Ideas to overheating MC 165 6 cyl 250 GM ??

[h20]

Engine temp stays around 140-150 while cruising but climbs to 200+ when idling.

What I?ve done
Replaced
-Impeller
-hose from impeller
-Water pump
-Tstat (140)
-Exhaust manifold

We replaced the manifold last after other items didn't resolve. Although there is no water in any cylinders a crack seemed likely because with the engine idling and the hose from the water pump to the manifold off a (engine was cooled off) we saw bubbling in the manifold... as if exhaust gas was entering water passage. But after replacing same result on the lake at idle.

Engine starts on first crank and runs smooth with good power.

Guys at the shop are perplexed as am I.
Any ideas as to what we might be missing?

 

[HT32BSX115]

Re: Ideas to overheating MC 165 6 cyl 250 GM ??

Howdy,


Remove the "hose from the impeller" (to the engine) and replace it with a same-size clear vinyl hose and look for bubbles.

Before you hook it up, run the engine at idle and let it run into a bucket and see how long it takes to fill it.


Regards,


Rick

 

[h20]

Re: Ideas to overheating MC 165 6 cyl 250 GM ??

Rick,

Sounds like a good idea. Just to confirm I'm following your thought, you are thinking the bubbles may be coming from the impeller at idle and air is leaking in somewhere which has less cooling and/or weaker flow rate? Sounds plausible to me.

Thanks, I'll let you know the result soon as I can get it setup.

 

[HT32BSX115]

Re: Ideas to overheating MC 165 6 cyl 250 GM ??

Yeah, that's one of the ways you can trouble shoot. You're looking for any air. It should be a steady stream of water and should fill a bucket fairly quickly.

You also might check the t-stat even though you replaced it. they don't fail very often......but I have seen non-working t-stat's right out of the box.


An engine running hot at idle is frequently a damaged impeller though........If you ran it dry even for a 30 sec, a new one could even be trashed.

You might also check for piece of a previous impeller failures down-stream of the pump. .....Although partially plugged hoses/passages usually don't result in hot running during idle....those usually result in hot running at higher power settings........

 

[Don S]

Re: Ideas to overheating MC 165 6 cyl 250 GM ??

The clear hose method will only work when the boat is in the water, not on muffs.
When on muffs, you will always get a lot of air. Even the volume tests MUST be done with the boat in the water. The pump in the drive will pump more than your garden hose will.

 

[HT32BSX115]

Re: Ideas to overheating MC 165 6 cyl 250 GM ??

The clear hose method will only work when the boat is in the water, not on muffs.

Yeah... I should'a said that!!!

you are thinking the bubbles may be coming from the impeller at idle and air is leaking in somewhere which has less cooling and/or weaker flow rate?

If you have any sort of a leak at the pump where it can suck air it will. Although in an Alpha sitting in the water, everything should be submerged and covered with water. There shouldn't be any bubbles at all.


My previous boat was a 1966 Mark Twain (150 Mercruiser, 230cu-in GMC I-6) that my dad bought new. I remember him always tapping (beating) on the thermostat housing (with a 12" "Crescent") thinking the thermostat was stuck! He even used to pull it and stick it in hot water to test and it always worked!

It would overheat at idle but not at RPMs above idle.

Every time it did that it was a problem with the raw water pump (impeller, housing, etc).

Doing the "flow-test" at the lake will probably confirm that you have poor output.

 

[h20]

Re: Ideas to overheating MC 165 6 cyl 250 GM ??

Haven't been able to do the clear tube test yet but did have the shop replace the housing which has the copper tube, old one showed heat damage. Flow out of the hose to the t-stat is very good.

Out of curiosity I decided to pull the riser which upon prior inspection was believed to be OK per the shop. However, I filled the water cavity then poured it out (direction of flow) to see how well if flowed. It seemed to flow out around the exhaust but when I tilted it back it still had a fairly large amount still in it which I poured out the end that connects to the manifold, lots of crud and a small rubbery piece of something. Rodded it out from both ends till no more scaling/crud flowing out but still some water remains. Is that normal or does it sound restricted?

I decided to dig in a bit further and pulled the head following shop manual. Hopefully I didn't overreach, haven't ever done this before.

Found a blockage in a passage way, poked at it with wire and blew it with compressor till cleared. Gasket looked OK. Lightly cleaned valves and combustion chamber as much as possible with tools available.

