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1986 25 foot Mercruiser 350 engine questions

V

Vai1953

New member
Hi All,
I'm new here, I've been a mechanic all my life. Owned boats for most of it, with outboard engines. I've had only two in board, and I'm working on the second one now. I bought the boat as a basket case, last year. The boat sat for 6 years. I've pumped out 70 gallons of gas, I've rebuilt the carb, replaced cap, rotor, plugs, new water pump on motor and in the lower unit, changed water separated,changed oil and filter, changed the Thermostat sensor. I've had it in the water about 6 times and each time I've been towed in. It would run great, but it felt like it didn't have the power it should, it would get up on plain and would run for about 10 minutes then start to loose power, and oil pressure would come down. Then stall out and not start again, would act like it was hot, but the reading on the dash would show 140 to 160. I would bring it home, the next day I would hook up muffs and start it and it would run great idle nice. One thing that keeps throwing me too, is that every time I check one or two plug wires were off on the port side only. I've tightened the connections to make sure they don't come off. I've checked the oil to make sure there isn't any water bubbles in the oil or any white clumps around the fill cap or on the stick. I'm thinking maybe some blow by coming from around the heads and exhaust manifold, kicking off the plug wires, a head gasket? There's no sound or any noise coming from around the head or exhaust manifold. Anyone have any ideas?
thanks for any help, Vince:(
 
Did you clean the fuel tank after the 70 gals were removed? checked the anti-syphon valve and the condition of the fuel line?

You need to figure if you are loosing fuel or loosing spark when it stalls out...not sure what "oil pressure would come down" means - drops 10 lbs or goes to zero...

on the plug wires, are the looms still there? proper set of wires or some hacked auto set?
 
Hi mako mark, what do you mean buy a anti siphon valve? The fuel line is good, I installed a shut off valve before the fuel filter and ran a wire thur the hose. No I didn't do anything with the tank except add a couple of cans of dry gas in it. There's no way to get the tank out to even look down the gauge hole. I put about 60 gallons of fresh gas in also.
When I start loosing power the oil pressure drops. I keep wondering if maybe a head gasket. Tomorrow, I'm pulling the valve covers to see if there are any lifters that are clogged or bent rocker arm. I keep hearing a click every once in a while.
 
Anti-syphon valve = a check valve, usually installed at the fuel tank pickup tube, for safety concerns...it is a restriction and any crud in the tank can stick in it and reduce the fuel flow.

A compression test wouldn't hurt but I think you'd be best served figuring out if your stall is due to fuel or spark...
 
As he said in his first response,

1. when this issue occurrs you must determine if it is fuel or ignition.

just radomly looking for issues wont help.

If an engine slows or slowly dies (stops running) of course the oil pressure will drop.......typically under 1000 rpms on a warm motor oil pressure can drop to 10 psi.....very normal and common especially for 1986 350.....


A compression test also sounds like a good thing at this point. Seeing as it has sat for so long. Good base point to see what the cyclinders/valves are doing.

Also find the gas vent line, Make sure it is clear of cob webs and bugs...........if it is clogged then a vacuum will occur in the gas tank........
 
Well did the compression test and it wasn't good, looks like the heads are coming off. Port bank were 130, 95, 95, 70, starboard bank wasn't to bad except for one cylinder, 135, 50, 150, 120. I'm going to pull them, do a valve job, check and clean lifters and push rods, have the heads checked for any cracks or other damage. The engine has sat for awhile I don't think the block or pistons would be the problem, but a possible crack in one of the heads is a good possibility.
 
Not for nothing my 85' engine went was really my fault torqued down a corner bolt too tight when i was installing a new head.

So i acquired a whole boat for just the engine, anyway it was a 86' this was about 5 years ago.

I had it out so i turned it over and pulled off the oil pan.

I did a mild rebuild.

Just sayin it might be time for your engine.
 
I would like to pull it and do all the work on the bench, but getting it out would be close to impossible. Sitting on the trailer she sits 8 and a half feet tall, if I end up haveing to pull it, going to take a day to make an A frame big enough to hook a chain to it for the come a long. It's something that I have thought about, but not looking forward to.
 
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