502 carb raw/open cooling system and thermostat help
 

I have a new-to-me Mercruiser 502 (carbureted) engine in my boat and it uses an open/raw water cooling system. When I took it out for the first time, I never saw movement on the water temp gauge. Pulled wire off the sender and grounded it, gauge pegged, so I know its good. I then discovered what I assume is why, there is no thermostat in the housing. Water I was boating in was probably in the 50s so that explains it I guess.

I assume I should put a 140 degree marine thermostat in there, right? My question/concern is why previous owners ran without a thermostat. Maybe there is a valid reason and I need to leave it out?

I think I understand how the cooling system is set up in my boat, so maybe this helps get some insight. Belt driven raw water pump sucks in cool water and pumps it to an oil cooler. From there it goes into the front of the block on either side of timing cover. Through the engine, out the t-stat housing, and into the exhaust.

If I install a thermostat, wont the exhaust get too hot since no water would be flowing through it until the t-stat opens?

Here the best photo I currently have, in case it helps.
https://cimg0.ibsrv.net/gimg/www.off...e1b293c504.jpg

160 thermostat. Post a pic of your thermostat housing and water lings and your exhaust manifolds. Most have a type thermostat housing that has multiple outlets and water is bypassed all the time to the exhaust manifolds.

I think I found my answer. Engine has a water crossover kit but there is no bypass. The ports in the crossover tube and tstat housing for the bypass are plugged, so it has to run without a tstat or the exhaust wont have water flow until it opens. If I install a tstat, I need to remove the plugs and install a bypass so exhaust sees water at all times. Now, the question is, do I really need a tstat.

Don’t put a tstat in your system.

Thank you. Any insight as to why I should leave it versus installing a tstat and a bypass?

I’m the wrong person to ask.
‘cause
for most engines i like the merc recirculating pump and tstst housing set up. Usually provides better temp (water and oil) and psi (water) control.

If you need to build some heat, you can add a restrictor in place of the thermostat. Moroso makes a few different sizes - you'll need to experiment to get the right balance between block and exhaust temp.

Thank you for the lead on the Moroso restrictors! $17 on amazon for a pack of 3 with diff size holes to find the right one. I will give it a go!

If that picture is representative of how your boat is configured you can not run a thermostat at all. Don't even think about a resistor plate. You will not get enough cool water to the exhaust. You will overheat your exhaust.

Engineers have designed your motor with a thermostat housing in mind to guess what,,,, house a thermostat. You want the motor to get to temp to achieve best performance. You can read on the internet the negatives of running a motor cold. There are no positives.

There are a number of thermostat housings on the market with internal bypass to insure the exhaust get water as the block heats up.. It will not work with that circulating pump elimination crossover pipe in the picture for what I can tell..

The right way would be to change out the housing but the restrictor should work. The Merc water pump is capable of moving a ton of water AND the flow of water is already restricted in the factory system anyway - shouldn't be a problem to experiment.

Actually, the right way would be put a circulating pump and bypass-style thermostat housing, but that's not the subject here (at least right now).

Now Ive read that experimenting with a restrictor plate could help get some heat into the engine (a good thing) but may create too much pressure in the block....and, as said above, will also not get enough water to the exhaust.

If you look at the tubing used to move water around versus what comes out of the pump, there shouldn't be an issue. Water pressure in the block shouldn't be an issue either - nothing is closed off and water is going to free flow. You're just slowing it down some to let some heat transfer occur. So long as the risers do not get too hot you'll be fine. You want the water temp at the thru-hulls to be about 140.

Would drilling a couple of 1/8 holes in the thermostat work?

Thanks for all the input. So here are my options/considerations. The more I read, the more conflicted I get.

1) Add a bypass to my crossover system and put a 140 thermostat in there with 2-3 small holes drilled into it. In this scenario, is there any concern with the cold bypass water that sits on the top side of the closed thermostat causing erratic temp issues? Basically counteracting the hot water on the bottom side of the thermostat that is telling it to open? A kit like this is what Im thinking:
https://www.hardin-marine.com/p-1335...th-bypass.aspx

2) Play around with the restrictor plates to try to get more heat transfer. If I dont need to worry about too much block pressure or not enough water flow to the exhaust, this sounds like the way for me to go.

