87 Mercury 70hp low compression and overheating

[Deepblue22]

Attempting to get this one ready and all sorts of issues but a good one to learn on. Compression is low in top cyl but with oil the comp jumps up so I think this might be stuck rings. Today I tried to run some fuel additive to see what would happen, if it could clear out potential carbon and unstick the rings. But instead the motor overheated after about 7mins at idle. The block was hot to touch esp near top cyl and the water stream was hot. The water pump is rebuilt and the water stream was not 100% of its power but I thought an ok amount of water for idle speed on a garden hose application. I removed the thermostat cover and pried out the thermostat. The thermostat was closed, and it was dry on both sides. No evidence of water in behind thermostat. Evidence of water around the poppet. I boiled the thermostat on the stove and it did open up properly. I dont have a clear understanding on the water flow path through the block but I assume there should be water present either in front of or behind the thermostat? Can anyone confirm that. Would this mean there could be a plugged passageway? Any ideas how I should proceed from here? SN is a729226. Thanks!

 

[JB]

Re: 87 Mercury 70hp low compression and overheating

Howdy.

Welcome to iboats.

How low is low compression, deepblue?

 

[studlymandingo]

Re: 87 Mercury 70hp low compression and overheating

Welcome to iboats.

We do need to know how low the cylinder is. Post readings for all cylinders.

If it IS a stuck ring, then just fuel additive alone won't free the rings. Do a thorough decarb. (If you use the search button above, you'll get several good threads on this.) I would pull the plugs, spray the cylinders thoroughly with Deep Creep, rotate the engine a few turns, and let it sit overnight. Follow the decarb steps using a SeaFoam/ Gasoline mix, run it out well.

As to the overheating, it could be a number of things; most obvious check is to verify that your water pump is in good shape, you already checked the t-stat, and you want to make sure the poppet is working properly. Has this motor been operated in salt water? Lots of people get some dirt dauber families in the cooling system over the off-season as well.

Post your readings, and we will go from there.

 

[Deepblue22]

Re: 87 Mercury 70hp low compression and overheating

Hi Guys,

The compression was 95top, and 120 in the other 2 cyl. With a shot of oil top went to 150 and the others stayed at 120. This was done with engine cold, I could not run it inside the shop. I may re-test w/ a warmed up engine at some point.

I have read most of the post on the seafoam on this board (ps the new format is great for searching). But seafoam I cannot get locally, at least sofar. I got something called small engine cleaner, it is green fluid and says to add 15mL to 1 gallon gas so that is what I have done. I ran it for 7mins, shut down for 1hr, and ran for another 7mins or so noticing smoke and some carbon evidence on the plugs so I think this is working to loosen up and burn off the carbon.

After that second test it overheated again, and I checked the thermostat was open and was slightly wet, so was the poppet area. This engine is slightly disassembled - I have the cowl mount off and also the water output spigot. I wonder if that spigot when installed would cause a constriction and increase in water pressure, causing more water to travel up to the top of the block? Maybe I will reinstall that and see what happens.

It has been in saltwater. It has also been sitting for 2 years. Is there any benefit to unbolting the exhaust cover or cylinder head, can I see into any water passageways that way to check for blockages? Will the gaskets be toast or any chance of reuse?

How do I test the poppet.

 

[studlymandingo]

Re: 87 Mercury 70hp low compression and overheating

You should always do your compression tests when the engine is warm. Where do you live? You should be able to get SeaFoam at any auto parts store, or even a big department store should have it. I don't know what type of small engine stuff you are using, but the decarb usually involves a really heavy dose of the decarb solution. Check on eBay, I would be willing to bet you can buy some SeaFoam and Deep Creep from someone there.

Before I began pulling stuff apart to check out the cooling system, I would probably go to a marine supply and get something to break down mineral and salt deposits and run it through the system.​

 

[Deepblue22]

Re: 87 Mercury 70hp low compression and overheating

Ran engine 3rd time. Reinstalled the water outlet spigot. Engine block got warm to touch but not enough for the alarm to go off.

Compression on warm cylinders: top 95, mid 110, bottom 115. Tested top with some oil added and did not change reading.

 

[Deepblue22]

Re: 87 Mercury 70hp low compression and overheating

What do you guys think about the compression readings, 95, 110, 115, would that be rebuild time or ??? It seems to start and idle alright.

 

[Don S]

Re: 87 Mercury 70hp low compression and overheating

I would get a can of Mercruy Power Tune and follow the directions on the can.
Your rings are probably clogged up do to carbon. Possibly from using the wrong oil. The Power Tune cleans it so the rings do their job.
The sea foam does the same thing. But have never used it myself.

 

[Deepblue22]

Re: 87 Mercury 70hp low compression and overheating

Ran a can of seafoam. Compression now 88, 100, 115. Removed exhaust cover. Top piston moves back and forth 1/4 inch but not completely disconnected from crankshaft. Mid piston looks black/burnt or else still has a lot of carbon on it. Bottom piston nothing stands out sofar. Looks like I got a project for the next couple of weekends.

 

[Clams Canino]

Re: 87 Mercury 70hp low compression and overheating

Ya - rip it down. And replace the waterpump and thermostat as an SOP - not a maybe.

-W