mercruiser 5.0 mpi hard start when warm

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shoestring

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this one has me scratching my head. 2005 5.0 mpi, 150 hrs. starts great cold, shut it off when warm wait 5 min- restarts perfect, wait 30 min i have to give it about 1/2 throttle and crank for 5 seconds before it fires, definately an over rich mixture. pulled all of the injectors out and sent them out for a flow test and inspection. none of them leaked by, perfect balance and spray patterns. new fuel filter, spark plugs. i checked the pressure regulator for a leaking diaphram, thats not it. but I did notice the fuel return line at the regulator was capped. it had the quick connect fitting on the return line but the end was smashed shut and welded. looks pretty factory. anyone have an idea?
 
Bondo

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Re: mercruiser 5.0 mpi hard start when warm

Ayuh,... How nasty lookin' are the sparkplugs,..??
 
Don S

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Re: mercruiser 5.0 mpi hard start when warm

Personally, I would hook it up to a diagnostic computer and fuel pressure gauge, and see if all the sensors, temps and pressures are within specs.
This is assuming all the basics are in good shape. Things like fuel filters (check for water), tuneup, compression, etc.
 
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shoestring

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Re: mercruiser 5.0 mpi hard start when warm

Bond-o said:
Ayuh,... How nasty lookin' are the sparkplugs,..??
Click to expand...

old plugs looked great, replaced them after a seafoam treatment. new plugs are clean, normal color. engine runs great otherwise, seems to be getting fuel from somewhere while it is not running.
 
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shoestring

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Re: mercruiser 5.0 mpi hard start when warm

basics are in good shape, needed an excuse to buy a diagnostic computer anyways. i have a feeling everything is ok, but i should check it out anyway.
 
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shoestring

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Re: mercruiser 5.0 mpi hard start when warm

Don S said:
Personally, I would hook it up to a diagnostic computer and fuel pressure gauge, and see if all the sensors, temps and pressures are within specs.
This is assuming all the basics are in good shape. Things like fuel filters (check for water), tuneup, compression, etc.
Click to expand...

I did have it put on a dyno last summer to check the tach for accuracy, tech ran thru everything and gave it a good bill of health. commented on how strong it ran.
 
Don S

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Re: mercruiser 5.0 mpi hard start when warm

shoestring said:
basics are in good shape, needed an excuse to buy a diagnostic computer anyways. i have a feeling everything is ok, but i should check it out anyway.
Click to expand...

If everything was OK, it wouldn't be hard starting.

Here is the computer you need, it does it all and won't break the bank http://www.rinda.com/marine/mtspec.htm#techmate

Do you have the OEM service manual to follow? If not, you are just tinkering, what is the full model number of your engine, or even the VP serial number.
 
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shoestring

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Re: mercruiser 5.0 mpi hard start when warm

i will have to get back with the engine number, i just ordered the computer.
Don S said:
If everything was OK, it wouldn't be hard starting.

Here is the computer you need, it does it all and won't break the bank http://www.rinda.com/marine/mtspec.htm#techmate

Do you have the OEM service manual to follow? If not, you are just tinkering, what is the full model number of your engine, or even the VP serial number.
Click to expand...
 
emilsr

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Re: mercruiser 5.0 mpi hard start when warm

Are you sure this isn't a vapor lock problem? Have you tried cracking open the hatch and/or running the blower 5-10 minutes before starting?
 
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shoestring

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Re: mercruiser 5.0 mpi hard start when warm

emilsr said:
Are you sure this isn't a vapor lock problem? Have you tried cracking open the hatch and/or running the blower 5-10 minutes before starting?
Click to expand...

no, i'm not sure it's not a vapor lock problem. i always run the blower for 5 min or so before start. when i have this problem, it stumbles to life, like it is flooded. but vaporlock could definately be possible. lean vs rich starting problems are very similar with just a little different feel. hard to tell with it in the water and quiet. I don't often bet but if i did i would bet it's rich. wondering if water may be pooling in the exhaust system somewhere causing some back pressure. it seems to be worse if i anchor in rough water. i can't get it to do it on the muffs.
 
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shoestring

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Re: mercruiser 5.0 mpi hard start when warm

Any thoughts on the capped fuel return line?
 
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shoestring

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Re: mercruiser 5.0 mpi hard start when warm

Don S said:
Post a picture of it and it's location.
Click to expand...

I will do that when i get home. it is on the fuel rail just below the fuel supply inlet connection. right at the vacuum operated fuel pressure regulator. I have the fuel rail off and at home. boat is stored out of town for the winter.
 
emilsr

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Re: mercruiser 5.0 mpi hard start when warm

shoestring said:
no, i'm not sure it's not a vapor lock problem. i always run the blower for 5 min or so before start. when i have this problem, it stumbles to life, like it is flooded. but vaporlock could definately be possible. lean vs rich starting problems are very similar with just a little different feel. hard to tell with it in the water and quiet. I don't often bet but if i did i would bet it's rich. wondering if water may be pooling in the exhaust system somewhere causing some back pressure. it seems to be worse if i anchor in rough water. i can't get it to do it on the muffs.
Click to expand...