Not trying to PooP on what you have but it is plumed all wrong. I am not even going to touch on the circulating pump thats a another issue. Most 454/502 motors have a thermostat housing with 4 outputs. 1 to each manifold and 1 to each riser. you have plugs in your risers. . This is he way it should look and have a 160 thermostat in it.

https://www.ebay.com/itm/40425773080...BoCvbAQAvD_BwE

If you look real close, it looks like his risers have wet joint.

they have a wet joint that’s plugged off with a blue plug

If your risers are staying cool, you can make what you have work - you cannot run a thermostat unless you have a bypass going from the crossover to the thermostat. The restrictors will work without having to change what you have.

If you want to run a thermostat,... your crossover has a bypass fitting and you have a plug on top of the t-stat housing. Put fittings in those two and connect them with a 3/4 to 1-inch hose. Water will flow through the path of least resistance. Once the thermostat opens, water will flow more from the engine side vs the bypass. Monitoring water pressure is a good idea (running a crossover or not).

Helpful information:

https://www.offshoreonly.com/forums/...er-bypass.html

https://www.cpperformance.com/p-1335...th-bypass.aspx

TZ

Risers are staying very cool.

Im going to put in a 140 thermostat, with relief holes in it, and either find fittings and a hose to make a bypass with what I have...or just get that Hardin/CPP kit you linked. I like that their kit has a larger 1" bypass hose where if I got fittings to make a bypass of what I currently have, it would be smaller and more restrictive than their 1" bypass.

The kit is a good plan.

I have used the Hardin Marine Tstat housing and it works well. If you look at the cut away of the housing, there is a divider inside that helps keep the cold water from affecting the Tstat function. I was skeptical at first but my water temps are rock solid at 150 degrees and the Tstat begins to open as quickly as it ever has.

Troy

Thats great to hear. I have the Hardin kit on the way to me now. Will try to install next week and give it a try. Thanks again everyone.

OK I just installed that nice Hardin kit, so now I have a crossover WITH a bypass, so the exhaust always has water even when thermostat is closed. I used the 140 marine thermostat that came with the kit, which already has a hole in it for steam.

Took it for a spin today. Water I was boating in was 65-70 degrees. While trying to warm it up and idling through the no wake zone, temp gauge never rose off 100. I then went to speed, about 40-50MPH and drove for a while. Temp gauge never rose off 100. As mentioned before, I dont think the gauge is bad because when I took the wire off the sender and grounded it, the gauge pegged. After driving for a while, I stopped and shut down the engine. Risers are staying cool to the touch. Crossover tube was cool-ish also, except the starboard side was a bit warmer than the port for some reason.

Anyway, I let it sit for a while with no water flow so the engine could absorb some of the heat. I then keyed my ignition on to check the temp gauge and it finally moved, granted it did not show more than 110-120. Again, this was after I shut the engine off and let it absorb some heat. Engine on, it dropped back to 100 and stayed there.

Think my temp sender at the intake manifold could be bad?
Im also going to remove the thermostat and heat it in water to be sure its opening at 140 and working correctly. But, I assume it is opening because if it wasnt then my temp gauge should've shot up...unless the sender is bad of course.

I've got the same Hardin set up. I have not had the boat out since but on muffs both motors will register almost 140 degrees at 3lbs water pressure at an idle.

You definitely have a sender or gauge issue. Unless your dumping water out of the block some other way it's got to come to temp at an idle or running.

