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Bizarre behaviour from Mercruiser 5.0l MPI which starts but then won't restart?

S

snapperscott01

New member
Hi all,

I have a bizarre thing happening with my 08 mercruiser MPI 5.0l. It will start perfect first time but then won't restart. It is doing some really weird things in the driveway as some days it will start 5 times in a row with no issue and the very next day it will start once and then not start again.

My fault finding to date has consisted of:
-checking both batteries which have been tested and are perfect
-voltage through the ignition fuse if fine no leakage and starts with over 10V although I haven't been able to test the voltage when it won't start yet.
-I have replaced the distributor and rotor which was in very poor condition completely covered in corrosion.

I have no other ideas of what could be wrong as some days in the driveway with the muffs on with the water on it starts perfectly over and over again and it won't cause the non-starting issue and other days it will start once and then won't start again.

I'm really confused on what could be going on here and my mechanic has no idea either.


Any ideas?
 
Ayuh,.... Is it crankin' over on the starter, 'n not firin' up,..??

Or not doin' Nothin', when ya turn the key,..??
 
bondo said:
Ayuh,.... Is it crankin' over on the starter, 'n not firin' up,..??

Or not doin' Nothin', when ya turn the key,..??
Click to expand...

Sorry yeah cranking over perfectly just not firing on the times that it doesn't. The bizarre bit is that it nearly always starts first time and then won't start 1 minute later. That tells me that everything that is should be working is working but then I can't explain why it won't start (but will happily crank over) again.
 
You`ll need to do a few things to help narrow it down. Easiest to do is pull the main engine harness plug and check for corrosion on the pins.
Then grease the pins and install it with the clamp. Just snug on the clamp, dont overdo it
Get a led light or a instrament light and wire it to the + side of the coil. You will have power when lit, when it dies check to see if its still lighted or went out with the key on.
If not lit ,your losing the 12v power feed to the engine. start at the Ign switch and work it back to the motor.
Check to see if there is a enclosed fuse holder with a Red wire on the top of the battery. This powers the ECM and without power it wont run.
Battery fumes are very corrosive
DSCN3463_zpstk1wqgac.jpg
 
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Bt Doctur said:
You`ll need to do a few things to help narrow it down. Easiest to do is pull the main engine harness plug and check for corrosion on the pins.
Then grease the pins and install it with the clamp. Just snug on the clamp, dont overdo it
Get a led light or a instrament light and wire it to the + side of the coil. You will have power when lit, when it dies check to see if its still lighted or went out with the key on.
If not lit ,your losing the 12v power feed to the engine. start at the Ign switch and work it back to the motor.
Check to see if there is a enclosed fuse holder with a Red wire on the top of the battery. This powers the ECM and without power it wont run.
Battery fumes are very corrosive
DSCN3463_zpstk1wqgac.jpg
Click to expand...

Motor has been starting perfectly all week in my driveway so i took it down to the river to see what would happen. It started perfectly and we took it for a 15min drive without any issues. Back at the ramp I turned the motor off and waited for the car and trailer to arrive and 5mins later when they arrived it wouldn't start again. Multimeter says still plus 11V all the time it was trying to start so its not the fuse on the starter motor. Anyway took it home and it wouldn't start until about another few minutes later when i tried it again with full throttle and it reluctantly started. Now it starts again perfectly without any throttle.

Any ideas?
 
Before you pull all your hair out trying to find this intermittent problem......when it gets to the point where it will not start, pull all the spark plugs and do a compression test. If it is slightly hanging a valve in the cylinder head, it will not start.


A little water on the valve stem, whether it be sea water or condensate, will cause this condition.

Condensate is caused by running your engine warming it up and shutting it down.
 
Descriptions still leave a bit to be desired.....sounds like the engine will crank over but no start....I'd be inclined to check the voltage @ fuel pump...could be the relay has given up the ghost for its contacts.....
 
makomark said:
Descriptions still leave a bit to be desired.....sounds like the engine will crank over but no start....I'd be inclined to check the voltage @ fuel pump...could be the relay has given up the ghost for its contacts.....
Click to expand...

Cheers Makomark. Yes the motor cranks perfectly just won't start. I'm assuming the voltage at the fuel pump relay should be around 11V? There is definitely fuel getting to the injectors because when it doesn't start the fuel smell is pretty strong and I can see it coming back through the throttle body out the air filter.


Scott.
 
Docksidemarineservices said:
Before you pull all your hair out trying to find this intermittent problem......when it gets to the point where it will not start, pull all the spark plugs and do a compression test. If it is slightly hanging a valve in the cylinder head, it will not start.


A little water on the valve stem, whether it be sea water or condensate, will cause this condition.

Condensate is caused by running your engine warming it up and shutting it down.
Click to expand...


If you can see fuel coming back through the Spark Arrestor, you need to do what I suggested.
 
