Anyone with overheating knowledge

"i have a 3.7 mercruiser, clos

"i have a 3.7 mercruiser, closed cooling system. motor is rebuilt and head rebuilt and trued up, etc. i was having trouble keeping coolant in cause it was blowing the riser gasket's but i finally got one to seal up good. so then it still had alot of pressure in the cooling system and was still pushing coolant out of the overflow tank, and would slowly push all of it out of the engine. so i took the thermostat out thinking it was stuck. went to the lake without a t'stat in it and still, ton's of pressure in the cooling system, only ran two small circle's around the landing area and it had pushed all the coolant out of the cap, thru a small hose to the overflow and out of the overflow tank. what is causing this pressure, could it be a head gasket??? but no coolant in the oil, and good oil pressure. i will check the compression tonight and try to post back soon, would a head gasket cause this or a cracked head??? also how do you pressure test the cooling system and how do you do a leak down test on the cylinder's i have a compression tester but don't know how it would do the leak down test>? any help is great and hope to get it fixed soon. thanks"

"Its not your t stat. Almost c

"Its not your t stat. Almost certainly a head gasket or a cracked head in my view. That engine uses an iron ford 460 head so i would guess head gasket first. Blown head gaskets don't always mean water in the oil or oil in the water, btw. report back on the compression test. It may not show anything. Think it's time to pull the head. Go back to the builder if the work is under warrany."

"Dave D.;

Call one of the d

"Dave D.;

Call one of the discount autoparts stores and ask if they have a radiator pressure tester you can use. If they do, tow the boat to it and attach the pressure tester cap in place of your boat's pressure cap. Make sure all your hose clamps are tight. Pump the gauge up to 16 lbs. or what the cap is listed at and see if it will hold pressure. If it holds steady for a few minutes without dropping at all, your cooling system and engine gaskets holding in the coolant are not leaking coolant. This test would seem to preclude that the engine water jacket is not leaking. If these tests are ok, see next one.

If they also have an exhaust gases analyzer, stick the probe into the coolant overflow tank and see if it detects exhaust gases. If it does, you have exhaust leaking into the water jacket of the block or head somehow. Could be a gasket, cracked head or block or a warped head.

No exhaust gases in the coolant? Do the next.

Before pulling the head, perform the tests D. Hayes suggested. If they are all good, it comes down to a fast overheating problem or a blowby gases pressure problem which should be found in the leakdown test.

Is the engine circulating pump in GOOD condition and is the belt turning the pump? The coolant has to be moving sufficiently enough thru the entire system or you can have a flash boil problem which creates pressure in the system.

Are you sure the heat exchanger is functioning properly and the raw water pump is providing the necessary flow of water?

Guy"

"Don't do the head, it&#39

"Don't do the head, it's the riser gasket surfaces. My 165 was doing the same thing, just use "JB Weld" (in Canada known as farmer weld)epoxy and file flat or buy a new exhaust riser. The former worked for me @ 6.95 for 2 tubes and hasn't O/H since.....Terry"

"David:

"but no coolant

"David:

"but no coolant in the oil" How did you verify it? Drain small amount of oil from oil pan?

If you run it on a hose in the driveway and remove the overflow cap, do you get bubbles or gurgling right away? Bubbles could be an indication of an exhaust gasket to water jacket leak. When coolant starts coming out, is it HOT? What does the temp. gauge read? Check the circulating pump belt tension and is it slipping or even turning the pump?
Guy"

"ok, not the riser, it's n

"ok, not the riser, it's new and got a great seal. the compression test shows cylinder's 1,2,and 3 at 160 each with maybe a pound plus or minus between the three... the fourth cylinder is at 125 psi. so i think maybe a head gasket, hope it's not a cracked head. with all four plugs out and the starter turning the motor the number four cylinder is like a choo choo train puffing white smoke/water vapor mix, is there any may to tell if it is a head gasket or cracked head while it is together or must the head be sent off for an x ray to tell..."

"Either way it's time to t

"Either way it's time to take it apart. Fog that cylinder with plenty of oil so you don't get surface rust on the cylinder walls in the mean time. Take the head off, examine the gasket carefully for signs of breached combustion chamber seal. As a precaution, send the head over to any good machine shop for evaluation."

yeah i was thinking about send

yeah i was thinking about sending the head off to get checked just to be safe... what does this normally cost to do???

"i don't remember, but it

"i don't remember, but it obviously depends on the work that needs to get done. Not expensive to check. I would view it as "you don't have a choice"... unless you are good at reading head gaskets and clearly see the issue."

"ok, head gasket what shot. se

"ok, head gasket what shot. seemed like a cheap gasket, it was all apart and had hole's in it where gasket material was., also the steel rings on number four were seperated and warped some. so a new gasket went on and it's back together. so far it's good, i got 175 psi on all cylinder's and oil and water seem to be ok, i did run it and change it twice before the good oil went in with a new filter each time to get all water out of the oil system. i will keep ya posted but hope all is finally good, also i believe that to be the cause of the riser gasket's that kept blowing, pressure was getting in coolant system and blowing them, just a thought..."