cooling issue after rebuild

[Sangster21]

I have just re installed a fresh rebuilt I6 250cu in mercruiser with MC 1 drive, vintage 1976. Got it timed and idle adjustment with a small hitch on the way. We were working on the idle adjustment, when we manually opened the throttle, it got stuck in an open position ( may not have been wide open) and raced the engine. I shut the ign. off quickly and sorted out the throttle linkage, turn out I had the idle cam installed incorrectly and caused the throttle to jam. We were on the muffs at the time. Went to sea trial today and engine ran hot... the thermostat opened, temp dropped than quickly started to to rise. I shut it off. The engine has a San Juan engineering cooling system, with engine coolant circulating through block and the manifold. Sea water goes from leg to heat exchanger and on to cool exhaust elbow and out. Any way, I felt the hose from the transom and it was hot!!?? What the hell? I pulled hose from transom plate (from leg) to heat exchanger and turned over engine to check for sea water flow..... there was exhaust gas and no water!! I am thinking the high throttle toasted my leg water pump impeller when it was on the muffs. Any one have some thoughts for me? The boat is on trailer, so relatively easy to drain gear oil and puller lower off to inspect the water pump.
Alan

 

[Bondo]

Re: cooling issue after rebuild

The boat is on trailer, so relatively easy to drain gear oil and puller lower off to inspect the water pump.

Ayuh,.... That's the obvious place to start,....

 

[Scott Danforth]

Re: cooling issue after rebuild

in addition to the raw water pump, you may want to check the glycol side of the heat exchanger. did you re-check the level after your initial fill? many times you have to re-add fluid as the system burbs

 

[Sangster21]

Re: cooling issue after rebuild

I drained the leg oil.... nice and green - good. Separated the lower from upper leg .... and we find a massacred impeller. It seems that a high RPM blip on the muffs is a very bad thing indeed. I will take a flashlight and look up into the upper housing to see if the water pocket up there looks overheated.... anything else I should look for while I have it apart? I will definately replace the entire water pump, housing and all - it is pretty toasted. I wonder if I should look at getting better quality muffs, the two I have (one for kicker, one for main) only feed water from one side of the muffs, I have seen others that have a hose that feeds water to both ears of the muffs, seems like a better design. I will also top up the glycol side when back together, it does need a bit of anti freeze, but obviously that was not my issue here! When I was doing sea trials ( for all of about 10 minutes) I had the pressure cap off trying to let it burp - then it got hot quick and we shut her off. End of sea trial.

 

[thumpar]

Re: cooling issue after rebuild

I would suggest backflushing the system to get all the missing pieces out.

 

[Sangster21]

Re: cooling issue after rebuild

OK - good idea. How would I do that? Attach a hose ( may be using a rad flush T) to the sea water outlet side of the heat exchanger and pushing water thru the heat exchanger back thru the transom plate and out through the leg? Sound right? Perhaps I would be better off to remove the heat exchanger completely and take end caps off and flush / dump out debris? Please advise.
Alan

 

[thumpar]

Re: cooling issue after rebuild

I don't have experience with heat exchanges but you want all the water to go backward so don't use a T. Just take off the hoses and backflush them where you can. Mine went on mine at the end of the summer. I took the hose off the pump (engine driven seawater pump) and put the hose from the house in the hose the goes to the thermostat housing. Got all the pieces of rubber out.

 

[Scott Danforth]

Re: cooling issue after rebuild

OK - good idea. How would I do that? Attach a hose ( may be using a rad flush T) to the sea water outlet side of the heat exchanger and pushing water thru the heat exchanger back thru the transom plate and out through the leg? Sound right? Perhaps I would be better off to remove the heat exchanger completely and take end caps off and flush / dump out debris? Please advise.
Alan

unbolt the end caps, dig out the pieces. you should be able to see thru the tubes with a flashlight. tubes are copper, so if you need to rod out the tubes, keep that in mind. should be 0.248" od tubes with 0.040 wall. then back-flush the line feeding the HX from the outdrive. Depending if you have a power steering cooler, or an oil cooler, those may need to be back-flushed and cleaned as well.

it has been my experience that the PS cooler does an effective job of straining and plugging prior to debris getting to the main HX.

 

[Sangster21]

Re: cooling issue after rebuild

Thanks for the advice Scott. It won't be much effort to unbolt the heat exchanger and remove it entirely..... er drat except that it is filled with coolant. Rats. Anyway, I think it is a good idea and worth the effort. I don't have power steering on the old rig, so I could well imagine any bits finding there way into the HX and sitting in the bottom. This particular cooler is positioned vertically not horizontally like many seem to.
Regards,
Alan

 

[Sangster21]

Re: cooling issue after rebuild

Well....crap. Update on the cooling issue. I have been busy as all hell with work, masters course work and a house renovation a two ferry trip away. Finally managed to put the boat in the water to have a go, runs fine, shifts fine, but I think we have a problem. After flushing the engine with fresh water, I thought to pull the dipstick and have a look at the oil. Oh poo, oil looks a bit brown and somewhat chocolaty. I better investigate this I thought. Tonight I go back and look at the oil... hmm seems clear and oily. I pressure tested the coolant system and it holds pressure! I am still unsure, last night it looked milkshakey, so I put the engine on muffs (my new quicksilver ones that positively clamp to both sides of the leg) and let it run, and run some more. I check the oil again, seems good and oily. I let it run some more and.... seems like a lot of blow by through the crankcase breather.... hmm sort of looks like steam.... seems like the fluid dripping off the spark arrestor looks a lot like coolant. Let it cool down enough to remove the pressure cap and test again, and there is definitely pressure loss.The coolant (that should look brand new) I collected looked pretty brown and gucky, sort of like what you would expect to find in a coolant system that was long overdue for replacement not coolant with 2 hours of run time. I think my next step is to remove the cylinder head to inspect. I imagine I will need to check the head for distortion (check for flat) and cracks. Should I send it out for magnafluxing or will a close visual inspection be adequate? I am hoping I can install a new head gasket and re torque and be good to go. What are the chances of this being the case? Thoughts?
Alan

 

[harleyman1975]

Re: cooling issue after rebuild

That Chevy six is known for warping after an overheat due to its length. If it was me I would check for true since it is off...even if it is a "two ferry" ride to machine shop.