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V6 overheating at low rpm

R

Rich Haverick

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"Hello, I have a 1990 Bayliner

"Hello, I have a 1990 Bayliner trophy hardtop with a 4.3 v/6 alpha one i/o mercruiser.
The temp. gauge indicates that it overheats at low rpm or idle on the lake or on the motor flusher.
When I rev the motor the temperature drops but soon after ir rises to 200 degrees .i shut it down then.
this is what is new as far as parts; impeller, water pump, thermostat, manifolds and exhaust risers. One thing that I noticed was that the elbow gasket (restrictor) has 2 slots and 2 holes.The one that I took off when changing the manifolds and risers had 4 slots. I modified the new gaskets like the other ones (4 slots) Would this make that much of a difference? Also, should I wait until the temp gauge reads a higher setting 200 or so and maybe give the t-stat more time to open? the t- sat is a 140 degree.It looks like water is flowing everywhere that it should but it still gets hot at idle or low rpm. It cools back down when I rev the motor to 3000 rpm or more. This has been a continuing problem and a frusrating one to say the least. The boat did come from a saltwater enviornment but I have gone through it and changed all parts that I thought that I should. The engine block doesn't seem to have any blockage and has been flushed out (just with a garden hose).
The motor has good compression in all cylinders and runs fine otherwise. Please help if you can. Thanks"
 
Is this a new problem after th

Is this a new problem after the maintenance or was all the maintenance done in efforts to correct this problem?

What did the old impeller look like? Was it missing any chunks? Were they recovered?
 
sounds like lack of flow. Must

sounds like lack of flow. Must be a restriction of some sort. how did the t stat housing look?
try a thermal temp gun to isolate hot spot.
Have you replaced the exhaust flappers?
 
This has been a problem all al

This has been a problem all along . The last time that the boat shop sais they corrected the problem was when they replaced the water circulating pump. I don't recall ever seeing myself the temperature gauge acting like it should.But last year they told me that when it stayed about 160 on the hose.. I checked the port exhaust shutter and it looked clean but should it hang up and stop wherever you turn it? I thought that they should move freely..I looked into the t-stat housing the best I could and it seems okay. I agree that it sounds like some sort of restriction but It isn't obvious to me.
 
"Yes, and the impeller is new

"Yes, and the impeller is new and working fine."
 
"I checked what hoses were hot

"I checked what hoses were hot and which ones weren't and it's like this; When at operating temp (or hotter) The raw water hose is cold going to the t-stat housing. The bypass hose that goes to the circulating pump is hot..The 2 middle hoses that goe to the ehaust elbows are cold, and the two rear hoses that go to the manifolds after the thermostat is open are (of course) hot. But take note; the hottest hose is the one going to the circulating pump on front of engine.( starboard side front.) Does this indicate a restricted t-stat housing? It seems to me that a hose that feeds into another only inches away can have that much of a difference in temperature.Thanks to all so far for your help"
 
"I think you need to replace t

"I think you need to replace the gaskets between the manifolds and the risers and leave them in their original configuration. It's affecting the flow of water leaving the t-stat housing to the manifolds and may be backing everything up. The slots in the gasket should be on the sides with the holes fore and aft. (If someone used the wrong gaskets in the past, it was probably in conjuction with removing the t-stat and leaving it out... not recommended.)

Other possibility I see is a blockage in the t-stat housing between the inlet water nozzle and the circ pump supply hose nozzle. There is a small passage there which allows inlet water to enter the engine's water stream.

Also, are you sure the t-stat is opening? Pull it out, drop in a pot of hot water on the stove and watch it... infant mortality with those things isn't all that uncommon."
 
"Thanks Troy, I'll try th

"Thanks Troy, I'll try that. Is there a way to visually see that port on the t-stat housing after removing it ?"
 
