Engine Overheating

[kwoolard]

I have an 1974 888/302 Mercruiser. A few months ago I removed the exhaust manifolds and risers, cleaned them in muriatic acid and replaced all hoses and the thermostat housing. I have center riser manifolds, however, the engine was originally equipped with the log style manifolds. Therefore, the cooling lines are still set up like the old style (3 lines per side - one to the back of the manifold - one to the front - and one to the riser). The boat had set up for about three years, that is what drove me to remove, clean, or replace the cooling parts. <br /><br />The motor is equipped with an un-orginal belt driven water pump. While cruising with the belt driven water pump supplying the water the engine runs about 180-190, which I feel is too hot. I have been told that for raw water cooled engines the temp should be around 165 max. I have a new 140 degree thermostat installed. Well, just out of curiosity, I decided to remove the belt driven water pump and use the sterndrive pump which is orginally the way it was set up. Ran the motor this past weekend and as soon as I got on plane the engine shot up to boiling point (212) !!!!<br /><br />What could be wrong? Water passage ways blocked off? Bad circulating pump? I know there is a little rust in the engine block, because when I run the motor on muffs in the yard it initially runs a little rusty water. What do I need to do?

 

[cuzner]

Re: Engine Overheating

Have you ever changed your impeller?<br /><br /> Jim

 

[yoced]

Re: Engine Overheating

Kwoolard - <br />You are running too hot, or your gauge is off.<br /><br />My first suggestion would be to talk to Osco as they are the source of half your cooling system there. Probably no one knows as much about this log to center-rise conversion as John Morgan at Osco. He may be a bit hard to talk to sometimes ("like pulling teeth to get info", I think Crazy Charlie said)) but he really does know his older conversion stuff.<br /><br />My second suggestion is that you are perhaps not routing the hoses quite right - I think that you may have your two manifold hoses front and rear, when what you want I think is in at rear and out front high. It is possible, making all these changes that you may have mixed up your hoses to your thermostat housing. <br /><br />You also said you changed T-Stat housing as well as the element? Changed to WHAT? This could severely affect the flow plan here.<br /><br />On a more 'duh' note, you could simply have your thermostat upside-down.<br /><br />I also question how you took the pump driven off the system? If you have a leg pump pumping into the T leading to the belt driven pump's pickup, then you take the RWP off the system, isn't your leg pump merely pumping water right down the tube to the raw-water pickup? Or did you install a check-valve? Or reroute?<br /><br />Also, do we know that the outdrive pump was ever working? Perhaps it had been disabled years ago?<br /><br />Again, I would strongly suggest calling Osco and talking to John. He cut his teeth on this conversion stuff. If calling him is hard, we can get you in touch with him via e-mail I think.

 

[kwoolard]

Re: Engine Overheating

Both the outdrive and the raw water pump impellers are new. I didn't explain how I set up the plumbing with both the raw water pump and stern drive pump. Initially, I ran the stern drive water pump right back out the transom, ie. it was only cooling the outdrive. When I disconnected the raw water pump I merely just removed the drive belt, I then routed the stern drive supply line which was initially routed back out the transom, to the thermostat housing inlet.<br /><br />I am 100% sure I have the water lines plumbed right. When referring to either the front of the manifold or the rear of the manifold I am looking from the end opposite the flywheel, the circulating pump side. Therefore, water enters the rear of the manifold(the flywheel end)and exits the front of the manifold(the circulating pump end). <br /><br />When I said I changed the T-Stat housing, I meant replaced with the exact same model. I don't think that with this particular T-Stat housing it is possible to install the thermostat upside-down.....I could be wrong. But if you are looking at the top of the T-Stat housing the spring on the thermostat is on the bottom(the same side as the raw water inlet line). Is that right??? Besides talking to OSCO about the manifold and riser set, what else could be wrong??

 

[yoced]

Re: Engine Overheating

Your element is in correctly, assuming (and it sounds like it) that it is placed between the housing body and the cover, rather than under the thermostat housing.<br /><br />I would still like to see you take the water out of the manifold front top rather than front low. With the block off gasket you have under the riser there is no way for the manifold to FILL with water. The way you have it plumbed (if I am understanding properly), you are merely pumping water through the manifold one end to the other. The top inches are now filled with STEAM, which will be a Very Bad Thing.

