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Still overheating and at my wits-end :-(


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Hello all,

Thought I'd start a new thread with this to bring everything together. 

Last year boat ran lovely but temperature gauge hardly ever moved. This winter I changed the water pump and performed a transom service. Put her back in the water and the temperature gauge started to rise quickly to overheating. At 1400 rpm it stayed around 140° but at idle rose quickly to overheating. The problem seems to be getting worst as at 1400 rpm it now overheats.

This is what I've found out so far...

  • Pulled off the incoming water pipe to the thermostat housing and I appear to have a reasonably good flow of raw water, even at idle
  • Changed thermostat
  • Changed circulation pump
  • Changed temperature gauge, sender unit and wire between both
  • Probed the sender unit with a temperature gun and the engine seems to be running around 140° - 150° which is where it needs to be but the gauge is reporting 220+°. The replacement gauge is doing exactly the same thing
  • I've taken the top off the thermostat housing when the gauge is reporting 220° and probed the water at 140°

I've taken the thermostat housing cover off and put a hosepipe in to make sure the engine is filled with water. With a moderate flow of water the actual block doesn't fill. Is that normal ?  I can't see any leaks, the bilge is dry and the engine still runs lovely even though the gauge shoots up quickly.

I'm at my wits-end and have a mechanic coming out on Saturday.

And one other thing....I'm now doubting myself. Should there have been a seal between the pipe that sticks up from the water pump, and the pipe that it slides over when you replace the lower half of the outdrive. There wasn't anything on mine when I took it out and I never put anything on when I replaced it.   

Than you for your endless patience.

A very sad Stu. 

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Your boat may have two water temperature sending units. One is for gauge indication, and the other is a simple switch that talks to the ECM. Perhaps you replaced the ECM sender/switch, and not the variable output sender that feeds the helm indication. Sounds like your engine is not actually overheating. W

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Sorry. Its a 5.7 Mercruiser, Alpha 1 outdrive in a 1990 Chaparral Signature 27.   @Wingnut I don't think it has an ECM and I can only see one sender unit. I've verified that the one I changed is the one that drives the gauge by earthing it out giving a hot reading. I'm actually down on the boat as I'm typing this and have had it running on a hosepipe with the top hose - raw water intake off and the flow still looks ok.   

I took this video the other day, so let me know what you think. Its upside down but the pipe I've got off is the correct raw water flow. Engine was at idle.

 

Thanks

Stu

 

 

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5 minutes ago, Stuart Adair said:

Sorry. Its a 5.7 Mercruiser, Alpha 1 outdrive in a 1990 Chaparral Signature 27.   @Wingnut I don't think it has an ECM and I can only see one sender unit. I've verified that the one I changed is the one that drives the gauge by earthing it out giving a hot reading. I'm actually down on the boat as I'm typing this and have had it running on a hosepipe with the top hose - raw water intake off and the flow still looks ok.   

I took this video the other day, so let me know what you think. Its upside down but the pipe I've got off is the correct raw water flow. Engine was at idle.

 

Thanks

Stu

 

 

That's not a lot of water volume even at idle. Is the Alpha raw water impeller fresh? Is the connecting hose from the drive to the transom assembly intact, not kinked, or restricted?  You are right, no ECM on a 1990. W

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I think you've answered what I suspected, but never wanted to hear. The boats in the marina so I have to pay to have her craned back out which I'll get sorted out. I'll strip the outdrive back off and check I've not done anything stupid like missed the 'O' ring between the drive and bell housing.  One further question that's praying on my mind. The solid plastic pipe that sticks up from the water pump slides over a pipe that sticks down from the outdrive. Should there be a seal on there ?  There definitely wasn't on mine and I'm going through my mind wondering where water pressure could be going.

Thanks 

Stu

 

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If your alpha is Gen 2, the black plastic coupling that sits on top of the water pump housing and the pipe from the upper half of the drive slides into is supposed to have 2 orings.  Merc #816597A1.

