Mercruiser 170 Rough Idle on Start and High Oil Pressure

[Adnieder15]

I recently purchased an '86 Chaparral 187 with the infamous Mercruiser 170. It's been well maintained over the years and serviced it this spring with new ignition, spark plugs, gimball bearings, and water pump. I also had the carburetor cleaned and adjusted by the mechanic (but not rebuilt).

The past week the motor has been idling very rough on start-up and sputters out if left in neutral. I have to choke it at 1500+ rpm for several minutes, if not drive it at 2000+ rpm for a couple minutes before it calms down and idles normally. Also at start-up, I noticed the oil pressure jumps immediately to 80 psi and stays there until the engine finally begins to idle normally.

On top of that, when the engine is initially idling rough I can also see oil or gas stains in the water, and occasionally smoke from the lower unit exhaust.

Any recommendations on where to look first? I know these engines tend to idle rough initially, but it seems somewhat extreme to have to drive the boat for several minutes before it can idle without shutting off.

Included is a video of the engine idling shortly after startup normally about two weeks ago before the issues began.

**Also, can anyone tell me what the white liquid container holds under the engine and spark plug cables? Bottom left of the picture.

 

[nola mike]

Ok ...
No such thing as cleaning the carb from the outside. 99/100x your problem is fuel delivery. Have you checked/cleaned filters? How old is the gas? Do you have a fuel/water separating canister filter (highly recommend to add one if not). The stains in the water and rough idle sound like it's running rich. At idle if you look down the barrel of the carb do you see fuel dribbling? That engine sounds like a tractor at idle, but it should idle like a happy tractor. Oh, and that's your coolant overflow tank. You need to get real familiar with the coolant system on this. You absolutely 100% need a factory service manual, and know off the top of your head that section 6 is the coolant system, section 5 is fuel, section 4 is electric, 4e is wiring diagrams (that may be a bit extreme, but you might have no choice).

 

[Eric 3.7LX Alpha 1]

I agree with Mike. Here is a diagram of the coolant flow for the engine, which shows where the coolant bottle connects to. I also have a TSB attached showing timing was lowered down to 4 degrees.

 

[Scott Danforth]

any good mechanic knows to clean a carburetor is to remove the carburetor, pull it apart, go thru every passage and then install new gaskets and re-assemble the carburetor.

as Mike stated there is no way to clean the carburetor and not "rebuild" it.

 

[Bondo]

I recently purchased an '86 Chaparral 187 with the infamous Mercruiser 170. It's been well maintained over the years and serviced it this spring with new ignition, spark plugs, gimball bearings, and water pump. I also had the carburetor cleaned and adjusted by the mechanic (but not rebuilt).

The past week the motor has been idling very rough on start-up and sputters out if left in neutral. I have to choke it at 1500+ rpm for several minutes, if not drive it at 2000+ rpm for a couple minutes before it calms down and idles normally. Also at start-up, I noticed the oil pressure jumps immediately to 80 psi and stays there until the engine finally begins to idle normally.

On top of that, when the engine is initially idling rough I can also see oil or gas stains in the water, and occasionally smoke from the lower unit exhaust.

Any recommendations on where to look first? I know these engines tend to idle rough initially, but it seems somewhat extreme to have to drive the boat for several minutes before it can idle without shutting off.

Included is a video of the engine idling shortly after startup normally about two weeks ago before the issues began.

**Also, can anyone tell me what the white liquid container holds under the engine and spark plug cables? Bottom left of the picture.

Ayuh,..... Welcome Aboard,...... Sounds like the choke is working, maybe abit to rich, hence the rough idle, til it warms, 'n the choke is pulled off,.....
As for the oil pressure,.... Cold oil is thick oil, hence the higher pressure, til it warms, 'n thins,.....

 

[Adnieder15]

Ok ...
No such thing as cleaning the carb from the outside. 99/100x your problem is fuel delivery. Have you checked/cleaned filters? How old is the gas? Do you have a fuel/water separating canister filter (highly recommend to add one if not). The stains in the water and rough idle sound like it's running rich. At idle if you look down the barrel of the carb do you see fuel dribbling? That engine sounds like a tractor at idle, but it should idle like a happy tractor. Oh, and that's your coolant overflow tank. You need to get real familiar with the coolant system on this. You absolutely 100% need a factory service manual, and know off the top of your head that section 6 is the coolant system, section 5 is fuel, section 4 is electric, 4e is wiring diagrams (that may be a bit extreme, but you might have no choice).

1) Good point about the carb. I know my mechanic took the carb apart and cleaned it, so not sure how a rebuild would have been any different.
2) I'll check the filters and see if they need cleaning. Don't know if there is a fuel/water separating canister filter.
3) the previous owner who used the boat the past 5 years constantly topped off the tank with '87 octane, so I ran the boat to under 1/4 tank before I refueled it with ethanol-free 91 octane, and have been using this octane since then.
3) I'll check the carb today for any fuel. Sounds like it could be worth it to take it apart and clean it out completely.
4) any recommendations where I can find the factory service manual for this model?

Thanks all. I sure chose an interesting boat as a first-time buyer. Time to study up on this motor.

 

[crownline210ls]

B.O.A.T.= Breakout another thousand. Good luck with it. Sounds like fuel issue. Find your filter and check it out maybe on the pump itself in the tank. And some fuel treatment and keep running good gas and see if it clears up. Don’t spend your summer working on it. Rub it