Have head back on and torqued however I now need to set the valve lash. Is it OK to do a few short turns with the starter so I can position #1 at TDC without hooking up the muffs? Or, is there a way to manually turn the crank, I can't see where to put a wrench on it with it mounted in the boat.

 

[HT32BSX115]

Re: Ideas to overheating MC 165 6 cyl 250 GM ??

You probably found the other common cause of overheating.

Blocked cooling passages in the block/head,/manifold/riser.

Since they're usually all exposed cast iron they begin to rust immediately. When enough rust clogs the passages, the water flow thought the whole system is reduced.

The only real fix is replace the parts.

Some people have used acid and/or mechanical means to clear the rust but remember that the rust comes from the part itself. The walls are getting thinner and thinner. Eventually there's rust-though and leaks into areas you do not want or cannot have water going.

Any blockage that reduces cooling water flow will contribute to or cause over heats.

a small rubbery piece of something

That's probably remnants of previous impeller failures....the impeller pieces have to go somewhere!

No problem turning the engine with the starter or a wrench with no water on the impeller. Just don't run it dry.

Have head back on and torqued however I now need to set the valve lash.

UM, you did replace the head gasket, right?

 

[h20]

Re: Ideas to overheating MC 165 6 cyl 250 GM ??

UM, you did replace the head gasket, right?[/QUOTE]

Thanks for the response, hope you are having a great 4th!

Yep, new head gasket went on.


Was able to turn from the pulley so I then adjusted all valves per shop manual and finished reassembling motor.

Cranked over and nothing. Fuel is there (see in carb and s-plugs are wet). Oil pressure not showing on gage (but I seems to be flowing because unfortunately oil is leaking from one of pushrod covers...yes, installed new gasket) so hopefully the pressure just isn't there yet because I only cranked for about 5 secs at a time. Stopped trying after about 3 attempts when it was apparent no spark happening.

I think it's the distributor, had difficulty getting it reinstalled as manual described.

Procedure I followed:
With distributor out I pulled #1 plug, rotated crank till pressure on finger in plug hole, advanced to TDC. According to manual I then positioned the rotor to point at #1 cap position, counter rotate rotor 1/8 turn past #1 then try to install... That is where I can't get it. To fully engage the cam, oil tang slot and fully seat the distributor I have to turn it almost 1/4 turn further which then leaves to way past #1 cap position. I could just rotate the base to align the plug position with where the rotor is pointing but that doesn't seem right, for one the plug wires don't route as they did.

Any suggestions?

 

[h20]

Re: Ideas to overheating MC 165 6 cyl 250 GM ??

Got engine back running. Replacement riser and hose from transom to tstat installed. Took out to lake and... still running hot at idle.

No bubbles showing when running clear tube, but the pressure or lack there of from the impeller was easily blocked by my thumb.

Dropped the lower unit hooked hose directly to the copper pipe in upper unit and flow was very good, even had it coming out the side overflow ports which it hasn't done when running off muffs with engine running.

Inspected the impeller, which was newly installed, to see if maybe the key was slipping or blades were damaged. Nothing, it looked just like a new impeller... stumped.

One observation I made and hope to get comment on from those more knowledgeable than myself is regarding impeller housing base. The housing base wasn't replaced and in looking at it I see a threaded hole on the side of the chamber that moves the water up the tube. All I can find in searching for info about it is that there are 2 base options for my year MC, with or without a motor flush adapter hole. Since there is about an 1/8 to a 1/4" space between the base housing and the lower unit case wall I'm wondering if this is the source of my pressure/flow loss ?? Anyone ever encountered this?

 

[h20]

Fixed** Re: Ideas to overheating MC 165 6 cyl 250 GM ??

Fixed** Re: Ideas to overheating MC 165 6 cyl 250 GM ??

The wrong impeller housing base was installed by previous owner(s) ... costly and time consuming misdiagnosis after my purchase.

Filled the flush out plug hole with a little JB Weld (housing was in good shape so elected not to pull apart further plus wait on another part to be shipped) and she runs perfect, 135-145 temps all day long.

Low flow/pressure at idle RPM was all due to either sucking in exhaust or pushing water out that hole but at faster RPMs there was enough added pressure to overcome and keep temp down and flow volume up.

Surprising this hasn't been an issue brought up in the past, hope this resolution post helps someone else avoid the same frustrations.