting last week were false, or maybe had something to do with the mercury switch being disconnected. We are set to do a sea trial this week anyway.

 

[Boatnut50]

Re: Mercury switch and Overheating

Well, as they say, "Back to the drawing board". We took the boat out for a trial yesterday afternoon and it didn't go well. We put the boat in the water and it started fine and really idled well. After warming it up for a little while it was time to try to run it. The temp gage showed it to be warm and not hot so I put it in gear to get it up on plane. I was careful to watch the tach and not get over the 3000 RPM that was recomended in the break-in procedures and the boat came up on plane pretty easily. Everything was looking fine and the boat was actually doing great. Then not more than a mile from the launch, the temp alarm went off and I immediately shut it down. The gage was pegged so we sat there to let it cool off. After about 5 min. I turned the key on and the alarm wasn't going off but the gage still showed 3/4 the way to hot. We waited a few more min and the gage went down to about 1/2 so we started it up and headed back. On the way back to the truck we went at slow speed and everything went fine, temp gage didn't move and boat ran well so we tried to get it up on plane again. Almost as soon as I applied power you could see the temp gage start to move (from a little more than 1/2 and I didn't let it get over 3/4) so I backed off and the gage then slowly went back down. We took the boat back to the shop and pulled the lower unit and checked the pitch of the blades on the impellor. When we changed the pitch the pump didn't work at all. All we could feel coming out of the pump was air, and it never did produce any water so we removed the lower unit again and put the blades back the way they were. The problem now is that we can't get it to run hot on the hose but it does in the water. The tell tale doesn't seem to be passing the water like I feel like it should (water is coming out, but not with much force). We have replaced both thermostats and the water pump. We took the thermostats out (one at a time) and put a water hose where they were while the foot was off and it seemed to pass water freely. The only thing that we know of in the cooling system that we haven't changed is the overpressure pretection valve. Keep in mind that I got this motor after someone fried a cylinder, and I don't know why. But when I changed the water pump the impellor was shredded and we suspect that overheating was the problem that caused it to burn the cylinder. Any suggestions you guys have will be appreciated.

 

[The Marine Doctor]

Re: Mercury switch and Overheating

The reason the engine will not overheat on the muff is because of water pressure by the hose and no backpressure on the exhaust.<br /><br />So<br /><br />Weak water pump.<br />Sticking Poppet Valve<br />T-stats sticking closed.<br />Blown headgasket<br /><br />TMD

 

[sundog]

Re: Mercury switch and Overheating

youve put in way to much time to quit on this one! I agree with marinedr... water pump would have been my strong guess, but it sounds like youve done about everything with that.

 

[robalofish]

Re: Mercury switch and Overheating

Several questions.<br /><br />I know you changed the thermostats but have you checked them in hot water to be sure the open correctly? There is a chance that one might be bad.<br /><br />Have you located all of the shredded pieces from the original pump?<br /><br />When you changed the water pump did you change the entire pump or just the impeller? <br /><br />Did you put the RTV sealant on the water dam when re-installing the water pump?<br /><br />Have you checked the temperature gauge sender in hot water with a thermometer?<br /><br />Have you checked and replaced the gaskets and diaphragm and grommets in the poppet valve?<br /><br />I’ll watch for your reply.<br /><br />RFish >()>

 

[Boatnut50]

Re: Mercury switch and Overheating

Thanks for the replies guys, and don't worry about me quitting Sundog, I am determined to get this thing figured out. Doc I don't know how else to prove or disprove the waterpump. I am going to repair or replace the poppit valve and check the thermostats for proper operation in hot water. The headgaskets are new and we took special care to torque them like they were supposed to be, but wouldn't there be other symptoms if they were blown. Robolo we had the engine completely dissasembled to rebuild it, and we didn't find any debris from the impellor. We changed the entire pump, housing as well as the plate and gaskets. As far as the water dam, I don't guess we did that and I don't think I even know where it is. If that is necessary, it might be my problem. Please educate me a little on that. Thanks again for all the help. With work and my other obligations I don't get to work on this thing but a few hours a week and every other weekend, and now it is starting to get cold down her so I might be slow at getting more sea trials, but I'm not giving up unless I destroy the motor (I'm being careful not to do that).