"I was working on copying a pi

"I was working on copying a picture out of the manual when I got distracted. Here you go:
<center><table border=1><tr><td>http://www.marineengine.com/discus/icons/attachment_icon.gif""" align=left alt=""application/octet-stream"">T-stat Housing (6 port)
[url=""][b]T-stat housing.htm[/b][/url] (9.6 k)</td></tr></table></center>"
 
"Should have a .pdf to begin w

"Should have a .pdf to begin with. Sorry for the delay.<center><table border=1><tr><td>http://www.marineengine.com/discus/icons/mime_pdf.gif""" align=left alt=""application/pdf"">
[url=""][b]T-stat housing.pdf[/b][/url] (41.3 k)</td></tr></table></center>"
 
"Well, I installed the proper

"Well, I installed the proper exhaust riser gaskets, Checked the t-stat in hot water(opening properly cleaned and checked the t-stat housing ,put things back together and still the overheating problem. I took a heat gun to all the hoses and the temps were as follows; inlet hose into t-sat, 64, outlet of t-stat housing to circ. pump 173, middle hoses to exhaust elbows 82 hoses from thermostat to engine manifolds 170 -+, and engine temp on manifold (bottom of t-stat housing 190. It's a sad day in my boating world, It looks like either my engine block is clogged up , or might it be a head gasket?(although I don't see any signs of that in the engine oil) Whatever the case, this powerplant is getting hot and my enthusiasm is dwindling.. I still don't think that the circ pump is pumping enough although it is new . It seems that the large diameter hose that feeds it should have more water going into it than it does.In any event, I am now looking for the next step into ending this torment..I have never been a quitter but I'm getting close. On a brighter note, thanks for the help and I would appreciate again more good advice.."
 
"It doesn't make a whole l

"It doesn't make a whole lot of sense that the water going to the exhaust elbows is significantly hotter than the inlet water. It should be a straight shot right through the t-stat housing. A few degrees, maybe, almost 20 seems a bit much.
.... an internal leak (corroded) in the t-stat housing? ...between the hot water coming out of the engine and the water supply to the elbows? This could also explain the reduced flow through the circ water supply hose if the water is short cycling out the elbows."
 
"Wanted to add some words of e

"Wanted to add some words of encouragment:
Hang in there Rich! You've done a great job eliminating likely possibilities and using the infra red thermometer for real time data was a great move. It's funny you called it a 'heat gun' at first which kinda scared me... typically use those to apply heat (like a Tim Taylor hair dryer). Lord knows we don't need any more of that right now. But once I saw the numbers, I knew what you were saying.

We'll get it figured out."
 
"Thanks Troy, I did notice tha

"Thanks Troy, I did notice that there is some corrosion in the t-stat housing where the o-ring sits . The small shoulder that the o -ring sits on (under the thermostat) has corrosion where the shoulder is gone in a small area about 1/4 in length. The 0-ring spans that spot but water might be going by.At this point I'd hate to buy this part (which isn't cheap by the way) and have the same outcome. p.s Thanks for the words of encourgement. It couldn't have come at a better time. And yes, I mean't an infared thermometer. I did get a chuckle out of that one.Lords knows I don't need to apply anymore Heat, that's for sure."
 
"Wait a minute, I just re-read

"Wait a minute, I just re-read the whole thread and picked up a something I thought was already checked.
If the raw water pump was not providing enough pressure at the inlet of t-stat housing, hot water coming out of the engine would back flow through the hole you just looked at, giving you exactly the indications you have.
There is something blocking flow from the raw water pump or the raw water pump has issues:
- Blockage at the power steering pump cooler in line from the raw water pump, behind the engine.
- Kinked or leaking supply hose at the transom. (The drive is in the full out (down) position right?)
- You said new impeller... who did the work? are you sure it's the right impeller, installed correctly (key in the keyway, water tubes lined up right, new o-rings, gaskets and seals, etc.)"
 
"I dropped the lower gearcase

"I dropped the lower gearcase myself and checked the impeller.Removed only the impeller and put it back in according to the shop manual. I was going to check the powersteering cooler when I had the manifolds off but I didn't. Can I start the motor cold and quickly check flow before and after this cooler?. You know Troy now that you mention it last year the mechanic That I (used to ) bring my boat to, put things back together and pinched the raw water line that feeds through the transom . I was on the lake and brought it right back (OVERHEATING) and while dropping it off i found the problem myself. That will be the first thing that I will check out.There is a real good chance that it is constricted in some way again. If we get this problem solved , I just might send you some lottery tickets."
 
