Overheating

[duhn]

I have a 1986 Merc 150 blackmax. Took it out last week and was running like a champ. As I was loading the boat on the trailer I noticed that there was no water coming from the pee hole. The motor obviously got hot. I don't know how hot it got. I replace the water pump and the thermostats. the t-stats were the problem because they didn't open up when I put them in boiling water. I got everything back together and it took a while for it to get started. After it started it was no problem starting afterwards. However, It's not running like it should. It takes a while for the motor to rev up. I ended up doing a compression check and all cylinders were between 105 to 115. What else is likely to get damaged from the heat?

Thanks

 

[redjmp]

Re: Overheating

It is possible that you melted some metal off the tops of your pistons. Pull your plugs and look for metal deposits on them. If you can get a boroscope to look inside the cylinders even better.

 

[duhn]

Re: Overheating

No metal deposits on the spark plugs. No boroscope to look at the pistons, don't even know where to start looking for a boroscope.

What else is likely to get damaged from overheating????

 

[redjmp]

Re: Overheating

Well if you don't have metal on your plugs thats a good thing! But your compression numbers are a little low. You probably scuffed up your cylinder walls a little. Or you might have warped a head and now the gasket isn't holding enough pressure... Usually the motor has to overheat enough to seize and stall before there's real serious damage.

 

[j_martin]

Re: Overheating

Well if you don't have metal on your plugs thats a good thing! But your compression numbers are a little low. You probably scuffed up your cylinder walls a little. Or you might have warped a head and now the gasket isn't holding enough pressure... Usually the motor has to overheat enough to seize and stall before there's real serious damage.

The actual compression numbers depend on the gauge, the altitude, and even the weather a little. You are right on the edge of acceptable variation, however. It could be just tired. If it's on a heavy boat, and propped on the steep side, it could let you down.

Electrical parts can be damaged in an overheat. That's where I'd look, starting with a spark, and stator charge coil output when cranking. (DVA meter is required)

hope it helps
John

 

[duhn]

Re: Overheating

Thanks for the responses. The compression numbers are consistant with the last check a year ago, +/- a few psi per cylinder. Yes I would say that this motor is tired. Alot of hours have been put on it even before I aquired it.

I will look into the electircal ignition again.

 

[duhn]

Re: Overheating

I have 2 sets of electrical ignition components except for the coils. Both sets were checked with meter, and checked out good. I ended up changing the stator, trigger and switch boxes.

I put the boat in the water because it always runs differently in the water under a load. It definetly ran better but not 100%. In neutral, it was slow to idle up to high rpm but it got there. In gear was another thing... max rpm was 1200. Alot of "lean sneezing" when I let off the throttle. There was fuel in the see through inline filter and the bulb primer was no help when I pumped it.

Any suggestions?
My luck the coils got damaged from the overheating, and I dont have another set to test with.

 

[CharlieB]

Re: Overheating

You really should do a proper spark test. Swapping out parts just because you have used parts laying around is no guarantee that you have good spark.

After confirming you have good spark, have you tried just 'bumping' the enrichener to see if the added fuel makes any improvement? If so that indicates a fueling problem and it may be time to clean carbs.

 

[duhn]

Re: Overheating

Thanks Charlie,
What would be the proper way to check for a good spark? Is there a special tool? I have conducted a spark test which seasoned mechanics would consider primitave and archaic. I started the motor with a plug removed from the block, used 2 pcs of wood as chopsticks and positioned the plug close to the block. All 6 cylinders had a spark jumping. I don't know if it was a "good" spark, but there was a spark.

 

[CharlieB]

Re: Overheating

Usually an adjustable spark gap tester is used, set to 7/16 inch. This large a gap forces the ignition to raise the voltage high enough to bridge the 7/16 gap roughly equal to that needed to bridge a .035 gap when under cyl compression.

It is possible to hold the plug wire at 7/16 from the engine to check for spark but most of us either cannot reliably gauge the distance well enough or do not like the possibility of getting BIT REPEATEDLY but holding a plug wire of a running engine!

Your local NAPA or likewise should be able to order you an adjustable spark tester cheap enough.