 

[kwoolard]

Re: Engine Overheating

Yoced, my water lines are run exactly as you say....in the manifold at the rear(flywheel end...low) and out at the front(circulating pump end.....high). Therefore, the manifold is filling completely with water.<br /><br />As far as the thermostat it is sitting in the thermostat housing. I wish I had a picture but assuming all thermostats looke alike....the thermostat has a pointed top and a spring on the bottom. The pointed top is pointing to the thermostat housing cover, while the end with the spring is in the thermostat housing(where the raw water enters). <br /><br />Hope that makes sense....I will try and run the engine without the thermostat. If doing so makes the engine run exceptionally cooler then I probably can rule out any blockages and assume the thermostat is defective or I have it in wrong.

 

[kwoolard]

Re: Engine Overheating

One more question I would like to ask....it may be an stupid one but I'll ask anyway.<br /><br />I am not sure if you are familiar with the thermostat housing on the 888 models but the firs thing that the cool water coming in from the lake sees is the thermostat. So....how will the thermostat ever open if it is constantly being cooled with 75 degree water?????

 

[yoced]

Re: Engine Overheating

I think you have the thermostat in correctly.<br /><br />The thermostat will open when the engine water is warm enough. While it looks like the Cold water is run directly into the sensing element, that is not quite the case. The Cold water is being diverted toward the Circ pump, which is busy pulling it into the engine. Some of the cold water is allowed to bleed into the Risers so that some water is going through the risers before the element opens. Most of the water underneath the thermostat element is coming from the manifolds, so it is a bit hotter. This hot water then mixes with the incoming cold water on the way to the Circ pump. When the element opens then most of the hot water from the manifolds goes overboard to the risers while most of the colder water from the raw water pump goes down to the circulator pump.<br /><br />Here is a picture diagram of the circulation:<br /><br />

<br /><br />So far it seems you have it all correct - so you may in fact have an unrelated heating problem (bad spark advance, for instance). One more common problem would be to have the manifold in and out mixed up on the T-stat housing itself. This can really screw up the water flow.

 

[kwoolard]

Re: Engine Overheating

Yoced, thanks for all your help. My water lines are set up as depicted in the diagram you supplied above. This afternoon I ran the engine in the yard on the muffs and used a laser temperature gun to check the temperature of all hoses in the cooling circuit. Before doing so as the engine was running I disconnected each hose individually to double check the direction of flow through each hose, and all checked good.<br /><br />Using the laser temp gun I checked each hose. With the engine running at approx 160 degrees the temperature of each hose read as follows:<br /><br />Water in from sterndrive: 80 <br />From bottom rear of T-Stat housing to rear of manifold: 145 (Both hoses reading about the same)<br />From front/top of manifold to rear of top cap on T-Stat housing: 155 (Both hoses reading about the same)<br />From front of top cap out to risers: 110 (Both hoses reading about the same)<br />Water from bottom of T-Stat housing to circulating pump: 135<br /><br />I know this isn't the exact temperature of the water inside the hose but it gives me an idea of what is running through them.<br /><br />From these numbers it is obvious that the water leaving the exhaust manifolds is not exiting the cooling circuit and is just mixing almost entirely with the incoming water from the sterndrive. And the water that is exiting through the risers and out the exhaust is primarily the water coming in from the sterndrive mixed in with a little from the exhaust manifolds.<br /><br />Out of curiosity I disconnected one of the hoses coming from the front/top of the manifold while the engine was running, which ideally when the T-stat is open should be exiting out the risers. When I did this, the engine temperature dropped to 145. This tells me that the incoming water from the sterndrive and the water coming from the exhaust manifolds are fighting against one another to exit through the thermostat. And the winner is the water coming from the sterndrive, while the water from the exhaust manifold is being recirculated. <br /><br />To me it seems like a poor design of the thermostat housing, but obviously it had to work for them to use this style. I am confused now as to what to do. I know that my thermostat is opening because I removed it when the engine was still reading about 160. It was opened when I removed it and it closed only a minute after I pulled it out.<br /><br />The only thing that I haven't checked is spark advance and the circulating pump. But while running at idle speed, would spark advance have that much effect on the cooling?? And if the circulating pump wasn't working would I be getting the flow through the engine and hoses with just the pressure from the sterndrive pump??<br /><br />I am baffled, what should I do??

