Black Max 150 overheating and water pressure

[Phantom309]

I recently bought a bass boat (1987 Procraft 1780) with a Mercury Black Max 150 XR2. After doing a bit of work adjusting the timing, we finally took it out on the water yesterday. At first it was running great, and we were cruising around getting to know the boat as well as the lake. (First time boaters aside from a jon boat in a small river).

While running on a plane at full throttle, the alarm went off. Checked the gauges, which honestly I had not been monitoring closely, and the temperature was high, as was water pressure at about 40psi. I immediately backed off the throttle and shut the motor off. Waited a few minutes and fired it up again. Temperature had gone down some and the alarm was not sounding any more. We kept it slow back to the dock as it was time for us to leave at that point. At idle, the water pressure is about 10 psi. Full throttle it was 30-40. I’ve read this is high.

From what reading I’ve done, it would seem that I have a blockage somewhere in the cooling system, reducing water flow which is causing the high pressure and temperature. Where should I start on trying to find the blockage? Poppet valves? Would a stuck poppet valves cause the high pressure at idle?

Is there an easy way to set up a secondary temperature and water pressure sensor? I’d like to verify that the gauges are correct. Most of the other gauges are either non functional or very inaccurate so I do not put much faith in them.

I plan to do the work myself. I’m a fairly decent auto mechanic, just pretty new to boats.

 

[Chris1956]

40PSi is not usual for that motor. The poppit should open at about 15PSI.

Since the motor is new to you, I would replace both thermostats and rebuild the waterpump. I would also repair the poppit valve. make sure you have the exact year of the motor. Thermostats and waterpump kits are pretty universal, but poppits changed over the years.

 

[Dukedog]

gotta disagree with chris.. 40 lbs is gonna be high on this one.. i would venture ta say 17/18 lbs is 'bout "average" for tha merc V6.... i'd start with poppet.... complete "kit".

there are two designs (old and new) for tha poppet.. you will need ta identify yours by how many bolts hold it to tha block.. if its two bolts it will be "old design".. if its four bolts it will be "new design".. tha "kit" will use same four bolt design gaskets for both designs so don't panic when ya open tha box....

 

[Phantom309]

gotta disagree with chris.. 40 lbs is gonna be high on this one.. i would venture ta say 17/18 lbs is 'bout "average" for tha merc V6.... i'd start with poppet.... complete "kit".

What about PSI at idle?

 

[Dukedog]

3 ta 7 lbs is plenty.. depends on idle rpm....

 

[Phantom309]

Another thing, can someone point me towards a good service manual? I have a Seloc that came with it but it covers a very wide variety of outboards and I find it not the most useful. Is there a recommended manual that is more specific to just the motor I have?

 

[Dukedog]

justa 'bout any aftermarket manual is gonna suck.. jmo.

part no.
OEM.. 90-976583... 100 bucks from a dealer but might try ebay?
thats sticker shock i know.. but if ya gonna do these things yourself it will put ya $$$ ahead....

 

[Phantom309]

Okay been messing with it this morning. Here’s what I found.
The poppet valve seems to be in good shape. No sticking, wear, or anything and the gaskets all look good.
Thermostats look good. Tested them in hot water and they open up right at 145 degrees.
Tested both temp sensors. Starboard side for high temp alarm, and port side for temp gauge. Both worked when grounded out. Also tested in hot water. The temp guage does not have numbers but it read about 3/4 in 170 degrees. Alarm did not go off at 170.
Verified the PSI guage by connecting the air line to my air compressor. It is spot on. All ports I can see look clean when I’m pulling stuff of. Also the hoses from thermostats to pee hole and water outlet are clear. No signs of clogging.

Is there a way to reverse flush the cooling system? Thinking if it had sucked something in then maybe I can push it back out the way it came.

Have not checked the impeller yet, but I don’t think I need to be too worried about it as the water pressure was high, and I have verified that my guage reads correctly. So it is pumping water for sure.

Another stat that I forgot to mention was that it always helps a good stream of water from the pee hole, even when it got hot the other day. Didn’t see any steam, which I’ve read would go along with a leaking gasket letting exhaust into the water.

 

[Dukedog]

where is tha pressure gauge hose attached to tha block?

in tha poppet assy. there is a plastic disc.. it will become slightly "warped" that is seldom noticed unless someone mentions it.. it can cause a "sticky situation"... we would fab new ones with a piece of aluminum or thin stainless stock to replace...

 

[Phantom309]

where is tha pressure gauge hose attached to tha block?

in tha poppet assy. there is a plastic disc.. it will become slightly "warped" that is seldom noticed unless someone mentions it.. it can cause a "sticky situation"... we would fab new ones with a piece of aluminum or thin stainless stock to replace...

It’s going into the top of the block, just behind the flywheel.

I’ll have to look closer at the poppet as I’m sure I could have missed that. Either way I am going to be replacing the parts I checked today.

 

[Phantom309]

where is tha pressure gauge hose attached to tha block?

in tha poppet assy. there is a plastic disc.. it will become slightly "warped" that is seldom noticed unless someone mentions it.. it can cause a "sticky situation"... we would fab new ones with a piece of aluminum or thin stainless stock to replace...

Pulled the poppet back apart and looked at it. The plastic disk does appear to be very slightly warped.
I ordered everything to completely rebuild the poppet valve, new water pump, and new thermostats. Also ordered the service manual. In my limited experience, any OEM service manual is worth its weight in gold so $100 didn’t really phase me much.

I was doing some reading on another forum, and saw someone suggest taking the engine off the lower unit and checking the water and exhaust passages in the exhaust plate. It was suggested that if the motor had been sitting for a while there could be critters that got in and caused a blockage. To my knowledge this motor did sit for over a year, and there were some mud/dirt dauber nests on the side of the block so I will not be ruling out that possibility.
I’m thinking I’ll look into that if still having problems after I replace everything I just ordered. Is that a good plan of attack for diagnosing this? Or would you suggest something else?

 

[Dukedog]

pulling tha PH is usually last resort if all else fails.....

 

[Phantom309]

New thermostats installed and poppet fully rebuilt. Took it on the water today. Psi at idle is 9-10, WOT it never went over 18.

At WOT, The temp would get up to about 7/8 on the gauge. Based on when I tested the temp gauge, that should be maybe 185F. Is that high for this motor? It never got high enough for the alarm to go off (which I know it works from last time). And I ran it at WOT for a lot longer period than I did last time when it got hot. Once I back off it would cool back down to about 5/8 on the guage pretty quickly.

Think I might have spun my prop. But that’s a different topic and I’ll be starting a new thread for it

 

[Faztbullet]

Temp gauges are inaccurate due to the sender mounting and location can cause different readings.. Put a piece of tape over it and rely on pressure reading as true indication of impeller/cooling operation. It will also run hotter if overpropped as engine is lugging

 

[Phantom309]

Temp gauges are inaccurate due to the sender mounting and location can cause different readings.. Put a piece of tape over it and rely on pressure reading as true indication of impeller/cooling operation. It will also run hotter if overpropped as engine is lugging

Yeah I started to wonder if that was the case. It was overpropped, and I just took it out with my new prop at a better pitch. It was reading just above 3/4 on the gauge this time, so better than before. Still had a good stream from tell tale, and psi from 15-18 at WOT, so I’d say my problems are probably resolved and I don’t need to worry about temp