Non start Mercury 5.0 V8

Tailsme

I have a 312 with twin mercury EFI 5.0 V8, on rare occasions (today being one) I try to start one of the engines and it will not fire up. It cranks over, I have fuel and spark but it will not start up, when I stop cranking the engine smokes out of the air filter breather at the top. This has happened on 1 occasion a few months ago and the next day I went out and the engine starts up no problems and I have had no issues since until today. I am thinking this is an intermittent electrical problem or sensor issue but do not know where to start. Any pointers would help. Thanks in advance. Darren

212rowboat

Swap cam position sensors with operational engine next. 

212rowboat

nah... it's not hard to get to at all believe it or not... it's actually easier than the Mopar way of doing it, which is under the distributor cap.

212rowboat

Smoke?  Or, air/fuel mixture? 

I'm wagering a/f.... I'd swap the fuel regulator from the other engine and see if the issue follows it... 

Tailsme

Hi drewactual, yes it is air/fuel mixture coming from the air filter breather. I will try the fuel regulator, I believe I read somewhere that these can be very temperamental. I have a busy week but will try this by next weekend and let you know. Thank you for your advise.

Tailsme

I tested the pressure on the non start engine and I get 36lbs, I get the same on the engine that starts so I guess this means it cannot be the fuel regulator? Any other ideas. Engine cranks over nicely but will not start, when I stop cranking I get air/fuel mixture blowing out the air filter breather so I must be getting spark and fuel. Is there a sensor I should be checking? Any advice is much appreciated, not sure where to go from here. Thanks

212rowboat

Swap coils with other engine and see if issue follows it. 

Tailsme

Changed the coils but did not solve the issue, still non start on 1 engine and the other engine starts with the other coil. Any other ideas?

Tailsme

Hi RinkerYan, cannot find any. Would this not stop me from getting a spark if there was? I must have a spark and fuel because I have gases coming out of the air filter breather at the top which I assume means I have fuel igniting in the pots?

212rowboat

That's right, RY... You'll be one of the only boats moving... You'll have to rename it Jenny, and go into the shrimp business.  

mvn

RinkerYan said:

I have only had Gas boats with points, etc. Never had an electronic controlled motor but I would think there would be one. I had one on my old boat. It was a red push button breaker for the whole motor. I think what you see is just compressed vapor. Now I have Diesels with no electronics. I will also be able to start them after an Electro Magnetic bomb goes off too.

RY,

Remember this really bad '70's movie?  A solar flare knocked out everything electrical.  You would have survived this one too!

Mark

Tailsme

Hi RinkerYan, Yes I have a red button and it is a breaker, I have reset the breaker but still nothing.

Tailsme

Hi Drewactual, Do you know where I will find the cam sensors? Sorry but I do not know what they would look like. Thanks

212rowboat

I would think the CPS (camshaft position sensor) the most likely, but also the CKPS (crank shaft position sensor) could also be causing the issue.

212rowboat

with the other engine right beside it, and these sensors being easy to swap- I'd swap 'em and see.... it could be, simply, a bad connection.

Tailsme

Thank drewacyual, I will try tomorrow and let you know, looks easy enough to get to.

Tailsme

So today I changed the crankshaft sensor from the good engine to the non start engine and got the engine going, strange thing is I put the crankshaft sensor from the non start engine to the other engine and this engine also started? I can only assume I had a bad connection although the connection seemed good on inspection. I have stopped and started the engines a couple of times since with success and will try again in the morning. Thanks for your help Drewactual.

Michael T

Nice work Drew! MT

212rowboat

If he hadn't said it runs sometimes, I would have guessed timing chain snapped.  something was causing bad timing, as evidenced by the build up and release of fuel and air mixture, and quite frankly no backfiring.  

Glad it was simple... 

Now... Stop and think a minute about how dangerous a situation that is and the damage that could be caused. Not only to the boat.  

Unspent a/f ain't nuthin to be playing around with...

Michael T

Oh yeah - 20:1 is bomb makin' territory :-( MT 

Dream_Inn

Very interesting topic and I want to make sure I learn something from it.  (also, Tails - great you figured it out!)  So, I see from the picture where it is, but details of exactly where it is would be nice.  The diagram seems to point to the pulley.   Tails, since you removed it maybe you can provide some details?  Thanks!  Always learning new things on here!!