 

[yoced]

Re: Engine Overheating

Sounds to me like you have everything correctly plumbed. <br /><br />I would hook the belt-driven pump back up and take your tests *while under load* if possible. But it doesn't really matter. If you do this, aim your thermometer at the metal parts rather than at the hoses. You may get a better reading.<br /><br />Also consider installing clear plastic hoses in spots to monitor flow.<br /><br />Are you SURE about your element temp? Is it Possible you have a 160* thermostat? Or an automotive 180*? You know your element opens, but not at what temp, right? Take it out and warm it up on the kitchen stove.<br /><br />Also, I think that you could simply have clogged risers. As I mentioned earlier this spring, the 3" risers have a tendency to clog up. This will slow down the whole water flow system.<br /><br />Beyond that, consider that you may have a perfectly operating coolant system which may be overcome by an overheating engine. Old engines like yours tend to be hotter. Carbon, timing, Champion spark plugs, clogged water passages; on and on. Maybe it is time to tackle the engine as the problem not the cooling system.<br /><br />If you get the bug to do it, consider moving to a one-hose system so that your water goes into the manifold and right out the riser, rather than throwing it back into the engine. You will run much cooler that way. I would have done that when I changed T-Stat housings, but oh well. Before you think of that, however, you want to be fairly sure that it is not an engine problem causing you condition. <br /><br />Above all else we want to treat the disease rather than the symptoms, if we can.

 

[kwoolard]

Re: Engine Overheating

I have checked the timing, about 10 deg btdc. The spark plugs are brand new, champion, and the wires are not crossed. The engine doesn't miss at all, runs great at idle and when cruising. There is no spark knocking under load either. The engine was rebuilt about 3 years ago, so I don't think that carbon build up would be the problem. Are champion spark plugs not the preferred plugs to use?? Could running a lean mixture cause this overheating?? If so, how lean is too lean?? To start the engine I have to move the throttle to squirt a little fuel down the throat and it cranks right up and idles about 700rpm. <br /><br />Anything else it could be? I would like to just run the hose from the exhaust manifold out the boat somehow keeping it from recirculating that hot water. But, before I do I would like to solve any causes of this overheating problem. Also, wouldn't running the hose from the exhaust manifold back overboard be as close to running without a thermostat installed?

 

[yoced]

Re: Engine Overheating

About the Champions: You have an older engine, and you are probably picking a spec out of an older book. When you go to the store to buy a similar plug they will cross reference it for you and give you a newer plug - all well and good if they cross ref by heat range. I have been told that several years ago the folks at Champion did something to change the numbering system and that direct numerical cross references will give you a plug with the wrong heat range. Something to think about, but I am not real sure about the story I have been given - and I have no direct knowledge.<br /><br />Lean will cause heat. If you can find a local mechanic who can read your plugs that would be nice.<br /><br />At this point could I suggest you go for the cheap trick and rig a different sender and temp guage up and go for a cruise. You may find you are not overheating at all?<br /><br />Beyond that you are out of my league for long distance diagnosis. Perhaps someone lurking on this post might jump in with some bright ideas?<br /><br />The one hose system I am talking about would mean changing your thermostat to a different style: majority of water is recirculated under the thermostat element until it gets warm enough to open then goes straight through the exhaust manifold and riser then overboard. You just would not be putting the hottest water (from the manifolds) back into the engine.<br /><br />Good Luck!

 

[steve n carol]

Re: Engine Overheating

Kwoolard, I have a 225s with a similar engine config. mine runs on the hot side as well with new log style mans and 3" risers. The stainless steel blockoff plate between the logs and the risers is to retain the water intil wamm up and operating temp is achieved. then w/the T-stat open, it will began circulating hot water through the risers. I don't like this completly eithier, and have thought about modifing them. But I don't want steam in the logs.??? I have an aftermarket TORKER intake manifold on this. I think there is a bypass hole,(cooling), missing from this manifold. The gasket has a hole for it as well as the mercruiser T-stat housing. I am glad you have looked into this problem, as I am learning from this. there are others here with this problem. 2 with the 888. I'll be monitoring....sl

 

[kwoolard]

Re: Engine Overheating

As I have stated before, I am not fond of the fact that the cold water coming in is mixing with the hot water coming from the exhaust manifolds. <br /><br />So, I figured that I could solve this by removing the thermostat entirely. Well, this weekend I tried that and ran the motor on the flushers and the "NEW" temp gauge did not read above 140 degrees. Well, took it out for a ride and cruising at 3400 RPM the gauge slowly crept up to 175 degrees and held there. At these high rpms should the temp get that high without a thermostat? Or is it due to the design of the T-Stat housing, where it is impossible for all of the hot water to exit? With it still running higher than its normal operating temperature without a thermostat, tells me there is still something wrong.

