250 ox66 backfire

[flyin fisher]

have an 01 250 ox66 i recently purchased and put about 35 hours on it. have noticed the last 2 times iv'e pulled the boat and flushed on the muffs that it will backfire 4 or 5 times during a 5 minute flush. when it does this this is a noticable pop and a puff of smoke from the exhaust. i also run a hose on the fitting on the cowl in addition to the muffs. also for some reason it idles at 1100 rpm out of the water but around 700 or 800 while in the water. I didn't know if this was normal or not. any help is greatly appreciated.

 

[rodbolt]

Re: 250 ox66 backfire

its broke

get it fixed before it burns a piston.

when was the last complete service?
the ECU does not care if its in or out of the water the idle speed will remain the same.
when was the last O2 sensor test/cleaning?
when was the last VST cleaning?
when was the last spark plug cap test?
install the diagnostic test lamp and check for any codes.
that motor has a tendancy to eat low pressure fuel pumps. test them carefully.
I would also test the themosensor output but a session with the test lamp will tell if its working or not.
if the stbd side Tstat is open it will tell the ECU the engine is cold and you get a rich/high idle.
will show up as a constant code 33.
a code 33 will be normal until the engine is warm,2 min or so, then it should revert to a code 1.
if it reverts to a code 1 we just eliminated a lot of chasing and can concentrate on issues the ECU does not monitor.

 

[flyin fisher]

Re: 250 ox66 backfire

thanks for the quick response rodbolt, As soon as I purchased the boat I went through the engine,
-new water pump
-new thermostats
-new bypass valve
-3 new diaphram pumps
-new racor, inline filter, and cleaned filter in vst
-new plugs
-lower unit oil
-adjust tps
I'll try the O2 sensor, spark plug caps, and the diagnostic lamp,

 

[flyin fisher]

Re: 250 ox66 backfire

Backfire fixed. After I received the gasket kit for the O2 sensor, I pulled it off and cleaned it, though it didn't appear to be too dirty. Also decieded to clean the plugs and in the process of removing one of the plug wires realized it wasn't on tight. Reinstalled plugs and plug wires and she ran fine. Guess it just goes to show you should never overlook the small things like loose connections.

 

[andy6374]

Re: 250 ox66 backfire

the ECU does not care if its in or out of the water the idle speed will remain the same.

Rodbolt I'm certainly not one to question your advice, but don't all outboards idle higher on the muffs due to lack of backpressure on the exhaust?

 

[rodbolt]

Re: 250 ox66 backfire

most yamahas dont have any back pressure issues as most dont use adjustable idle air, motor hieght may affect some if the exhaust reliefs are under water, EFI does NOT rely on engine vacum to move fuel and idle speed is controled by the ECU mapping and is NOT backpresure sensitive per se.
the oxygen sensor is, so it may hunt more(HPDI) out of the water than in.
but the OX66 motors will idle the same in or out of the water.
I cant see a loose plug cap inducing a backfire on an OX66 motor.
a dirty O2 sensor can and you CANNOT see the O2 sensor element without cutting off the protective shield.
the only way to test an O2 sensor is with the O2 sensor test harness and the marine tech manual, not the service manual.
the tech bulliteins are not in the workshop manual yet.
on the OX66 motor the ECU cannot tell if the signal is good nor bad but it will react anyway.
if the sensor is falsely telling the ECU its rich the ECU WILL lean out the amount of fuel/oil injected(shortened injector ON time) and operation above 4000 RPM can lead to piston filure.
on the 3.1 motors it does not take many lean sneezes to wipe a lift pump diaphram.
andy, this aint a jonny rude that you set at 1100 on the muffs to obtain 750 or so in the water .
this is an electronically controled 3.1L engine that it either works as designed or its broke.
the default RPM on the 3.1 OX 66 is about 1000 and timing is usully fixed at 7degrees BTDC.
some fault codes will fix injection rich some just fix timing. right now I am in Caracas Venezuela and cannot recite from memory which fuel/ign systems do which.
a faulty T-stat or a failing temp sensor wil cause an excessive 1100 RPM idle but will show up as a code 33, 33 means the engine is cold and in warmup mode. after the temp sensor hits about 130 degrees or so it will revert to code 1 which indicates normal or it wil display the next lowest numerical code in the memory.
that code must be cleared before anyother codes can be accessed.
for instance a code 13 and a 15 can be stored but until the cause of code 13 is corrected you wont see the code 15.

 

[andy6374]

Re: 250 ox66 backfire

andy, this aint a jonny rude that you set at 1100 on the muffs to obtain 750 or so in the water .
this is an electronically controled 3.1L engine that it either works as designed or its broke.

8)
One of days I am going to get a new engine....got lots of learning to do. The easy day of no ECU's and plain carburation will be gone.

 

[rodbolt]

Re: 250 ox66 backfire

andy
pretty much so. actually EFI is much simpler than carbs however I can neglect my carbed motor and for 1.5 hours and a couple carb kits be bck in action. let the medium pressure pump go bad due to stale fuel and its 750 dollarsin parts and your back in action.
ill see how my new 175 E-Tec does, it should be ready when I get back from caracas.