3.1 225 OX66 nightmare hard start, dies, no WOT

[wiredrods]

I need a hero. I was an OB tech for 20 years, even with Yamaha in the late 90's when the ox66 came out. This one has me stumped. Cold start, cranks up and dies. Fuel pressure never varies from 35 psi. VST clean, sent out injectors for pro clean at Flagship after I cleaned them myself, tested perfect and no change from when sent in! blinky winky no codes. Set tps after checking, no change. Removed O2 sensor, cleaned and checked (although I was under the understanding this does not affect cold start) even left unplugged, no change. Replaced all three low pressure pumps, no change. Starts up, will run at low idle (600 rpm) cold ( no cold start higher rpm's), goose the throttle and it falls flat and quits. Sometimes it will rev right up! Changed plug wires ohmed out the old ones, perfect. Ohmed the coils, perfect. after reading a few posts, I unhooked the knock sensor and started, no change. So, I have eliminated the fuel system, compression and spark. From my understanding, the O2 sensor if unplugged will give a rich signal, so cold start should be fine. The knock sensor is not in the loop until started, so that doesn't make sense. Coolant sensor maybe thinking it's warm? Once hot, you leave it sit for an hour and same thing. I keep thinking an ECU, but Yamaha's ignition is so bullet proof! HELP!!!

 

[rodbolt]

Re: 3.1 225 OX66 nightmare hard start, dies, no WOT

have you tried testing the main relay output when it dies?
how about stator output?
your issues sound electrical to me but not like a CDI failure.
without a model number I cant say anymore.
do you know the difference between CCS and Non CCS 3.1l motors?
do you have a Kv tester?
all outboard techs should.

 

[wiredrods]

Re: 3.1 225 OX66 nightmare hard start, dies, no WOT

That's what's weird...doesn't just shut off, kinda just slows down to a point it won't stay running. It's a V-Max if that helps...I do have a Kv meter, that rediculously high priced Mercury marine magic one. Mercs are easy, though...usually a vented block or a stator is the problem with them!

Another weird thing it does: occasionally, at about 800 rpm, it sounds as if it is trying to clear a cylinder...like an old flooded Johnson. Da, dddd, Da then it will gain a couple hundred and fall back off. The 3.1 wasn't around when I left, so no idea on CCS, non-CCS motors.

Weirder still, at 3-4000 it runs perfect! The ocassional miss points at electrical, but the dies off has me stumped. I gave up for today, missed the good fishing tide anyways. One other post said grounds...typical stuff that I already checked, but...I'm going there again. I taught this stuff, and the simple was 90% the problem. I keep reverting to the basics and have to be missing something.

 

[rodbolt]

Re: 3.1 225 OX66 nightmare hard start, dies, no WOT

mercury never made a kv tester. they did have an analog DVA analog meter.
the Vmax does not use CCS.
if you unplug the knock sensor the engine should idle about 1100 RPM.
test primary CDI outputs with your merc meter.
it aint hard,looks hard but it aint.
its an incredibly simple system.
the CTS is d irt simple to test with an ohm meter.
but it sounds like there is an electrical issu
care to post a model number so we get an idea of what we are disscussing?

 

[wiredrods]

Re: 3.1 225 OX66 nightmare hard start, dies, no WOT

Sorry it took so long, boat at a friends. VX225TLRC Reset TPS, no change...idling cold I unhook temp sender, idle goes up to 800 and seems smoother, I assume because it is going rich. Unhook knock sensor, goes up about 100 rpm, no to 1100. Unplugged o2 sensor makes no difference. In neutral, engine also seems to go flat at about 1200 rpm, like it is bogging or going lean?

 

[rodbolt]

Re: 3.1 225 OX66 nightmare hard start, dies, no WOT

if rail pressure remains good its either fuel or ign.
test the ign with a DVA type peak reading meter and check the injector filters.
ign timing is computer controlled, however if anything fails the system reverts to a locked base timing of 7*BTDC.
depending on the failure it may or may not adavnce further.
the O2 sensor isnt really monitored but if you unplug it fuel is fixed rich at low speed and lean above 4000.
but without test equipment sometimes its best to quit while your ahead and have it fixed before it dies.

 

[wiredrods]

Re: 3.1 225 OX66 nightmare hard start, dies, no WOT

Thanks Rodbolt, turned out to be the cts...