Mercruiser 5.7 v8 Engine Overheating

Symptoms: During 2020 we were out in our Bayliner 2855 (1994) trying to get her up on the plane, when we heard a loud siren go off from the dashboard. Having no clue what it was we slowed down and the siren stopped. We discovered that it could be either the oil pressure or an overheating problem. We asked the local mechanic in Torquay and he said did the oil pressure or temperature gauge change? Thinking it was just a one off we took her out again and the same issue occurred with the temperature gauge shooting up and the siren going off.

We spoke to the local mechanic again and he said it’s probably the manifold and risers that need replacing, but it would probably be cheaper to buy a new engine (it seemed like his standard reply to everything), as OEMs cost £3k for the parts alone. We tried to find a mechanic to take a look, we had two booked in but they never turned up. So this is when we tried to fix it ourselves! We replaced both manifolds and risers with non-OEM parts given the recommendation of the mechanic. However, on replacing them, it was clear there was nothing wrong with them (see section below) and we should have done a better job of diagnosing the problem ourselves!

Symptoms Summary:

• Engine overheats at WOT trying to get on the plane
• The starboard riser gets extremely hot when compared to the port riser (I only discovered this after we had replaced the manifold and risers, so I’m assuming that was the case before replacing the risers).

Mercruiser Overheating Checklist (raw water):

Diagnostics check – It was time to get on and determine what the issues were with the engine and I set about trying to find the problem. Our engine is raw water cooled using a alpha gen II leg. As the boat is always in the water I wanted to complete all the engine checks before having to pay for it to be lifted out. Following the checklist:

• Check the belts and they were fine – tick
• Temp Sensor checked and that was fine – as the starboard manifold was over heating at idle. The starboard riser was overheating, which wasn’t always picked up by sensor as the temperature of the water from the engine was fine when both port and starboard raw water was mixed before reaching the sensor – tick.
• Issues with pick-up…couldn’t check until the boat was lifted out of the water. UPDATE: The boat was lifted out in April 2021 and oh boy were there many barnacles living on the water intake. These were cleaned off (see pictures below). However, this wasn’t the root cause of the issue – see below for how it was solved – tick.
• Pick-up pump – with the alpha one gen II that is in the leg. It was supposedly replaced in March 2020. I replaced the impeller in April 2021 – but it looked in good working order – so I doubt it was that – however, there was water in the leg so I installed a new gear gasket – tick
• Hoses checked within the engine bay. However, I forgot to check the hose going into the oil power steering unit, that comes from the pick-up through the engine (just below the port manifold) but that seemed clear but I backed washed it (this might have been the issue) – tick.
• Air leaks – well this is possible due to the thermostat leaking air and maybe not enough circulation due to a faulty water circulating pump (so we decide to replace both – see sections below). However, the problem still remained – tick .
• No ice! – Tick.
• Circulating pump replaced – didn’t fix the issue – tick.
• Thermostat replaced – didn’t fix the issue – tick.
• Ignition timing checked – fine – tick.
• Checked wires for firing order and spark plugs – all fine – see section below – tick.

Solved – now that was strange. After all the checks above the problem still occurred with an overheating starboard manifold. However, when moving the boat to where it had to be craned out – not a long distance, the engine no longer over heated! Or should I say the starboard manifold didn’t overheat. The issue is – well I can’t say what stopped it overheating, but maybe the force of the water through the pick-up help dislodge something following all the changes. I still serviced the leg and added new bellows (not related to the overheating at all, but they had been in there for over 4 years) but the problem has been resolved (note even before cleaning the barnacles off the leg). Took the boat out for a spin after the leg service and antifoul and it was flying along with no overheating issues – phew!

How to tell if something is broken – Diagnostics: I started by checking the pipes to and from the thermostat housing, and although they were all a little stiff they didn’t have any blockages. Next I purchased some clear hose to connect from the thermostat out to both manifolds to check the flow. As you can see there isn’t much flow (the port side had a solid flow, where the starboard side has lots of air in it). NOTE this isn’t at wide open throttle though…forgot to record that video, but when at WOT the water oscillated more with no continuous flow as found with the Port side.

At this stage I replaced the water circulating pump and thermostat and put new gaskets on to see if it was either the part of bad gaskets (see below for more detail). After doing this a tried the clear hose again and this was the result…pretty much the same, but I would say slightly better flow, but still not enough water in the hose to cool down the starboard riser. So the issue remains!

As well as looking the flow in I also looked at the flow out and checked the flappers on both sides of the boat and they both seemed to be working fine.

Next step was to buy a longer clear hose to run from the raw cooling inlet to the thermostat. The one I bought didn’t connect properly and I wasn’t sure if it gave a true reading as it had a big kink in the hose. So this is where I’m at with the journey…will report back later in Sept 2020, once the new hose arrives.

Replacing Mercruiser Manifold and Riser

After reading the Mercruiser Manual and watching lots of YouTube videos (this one was the best – https://youtu.be/AqbKaqUrbK0 ) I set about removing the manifold and riser, starting with the easier port side as nothing was attached to that riser. All the bolts came out very easily using a torque wrench. I used the torque wrench to see if they were torqued correctly to start with. The manifold was heavy and did need two pair of hands to avoid breaking the spark plugs. On inspection the gaskets looked fine and there were no blockages. They came up like new after scraping off the old gaskets. Now I have a spare set (unless you want to buy them!).

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Replacing Mercruiser Water Circulating Pump

Here’s a video of what the engine block looks like with both the water circulating pump and thermostat housing removed.

This is what the removed pump looked liked…it was in pretty good condition, but I think the rubber gasket had gone and there was some build up on the back plate.

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Replacing Mercruiser Thermostat and Gasket – the next job was to remove the thermostat housing and check that. It looked like the gasket might have had a small gap, but apart from that it all seemed fine and had no blockages. I tested thermostat in hot water and it worked fine (140f thermostat), but I replaced it with a new one (160f). Mercruiser changed to 160f the new injectors and it doesn’t seem to make a difference between the two for a Carb engine. I also want to run a calorifier, so I slightly higher engine temp will help warm up the hot water tank.

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Checking the spark plugs

I checked the spark plugs to give an indication to whether there might be head gasket issue or cracked cylinder – a further test I need to do is a compression test on each cylinder – hopefully later in Sept I will get round to this. Looking at the spark plugs they are seem to be fine. See below

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The Mercruiser 5.7l Diagnostic Tool Kit

Tools you will need to undertake some of the diagnostics are listed below:

• Clear hose – size you need from the thermostat to the manifolds – 28mm in diameter, but I used 32mm. I got them from ebay.
• Clear hose from the raw water intake to the thermostat 34mm diameter, but I used 35mm hose. I got them from ebay.

Mercruiser 5.7 Engine Cooling System

Servicing the Leg