Suzuki DF140 Intermittent RPM Drop

[mirage]

Hi All,

I have a KC2400 with 3 year old Suzuki DF 140's on it. The Starboard engine is fine but the port engine drops RPM suddenly every 3 or 4 minutes. Originally this problem only occured a few months ago with the rpm above 5000. The rpm would rapidly drop off to about 3000rpm for a second or two and then rapidly recover again. But now it's doing it at rpms around or above 4300 and dropping back to about 2000rpm. Only for about a second or two and then recovers quickly. It's a major pain as now it's the rpm I want to cruise at. Between about 4000 and 4100 it is OK.

To me it seems like a fuel delivery problem, vapour lock, water, blocked filter, kinked fuel hose but I can't find anything obvious. I've had a talk to the mobile mechanic I use and he had a list of things to do including swap the batteries over and place your hand over the fuel tank filler and see if there is an air lock when it's doing it. Always been too rough to do that!

Was wondering if anyone has had a similar problem with a Suzuki or any other brand.

Thanks, Scotty.

[Roughasguts]

Sounds like some sort of limp mode, over heat sensor, water pressure sensor or similar being faulty.

Find out where they are on your motor and swap them over on your good motor and track down which one you need to replace.

Cheers.

[mirage]

Thanks RasGuts, hadn't thought of that one. Never been any warning lights or "beeps" but I guess it could be that.
If I cant find something obvious by Monday I'm taking the boat up to the dealer at Noosa and getting the diagnostic gear hooked up while we take it for a run, so I might leave that one to them and suggest it.
To me it still seems like it should be something obvious with the fuel delivery.

Forgot to mention too, it doesn't seem to do it until sitting at 4300rpm for 10 or 15 minutes. I guess everything is nice and warm then!

[Spaniard_King]

Mirage,

I hvae seen the knock sensor do this on the modern 4 stroke to guard againt poor fuel quality it retards the timing which pulls rpm out of the engine.. and sudenly too.

Other wise you would be looking at a fuel supply issue. best way would be to run it on a portable fuel tank (use 98 octane to give the best possible chance of keeping the knock sensor happy) Do this to Isolate the problem to either the boat or the engine.

Water in the fuel will also pull rpm out of the engine so I would drain the vapor seperator and ensure all other filters are free from water.

Also for the price of a new set of plugs I would throw them in as well. BKR6E for your engine.
cheers

[mirage]

Thanks very much Garry. Never heard of the knock sensor.
Yeah the rpm drop is Very sudden. There is no sign of water in the filter but I'll check it again.
Scotty.

[Monza]

Sounds electrical - like it may be dropping a cylinder.

Start with the simple stuff - spark plug / leads. I had a Suzuki four that suddenly dropped rpms when taken above 4000rpm when warm. Turned out to be a faulty spark plug.

[Why Not]

Maybe get your engine into the Suzuki service centre and have them do a computer download for a diagnostic for you? Yes its dollars; but if it sorts it out then your a happy camper a lot quicker.

Why Not

[Spaniard_King]

Maybe get your engine into the Suzuki service centre and have them do a computer download for a diagnostic for you? Yes its dollars; but if it sorts it out then your a happy camper a lot quicker.

Why Not

Yes that is an option but there is a mirid of faults that do not show up on an ECU

Generally if a sensor is out of range you will get an allarm I don't think he is gettin an alarm so most likely it wont show up on the ECU/laptop.

[krb1962]

I have 115 suzukis & occasionally have a similar issue when the boat has been sitting for a while until the fuel line is bled of air. you may be sucking air.
cheers

[tigermullet]

Or is it one of those mongrel resistor type plugs just beginning to break down?

[mirage]

Thanks everyone for your replies. Spark plugs are another one I haven't thought of, will ask the question on them!
Anyway, going up to Noosa tomorrow morning and try and get it sorted. Will let you know if anything was found.
Scotty.

[mirage]

Forgot to add, I checked again today. Definately no water in the tank and no pressure lock in the fuel tank!

[julian1]

Check fuel lines for any kniks or collapses hose also check fuel primer bulb could be collapsing internally

[mirage]

Thanks Julian, After all the trouble shooting turns out it was the primer bulb!! Learn something new every day!!
Scotty.

[julian1]

Thanks Julian, After all the trouble shooting turns out it was the primer bulb!! Learn something new every day!!
Scotty.


No worries Scotty glad to be of assistance, sometimes itcan be the simple things that can take ages to fault find !!

cheer's

Julian

[ozscott]

Glad you got it sorted mate. I have had the odd problem in the past related to primer bulb. I replace mine every 2 years now with a new US made goo quality one. I know of people who do it yearly and some never until having to prime they cant...I find 2 years works well.

Cheers

[Noelm]

Thanks Julian, After all the trouble shooting turns out it was the primer bulb!! Learn something new every day!!
Scotty.
glad you sorted it out, but if you read Spaniard Kings troubleshooting way way back, he has suggested you try another complete portable tank and hose, to isolate the problem, it would have been fixed long ago if this was tried.

[mirage]

True. I work overseas and don't have a portable tank. Anyway, problem solved.