Water uptake.

[Jon Tallis]

Originally Posted by A1an

can you post a picture of the exhaust/water jacket now you have the side plate off?

Paul,
There's another possible cause that could lead to the telltale getting very hot very quickly - an exhaust leak into cooling water. It wouldn't lead to an engine overheat necessarily but could fool the sensor into thinking there was, dependant on the position of the sensor in relation to the leak.
Like A1an says, post some pics including the exhaust water jacket and we'll all have a look.
Also, do you have a manual for the engine? If not, please post what year it is and I may have one.

Re your comment on being stupid for running pre-mix - that's not the case at all - you put your trust in someone who claimed to know what they were talking about..... we'll all have our own view on whether your mechanic was talking bollo**s or not.......

[A1an]

Temp sensor is at the top of the head on that engine.

[Fenlander]

>>>an exhaust leak into cooling water.

Which is course what Tim had led to believe as a good possibility a few posts earlier this am. Certainly with cars and head gaskets it can cause very rapid and often expensive problems.

[TmMorris]

Agree Jon. If the telltale is exiting from the side plate next to the exhaust and there is a leak there then it could be that the engine is being cooled if the sensor is also in that plate.

My hesitance though is that it wouldn't seem logical for the heat sensor to be away from the block/head and the main water flow as that is where the primary risk is. Also, as the telltale plate in in series between the main entry point and the engine block , even if the leak were at that point there would be too much water flow for the telltale to get too hot to touch.

In my mind the water is getting up to the plate with the tell tale but being prevented from proceeding via the thermostat into the main cooling galleries so it is stuck in that first area and getting super heated.

A temp gun on the block would give key data as would a comp test.

If the head has been off then as well as an image of the exhaust water jacket an image of the head gasket area would be useful as it may show a blow internally between a key gallery and a piston liner where gas is escaping and halting the full flow of water around the head.

[Fenlander]

>>>I had a merc 50 which alarmed out higher revs. did all the temp and water tests and found that the temp sensor was faulty..... just a thought

Just re-read the whole thread to see if something has been overlooked... the possibility of the overheat sensor being defective which I see RoyP mentioned much earlier as above.

The OP's reply was...

>>>I've checked the opening temperature of the thermostat and I checked the tell tail just before the over temperature alarm went off, 50+°C.

If the maximum temp the OP is seeing is 30-40degC under the stat opening temperature is it really overheating?

Jon Tallis again mentioned if the sensor was OK but I can't see this was answered??

By chance this AM I've just sorted Mrs F's Mercedes after it went into limp home mode and showed off the scale on the temp gauge with an overheat warning on the dash.

I diagnosed to find the temp reading to the gauge and ECU was 102degC when the engine hadn't been run for 12hrs and was at an ambient of 25degC. A new coolant temp sensor fitted and it's all OK.

So if the OP hasn't addressed this possibility it's worth considering.

[TmMorris]

Yup. I think a few of us have mooted checking the temp sender as well as putting a temp gun on the cylinder block. The impression I was getting was that the telltale water was almost too hot to touch though besides, if there is good flow I'm not sure I'd expect to find much heat in that water anyway?

Years ago, I had the fault warning light come on on my VW only to discover that what was at fault was the fault warning system! So you never know though.

[Fenlander]

Yes I see the mention of something like a non contact thermometer to get some actual temperatures. In truth this is all getting quite involved and it would be better going forward with a faulty sensor crossed off the what-if list.

[paintman]

Originally Posted by Jon Tallis

All good info but the OP's engine isn'ta Merc V6, it's a Yam 70

Yes. I was aware of that.

We currently have no idea whether the OPs engine introduces the oil into the fuel before the carb - in which case it will arrive at and pass through the jets as premix - or at another point without passing through the carb jets.

Converting an oil injected engine to premix doesn't automatically mean the engine will have problems - as with the Mercury engines where the kit is a factory item not a bodge job which seems to be the case with the OP's'conversion'.

But you will also note that I suggested the OP reinstate his system to see if that makes any difference and I for one will be interested in that outcome.

[A1an]

I’ll bet the square root of fuck all that this isn’t a fuel problem.

[spartacus]

Originally Posted by A1an

I’ll bet the square root of fuck all that this isn’t a fuel problem.

Telling it like it is.

To the OP, post some pictures of the cylinder head and water jacket.

You can test the thermostat too in a jug of boiling water to see it opens. Sometimes an issue with pattern parts.

Once the head is back on (checking it's not warped with a steel rule) with a new gasket, use new bolts and start from inside out to tighten and then torque down to factory setting, that part is critical.

Post up compression on all cylinders after that.

Any pictures of the lower leg and waterpump housing would be useful.

[TmMorris]

Originally Posted by A1an

I’ll bet the square root of fuck all that this isn’t a fuel problem.

Not now. It may have been the initial trigger for the overheat, a weak water pump or a dirt blockage of the water ways but I think we are well beyond the primary cause of any over heating and now clearly needing to diagnose a secondary issue.

The OP has tested the thermostat and for physical blockages as these are the two main things that would throw up the symptoms of a super hot telltale from water being halted at the exhaust jacket and not going on to the head and block. All seems spot on as does the water pump as I believe the OP was going to spin that up on a drill.

The only thing I can therefor think of is a gas leak internally from the head into a water gallery that is creating a flow blockage and so the overheating. The initial cause for the first overheating may still be there, may have been cured in the fettling so far or may be cured when the major issues is remedied.

It's worth testing the temp sender but if the telltale is getting this hot then I suspect the sender is working well. And if the water pump is working properly and is pushing enough water into the engine then some form of 'blockage' is the only thing I can think of that would cause the water in the chamber with the telltale to get that hot, that quickly.

[paintman]

Jut a couple of thoughts on the last couple of posts. And yes, I do know they're not outboard motor related - although both engines have been used in boats over the years.

Pattern thermostats.
There was an issue with pattern thermostats on the Ford V6 Essex which were missing a bleed hole and air got trapped preventing water from contacting the stat. Would sit at tick-over and boil. I'm aware of that as it was a Scimitar issue and covered in the owner's club magazine 'Slice' & I had one at the time.

Faulty temp senders.
Have had a couple of these fail on my Range Rover - 1986 3.5EFi. Either wouldn't work at all or the gauge goes off the top of the scale and squeaky bum time!

Long experience - and that of others - has taught me that pattern or 'good quality aftermarket' - parts can sometimes be very good but sometimes shouldn't be touched with a bargepole.
The bit in quotes is a favourite of Land Rover aftermarket parts sellers.

If a critical part or having to replace it would mean lots of work I'll go genuine every time.

[jwalker]

Erm, you did wind the blades of the water pump impeller in the correct direction when you assembled the pump didn't you?

[spartacus]

There's a woodruff key that fits on the impeller. If it doesn't engage properly then it will spin on the driveshaft - but I doubt it would even pump water. Anyway - double check.

[TonyM]

Phew, that was an epic read 😀. Im sure I read earlier on that the engine needs to be run with the choke slightly open which would indicate it’s running lean, when running lean the tell tale is often warm/hot to the touch.
It seems there is good water pressure and flow through the engine, thermostat works ok, what actually changed on the engine from when it ran properly to when it indicates over temping?
Cheers