 

[robalofish]

Re: Mercury switch and Overheating

Directly in front of the water pump is the water dam this should be sealed with the RTV to prevent the exaust from entering the pump intake and causing the pump to suck air/exaust. (sorry just noticed typo).<br />Im not sure your model has this but with your serial number I/you can check the parts breakdown on Mercury's site and see.<br /> http://www.mercurypartsexpress.com/ <br /><br />RFish >()>

 

[jim dozier]

Re: Mercury switch and Overheating

Just out of curiosity in your comments above you describe changing the impeller pitch. I am not aware of any adjustable pitch water pump impellers. Could you expand on this? I agree that you need to seal the exhaust dam from the water pump. Also, are you certain that you got good seal on the grommet on the water pick up tube. Sometimes if things don't go well the grommet doesn't sit right. I am also concerned that you did not find any pieces of the old impeller in your rebuild. While you have the lower unit off again, you might try backflushing the block from the thermostat housing out the water tube in the middle unit. Put a bucket under the tube and see if any old impeller pieces come out. Try compressed air also. Then clamp a garden hose on the water tube and forward flush it out the thermostat housing. Check the top grommet of the water tube with a flash light while your're doing this for any leaks. You might recheck your ignition timing, if the engine has too much ignition advance you may cause overheating and detonation.

 

[Boatnut50]

Re: Mercury switch and Overheating

Jim, when I used the word pitch I think my terminology is probably wrong. I probably should have said "cup" as you have to cup the blades one way or another because they are too long to fit in the housing straight. As far as touching the gromet I can't be sure that it is making a good seal but when we reinstall the lower unit it won't go completely together by hand and we have to pull up the final 1/4" maybe by using the bolts. This I was contributing to the gromet, althought I could be wrong about that. What I meant about not finding any pieces of the impeller was that I didn't find any other than in the pump housing. I wish I had tried to reconstruct the old impeller now, but it seemed to be all there. It was completely separated from the shaft though. We don't have access to compressed air right now so we can't easily blow out the passages but we used water while the foot was off and put the hose in the tube where the pump would go and got water out of each thermostat hole (one at a time). We then put the hose in the thermostat hole (again one at a time) and got water out of the hole to the pump and yes we were watching closely for something to come out with the water. I probably won't get a chance to work on it till next week, but the more I study and research this thing, the more I feel like my problem is the pottet valve. I'm going to look for this dam and seal it and rebuild or replace the valve. The timing could well be my problem too but it starts and runs so well I kind of put that out of my mind.

 

[jim dozier]

Re: Mercury switch and Overheating

In regards to the impeller direction, either rotate the driveshaft in the direction of engine rotation as you lower the housing onto the impeller or rotate the housing in the opposite direction of engine rotations as its is lowered onto the impeller. It is important to get them pointed in the right direction.

 

[mellowyellow]

Re: Mercury switch and Overheating

if the old impeller was falling apart there may<br />still be pieces of it floating around and partially<br />blocking the cooling system.<br />good luck,<br />M.Y.

 

[Jerry M.]

Re: Mercury switch and Overheating

Since it seems to overheat under power, have you checked the timing?<br /><br />Jerry

 

[pofarm]

Re: Mercury switch and Overheating

I'm having the same overheating problem with mine. I just got it out of the shop from a complete powerhead rebuild. At idle it doesn't overheat, but run it over 2000 rpm for a few minutes and the alarm sounds. I don't have a temp gauge but my water pressure gauge shows 15# @ idle and 30# @ 3000 rpm. It is back in the shop now to get checked out.

 

[Boatnut50]

Re: Mercury switch and Overheating

Profarm, if you read this, let me know what they tell you at the shop, it sounds like you have the same problem that I have.

 

[Dhadley]

Re: Mercury switch and Overheating

A shop here in town ran into that and a Merc guy told him to reduce the spring pressure on the poppet valve. They cut 2 rounds off the spring and it cured the problem. The Merc guy said its common to do that in "warm water climates". He suggested to cut one round and test and then cut another if needed but the guys at the shop were a bit impatient.<br /><br />I had another guy tell me his Merc dealer says there are two springs. One standard and a "tropical" spring. Dont know about that. Thats the only guy I ever heard mention that.<br /><br />Good luck!