It was a common problem on certain models of Merc MPI V-8's; primarily the 454 and 502 from '97 up. The temp fix was to add an auxilary fuel pump and inline filter to increase pressure (later they re-engineered the system). I'm not aware of it being a "common" problem on the 5.0mpi, but when we had one we had this happen a couple of times in very hot weather after a long run and letting it sit for 20-30 minutes. The first time I opened the hatch, dove in the bilge and of course found nothing and it fired right up. After that I just cracked the hatch open on hot days until the heat dissipated.

If none of your other checks turn up any problems I'd almost bet this was the problem. All that cast iron under the hatch can hold a lot of heat....more than enough to vaporize the fuel in the lines if it isn't under a good bit of pressure.
 
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shoestring

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Re: mercruiser 5.0 mpi hard start when warm

that does make sense. it is pretty hot in the compartment after a good run. there is not alot of air exchange with the blower off. both grills are on the stern, nothing on the sides. plus the fuel doesn't return to the fuel cooler, seems like that would be the point of it.
emilsr said:
It was a common problem on certain models of Merc MPI V-8's; primarily the 454 and 502 from '97 up. The temp fix was to add an auxilary fuel pump and inline filter to increase pressure (later they re-engineered the system). I'm not aware of it being a "common" problem on the 5.0mpi, but when we had one we had this happen a couple of times in very hot weather after a long run and letting it sit for 20-30 minutes. The first time I opened the hatch, dove in the bilge and of course found nothing and it fired right up. After that I just cracked the hatch open on hot days until the heat dissipated.

If none of your other checks turn up any problems I'd almost bet this was the problem. All that cast iron under the hatch can hold a lot of heat....more than enough to vaporize the fuel in the lines if it isn't under a good bit of pressure.
Click to expand...
 
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shoestring

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Re: mercruiser 5.0 mpi hard start when warm

update, the cap on the fuel return line at the fuel rail is factory. the regulator is now at the pump. still has a vacuum line at the rail but not sure how it works, if it even does. my engine # is 0w065640. not sure if the o's are o's or zeros. Don s., I would like to pick your brain on the internal workings of the ecm before i dive into it.
 
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wakeboardbob

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Re: mercruiser 5.0 mpi hard start when warm

shoestring said:
old plugs looked great, replaced them after a seafoam treatment. new plugs are clean, normal color. engine runs great otherwise, seems to be getting fuel from somewhere while it is not running.
Click to expand...




Almost sounds like my 2001 EFI 5.0 It had hydrolock on the fuel after sitting for 30 min, but only in the winter (fuel formulations maybe) Merc did a service bulletin, add an additional fuel pump in line with current and it never happened again. Next time your out and stop open the engine hatch to let the heat out, see if it does it again. Mine was not real consistant but the hydrolock mod fixed it.
 
captain zac

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Re: mercruiser 5.0 mpi hard start when warm

Not an engine guy but

My buddy bought a new 1999 Sea ray 290 twin 5.0's
had same problem or atleast it sounds the same
factory came to the boat and installed 2 electric fuel pumps
boat now has 4 fuel pumps
no problems since
 
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telstar1

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Re: mercruiser 5.0 mpi hard start when warm

On pickups,Ive found a fuel pressure gauge temporarily or perm. hooked up to the fuel rail to be invaluable in figuring this type of stuff out. Tells you at all times whether there is fuel at the injectorsor not. For a temp.,diagnostic hookup,just remove the schraeder valve on fuiel rail(assuning there is one) and clamp a 1/4 inch hose on, run to a pressure gauge.
 
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shoestring

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Re: mercruiser 5.0 mpi hard start when warm

wakeboardbob said:
Almost sounds like my 2001 EFI 5.0 It had hydrolock on the fuel after sitting for 30 min, but only in the winter (fuel formulations maybe) Merc did a service bulletin, add an additional fuel pump in line with current and it never happened again. Next time your out and stop open the engine hatch to let the heat out, see if it does it again. Mine was not real consistant but the hydrolock mod fixed it.
Click to expand...

I'm going to try opening the hatch to see if this solves the problem. if it does, I am going to install a second bilge blower with the inlet near the top of the engine. the way the one is configured now doesn't do much for removing heat. My engine cover is a sun pad and keeping it open is going to attract grief from the girls sun bathing. it is valuable real estate when anchored. I have read the service bulletin and i will add the fuel pump if other things don't help.
 
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