"Rich:

"small shoulder

"Rich:

"small shoulder that the o -ring sits on (under the thermostat) has corrosion where the shoulder is gone in a small area about 1/4 in length"

Troy has you on the path to success. You really need to replace the T'stat housing assy. due to the heavy corrosion and damage it's caused.

This page will show the part numbers for both old and new designs.

http://www.mercruiserparts.com/selectDocs.asp?doc_nbr=17468

old design: 87290A4
new design: 16413A3"
 
"Hey Guys, I just checked the

"Hey Guys, I just checked the water feed hose that goes from the raw water pump through the transom and it is kinked in a v shape to the point that I am surprised that any water is going through it at all. I don't believe they even fixed it from last time this happened.It looks like a real bear to get at. This might be the answer.. Any suggestions on the best way to approach this? No wonder why it hasn't been cooling. I hope that this is it. Thanks Guy and Troy Happy 4th"
 
Can you see the screw clamps o

Can you see the screw clamps on the hose? Can you reach them w/a long screwdriver to remove them? Installing new hose; use soapy water on fittings and soak hose end in hot tap water to soften it so it will expand over fittings. Someone said once that you need to put a slight twist in it so it will work? Not sure. Use new SS clamps.
 
"That kink has gotta be it Ric

"That kink has gotta be it Rich... or at least 99% of the problem.

I think you're looking at pulling the drive off and removing at least the bell housing to get to the water hose between the bell housing and the transom plate.

Consider what Guy is saying about the T-stat housing too. That's not likely the source of your agony now, but you may be revisiting that sometime down the road."
 
"When you replace the linked w

"When you replace the linked water hose, make sure your replacement is not the flimsy type. Inspect the 3/4" hose personally before you buy to make sure it is high quality and fairly stiff or it will flatten out again. I bought a 3 foot length so if I screwed the cut I would have spare at the house.
The manual for my 4.3L said to cut hose at 13 & 7/8" length, but at that length it will flatten more than I liked. I cut my hose (Gates multi-ply) down to 13 & 1/2". It is long enough and water temps dropped about 5 to 10 degrees. It seems like a heavier flow of water, although I never actually measured the volume. I did not worry about any 'twist' as a trial manuver showed no problem."
 
"I'd go with the pre-cut m

"I'd go with the pre-cut molded hose (i.e., mercruiser part) as well... it's in the shape the engineers considered best for the application, and therfore less likely to kink."
 
"I removed the outdrive and at

"I removed the outdrive and attempted to remove the bellhousing until I ran into my present obstacle.. Not having the proper 12 point star bit to remove the pins. I will try to get one today, Maybe a grade 5 bolthead will work. I don't like doing things like that but at the present time this is is the only thing that is delaing me..
I did buy the proper mercruiser hose ( thick walled) And I think that I will trim it to 13 1/2 inches. I just wish that I could insert a spring into it like a radiator hose so it can't kink.
Thanks for all the great advice.It's all very helpfull"
 
"Here's the part number fo

"Here's the part number for the tool, if it helps:
Hinge Pin Tool (91-78310)

Don't put a spring in the hose... will only end up being a source of another problem later.

As far as preventing it from kinking again, you've taken the first, probably most important step... you got the right part.

For future care and feeding, keep the drive in the full down position whenever the boat is not actively being moved on the trailer. The boots will last much longer as well.

Speaking of the boots, unless they are absolutely perfect, you're going to replace them since you have the drive off anyway, right?"
 
"Yes, The boots are new from

"Yes, The boots are new from last year and are very rubbery and clean to the touch.. They look real good. I'll stay on top of things myself from now on. I do have another question though Troy, I usually fill the block with antifreeze and do the fogging for winterizing. Is it better to just drain everything instead . I would think that there would still be condensation inside the motor ."
 
"Filling with anitfreeze is be

"Filling with anitfreeze is best. Use the enviromentally safe (non-ethylene glycol based) stuff.
I use my boat year-round now, but when I used to winterize, I used a 5 gallon jug with the anti-freeze in it, connected to the rabbit ears. I would start the engine and as soon as I saw the pink stuff coming out of the idle ports, I would let it go another couple of seconds, then shoot the fogging oil into the carb."
 
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