 

[steve n carol]

Re: Engine Overheating

There are lots that I do not know. I do know that a thermostat is necessary to maintain an even temperature, hot or cool, an even temp.<br />removing the T-stat or exiting a forward manifold hose overboard, while it sounds like a good idea, (run coolest), the theory is to maintain an constant, optimum and efficient operating temperature, thus the T-stat. By not employing the T-stat, and in combination, not re-routing hot water back into the engine, to mix with the cool lake/sea water, the engine will be running at uneven temps within the block, at a cooler, un-efficient temp. Today I found that mine had a piece of 3/8" plastic tubing, about 2" long, (broken on each end), lodged in the fresh water supply line that comes in from the outdrive. (I have the outdrive off for seals). and backflushed the supply line and this piece "popped" out. don't know what it's function is or where it came from. but now it runs steady @ 170*. I don't possess an infered temp gun, instead I opened a petcock here and there, also on the block and checked the temps with a thermomoter. (medium rare/poultry) the gauage in the dash is right on....let me know know how you do...sl

 

[steve n carol]

Re: Engine Overheating

Well, took it out for a ride and cruising at 3400 RPM the gauge slowly crept up to 175 degrees and held there. At these high rpms should the temp get that high without a thermostat?

by the way, I assume that the temp of the water that you were running on was warm. consider....what the tempertature results would be in cold water??..sl

 

[yoced]

Re: Engine Overheating

Remember, in your design the thermostat still comes after the manifolds. Running without thermostat may keep the engine from coming up to temperature for a little longer, but it will do nothing to keep the temp down once it has come up. This is especially true due to the design of this housing. If you have any water flow problems in your block then more of your COLD water will go overboard than your hot.<br /><br />Perhaps more importantly, you thermostat cannot let any more water overboard than the riser will let out. In this case an open thermostat element probably allows more water flow than the riser or hoses can deal with anyway. You only have 3/4" of internal flow per side. Either a wide open element or just plain not there, the feed to these just the same. And a blocked riser or hose to the risers will lower that further.

 

[kwoolard]

Re: Engine Overheating

Thanks for all the help! An un-mentioned, yet maybe important factor, is my WOT rpm's. The boat is a 23' 1974 Sportcraft hull, and again, being pushed by a 302 888 Mercruiser. When at WOT the engine is only turning 3500 rpm's. According to my manual, the WOT range is 3800-4200. I am definitely not reaching that, and I only have one other rider in the boat along with less than half a tank of fuel. I am using a 19 inch prop, so if figured that I may have to high a pitch of prop, which my be causing the engine to lug and thus cause my overheating problem. I dunno?? <br /><br />Today I borrowed a 14.5" x 17" pitch prop. I will try this out soon along with my belt driven raw water pump, and the thermostat installed to see if that will make a difference in both the WOT obtained and my operating temperature.

 

[kwoolard]

Re: Engine Overheating

A couple more things I would like to ask. With the center riser set up and three hose system that I have, is it normal to have some exhaust back pressure?? The reason I ask is because when I disconnected the hose from the top of the thermostat housing to the riser I could feel some exhaust coming back out the hose fitting. Is this normal to have a little backpressure?<br /><br />Also, does anyone know the type of spark plugs I should be using for this motor? I have been told that the wrong heat range on a spark plug could cause the engine to overheat.<br /><br />One more thing, the engine was rebuilt about 5 years ago and I noticed that the firing order that is stamped on my intake does not match any firing order that is listed in my manual for an 888. I am not sure if it was rebuilt as a street 302. Having said that, are there different plugs that should be used for an automotive 302 and a marine 302/888?

 

[steve n carol]

Re: Engine Overheating

merc book says;<br />"firing order-L/H rotation, 13726548<br /> " " -R/H rotation, 18456273<br />spark plug type-5/8" hex- 14mm plug- AC-MR43-T, except as follows: 888(only), serial No. 4615130-thru-461627 and above 4616267, use AC-CR83-T.<br />13/16" hex- 18mm plug- AC-CR83-T"<br /><br />thats what the book says, in my mind, I don't see how the #'s work, cause the second number is less than the first...? <br /><br />the h2o hose will pulsate. this is/could be caused from normal internal hose pressures, UNLESS your'e getting exhaust gasses exiting thru this hose. <br /><br />my book does not show center risers on the 888 exhaust system....sl