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5 minutes ago, SG Boater said:

If your alpha is Gen 2, the black plastic coupling that sits on top of the water pump housing and the pipe from the upper half of the drive slides into is supposed to have 2 orings.  Merc #816597A1.

Hi, Its a gen 1. Does that have the 'O' rings too ?  I'm racking my brains back to doing the job and I'm sure I never spotted any seals between the two pipes. That could be where I've messed up perhaps ?

Stu

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There should be a new guide tube and grommet in the kit. Did you maybe miss the grommet and it could be leaking at the water tube and not pumping much water?

  • Guide Tube GLM # 12820, OEM # 23-29804
  • Grommet GLM # 12830, OEM # 26-38970
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21 hours ago, JGuilesBand said:

I use Cecil Marine for the majority of my parts but I'm a Volvo Penta guy. I believe they carry Mercruiser stuff too. Great Lakes Skipper is always a good choice too. 

And Defender and Hodges.

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Thank you all. I've managed to find what I want at Repower Marine in the UK.  What I never fully appreciated until I spoke to them is that the copper tube slides all the way down inside the plastic tube to push on a seal that sites under the plastic tube, and its that seal I've missed off.  Boat comes out the water on Monday and I've asked a local Marine Engineer to take a look. 

Thanks

Stu

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I've got the new water pump kit today including the new water tube. What I can't find anywhere on Youtube or in diagrams is a clear view of what seals should go to seal the copper water tube to the plastic water tube, or if there is meant to be a seal under the plastic water tube. I think I'm going to give the job to the marine engineer at the boatyard to sort but I'd be really interested if a diagram exists.

Thanks

Stu

 

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You should see a black rubber seal in the top of the water pump housing.This is what the copper tube slides into. The plastic guide just get pressed into the top cover, it is only  guide, nothing to seal

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Well she's back out of the water and on hard standing. I had the local marina mechanic round for a quick look yesterday. His initial thoughts are there's an issue with the water pump but we'll wait and see. He's fully booked until the end of next week, and as I'm working away I'll leave the job to him to sort.

Look at the mess 3 months in the River Avon has made of my nice shiny outdrive and prop :(

Will keep you updated...

https://photos.app.goo.gl/oTPNjipByFiA3EjP8

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I'm not a man ashamed to confess my sins and lessons learnt in the hope others are helped...Just go gently on me with your "I told you so:rolleyes:

I made a catalogue of errors ...

1. Water pump wast not from a reputable supplier. There was no water tube (I never knew there should be at the time) and the seal between the water outlet pipe and copper tube was missing. Due to my inexperience I never spotted this. My new pump was from Repower Marine and had the water tube included.

2. My old water tube was cracked and instead of ordering a new one I found some UPVC pipe that appeared to be the same dimensions. However, coupled with the lack of a seal on the water pump there was nothing to stop the water gushing between the copper pipe and my bodged-UPVC pipe.

3. The dodgy water pump I fitted has a poor quality rubber rotor. The new one was thicker and took considerable effort to get in, whilst the old one just slipped in. This can be seen from the photo as the rotor is already distorted after only about 1 hour of use.

4. I should have ran the boat to full heat on muffs before she went back in the water. I would have realised that the risers we're hotter than usual and therefore spotted the error. I can now put my hands on the risers and keep them there without issue.

5. I should have spotted that there was little water coming out of the exhaust outlets on the outdrive. Instead most was coming out of the prop which I thought was normal.

Although there's a 140° thermostat fitted the gauge sits at 175° and despite my best efforts running the boat up river, then letting it idle for 20 minutes I can't get it to go any higher. I guess that on my boat 175° on the gauge = 140° in real life. I can now comfortably put my hands on the risers.

Anyway, lots of lessons learnt. But I'm now back in the water so let the Gin flow.

Stu :D

https://photos.app.goo.gl/rzYS2u8CMXNTU8Di9

https://photos.app.goo.gl/J23N1np1JSc6risM7

 

 

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