His system is plumbed correctly for what it is. Its a crossover system with no bypass and that is how they are plumbed.
The 4 hose type of Tstat housing has not been used since the late 90's. The newer ones have 2 hoses that go to lower exhaust manifold inlets and the Tstat housing has an internal bypass.
ALL Merc black carbed raw water cooled engines used 140* T stats as do all Merc Racing Carb engines and even some of the EFI ones.
The Merc black 454/502mpi's that use the 2 hose Tstat housing use 160* Tstats

Like SB, I'm not a fan of crossovers. I would have gone back to circ pump and 2 hose Tstat housing with a 140* Tstat

Thanks for the replies, everyone. And, I agree with this, thanks for confirming. It must indeed be a sender or gauge issue, as the water inside block has to be coming up to temp now that I have a thermostat in there. The water has nowhere else to go until the tstat opens, so it must be getting hot.

My existing sender was installed with loctite by a previous owner. Going to take a heat gun to warm it up and unscrew. Since Im going to put a heat gun to it anyway, I might as well heat it up to 180 or so and then key my ignition on to see if the temp gauge reflects accurately.

From everything I've read, if I ground the sender wire and the temp gauge pegs (like it did), that should mean the gauge is OK, right? This is reason Im thinking sender issue.

have you tried grounding the body of the sender, with all that goop on the threads it might not be making contact, I didnt read the whole thread

I have not tried grounding the body of the sender. Is it as simple as using a wire and touching one end to the sender body and the other end to ground? And that should peg the gauge also right?

If it doesnt peg then bad sender?
If it does peg, then the sender just isnt making good contact to ground?

leave it hooked up the way it is and run a ground wire to it and run the engine up to temp and see if it moves

Yes.

no.
it just makes sure your sender doesn’t have bad ground thru the threads.

Seems like I've pinned it down to a bad temp sender. Went out to the boat today, put a heat gun on the sender for a while to get it hot, temp gauge never moved. But, at least it heated the thread sealant up enough that I was finally able to remove it. It was in there good. Looks super crusty, right?!

Anyway, the new sender I previously bought is apparently the wrong one, too small to thread in. I connected the wire to it anyway and grounded the body, then put the heat gun to it and the gauge moved appropriately, which seems to confirm my original sender is bad.

Current (bad) sender appears to be a Datcon brand part # 02020-00. For the life of me I cannot an equivalent cross reference for this with Autozone, Napa or O'Reilly, nor online. Anyone ever run into this? Autozone guy I spoke with THINKS their TU201 will work but they dont have it in stock and there are no specs online. Maybe Im stuck buying another Datcon online and waiting for it in the mail. It seems the type I need is a 100-240F with 1/2" NPT threads


https://cimg0.ibsrv.net/gimg/www.off...56ff4c2ed5.jpg

Don't know what gauge you have, so let's assume you have Gaffrig water temp gauge with a range of 0 to 260. You have to contact Gaffrig and get the sender to match. You also need to tell them what size NPT thread you want.

Your done.

Im not sure which brand gauge either, as it has no name on it, at least not on the front side. Id have to take apart more of the boat to get to the back of the gauge and look for a brand name. Only my tach and speedo show the brand (Faria), but the rest of the gauges have no name. I suppose I better just order the same Datcon sender that was in it then.

That's odd. Typically manufacture's want the advertisement. There may very well be something on the back.
​Did it ever work? If so and you can get that sender online I would start there. If not I would start with a sender designed to operate in that heat range.

Im sure it worked at some point. The boat is a 1991 model. But, I just acquired it a couple months ago and it has not worked since Ive had it. I ordered the same Datcon sender that was in it but wont have it for a week or so. In the meantime, I am going to pick up the Autozone sender # TU201 tomorrow, then go hook the wire up, ground the body, and put the heat gun on it to see what happens. Hoping I can compare what my dash gauge reads to what the temp of the sender is after applying some heat. I have an IR/laser temp gun to check this. Thanks for the replies!

​​​​​​ You really just care about overheating. AutoZone should be able to tell you the heat range of that sender. That's what you want. One size does not fit all.

Got the TU201 from Autozone but its too small. They also couldnt tell me the heat range, very stupid. Anyway, guess Ill wait for the Datcon since I know its the right size and the range is listed as 100-240, which also matches the range on my temp gauge.

That's crazy, they are just being lazy. With a little effort they could find it out. Did you try NAPA? It was probably 1/8 pipe. They make adapters to step it up to work with your intake.