F115 carbon fouled spark plug, idle problem. Help.

[twirk]

I have an otherwise well kept/cared for yamaha f115.

The problems I am experiencing are:

- problems starting, sometimes takes too long to turn over
- fluctuating idle (between 700-900rpm)
- sometime engine dies at idle
- engine dies when going from trolling to higher rpms (true for reverse as well as forward)
- top rpm is 5400rpm (my boat is an approx. 800kg 19 foot open boat, excluding engine)
- top spark plug is carbon fouled (black soot) (top meaning the cylinder that is closest to the sky)

Steps I have taken:

- sent fuel injectors off for ultra sonic cleaning. Came back spec'd as new.
- cleaned vapor separator tank and filters.
- swapped spark plugs to see if the spark plug was faulty. It was not, hence I isolated the problem to the top cylinder.

I feel I have isolated the problem to be at the top cylinder. Electronics, fuel injector or something like that may be the problem. That is my thinking right now.

Any ideas what could be happening here?

 

[Capt Ken]

Re: F115 carbon fouled spark plug, idle problem. Help.

Have you had it checked with the diagnostic system to see if a sensor is out of wack? Has anyone messed with the intake balance adjustment? What is your fuel pressure?

 

[twirk]

Re: F115 carbon fouled spark plug, idle problem. Help.

Have you had it checked with the diagnostic system to see if a sensor is out of wack? Has anyone messed with the intake balance adjustment? What is your fuel pressure?

I haven't had a diagnostic check done. I don't believe anybody here in Greenland has such a tool.

Nobody has messed with the intake balance adjustment. I went on a long journey last year and the engine began giving me problems on the last leg of my tour. Unless something actually rattled loose nobody has toyed with the engine (I am not even sure what the intake balance adjustment is)

I have not tested the fuel pressure. I will see if I can get a hold of a pressure meter for that.

 

[twirk]

Re: F115 carbon fouled spark plug, idle problem. Help.

I also have a mariner 115 efi (which is the same engine as the Yamaha f115) that has a busted power head. All the other parts are in perfect order though.

My plan was to begin to swap parts over from the mariner to the Yamaha until the problem is fixed.

Perhaps starting with the ignition coils.

 

[Capt Ken]

Re: F115 carbon fouled spark plug, idle problem. Help.

I guess things are different over there. Here in the states, Mariner never sold a four stroke and the last year for Mariner was 99

 

[twirk]

Re: F115 carbon fouled spark plug, idle problem. Help.

I guess things are different over there. Here in the states, Mariner never sold a four stroke and the last year for Mariner was 99

Basically the mercury 115 efi, mariner 115 efi and the Yamaha f115 are the same machine. They are all four stroke. I think they only sell the mariner motors in Europe.

 

[twirk]

Re: F115 carbon fouled spark plug, idle problem. Help.

What is your fuel pressure?

Capt Ken. I got pressure gauge equipment and here are some results.

I used a normal air pressure gauge for car tries for the fuel pressure test. The seal SEEMED to me, to be good, but I am not sure that the connector type was the proper size for the fuel rail valve.
• fuel pressure is 30 psi at idle.

I did the following test incorrectly I believe. I did NOT remove all spark plugs and did not run WOT. I ran at idle. I will run the test again correctly and post results.
• cylinder compression is around 120-135 psi at idle, in every cylinder.

My thought now is that the VST is clogged. I took it apart a few weeks back and cleaned out the VST filter. There was just a little bit of debris, nothing serious. But I believe I filled my tank with debris filled fuel when I was in North Greenland. Which could explain why problems began arising when returning from North Greenland.

There might be crud in the fuel filter on the back of the engine, restricting fuel flow. I've changed my external water separator filter, so that I know is not a problem.

My fuel pressure regulator might be faulty. It was VERY rusted. I can exchange that and see what happens.

Right now I have the boat in my shop with the lower drive in a barrel of water so I can run tests on it.

 

[Capt Ken]

Re: F115 carbon fouled spark plug, idle problem. Help.

VST pressure should be 40+ psi and normally in the higher 40's.

 

[twirk]

Re: F115 carbon fouled spark plug, idle problem. Help.

VST pressure should be 40+ psi and normally in the higher 40's.

Thanks Capt Ken.

Today I will try switching out the high pressure VST pump to see if that clears things up.

Could the Pressure regulator (that little hat thingy on top of the fuel rail) be stuck and therefore letting pressure out? Do you think that is a possibility?

Do you think a clogged fuel filter (on the back of the engine in that clear bulb with the red floater) could be an issue? I don't see any visible debris in the filter though.

 

[Capt Ken]

Re: F115 carbon fouled spark plug, idle problem. Help.

Those regulators get rusty but have not seen one fail yet. You haven't replaced the filter on the engine?

 

[twirk]

Re: F115 carbon fouled spark plug, idle problem. Help.

Today I:

• replaced high pressure fuel pump
• replaced pressure regulator
• replaced ignition coils
• checked for leaks
• run the engine without fuel filter on back of engine. Just to see if obstruction was the problem.
• When I turn the key one notch, the fuel pressure spikes at around 40 psi and drops to 36psi (or so). When I start the engine it drops to around 30 psi and stays there.

So, the problem persists.