Need to look @ the feed to the ECM and Coil. If it cranks OK starter motor fuse not an issue. Make sure all volt measurement are made with meter negative to block ground. If compression is good and you have fuel what is left is no spark. Since you have fuel smell, too dangerous to use the old trick of seeing if you spark to ground from a disconnected plug wire, so... if you have a timing light, hook it up and see if it fires ( once every other crank revolution)
 
sandkicker said:
Need to look @ the feed to the ECM and Coil. If it cranks OK starter motor fuse not an issue. Make sure all volt measurement are made with meter negative to block ground. If compression is good and you have fuel what is left is no spark. Since you have fuel smell, too dangerous to use the old trick of seeing if you spark to ground from a disconnected plug wire, so... if you have a timing light, hook it up and see if it fires ( once every other crank revolution)
Click to expand...

Problem is I can't replicate the problem.

One thing i have noticed is that the seal around the cap and rotor is pretty worn? When I replaced the cap and rotor a few weeks ago I noticed that the seal around the sensor/plug or whatever the thing is that comes into the cap was really worn so I think that air is probably getting into the cap and rotor. Would it be likely that air getting in could cause any spark issues?
 
The sensor on your distributor assembly, is there a wiring harness plugged into it or is it open and not being used like the open one in the following link, http://www.marineengine.com/boat-forum/attachment.php?attachmentid=12698&d=1453875235&thumb=1

Condensation build up inside the distributor could be a concern in high humidity areas or rain, swimming, etc. water runoff too. http://www.marineengine.com/newparts/part_details.php?pnum=MER898141

What stern drive do you have? Bravo or Alpha?

Like mentioned, connect a timing light up to first the high tension coil wire running between the distributor cap and ignition coil and watch the flashes for consistency at each time you crank the engine whether you think it's going to start or not time wise. If the light flashes but it won't start, move the timing light inductive pickup sensor to each plug wire down by the spark plugs and check for flash consistency as the rotor turns at cranking speed.

What did the spark plugs look like when you did the engine cylinder compression test? Did you try new spark plugs while you had them out?

Next time it won't start, try disconnecting the gray wire off the back of tachometer gauge and see if it may start more consistently....But before you disconnect the gray wire, look to see if the RPM gauge is moving up to almost 400 RPM during cranking/before actually starting/running. If the RPM gauge is not moving then there may be something wrong with the crank reference single. An internally failing tachometer could cause a no start/spark condition.

Before all that, try cleaning your flame arrestor, throttle body assembly, IAC hole (idle air control valve) and order and install a new IAC muffler which should be item number 6 in the following link below but having your engine serial number would help confirm the accuracy if this is your throttle body as designs have changed over the years starting in n around your 2008 model year.

^ As another test when it won't start, advance the throttle only to around quarter throttle to allow additional air to the engine to see if it helps any.

^^ if all good you'll want to check fuel pressure to see where it's at before and during the event.

Does it seem to matter if the engine is hot, warm or cold when it won't start?
What were the compression numbers?

http://www.marineengine.com/parts/m...310000-thru-0w649999/throttle-body-mechanical
 
snapperscott01 said:
There is definitely fuel getting to the injectors because when it doesn't start the fuel smell is pretty strong and I can see it coming back through the throttle body out the air filter.


Scott.
Click to expand...

This will only happen two ways. One way is if a counter rotating starter was installed (not your problem as the engine will start and run, unless for some bizarre reason your starter decides to spin backwards sometimes, may be worth checking, I have seen stranger things) Or a hung intake valve.
 
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Have you tried manually holding the throttle butterfly wide open as someone cranks on the starter. Sounds like your just flooding to me? Possible leaking injector/injectors when you shut it down the fuel keeps spraying out the injector and floods the manifold. If it pops when you give it more air have the injectors cleaned and tested or replace them.
 
kimcrwbr1 said:
Have you tried manually holding the throttle butterfly wide open as someone cranks on the starter. Sounds like your just flooding to me? Possible leaking injector/injectors when you shut it down the fuel keeps spraying out the injector and floods the manifold. If it pops when you give it more air have the injectors cleaned and tested or replace them.
Click to expand...

Yes I agree it is just me flooding it when I've been trying to get it started. Biggest problem now is I just can't get it to do it again.
 
Another possibility is was it slugging water, do you have a separator/filter in the fuel supply? When if it happens again pull a couple plugs and see if they are wet and see if it burns with a lighter.
Also check for fuel in the fuel pressure regulator vacuum line to the intake manifold possible leaking regulator sucking raw fuel into the manifold.?
 
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kimcrwbr1 said:
Another possibility is was it slugging water, do you have a separator/filter in the fuel supply? When if it happens again pull a couple plugs and see if they are wet and see if it burns with a lighter.
Also check for fuel in the fuel pressure regulator vacuum line to the intake manifold possible leaking regulator sucking raw fuel into the manifold.?
Click to expand...

Bad fuel was my first suspect as the only thing that had changed was i filled the boat up with fuel. But I took the separator off and emptied it and there was no water in it. So I can rule that one out.
 
Docksidemarineservices said:
Before you pull all your hair out trying to find this intermittent problem......when it gets to the point where it will not start, pull all the spark plugs and do a compression test. If it is slightly hanging a valve in the cylinder head, it will not start.


A little water on the valve stem, whether it be sea water or condensate, will cause this condition.

Condensate is caused by running your engine warming it up and shutting it down.
Click to expand...
Question regarding the condensate cause. Will it evaporate over time so the motor starts again? Or do the plugs need to be pulled to verify.
 
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